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Template rendering, notifier, and supervisor for @hashicorp Consul and Vault data.

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Consul Template

CircleCI Go Documentation

This project provides a convenient way to populate values from Consul into the file system using the consul-template daemon.

The daemon consul-template queries a Consul or Vault cluster and updates any number of specified templates on the file system. As an added bonus, it can optionally run arbitrary commands when the update process completes. Please see the examples folder for some scenarios where this functionality might prove useful.

The documentation in this README corresponds to the master branch of Consul Template. It may contain unreleased features or different APIs than the most recently released version.

Please see the Git tag that corresponds to your version of Consul Template for the proper documentation.

Table of Contents

Community Support

If you have questions about how consul-template works, its capabilities or anything other than a bug or feature request (use github's issue tracker for those), please see our community support resources.

Community portal: https://discuss.hashicorp.com/c/consul

Other resources: https://www.consul.io/community.html

Additionally, for issues and pull requests, we'll be using the đź‘Ť reactions as a rough voting system to help gauge community priorities. So please add đź‘Ť to any issue or pull request you'd like to see worked on. Thanks.

Installation

  1. Download a pre-compiled, released version from the Consul Template releases page.

  2. Extract the binary using unzip or tar.

  3. Move the binary into $PATH.

To compile from source, please see the instructions in the contributing section.

Quick Example

This short example assumes Consul is installed locally.

  1. Start a Consul cluster in dev mode:

    $ consul agent -dev

  2. Author a template in.tpl to query the kv store:

    {{ key "foo" }}

  3. Start Consul Template:

    $ consul-template -template "in.tpl:out.txt" -once

  4. Write data to the key in Consul:

    $ consul kv put foo bar

  5. Observe Consul Template has written the file out.txt:

    $ cat out.txt
bar

For more examples and use cases, please see the examples folder in this repository.

Usage

For the full list of options:

$ consul-template -h

Command Line Flags

The CLI interface supports all options in the configuration file and vice-versa. Here are a few examples of common integrations on the command line.

Render the template on disk at /tmp/template.ctmpl to /tmp/result:

$ consul-template \
    -template "/tmp/template.ctmpl:/tmp/result"

Render multiple templates in the same process. The optional third argument to the template is a command that will execute each time the template changes.

$ consul-template \
    -template "/tmp/nginx.ctmpl:/var/nginx/nginx.conf:nginx -s reload" \
    -template "/tmp/redis.ctmpl:/var/redis/redis.conf:service redis restart" \
    -template "/tmp/haproxy.ctmpl:/var/haproxy/haproxy.conf"

Render a template using a custom Consul and Vault address:

$ consul-template \
    -consul-addr "10.4.4.6:8500" \
    -vault-addr "https://10.5.32.5:8200"

Render all templates and then spawn and monitor a child process as a supervisor:

$ consul-template \
  -template "/tmp/in.ctmpl:/tmp/result" \
  -exec "/sbin/my-server"

For more information on supervising, please see the Consul Template Exec Mode documentation.

Configuration File Format

Configuration files are written in the HashiCorp Configuration Language. By proxy, this means the configuration is also JSON compatible.

# This denotes the start of the configuration section for Consul. All values
# contained in this section pertain to Consul.
consul {
  # This block specifies the basic authentication information to pass with the
  # request. For more information on authentication, please see the Consul
  # documentation.
  auth {
    enabled  = true
    username = "test"
    password = "test"
  }

  # This is the address of the Consul agent. By default, this is
  # 127.0.0.1:8500, which is the default bind and port for a local Consul
  # agent. It is not recommended that you communicate directly with a Consul
  # server, and instead communicate with the local Consul agent. There are many
  # reasons for this, most importantly the Consul agent is able to multiplex
  # connections to the Consul server and reduce the number of open HTTP
  # connections. Additionally, it provides a "well-known" IP address for which
  # clients can connect.
  address = "127.0.0.1:8500"

  # This is the ACL token to use when connecting to Consul. If you did not
  # enable ACLs on your Consul cluster, you do not need to set this option.
  #
  # This option is also available via the environment variable CONSUL_TOKEN.
  token = "abcd1234"

  # This controls the retry behavior when an error is returned from Consul.
  # Consul Template is highly fault tolerant, meaning it does not exit in the
  # face of failure. Instead, it uses exponential back-off and retry functions
  # to wait for the cluster to become available, as is customary in distributed
  # systems.
  retry {
    # This enabled retries. Retries are enabled by default, so this is
    # redundant.
    enabled = true

    # This specifies the number of attempts to make before giving up. Each
    # attempt adds the exponential backoff sleep time. Setting this to
    # zero will implement an unlimited number of retries.
    attempts = 12

    # This is the base amount of time to sleep between retry attempts. Each
    # retry sleeps for an exponent of 2 longer than this base. For 5 retries,
    # the sleep times would be: 250ms, 500ms, 1s, 2s, then 4s.
    backoff = "250ms"

    # This is the maximum amount of time to sleep between retry attempts.
    # When max_backoff is set to zero, there is no upper limit to the
    # exponential sleep between retry attempts.
    # If max_backoff is set to 10s and backoff is set to 1s, sleep times
    # would be: 1s, 2s, 4s, 8s, 10s, 10s, ...
    max_backoff = "1m"
  }

  # This block configures the SSL options for connecting to the Consul server.
  ssl {
    # This enables SSL. Specifying any option for SSL will also enable it.
    enabled = true

    # This enables SSL peer verification. The default value is "true", which
    # will check the global CA chain to make sure the given certificates are
    # valid. If you are using a self-signed certificate that you have not added
    # to the CA chain, you may want to disable SSL verification. However, please
    # understand this is a potential security vulnerability.
    verify = false

    # This is the path to the certificate to use to authenticate. If just a
    # certificate is provided, it is assumed to contain both the certificate and
    # the key to convert to an X509 certificate. If both the certificate and
    # key are specified, Consul Template will automatically combine them into an
    # X509 certificate for you.
    cert = "/path/to/client/cert"
    key  = "/path/to/client/key"

    # This is the path to the certificate authority to use as a CA. This is
    # useful for self-signed certificates or for organizations using their own
    # internal certificate authority.
    ca_cert = "/path/to/ca"

    # This is the path to a directory of PEM-encoded CA cert files. If both
    # `ca_cert` and `ca_path` is specified, `ca_cert` is preferred.
    ca_path = "path/to/certs/"

    # This sets the SNI server name to use for validation.
    server_name = "my-server.com"
  }
}

# This is the signal to listen for to trigger a reload event. The default
# value is shown below. Setting this value to the empty string will cause CT
# to not listen for any reload signals.
reload_signal = "SIGHUP"

# This is the signal to listen for to trigger a graceful stop. The default
# value is shown below. Setting this value to the empty string will cause CT
# to not listen for any graceful stop signals.
kill_signal = "SIGINT"

# This is the maximum interval to allow "stale" data. By default, only the
# Consul leader will respond to queries; any requests to a follower will
# forward to the leader. In large clusters with many requests, this is not as
# scalable, so this option allows any follower to respond to a query, so long
# as the last-replicated data is within these bounds. Higher values result in
# less cluster load, but are more likely to have outdated data.
max_stale = "10m"

# This is the log level. If you find a bug in Consul Template, please enable
# debug logs so we can help identify the issue. This is also available as a
# command line flag.
log_level = "warn"

# This is the path to store a PID file which will contain the process ID of the
# Consul Template process. This is useful if you plan to send custom signals
# to the process.
pid_file = "/path/to/pid"

# This is the quiescence timers; it defines the minimum and maximum amount of
# time to wait for the cluster to reach a consistent state before rendering a
# template. This is useful to enable in systems that have a lot of flapping,
# because it will reduce the the number of times a template is rendered.
wait {
  min = "5s"
  max = "10s"
}

# This denotes the start of the configuration section for Vault. All values
# contained in this section pertain to Vault.
vault {
  # This is the address of the Vault leader. The protocol (http(s)) portion
  # of the address is required.
  address = "https://vault.service.consul:8200"

  # This is a Vault Enterprise namespace to use for reading/writing secrets.
  #
  # This value can also be specified via the environment variable VAULT_NAMESPACE.
  namespace = "foo"

  # This is the token to use when communicating with the Vault server.
  # Like other tools that integrate with Vault, Consul Template makes the
  # assumption that you provide it with a Vault token; it does not have the
  # incorporated logic to generate tokens via Vault's auth methods.
  #
  # This value can also be specified via the environment variable VAULT_TOKEN.
  # When using a token from Vault Agent, the vault_agent_token_file setting
  # should be used instead, as that will take precedence over this field.
  token = "abcd1234"

  # This tells Consul Template to load the Vault token from the contents of a file.
  # If this field is specified:
  # - by default Consul Template will not try to renew the Vault token, if you want it
  # to renew you will need to specify renew_token = true as below.
  # - Consul Template will periodically stat the file and update the token if it has
  # changed.
  # vault_agent_token_file = "/tmp/vault/agent/token"

  # This tells Consul Template that the provided token is actually a wrapped
  # token that should be unwrapped using Vault's cubbyhole response wrapping
  # before being used. Please see Vault's cubbyhole response wrapping
  # documentation for more information.
  unwrap_token = true

  # This option tells Consul Template to automatically renew the Vault token
  # given. If you are unfamiliar with Vault's architecture, Vault requires
  # tokens be renewed at some regular interval or they will be revoked. Consul
  # Template will automatically renew the token at half the lease duration of
  # the token. The default value is true, but this option can be disabled if
  # you want to renew the Vault token using an out-of-band process.
  #
  # Note that secrets specified in a template (using {{secret}} for example)
  # are always renewed, even if this option is set to false. This option only
  # applies to the top-level Vault token itself.
  renew_token = true

  # This section details the retry options for connecting to Vault. Please see
  # the retry options in the Consul section for more information (they are the
  # same).
  retry {
    # ...
  }

  # This section details the SSL options for connecting to the Vault server.
  # Please see the SSL options in the Consul section for more information (they
  # are the same).
  ssl {
    # ...
  }
}

# This block defines the configuration for connecting to a syslog server for
# logging.
syslog {
  # This enables syslog logging. Specifying any other option also enables
  # syslog logging.
  enabled = true

  # This is the name of the syslog facility to log to.
  facility = "LOCAL5"
}

# This block defines the configuration for de-duplication mode. Please see the
# de-duplication mode documentation later in the README for more information
# on how de-duplication mode operates.
deduplicate {
  # This enables de-duplication mode. Specifying any other options also enables
  # de-duplication mode.
  enabled = true

  # This is the prefix to the path in Consul's KV store where de-duplication
  # templates will be pre-rendered and stored.
  prefix = "consul-template/dedup/"
}

# This block defines the configuration for exec mode. Please see the exec mode
# documentation at the bottom of this README for more information on how exec
# mode operates and the caveats of this mode.
exec {
  # This is the command to exec as a child process. There can be only one
  # command per Consul Template process.
  command = "/usr/bin/app"

  # This is a random splay to wait before killing the command. The default
  # value is 0 (no wait), but large clusters should consider setting a splay
  # value to prevent all child processes from reloading at the same time when
  # data changes occur. When this value is set to non-zero, Consul Template
  # will wait a random period of time up to the splay value before reloading
  # or killing the child process. This can be used to prevent the thundering
  # herd problem on applications that do not gracefully reload.
  splay = "5s"

  env {
    # This specifies if the child process should not inherit the parent
    # process's environment. By default, the child will have full access to the
    # environment variables of the parent. Setting this to true will send only
    # the values specified in `custom_env` to the child process.
    pristine = false

    # This specifies additional custom environment variables in the form shown
    # below to inject into the child's runtime environment. If a custom
    # environment variable shares its name with a system environment variable,
    # the custom environment variable takes precedence. Even if pristine,
    # whitelist, or blacklist is specified, all values in this option
    # are given to the child process.
    custom = ["PATH=$PATH:/etc/myapp/bin"]

    # This specifies a list of environment variables to exclusively include in
    # the list of environment variables exposed to the child process. If
    # specified, only those environment variables matching the given patterns
    # are exposed to the child process. These strings are matched using Go's
    # glob function, so wildcards are permitted.
    whitelist = ["CONSUL_*"]

    # This specifies a list of environment variables to exclusively prohibit in
    # the list of environment variables exposed to the child process. If
    # specified, any environment variables matching the given patterns will not
    # be exposed to the child process, even if they are whitelisted. The values
    # in this option take precedence over the values in the whitelist.
    # These strings are matched using Go's glob function, so wildcards are
    # permitted.
    blacklist = ["VAULT_*"]
  }

  # This defines the signal that will be sent to the child process when a
  # change occurs in a watched template. The signal will only be sent after the
  # process is started, and the process will only be started after all
  # dependent templates have been rendered at least once. The default value is
  # nil, which tells Consul Template to stop the child process and spawn a new
  # one instead of sending it a signal. This is useful for legacy applications
  # or applications that cannot properly reload their configuration without a
  # full reload.
  reload_signal = ""

  # This defines the signal sent to the child process when Consul Template is
  # gracefully shutting down. The application should begin a graceful cleanup.
  # If the application does not terminate before the `kill_timeout`, it will
  # be terminated (effectively "kill -9"). The default value is "SIGINT".
  kill_signal = "SIGINT"

  # This defines the amount of time to wait for the child process to gracefully
  # terminate when Consul Template exits. After this specified time, the child
  # process will be force-killed (effectively "kill -9"). The default value is
  # "30s".
  kill_timeout = "2s"
}

# This block defines the configuration for a template. Unlike other blocks,
# this block may be specified multiple times to configure multiple templates.
# It is also possible to configure templates via the CLI directly.
template {
  # This is the source file on disk to use as the input template. This is often
  # called the "Consul Template template". This option is required if not using
  # the `contents` option.
  source = "/path/on/disk/to/template.ctmpl"

  # This is the destination path on disk where the source template will render.
  # If the parent directories do not exist, Consul Template will attempt to
  # create them, unless create_dest_dirs is false.
  destination = "/path/on/disk/where/template/will/render.txt"

  # This options tells Consul Template to create the parent directories of the
  # destination path if they do not exist. The default value is true.
  create_dest_dirs = true

  # This option allows embedding the contents of a template in the configuration
  # file rather then supplying the `source` path to the template file. This is
  # useful for short templates. This option is mutually exclusive with the
  # `source` option.
  contents = "{{ keyOrDefault \"service/redis/maxconns@east-aws\" \"5\" }}"

  # This is the optional command to run when the template is rendered. The
  # command will only run if the resulting template changes. The command must
  # return within 30s (configurable), and it must have a successful exit code.
  # Consul Template is not a replacement for a process monitor or init system.
  command = "restart service foo"

  # This is the maximum amount of time to wait for the optional command to
  # return. Default is 30s.
  command_timeout = "60s"

  # Exit with an error when accessing a struct or map field/key that does not
  # exist. The default behavior will print "<no value>" when accessing a field
  # that does not exist. It is highly recommended you set this to "true" when
  # retrieving secrets from Vault.
  error_on_missing_key = false

  # This is the permission to render the file. If this option is left
  # unspecified, Consul Template will attempt to match the permissions of the
  # file that already exists at the destination path. If no file exists at that
  # path, the permissions are 0644.
  perms = 0600

  # This option backs up the previously rendered template at the destination
  # path before writing a new one. It keeps exactly one backup. This option is
  # useful for preventing accidental changes to the data without having a
  # rollback strategy.
  backup = true

  # These are the delimiters to use in the template. The default is "{{" and
  # "}}", but for some templates, it may be easier to use a different delimiter
  # that does not conflict with the output file itself.
  left_delimiter  = "{{"
  right_delimiter = "}}"

  # These are functions that are not permitted in the template. If a template
  # includes one of these functions, it will exit with an error.
  function_blacklist = []

  # If a sandbox path is provided, any path provided to the `file` function is
  # checked that it falls within the sandbox path. Relative paths that try to
  # traverse outside the sandbox path will exit with an error.
  sandbox_path = ""

  # This is the `minimum(:maximum)` to wait before rendering a new template to
  # disk and triggering a command, separated by a colon (`:`). If the optional
  # maximum value is omitted, it is assumed to be 4x the required minimum value.
  # This is a numeric time with a unit suffix ("5s"). There is no default value.
  # The wait value for a template takes precedence over any globally-configured
  # wait.
  wait {
    min = "2s"
    max = "10s"
  }
}

Note that not all fields are required. If you are not retrieving secrets from Vault, you do not need to specify a Vault configuration section. Similarly, if you are not logging to syslog, you do not need to specify a syslog configuration.

For additional security, tokens may also be read from the environment using the CONSUL_TOKEN or VAULT_TOKEN environment variables respectively. It is highly recommended that you do not put your tokens in plain-text in a configuration file.

Instruct Consul Template to use a configuration file with the -config flag:

$ consul-template -config "/my/config.hcl"

This argument may be specified multiple times to load multiple configuration files. The right-most configuration takes the highest precedence. If the path to a directory is provided (as opposed to the path to a file), all of the files in the given directory will be merged in lexical order, recursively. Please note that symbolic links are not followed.

Commands specified on the CLI take precedence over a config file!

Templating Language

Consul Template parses files authored in the Go Template format. If you are not familiar with the syntax, please read Go's documentation and examples. In addition to the Go-provided template functions, Consul Template provides the following functions:

API Functions

API functions interact with remote API calls, communicating with external services like Consul and Vault.

caLeaf

Query Consul for the leaf certificate representing a single service.

{{ caLeaf "<NAME>" }}

For example:

{{ with caLeaf "proxy" }}{{ .CertPEM }}{{ end }}

renders

-----BEGIN CERTIFICATE-----
MIICizCCAjGgAwIBAgIBCDAKBggqhkjOPQQDAjAWMRQwEgYDVQQDEwtDb25zdWwg
...
lXcQzfKlIYeFWvcAv4cA4W258gTtqaFRDRJ2i720eQ==
-----END CERTIFICATE-----

The two most useful fields are .CertPEM and .PrivateKeyPEM. For a complete list of available fields, see consul's documentation on LeafCert.

caRoot

Query Consul for all connect trusted certificate authority (CA) root certificates.

{{ caRoots }}

For example:

{{ range caRoots }}{{ .RootCertPEM }}{{ end }}

renders

-----BEGIN CERTIFICATE-----
MIICWDCCAf+gAwIBAgIBBzAKBggqhkjOPQQDAjAWMRQwEgYDVQQDEwtDb25zdWwg
...
bcA+Su3r8qSRppTlc6D0UOYOWc1ykQKQOK7mIg==
-----END CERTIFICATE-----

The most useful field is .RootCertPEM. For a complete list of available fields, see consul's documentation on CARootList.

connect

Query Consul for connect-capable services based on their health.

{{ connect "<TAG>.<NAME>@<DATACENTER>~<NEAR>|<FILTER>" }}

Syntax is exactly the same as for the service function below.

{{ range connect "web" }}
server {{ .Name }} {{ .Address }}:{{ .Port }}{{ end }}

renders the IP addresses of all healthy nodes with a logical connect-capable service named "web":

server web01 10.5.2.45:21000
server web02 10.2.6.61:21000
datacenters

Query Consul for all datacenters in its catalog.

{{ datacenters }}

For example:

{{ range datacenters }}
{{ . }}{{ end }}

renders

dc1
dc2

An optional boolean can be specified which instructs Consul Template to ignore datacenters which are inaccessible or do not have a current leader. Enabling this option requires an O(N+1) operation and therefore is not recommended in environments where performance is a factor.

// Ignores datacenters which are inaccessible
{{ datacenters true }}
file

Read and output the contents of a local file on disk. If the file cannot be read, an error will occur. When the file changes, Consul Template will pick up the change and re-render the template.

{{ file "<PATH>" }}

For example:

{{ file "/path/to/my/file" }}

renders

file contents

This does not process nested templates. See executeTemplate for a way to render nested templates.

key

Query Consul for the value at the given key path. If the key does not exist, Consul Template will block rendering until the key is present. To avoid blocking, use keyOrDefault or keyExists.

{{ key "<PATH>@<DATACENTER>" }}

The <DATACENTER> attribute is optional; if omitted, the local datacenter is used.

For example:

{{ key "service/redis/maxconns" }}

renders

15
keyExists

Query Consul for the value at the given key path. If the key exists, this will return true, false otherwise. Unlike key, this function will not block if the key does not exist. This is useful for controlling flow.

{{ keyExists "<PATH>@<DATACENTER>" }}

The <DATACENTER> attribute is optional; if omitted, the local datacenter is used.

For example:

{{ if keyExists "app/beta_active" }}
  # ...
{{ else }}
  # ...
{{ end }}
keyOrDefault

Query Consul for the value at the given key path. If the key does not exist, the default value will be used instead. Unlike key, this function will not block if the key does not exist.

{{ keyOrDefault "<PATH>@<DATACENTER>" "<DEFAULT>" }}

The <DATACENTER> attribute is optional; if omitted, the local datacenter is used.

For example:

{{ keyOrDefault "service/redis/maxconns" "5" }}

renders

5

Note that Consul Template uses a multi-phase execution. During the first phase of evaluation, Consul Template will have no data from Consul and thus will always fall back to the default value. Subsequent reads from Consul will pull in the real value from Consul (if the key exists) on the next template pass. This is important because it means that Consul Template will never "block" the rendering of a template due to a missing key from a keyOrDefault. Even if the key exists, if Consul has not yet returned data for the key, the default value will be used instead.

ls

Query Consul for all top-level kv pairs at the given key path.

{{ ls "<PATH>@<DATACENTER>" }}

The <DATACENTER> attribute is optional; if omitted, the local datacenter is used.

For example:

{{ range ls "service/redis" }}
{{ .Key }}:{{ .Value }}{{ end }}

renders

maxconns:15
minconns:5
safeLs

Same as ls, but refuse to render template, if the KV prefix query return blank/empty data.

This is especially useful, for rendering mission critical files, that are being populated by consul-template.

For example:

/root/.ssh/authorized_keys
/etc/sysconfig/iptables

Using safeLs on empty prefixes will result in template output not being rendered at all.

To learn how safeLs was born see CT-1131 C-3975 and CR-82.

node

Query Consul for a node in the catalog.

{{node "<NAME>@<DATACENTER>"}}

The <NAME> attribute is optional; if omitted, the local agent node is used.

The <DATACENTER> attribute is optional; if omitted, the local datacenter is used.

For example:

{{ with node }}
{{ .Node.Address }}{{ end }}

renders

10.5.2.6

To query a different node:

{{ with node "node1@dc2" }}
{{ .Node.Address }}{{ end }}

renders

10.4.2.6

To access map data such as TaggedAddresses or Meta, use Go's text/template map indexing.

nodes

Query Consul for all nodes in the catalog.

{{ nodes "@<DATACENTER>~<NEAR>" }}

The <DATACENTER> attribute is optional; if omitted, the local datacenter is used.

The <NEAR> attribute is optional; if omitted, results are specified in lexical order. If provided a node name, results are ordered by shortest round-trip time to the provided node. If provided _agent, results are ordered by shortest round-trip time to the local agent.

For example:

{{ range nodes }}
{{ .Address }}{{ end }}

renders

10.4.2.13
10.46.2.5

To query a different data center and order by shortest trip time to ourselves:

{{ range nodes "@dc2~_agent" }}
{{ .Address }}{{ end }}

To access map data such as TaggedAddresses or Meta, use Go's text/template map indexing.

secret

Query Vault for the secret at the given path.

{{ secret "<PATH>" "<DATA>" }}

The <DATA> attribute is optional; if omitted, the request will be a vault read (HTTP GET) request. If provided, the request will be a vault write (HTTP PUT/POST) request.

For example:

{{ with secret "secret/passwords" }}
{{ .Data.wifi }}{{ end }}

renders

FORWARDSoneword

To access a versioned secret value (for the K/V version 2 backend):

{{ with secret "secret/passwords?version=1" }}
{{ .Data.data.wifi }}{{ end }}

When omitting the ?version parameter, the latest version of the secret will be fetched. Note the nested .Data.data syntax when referencing the secret value. For more information about using the K/V v2 backend, see the Vault Documentation.

When using Vault versions 0.10.0/0.10.1, the secret path will have to be prefixed with "data", i.e. secret/data/passwords for the example above. This is not necessary for Vault versions after 0.10.1, as consul-template will detect the KV backend version being used. The version 2 KV backend did not exist prior to 0.10.0, so these are the only affected versions.

An example using write to generate PKI certificates:

{{ with secret "pki/issue/my-domain-dot-com" "common_name=foo.example.com" }}
{{ .Data.certificate }}{{ end }}

The parameters must be key=value pairs, and each pair must be its own argument to the function:

Please always consider the security implications of having the contents of a secret in plain-text on disk. If an attacker is able to get access to the file, they will have access to plain-text secrets.

Please note that Vault does not support blocking queries. As a result, Consul Template will not immediately reload in the event a secret is changed as it does with Consul's key-value store. Consul Template will renew the secret with Vault's Renewer API. The Renew API tries to use most of the time the secret is good, renewing at around 90% of the lease time (as set by Vault).

Also consider enabling error_on_missing_key when working with templates that will interact with Vault. By default, Consul Template uses Go's templating language. When accessing a struct field or map key that does not exist, it defaults to printing <no value>. This may not be the desired behavior, especially when working with passwords or other data. As such, it is recommended you set:

template {
  error_on_missing_key = true
}

You can also guard against empty values using if or with blocks.

{{ with secret "secret/foo"}}
{{ if .Data.password }}
password = "{{ .Data.password }}"
{{ end }}
{{ end }}
secrets

Query Vault for the list of secrets at the given path. Not all endpoints support listing.

{{ secrets "<PATH>" }}

For example:

{{ range secrets "secret/" }}
{{ . }}{{ end }}

renders

bar
foo
zip

To iterate and list over every secret in the generic secret backend in Vault:

{{ range secrets "secret/" }}
{{ with secret (printf "secret/%s" .) }}{{ range $k, $v := .Data }}
{{ $k }}: {{ $v }}
{{ end }}{{ end }}{{ end }}

You should probably never do this.

Please also note that Vault does not support blocking queries. To understand the implications, please read the note at the end of the secret function.

service

Query Consul for services based on their health.

{{ service "<TAG>.<NAME>@<DATACENTER>~<NEAR>|<FILTER>" }}

The <TAG> attribute is optional; if omitted, all nodes will be queried.

The <DATACENTER> attribute is optional; if omitted, the local datacenter is used.

The <NEAR> attribute is optional; if omitted, results are specified in lexical order. If provided a node name, results are ordered by shortest round-trip time to the provided node. If provided _agent, results are ordered by shortest round-trip time to the local agent.

The <FILTER> attribute is optional; if omitted, only health services are returned. Providing a filter allows for client-side filtering of services.

For example:

The example above is querying Consul for healthy "web" services, in the "east-aws" data center. The tag and data center attributes are optional. To query all nodes of the "web" service (regardless of tag) for the current data center:

{{ range service "web" }}
server {{ .Name }} {{ .Address }}:{{ .Port }}{{ end }}

renders the IP addresses of all healthy nodes with a logical service named "web":

server web01 10.5.2.45:2492
server web02 10.2.6.61:2904

To access map data such as NodeTaggedAddresses or NodeMeta, use Go's text/template map indexing.

By default only healthy services are returned. To list all services, pass the "any" filter:

{{ service "web|any" }}

This will return all services registered to the agent, regardless of their status.

To filter services by a specific set of healths, specify a comma-separated list of health statuses:

{{ service "web|passing,warning" }}

This will returns services which are deemed "passing" or "warning" according to their node and service-level checks defined in Consul. Please note that the comma implies an "or", not an "and".

Note: Due to the use of dot . to delimit TAG, the service command will not recognize service names containing dots.

Note: There is an architectural difference between the following:

{{ service "web" }}
{{ service "web|passing" }}

The former will return all services which Consul considers "healthy" and passing. The latter will return all services registered with the Consul agent and perform client-side filtering. As a general rule, do not use the "passing" argument alone if you want only healthy services - simply omit the second argument instead.

services

Query Consul for all services in the catalog.

{{ services "@<DATACENTER>" }}

The <DATACENTER> attribute is optional; if omitted, the local datacenter is used.

For example:

{{ range services }}
{{ .Name }}: {{ .Tags | join "," }}{{ end }}

renders

node01 tag1,tag2,tag3
tree

Query Consul for all kv pairs at the given key path.

{{ tree "<PATH>@<DATACENTER>" }}

The <DATACENTER> attribute is optional; if omitted, the local datacenter is used.

For example:

{{ range tree "service/redis" }}
{{ .Key }}:{{ .Value }}{{ end }}

renders

minconns 2
maxconns 12
nested/config/value "value"

Unlike ls, tree returns all keys under the prefix, just like the Unix tree command.

safeTree

Same as tree, but refuse to render template, if the KV prefix query return blank/empty data.

This is especially useful, for rendering mission critical files, that are being populated by consul-template.

For example:

/root/.ssh/authorized_keys
/etc/sysconfig/iptables

Using safeTree on empty prefixes will result in template output not being rendered at all.

To learn how safeTree was born see CT-1131 C-3975 and CR-82.

Scratch

The scratchpad (or "scratch" for short) is available within the context of a template to store temporary data or computations. Scratch data is not shared between templates and is not cached between invocations.

scratch.Key

Returns a boolean if data exists in the scratchpad at the named key. Even if the data at that key is "nil", this still returns true.

{{ scratch.Key "foo" }}
scratch.Get

Returns the value in the scratchpad at the named key. If the data does not exist, this will return "nil".

{{ scratch.Get "foo" }}
scratch.Set

Saves the given value at the given key. If data already exists at that key, it is overwritten.

{{ scratch.Set "foo" "bar" }}
scratch.SetX

This behaves exactly the same as Set, but does not overwrite if the value already exists.

{{ scratch.SetX "foo" "bar" }}
scratch.MapSet

Saves a value in a named key in the map. If data already exists at that key, it is overwritten.

{{ scratch.MapSet "vars" "foo" "bar" }}
scratch.MapSetX

This behaves exactly the same as MapSet, but does not overwrite if the value already exists.

{{ scratch.MapSetX "vars" "foo" "bar" }}
scratch.MapValues

Returns a sorted list (by key) of all values in the named map.

{{ scratch.MapValues "vars" }}

Helper Functions

Unlike API functions, helper functions do not query remote services. These functions are useful for parsing data, formatting data, performing math, etc.

base64Decode

Accepts a base64-encoded string and returns the decoded result, or an error if the given string is not a valid base64 string.

{{ base64Decode "aGVsbG8=" }}

renders

hello
base64Encode

Accepts a string and returns a base64-encoded string.

{{ base64Encode "hello" }}

renders

aGVsbG8=
base64URLDecode

Accepts a base64-encoded URL-safe string and returns the decoded result, or an error if the given string is not a valid base64 URL-safe string.

{{ base64URLDecode "aGVsbG8=" }}

renders

hello
base64URLEncode

Accepts a string and returns a base-64 encoded URL-safe string.

{{ base64Encode "hello" }}

renders

aGVsbG8=
byKey

Accepts a list of pairs returned from a tree call and creates a map that groups pairs by their top-level directory.

For example:

groups/elasticsearch/es1
groups/elasticsearch/es2
groups/elasticsearch/es3
services/elasticsearch/check_elasticsearch
services/elasticsearch/check_indexes

with the following template

{{ range $key, $pairs := tree "groups" | byKey }}{{ $key }}:
{{ range $pair := $pairs }}  {{ .Key }}={{ .Value }}
{{ end }}{{ end }}

renders

elasticsearch:
  es1=1
  es2=1
  es3=1

Note that the top-most key is stripped from the Key value. Keys that have no prefix after stripping are removed from the list.

The resulting pairs are keyed as a map, so it is possible to look up a single value by key:

{{ $weights := tree "weights" }}
{{ range service "release.web" }}
  {{ $weight := or (index $weights .Node) 100 }}
  server {{ .Node }} {{ .Address }}:{{ .Port }} weight {{ $weight }}{{ end }}
byTag

Takes the list of services returned by the service or services function and creates a map that groups services by tag.

{{ range $tag, $services := service "web" | byTag }}{{ $tag }}
{{ range $services }} server {{ .Name }} {{ .Address }}:{{ .Port }}
{{ end }}{{ end }}
byMeta

Takes a list of services returned by the service or services and returns a map that groups services by ServiceMeta values. Multiple service meta keys can be passed as a comma separated string. |int can be added to a meta key to convert numbers from service meta values to padded numbers in printf "%05d" % value format (useful for sorting as Go Template sorts maps by keys).

Example:

If we have the following services registered in Consul:

{
  "Services": [
     {
       "ID": "redis-dev-1",
       "Name": "redis",
       "ServiceMeta": {
         "environment": "dev",
         "shard_number": "1"
       },
       ...
     },
     {
       "ID": "redis-prod-1",
       "Name": "redis",
       "ServiceMeta": {
         "environment": "prod",
         "shard_number": "1"
       },
       ...
     },
     {
       "ID": "redis-prod-2",
       "Name": "redis",
       "ServiceMeta": {
         "environment": "prod",
         "shard_number": "2",
       },
       ...
     }
   ]
}
{{ service "redis|any" | byMeta "environment,shard_number|int" | toJSON }}

The code above will produce a map of services grouped by meta:

{
  "dev_00001": [
    {
      "ID": "redis-dev-1",
      ...
    }
  ],
  "prod_00001": [
    {
      "ID": "redis-prod-1",
      ...
    }
  ],
  "prod_00002": [
    {
      "ID": "redis-prod-2",
      ...
    }
  ]
}
contains

Determines if a needle is within an iterable element.

{{ if .Tags | contains "production" }}
# ...
{{ end }}
containsAll

Returns true if all needles are within an iterable element, or false otherwise. Returns true if the list of needles is empty.

{{ if containsAll $requiredTags .Tags }}
# ...
{{ end }}
containsAny

Returns true if any needle is within an iterable element, or false otherwise. Returns false if the list of needles is empty.

{{ if containsAny $acceptableTags .Tags }}
# ...
{{ end }}
containsNone

Returns true if no needles are within an iterable element, or false otherwise. Returns true if the list of needles is empty.

{{ if containsNone $forbiddenTags .Tags }}
# ...
{{ end }}
containsNotAll

Returns true if some needle is not within an iterable element, or false otherwise. Returns false if the list of needles is empty.

{{ if containsNotAll $excludingTags .Tags }}
# ...
{{ end }}
env

Reads the given environment variable accessible to the current process.

{{ env "CLUSTER_ID" }}

This function can be chained to manipulate the output:

{{ env "CLUSTER_ID" | toLower }}

Reads the given environment variable and if it does not exist or is blank use a default value, ex 12345.

{{ or (env "CLUSTER_ID") "12345" }}
executeTemplate

Executes and returns a defined template.

{{ define "custom" }}my custom template{{ end }}

This is my other template:
{{ executeTemplate "custom" }}

And I can call it multiple times:
{{ executeTemplate "custom" }}

Even with a new context:
{{ executeTemplate "custom" 42 }}

Or save it to a variable:
{{ $var := executeTemplate "custom" }}
explode

Takes the result from a tree or ls call and converts it into a deeply-nested map for parsing/traversing.

{{ tree "config" | explode }}

Note: You will lose any metadata about the key-pair after it has been exploded. You can also access deeply nested values:

{{ with tree "config" | explode }}
{{ .a.b.c }}{{ end }}

You will need to have a reasonable format about your data in Consul. Please see Go's text/template package for more information.

explodeMap

Takes the value of a map and converts it into a deeply-nested map for parsing/traversing, using the same logic as explode.

{{ scratch.MapSet "example", "foo/bar", "a" }}
{{ scratch.MapSet "example", "foo/baz", "b" }}
{{ scratch.Get "example" | explodeMap | toYAML }}
indent

Indents a block of text by prefixing N number of spaces per line.

{{ tree "foo" | explode | toYAML | indent 4 }}
in

Determines if a needle is within an iterable element.

{{ if in .Tags "production" }}
# ...
{{ end }}
loop

Accepts varying parameters and differs its behavior based on those parameters.

If loop is given one integer, it will return a goroutine that begins at zero and loops up to but not including the given integer:

{{ range loop 5 }}
# Comment{{end}}

If given two integers, this function will return a goroutine that begins at the first integer and loops up to but not including the second integer:

{{ range $i := loop 5 8 }}
stanza-{{ $i }}{{ end }}

which would render:

stanza-5
stanza-6
stanza-7

Note: It is not possible to get the index and the element since the function returns a goroutine, not a slice. In other words, the following is not valid:

# Will NOT work!
{{ range $i, $e := loop 5 8 }}
# ...{{ end }}
join

Takes the given list of strings as a pipe and joins them on the provided string:

{{ $items | join "," }}
trimSpace

Takes the provided input and trims all whitespace, tabs and newlines:

{{ file "/etc/ec2_version" | trimSpace }}
parseBool

Takes the given string and parses it as a boolean:

{{ "true" | parseBool }}

This can be combined with a key and a conditional check, for example:

{{ if key "feature/enabled" | parseBool }}{{ end }}
parseFloat

Takes the given string and parses it as a base-10 float64:

{{ "1.2" | parseFloat }}
parseInt

Takes the given string and parses it as a base-10 int64:

{{ "1" | parseInt }}

This can be combined with other helpers, for example:

{{ range $i := loop key "config/pool_size" | parseInt }}
# ...{{ end }}
parseJSON

Takes the given input (usually the value from a key) and parses the result as JSON:

{{ with $d := key "user/info" | parseJSON }}{{ $d.name }}{{ end }}

Note: Consul Template evaluates the template multiple times, and on the first evaluation the value of the key will be empty (because no data has been loaded yet). This means that templates must guard against empty responses.

parseUint

Takes the given string and parses it as a base-10 int64:

{{ "1" | parseUint }}
plugin

Takes the name of a plugin and optional payload and executes a Consul Template plugin.

{{ plugin "my-plugin" }}

The plugin can take an arbitrary number of string arguments, and can be the target of a pipeline that produces strings as well. This is most commonly combined with a JSON filter for customization:

{{ tree "foo" | explode | toJSON | plugin "my-plugin" }}

Please see the plugins section for more information about plugins.

regexMatch

Takes the argument as a regular expression and will return true if it matches on the given string, or false otherwise.

{{ if "foo.bar" | regexMatch "foo([.a-z]+)" }}
# ...
{{ else }}
# ...
{{ end }}
regexReplaceAll

Takes the argument as a regular expression and replaces all occurrences of the regex with the given string. As in go, you can use variables like $1 to refer to subexpressions in the replacement string.

{{ "foo.bar" | regexReplaceAll "foo([.a-z]+)" "$1" }}
replaceAll

Takes the argument as a string and replaces all occurrences of the given string with the given string.

{{ "foo.bar" | replaceAll "." "_" }}

This function can be chained with other functions as well:

{{ service "web" }}{{ .Name | replaceAll ":" "_" }}{{ end }}
split

Splits the given string on the provided separator:

{{ "foo\nbar\n" | split "\n" }}

This can be combined with chained and piped with other functions:

{{ key "foo" | toUpper | split "\n" | join "," }}
timestamp

Returns the current timestamp as a string (UTC). If no arguments are given, the result is the current RFC3339 timestamp:

{{ timestamp }} // e.g. 1970-01-01T00:00:00Z

If the optional parameter is given, it is used to format the timestamp. The magic reference date Mon Jan 2 15:04:05 -0700 MST 2006 can be used to format the date as required:

{{ timestamp "2006-01-02" }} // e.g. 1970-01-01

See Go's time.Format for more information.

As a special case, if the optional parameter is "unix", the unix timestamp in seconds is returned as a string.

{{ timestamp "unix" }} // e.g. 0
toJSON

Takes the result from a tree or ls call and converts it into a JSON object.

{{ tree "config" | explode | toJSON }}

renders

{"admin":{"port":"1234"},"maxconns":"5","minconns":"2"}

Note: Consul stores all KV data as strings. Thus true is "true", 1 is "1", etc.

toJSONPretty

Takes the result from a tree or ls call and converts it into a pretty-printed JSON object, indented by two spaces.

{{ tree "config" | explode | toJSONPretty }}

renders

{
  "admin": {
    "port": "1234"
  },
  "maxconns": "5",
  "minconns": "2",
}

Note: Consul stores all KV data as strings. Thus true is "true", 1 is "1", etc.

toLower

Takes the argument as a string and converts it to lowercase.

{{ key "user/name" | toLower }}

See Go's strings.ToLower for more information.

toTitle

Takes the argument as a string and converts it to titlecase.

{{ key "user/name" | toTitle }}

See Go's strings.Title for more information.

toTOML

Takes the result from a tree or ls call and converts it into a TOML object.

{{ tree "config" | explode | toTOML }}

renders

maxconns = "5"
minconns = "2"

[admin]
  port = "1134"

Note: Consul stores all KV data as strings. Thus true is "true", 1 is "1", etc.

toUpper

Takes the argument as a string and converts it to uppercase.

{{ key "user/name" | toUpper }}

See Go's strings.ToUpper for more information.

toYAML

Takes the result from a tree or ls call and converts it into a pretty-printed YAML object, indented by two spaces.

{{ tree "config" | explode | toYAML }}

renders

admin:
  port: "1234"
maxconns: "5"
minconns: "2"

Note: Consul stores all KV data as strings. Thus true is "true", 1 is "1", etc.

sockaddr

Takes a quote-escaped template string as an argument and passes it on to hashicorp/go-sockaddr templating engine.

{{ sockaddr "GetPrivateIP" }}

See hashicorp/go-sockaddr documentation for more information.

Math Functions

The following functions are available on floats and integer values.

add

Returns the sum of the two values.

{{ add 1 2 }} // 3

This can also be used with a pipe function.

{{ 1 | add 2 }} // 3
subtract

Returns the difference of the second value from the first.

{{ subtract 2 5 }} // 3

This can also be used with a pipe function.

{{ 5 | subtract 2 }} // 3

Please take careful note of the order of arguments.

multiply

Returns the product of the two values.

{{ multiply 2 2 }} // 4

This can also be used with a pipe function.

{{ 2 | multiply 2 }} // 4
divide

Returns the division of the second value from the first.

{{ divide 2 10 }} // 5

This can also be used with a pipe function.

{{ 10 | divide 2 }} // 5

Please take careful note of the order or arguments.

modulo

Returns the modulo of the second value from the first.

{{ modulo 2 5 }} // 1

This can also be used with a pipe function.

{{ 5 | modulo 2 }} // 1

Please take careful note of the order of arguments.

Plugins

Authoring Plugins

For some use cases, it may be necessary to write a plugin that offloads work to another system. This is especially useful for things that may not fit in the "standard library" of Consul Template, but still need to be shared across multiple instances.

Consul Template plugins must have the following API:

$ NAME [INPUT...]

  • NAME - the name of the plugin - this is also the name of the binary, either a full path or just the program name. It will be executed in a shell with the inherited PATH so e.g. the plugin cat will run the first executable cat that is found on the PATH.

  • INPUT - input from the template. There will be one INPUT for every argument passed to the plugin function. If the arguments contain whitespace, that whitespace will be passed as if the argument were quoted by the shell.

Important Notes

  • Plugins execute user-provided scripts and pass in potentially sensitive data from Consul or Vault. Nothing is validated or protected by Consul Template, so all necessary precautions and considerations should be made by template authors

  • Plugin output must be returned as a string on stdout. Only stdout will be parsed for output. Be sure to log all errors, debugging messages onto stderr to avoid errors when Consul Template returns the value. Note that output to stderr will only be output if the plugin returns a non-zero exit code.

  • Always exit 0 or Consul Template will assume the plugin failed to execute

  • Ensure the empty input case is handled correctly (see Multi-phase execution)

  • Data piped into the plugin is appended after any parameters given explicitly (eg {{ "sample-data" | plugin "my-plugin" "some-parameter"}} will call my-plugin some-parameter sample-data)

Here is a sample plugin in a few different languages that removes any JSON keys that start with an underscore and returns the JSON string:

#! /usr/bin/env ruby
require "json"

if ARGV.empty?
  puts JSON.fast_generate({})
  Kernel.exit(0)
end

hash = JSON.parse(ARGV.first)
hash.reject! { |k, _| k.start_with?("_")  }
puts JSON.fast_generate(hash)
Kernel.exit(0)
func main() {
  arg := []byte(os.Args[1])

  var parsed map[string]interface{}
  if err := json.Unmarshal(arg, &parsed); err != nil {
    fmt.Fprintln(os.Stderr, fmt.Sprintf("err: %s", err))
    os.Exit(1)
  }

  for k, _ := range parsed {
    if string(k[0]) == "_" {
      delete(parsed, k)
    }
  }

  result, err := json.Marshal(parsed)
  if err != nil {
    fmt.Fprintln(os.Stderr, fmt.Sprintf("err: %s", err))
    os.Exit(1)
  }

  fmt.Fprintln(os.Stdout, fmt.Sprintf("%s", result))
  os.Exit(0)
}

Caveats

Docker Image Use

The Alpine Docker image is configured to support an external volume to render shared templates to. If mounted you will need to make sure that the consul-template user in the docker image has write permissions to the directory. Also if you build your own image using these you need to be sure you have the permissions correct.

The consul-template user in docker has a UID of 100 and a GID of 1000.

This effects the in image directories /consul-template/config, used to add configuration when using this as a parent image, and /consul-template/data, exported as a VOLUME as a location to render shared results.

Previously the image initially ran as root in order to ensure the permissions allowed it. But this ran against docker best practices and security policies.

If you build your own image based on ours you can override these values with --build-arg parameters.

Dots in Service Names

Using dots . in service names will conflict with the use of dots for TAG delineation in the template. Dots already interfere with using DNS for service names, so we recommend avoiding dots wherever possible.

Once Mode

In Once mode, Consul Template will wait for all dependencies to be rendered. If a template specifies a dependency (a request) that does not exist in Consul, once mode will wait until Consul returns data for that dependency. Please note that "returned data" and "empty data" are not mutually exclusive.

When you query for all healthy services named "foo" ({{ service "foo" }}), you are asking Consul - "give me all the healthy services named foo". If there are no services named foo, the response is the empty array. This is also the same response if there are no healthy services named foo.

Consul template processes input templates multiple times, since the first result could impact later dependencies:

{{ range services }}
{{ range service .Name }}
{{ end }}
{{ end }}

In this example, we have to process the output of services before we can lookup each service, since the inner loops cannot be evaluated until the outer loop returns a response. Consul Template waits until it gets a response from Consul for all dependencies before rendering a template. It does not wait until that response is non-empty though.

Note: Once mode implicitly disables any wait/quiescence timers specified in configuration files or passed on the command line.

Exec Mode

As of version 0.16.0, Consul Template has the ability to maintain an arbitrary child process (similar to envconsul). This mode is most beneficial when running Consul Template in a container or on a scheduler like Nomad or Kubernetes. When activated, Consul Template will spawn and manage the lifecycle of the child process.

This mode is best-explained through example. Consider a simple application that reads a configuration file from disk and spawns a server from that configuration.

$ consul-template \
    -template "/tmp/config.ctmpl:/tmp/server.conf" \
    -exec "/bin/my-server -config /tmp/server.conf"

When Consul Template starts, it will pull the required dependencies and populate the /tmp/server.conf, which the my-server binary consumes. After that template is rendered completely the first time, Consul Template spawns and manages a child process. When any of the list templates change, Consul Template will send a configurable reload signal to the child process. Additionally, Consul Template will proxy any signals it receives to the child process. This enables a scheduler to control the lifecycle of the process and also eases the friction of running inside a container.

A common point of confusion is that the command string behaves the same as the shell; it does not. In the shell, when you run foo | bar or foo > bar, that is actually running as a subprocess of your shell (bash, zsh, csh, etc.). When Consul Template spawns the exec process, it runs outside of your shell. This behavior is different from when Consul Template executes the template-specific reload command. If you want the ability to pipe or redirect in the exec command, you will need to spawn the process in subshell, for example:

exec {
  command = "/bin/bash -c 'my-server > /var/log/my-server.log'"
}

Note that when spawning like this, most shells do not proxy signals to their child by default, so your child process will not receive the signals that Consul Template sends to the shell. You can avoid this by writing a tiny shell wrapper and executing that instead:

#!/usr/bin/env bash
trap "kill -TERM $child" SIGTERM

/bin/my-server -config /tmp/server.conf
child=$!
wait "$child"

Alternatively, you can use your shell's exec function directly, if it exists:

#!/usr/bin/env bash
exec /bin/my-server -config /tmp/server.conf > /var/log/my-server.log

There are some additional caveats with Exec Mode, which should be considered carefully before use:

  • If the child process dies, the Consul Template process will also die. Consul Template does not supervise the process! This is generally the responsibility of the scheduler or init system.

  • The child process must remain in the foreground. This is a requirement for Consul Template to manage the process and send signals.

  • The exec command will only start after all templates have been rendered at least once. One may have multiple templates for a single Consul Template process, all of which must be rendered before the process starts. Consider something like an nginx or apache configuration where both the process configuration file and individual site configuration must be written in order for the service to successfully start.

  • After the child process is started, any change to any dependent template will cause the reload signal to be sent to the child process. If no reload signal is provided, Consul Template will kill the process and spawn a new instance. The reload signal can be specified and customized via the CLI or configuration file.

  • When Consul Template is stopped gracefully, it will send the configurable kill signal to the child process. The default value is SIGTERM, but it can be customized via the CLI or configuration file.

  • Consul Template will forward all signals it receives to the child process except its defined reload_signal and kill_signal. If you disable these signals, Consul Template will forward them to the child process.

  • It is not possible to have more than one exec command (although each template can still have its own reload command).

  • Individual template reload commands still fire independently of the exec command.

De-Duplication Mode

Consul Template works by parsing templates to determine what data is needed and then watching Consul for any changes to that data. This allows Consul Template to efficiently re-render templates when a change occurs. However, if there are many instances of Consul Template rendering a common template there is a linear duplication of work as each instance is querying the same data.

To make this pattern more efficient Consul Template supports work de-duplication across instances. This can be enabled with the -dedup flag or via the deduplicate configuration block. Once enabled, Consul Template uses leader election on a per-template basis to have only a single node perform the queries. Results are shared among other instances rendering the same template by passing compressed data through the Consul K/V store.

Please note that no Vault data will be stored in the compressed template. Because ACLs around Vault are typically more closely controlled than those ACLs around Consul's KV, Consul Template will still request the secret from Vault on each iteration.

When running in de-duplication mode, it is important that local template functions resolve correctly. For example, you may have a local template function that relies on the env helper like this:

{{ key (env "KEY") }}

It is crucial that the environment variable KEY in this example is consistent across all machines engaged in de-duplicating this template. If the values are different, Consul Template will be unable to resolve the template, and you will not get a successful render.

Termination on Error

By default Consul Template is highly fault-tolerant. If Consul is unreachable or a template changes, Consul Template will happily continue running. The only exception to this rule is if the optional command exits non-zero. In this case, Consul Template will also exit non-zero. The reason for this decision is so the user can easily configure something like Upstart or God to manage Consul Template as a service.

If you want Consul Template to continue watching for changes, even if the optional command argument fails, you can append || true to your command. Note that || is a "shell-ism", not a built-in function. You will also need to run your command under a shell:

$ consul-template \
  -template "in.ctmpl:out.file:/bin/bash -c 'service nginx restart || true'"

In this example, even if the Nginx restart command returns non-zero, the overall function will still return an OK exit code; Consul Template will continue to run as a service. Additionally, if you have complex logic for restarting your service, you can intelligently choose when you want Consul Template to exit and when you want it to continue to watch for changes. For these types of complex scripts, we recommend using a custom sh or bash script instead of putting the logic directly in the consul-template command or configuration file.

Command Environment

The current processes environment is used when executing commands with the following additional environment variables:

  • CONSUL_HTTP_ADDR
  • CONSUL_HTTP_TOKEN
  • CONSUL_HTTP_AUTH
  • CONSUL_HTTP_SSL
  • CONSUL_HTTP_SSL_VERIFY

These environment variables are exported with their current values when the command executes. Other Consul tooling reads these environment variables, providing smooth integration with other Consul tools (like consul maint or consul lock). Additionally, exposing these environment variables gives power users the ability to further customize their command script.

Multi-phase Execution

Consul Template does an n-pass evaluation of templates, accumulating dependencies on each pass. This is required due to nested dependencies, such as:

{{ range services }}
{{ range service .Name }}
  {{ .Address }}
{{ end }}{{ end }}

During the first pass, Consul Template does not know any of the services in Consul, so it has to perform a query. When those results are returned, the inner-loop is then evaluated with that result, potentially creating more queries and watches.

Because of this implementation, template functions need a default value that is an acceptable parameter to a range function (or similar), but does not actually execute the inner loop (which would cause a panic). This is important to mention because complex templates must account for the "empty" case. For example, the following will not work:

{{ with index (service "foo") 0 }}
# ...
{{ end }}

This will raise an error like:

<index $services 0>: error calling index: index out of range: 0

That is because, during the first evaluation of the template, the service key is returning an empty slice. You can account for this in your template like so:

{{ with service "foo" }}
{{ with index . 0 }}
{{ .Node }}{{ end }}{{ end }}

This will still add the dependency to the list of watches, but will not evaluate the inner-if, avoiding the out-of-index error.

Running and Process Lifecycle

While there are multiple ways to run Consul Template, the most common pattern is to run Consul Template as a system service. When Consul Template first starts, it reads any configuration files and templates from disk and loads them into memory. From that point forward, changes to the files on disk do not propagate to running process without a reload.

The reason for this behavior is simple and aligns with other tools like haproxy. A user may want to perform pre-flight validation checks on the configuration or templates before loading them into the process. Additionally, a user may want to update configuration and templates simultaneously. Having Consul Template automatically watch and reload those files on changes is both operationally dangerous and against some of the paradigms of modern infrastructure. Instead, Consul Template listens for the SIGHUP syscall to trigger a configuration reload. If you update configuration or templates, simply send HUP to the running Consul Template process and Consul Template will reload all the configurations and templates from disk.

Debugging

Consul Template can print verbose debugging output. To set the log level for Consul Template, use the -log-level flag:

$ consul-template -log-level info ...
<timestamp> [INFO] (cli) received redis from Watcher
<timestamp> [INFO] (cli) invoking Runner
# ...

You can also specify the level as debug:

$ consul-template -log-level debug ...
<timestamp> [DEBUG] (cli) creating Runner
<timestamp> [DEBUG] (cli) creating Consul API client
<timestamp> [DEBUG] (cli) creating Watcher
<timestamp> [DEBUG] (cli) looping for data
<timestamp> [DEBUG] (watcher) starting watch
<timestamp> [DEBUG] (watcher) all pollers have started, waiting for finish
<timestamp> [DEBUG] (redis) starting poll
<timestamp> [DEBUG] (service redis) querying Consul with &{...}
<timestamp> [DEBUG] (service redis) Consul returned 2 services
<timestamp> [DEBUG] (redis) writing data to channel
<timestamp> [DEBUG] (redis) starting poll
<timestamp> [INFO] (cli) received redis from Watcher
<timestamp> [INFO] (cli) invoking Runner
<timestamp> [DEBUG] (service redis) querying Consul with &{...}
# ...

FAQ

Q: How is this different than confd?
A: The answer is simple: Service Discovery as a first class citizen. You are also encouraged to read this Pull Request on the project for more background information. We think confd is a great project, but Consul Template fills a missing gap. Additionally, Consul Template has first class integration with Vault, making it easy to incorporate secret material like database credentials or API tokens into configuration files.

Q: How is this different than Puppet/Chef/Ansible/Salt?
A: Configuration management tools are designed to be used in unison with Consul Template. Instead of rendering a stale configuration file, use your configuration management software to render a dynamic template that will be populated by Consul.

Contributing

To build and install Consul-Template locally, you will need to install Go.

Clone the repository:

$ git clone https://github.com/hashicorp/consul-template.git

To compile the consul-template binary for your local machine:

$ make dev

This will compile the consul-template binary into bin/consul-template as well as your $GOPATH and run the test suite.

If you want to compile a specific binary, set XC_OS and XC_ARCH or run the following to generate all binaries:

$ make build

If you want to run the tests, first install consul and vault locally, then:

$ make test

Or to run a specific test in the suite:

go test ./... -run SomeTestFunction_name

About

Template rendering, notifier, and supervisor for @hashicorp Consul and Vault data.

www.hashicorp.com/

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