Tern - The SQL Fan's Migrator

Tern is a standalone migration tool for PostgreSQL. It includes traditional migrations as well as a separate optional workflow for managing database code such as functions and views.

Features

Multi-platform
Stand-alone binary
SSH tunnel support built-in
Data variable interpolation into migrations

Installation

Go versions up to and including 1.17:

go get -u github.com/jackc/tern

Go versions 1.17 and higher:

go install github.com/jackc/tern@latest

Creating a Tern Project

To create a new tern project in the current directory run:

tern init

Or to create the project somewhere else:

tern init path/to/project

Tern projects are composed of a directory of migrations and optionally a config file. See the sample directory for an example.

Configuration

Database connection settings can be specified via the standard PostgreSQL environment variables, via program arguments, or in a config file. By default tern will look in the current directory for the config file tern.conf and the migrations.

The tern.conf file is stored in the ini format with two sections, database and data. The database section contains settings for connection to the database server.

Values in the data section will be available for interpolation into migrations. This can help in scenarios where migrations are managing permissions and the user to which permissions are granted should be configurable.

If all database settings are supplied by PG* environment variables or program arguments the config file is not required. In particular, using the PGSERVICE can reduce or eliminate the need for a configuration file.

The entire tern.conf file is processed through the Go standard text/template package. Sprig functions are available.

Example tern.conf:

[database]
# host supports TCP addresses and Unix domain sockets
# host = /private/tmp
host = 127.0.0.1
# port = 5432
database = tern_test
user = jack
password = {{env "MIGRATOR_PASSWORD"}}
# version_table = public.schema_version
#
# sslmode generally matches the behavior described in:
# http://www.postgresql.org/docs/9.4/static/libpq-ssl.html#LIBPQ-SSL-PROTECTION
#
# There are only two modes that most users should use:
# prefer - on trusted networks where security is not required
# verify-full - require SSL connection
# sslmode = prefer
#
# "conn_string" accepts two formats; URI or DSN as described in:
# https://www.postgresql.org/docs/current/libpq-connect.html#LIBPQ-CONNSTRING
#
# This property is lenient i.e., it does not throw error
# if values for both "conn_string" and "host/port/.." are
# provided. In this case, the individual properties will
# override the correspoding part in the "conn_string".
#
# URI format:
# conn_string = postgresql://other@localhost/otherdb?connect_timeout=10&application_name=myapp
# DSN format:
# conn_string = host=localhost port=5432 dbname=mydb connect_timeout=10

# Proxy the above database connection via SSH
# [ssh-tunnel]
# host =
# port = 22
# user defaults to OS user
# user =
# password is not required if using SSH agent authentication
# password =

[data]
prefix = foo
app_user = joe

This flexibility configuration style allows handling multiple environments such as test, development, and production in several ways.

Separate config file for each environment
Environment variables for database settings and optionally one config file for shared settings
Program arguments for database settings and optionally one config file for shared settings

Migrations

To create a new migration:

tern new name_of_migration

This will create a migration file with the given name prefixed by the next available sequence number (e.g. 001, 002, 003).

The migrations themselves have an extremely simple file format. They are simply the up and down SQL statements divided by a magic comment.

---- create above / drop below ----

Example:

create table t1(
  id serial primary key
);

---- create above / drop below ----

drop table t1;

If a migration is irreversible such as a drop table, simply delete the magic comment.

drop table widgets;

To interpolate a custom data value from the config file prefix the name with a dot and surround the whole with double curly braces.

create table {{.prefix}}config(
  id serial primary key
);

Migrations are read from files in the migration directory in the order of the numerical prefix. Each migration is run in a transaction.

Any SQL files in subdirectories of the migration directory, will be available for inclusion with the template command. This can be especially useful for definitions of views and functions that may have to be dropped and recreated when the underlying table(s) change.

// Include the file shared/v1_001.sql. Note the trailing dot.
// It is necessary if the shared file needs access to custom data values.
{{ template "shared/v1_001.sql" . }}
);

Tern uses the standard Go text/template package so conditionals and other advanced templating features are available if needed. See the package docs for details. Sprig functions are also available.

Migrating

To migrate up to the last version using migrations and config file located in the same directory simply run tern:

tern migrate

To migrate up or down to a specific version:

tern migrate --destination 42

To migrate up N versions:

tern migrate --destination +3

To migrate down N versions:

tern migrate --destination -3

To migrate down and rerun the previous N versions:

tern migrate --destination -+3

To use a different config file:

tern migrate --config path/to/tern.json

To use a different migrations directory:

tern migrate --migrations path/to/migrations

Renumbering Conflicting Migrations

When migrations are created on multiple branches the migrations need to be renumbered when the branches are merged. The tern renumber command can automatically do this. On the branch with the only migrations to keep at the lower numbers run tern renumber start. Merge the branches. Then run tern renumber finish.

$ git switch master
Switched to branch 'master'
$ ls
001_create_users.sql
002_add_last_login_to_users.sql

$ git switch feature
Switched to branch 'feature'
$ ls
001_create_users.sql
002_create_todos.sql

# Both branches have a migration number 2.

# Run tern renumber start on the branch with the migrations that should come first.

$ git switch master
Switched to branch 'master'
$ tern renumber start

# Then go to the branch with migrations that should come later and merge or rebase.

$ git switch feature
$ git rebase master
Successfully rebased and updated refs/heads/feature.
$ ls
001_create_users.sql
002_add_last_login_to_users.sql
002_create_todos.sql

# There are now two migrations with the same migration number.

$ tern renumber finish
$ ls
001_create_users.sql
002_add_last_login_to_users.sql
003_create_todos.sql

# The migrations are now renumbered in the correct order.

Code Packages

The migration paradigm works well for creating and altering tables, but it can be unwieldy when dealing with database code such as server side functions and views. For example, consider a schema where view c depends on view b which depends on view a. A change to a may require the following steps:

Drop c
Drop b
Drop a
Create a
Create b
Create c

In addition to the challenge of manually building such a migration it is difficult to use version control to see the changes in a particular database object over time when its definition is scattered through multiple migrations.

A solution to this is code packages. A code package is a directory with an install.sql file that contains the instructions to completely drop and recreate a set of database code. The command code install can be used to directly install a code package (especially useful during development) and the code snapshot command can be used to make a single migration that installs that code package.

For example given a directory code containing the following files:

-- install.sql
drop schema if exists code cascade;
create schema code;

{{ template "a.sql" . }}
{{ template "b.sql" . }}
{{ template "c.sql" . }}

-- a.sql
create view code.a as select ...;

-- b.sql
create view code.b as select * from code.a where ...;

-- c.sql
create view code.c as select * from code.b where ...;

Then this command would install the package into the database.

tern code install path/to/code --config path/to/tern.conf

And this command would create a migration from the current state of the code package.

tern code snapshot path/to/code --migrations path/to/migrations

Code packages have access to data variables defined in your configuration file as well as functions provided by Sprig.

It is recommended but not required for each code package to be installed into its own PostgreSQL schema. This schema could be determined by environment variable as part of a blue / green deployment process.

Template Tips

The env function can be used to read process environment variables.

drop schema if exists {{ env "CODE_SCHEMA" }} cascade;
create schema {{ env "CODE_SCHEMA" }};

The Sprig dictionary functions can be useful to call templates with extra parameters merged into the . value.

{{ template "_view_partial.sql" (merge (dict "view_name" "some_name" "where_clause" "some_extra_condition=true") . ) }}

SSH Tunnel

Tern includes SSH tunnel support. Simply supply the SSH host, and optionally port, user, and password in the config file or as program arguments and Tern will tunnel the database connection through that server. When using a SSH tunnel the database host should be from the context of the SSH server. For example, if your PostgreSQL server is pg.example.com, but you only have SSH access, then your SSH host would be pg.example.com and your database host would be localhost.

Tern will automatically use an SSH agent or ~/.ssh/id_rsa if available.

Embedding Tern

All the actual functionality of tern is in the github.com/jackc/tern/migrate library. If you need to embed migrations into your own application this library can help.

Running the Tests

To run the tests tern requires two test databases to run migrations against.

Create a new database for main tern program tests.
Open testdata/tern.conf.example
Enter the connection information.
Save as testdata/tern.conf.
Create another database for the migrate library tests.
Run tests with the connection string for the migrate library tests in the MIGRATE_TEST_CONN_STRING environment variable

MIGRATE_TEST_CONN_STRING="host=/private/tmp database=tern_migrate_test" go test ./...

Prior Ruby Gem Version

The projects using the prior version of tern that was distributed as a Ruby Gem are incompatible with the version 1 release. However, that version of tern is still available through RubyGems and the source code is on the ruby branch.

Version History

1.13.0 (April 21, 2022)

Add conn string connection config option (vivek-shrikhande)
Add Filename to MigrationPgError (davidmdm)

1.12.5 (June 12, 2021)

Look for SSH keys in ~/.ssh/id_rsa (Miles Delahunty)

1.12.4 (February 27, 2021)

Use user's known_hosts file when connecting via SSH

1.12.3 (December 24, 2020)

Fix reported version number

1.12.2 (December 23, 2020)

Fix setting port from config file
Fix non-schema qualified version table not in public but in search path (Tynor Fujimoto)

1.12.1 (June 27, 2020)

Update to latest version of pgx.

1.12.0 (June 26, 2020)

Command code install no longer outputs compiled SQL.
Add code compile command to print compiled SQL code package.
Better error reporting for code install.

1.11.0 (April 10, 2020)

Add Sprig functions to configuration file and migrations.
Add SQL code management distinct from migrations.

1.10.2 (March 28, 2020)

CLI now handles SIGINT (ctrl+c) and attempts to cancel in-progress migration before quitting

1.10.1 (March 24, 2020)

Fix default CLI version-table argument overriding config value

1.10.0 (March 7, 2020)

Better locking to protect against multiple concurrent migrators on first run
Update pgx version to support PostgreSQL service files

1.9.1 (February 1, 2020)

Look for version table in all schemas in search_path instead of just the top schema

1.9.0 (February 1, 2020)

Default version table is explicitly in public schema
Update to pgx v4 (Alex Gaynor)

1.8.2 (July 19, 2019)

Show PostgreSQL error details
Rename internal error type

1.8.1 (April 5, 2019)

Issue reset all after every migration
Use go modules instead of Godep / vendoring

1.8.0 (February 26, 2018)

Update to latest version of pgx (PostgreSQL driver)
Support PGSSLROOTCERT
Fix typos and internal cleanup
Refactor internals for easier embedding (hsyed)

1.7.1 (January 30, 2016)

Simplify SSH tunnel code so it does not listen on localhost

1.7.0 (January 17, 2016)

Add SSH tunnel support

1.6.1 (January 16, 2016)

Fix version output
Evaluate config files through text/template with ENV

1.6.0 (January 15, 2016)

Optionally read database connection settings from environment
Accept database connection settings via program arguments
Make config file optional

1.5.0 (October 1, 2015)

Add status command
Add relative migration destinations
Add redo migration destinations

1.4.0 (May 15, 2015)

Add TLS support

1.3.3 (May 1, 2015)

Fix version output

1.3.2 (May 1, 2015)

Better error messages

1.3.1 (December 24, 2014)

Fix custom version table name

1.3.0 (December 23, 2014)

Prefer host config whether connecting with unix domain socket or TCP

1.2.2 (May 30, 2014)

Fix new migration short name

1.2.1 (May 18, 2014)

Support socket directory as well as socket file

1.2.0 (May 6, 2014)

Move to subcommand interface
Require migrations to begin with ascending, gapless numbers
Fix: migrations directory can contain other files
Fix: gracefully handle invalid current version
Fix: gracefully handle migrations with duplicate sequence number

1.1.1 (April 22, 2014)

Do not require user -- default to OS user

1.1.0 (April 22, 2014)

Add sub-template support
Switch to ini for config files
Add custom data merging

1.0.0 (April 19, 2014)

Total rewrite in Go

0.7.1

Print friendly error message when database error occurs instead of stack trace.

0.7.0

Added ERB processing to SQL files

Files

README.markdown

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