Addition of .json as variable file format type.

Variable files can now be written in json using the .json file
extension and format. Documentation has also been updated to show
example.

Closes hashicorp#208

command/deploy.go

@@ -74,7 +74,7 @@ General Options:
   -var-file=<file>
     Used in conjunction with the -job-file will deploy a templated job to your
     Nomad cluster. You can repeat this flag multiple times to supply multiple var-files.
-    [default: levant.(yaml|yml|tf)]
+    [default: levant.(json|yaml|yml|tf)]
 `
 	return strings.TrimSpace(helpText)
 }

command/render.go

@@ -45,7 +45,7 @@ General Options:
 
   -var-file=<file>
     The variables file to render the template with. You can repeat this flag multiple
-    times to supply multiple var-files. [default: levant.(yaml|yml|tf)]
+    times to supply multiple var-files. [default: levant.(json|yaml|yml|tf)]
 `
 	return strings.TrimSpace(helpText)
 }

docs/templates.md

@@ -4,54 +4,98 @@ Alongside enhanced deployments of Nomad jobs; Levant provides templating functio
 
 ### Template Substitution
 
-Levant currently supports `.tf`, `.yaml` and `.yml` file extensions for the declaration of template variables and uses opening and closing double squared brackets `[[ ]]` within the templated job file. This is to ensure there is no clash with existing Nomad interpolation which uses the standard `{{ }}` notation.
+Levant currently supports `.json`, `.tf`, `.yaml`, and `.yml` file extensions for the declaration of template variables and uses opening and closing double squared brackets `[[ ]]` within the templated job file. This is to ensure there is no clash with existing Nomad interpolation which uses the standard `{{ }}` notation.
 
-Example Job Template:
+#### JSON
+
+JSON as well as YML provide the most flexible variable file format. It allows for descriptive and well organised jobs and variables file as shown below.
+
+Example job template:
+```hcl
+resources {
+    cpu    = [[.resources.cpu]]
+    memory = [[.resources.memory]]
+
+    network {
+        mbits = [[.resources.network.mbits]]
+    }
+}
+```
+
+Example variable file:
+```json
+{
+    "resources":{
+        "cpu":250,
+        "memory":512,
+        "network":{
+            "mbits":10
+        }
+    }
+}
+```
+
+#### Terraform
+
+Terraform (.tf) is probably the most inflexible of the variable file formats but does provide an easy to follow, descriptive manner in which to work. It may also be advantageous to use this format if you use Terraform for infrastructure as code thus allow you to use a consistant file format.
+
+Example job template:
 ```hcl
 resources {
-    cpu    = [[.cpu]]
-    memory = [[.memory]]
+    cpu    = [[.resources_cpu]]
+    memory = [[.resources_memory]]
 
     network {
-        mbits = [[.mbits]]
+        mbits = [[.resources_network_mbits]]
     }
 }
 ```
 
-`.tf` variables file:
+Example variable file:
 ```hcl
-variable "cpu" {
-  default = 250
+variable "resources_cpu" {
+  description = "the CPU in MHz to allocate to the task group"
+  type        = "string"
+  default     = 250
 }
 
-variable "memory" {
-  default = 512
+variable "resources_memory" {
+  description = "the memory in MB to allocate to the task group"
+  type        = "string"
+  default     = 512
 }
 
-variable "mbits" {
-  default = 10
+variable "resources_network_mbits" {
+  description = "the network bandwidth in MBits to allocate"
+  type        = "string"
+  default     = 10
 }
 ```
 
-`.yaml` or `.yml` variables file:
-```yaml
-cpu: 250
-memory: 512
-mbits: 10
-```
+#### YAML
 
-Render:
+Example job template:
 ```hcl
 resources {
-    cpu    = 250
-    memory = 512
+    cpu    = [[.resources.cpu]]
+    memory = [[.resources.memory]]
 
     network {
-        mbits = 10
+        mbits = [[.resources.network.mbits]]
     }
 }
 ```
 
+Example variable file:
+```yaml
+---
+resources:
+  cpu: 250
+  memory: 512
+  network:
+    mbits: 10
+```
+
 ### Template Functions
 
 Levant's template rendering supports a number of functions which provide flexibility when deploying jobs. As with the variable substitution, it uses opening and closing double squared brackets `[[ ]]` as not to conflict with Nomad's templating standard. Levant parses job files using the [Go Template library](https://golang.org/pkg/text/template/) which makes available the features of that library as well as the functions described below.

helper/files.go

@@ -31,6 +31,10 @@ func GetDefaultVarFile() (varFile string) {
 		log.Debug().Msg("helper/files: using default var-file `levant.yml`")
 		return "levant.yml"
 	}
+	if _, err := os.Stat("levant.json"); !os.IsNotExist(err) {
+		log.Debug().Msg("helper/files: using default var-file `levant.json`")
+		return "levant.json"
+	}
 	if _, err := os.Stat("levant.tf"); !os.IsNotExist(err) {
 		log.Debug().Msg("helper/files: using default var-file `levant.tf`")
 		return "levant.tf"

template/render.go

@@ -2,6 +2,7 @@ package template
 
 import (
 	"bytes"
+	"encoding/json"
 	"fmt"
 	"io/ioutil"
 	"path"
@@ -70,6 +71,8 @@ func RenderTemplate(templateFile string, variableFiles []string, addr string, fl
 			variables, err = t.parseTFVars(variableFile)
 		case yamlVarExtension, ymlVarExtension:
 			variables, err = t.parseYAMLVars(variableFile)
+		case jsonVarExtension:
+			variables, err = t.parseJSONVars(variableFile)
 		default:
 			err = fmt.Errorf("variables file extension %v not supported", ext)
 		}
@@ -98,6 +101,21 @@ func RenderTemplate(templateFile string, variableFiles []string, addr string, fl
 	return
 }
 
+func (t *tmpl) parseJSONVars(variableFile string) (variables map[string]interface{}, err error) {
+
+	jsonFile, err := ioutil.ReadFile(variableFile)
+	if err != nil {
+		return
+	}
+
+	variables = make(map[string]interface{})
+	if err = json.Unmarshal(jsonFile, &variables); err != nil {
+		return
+	}
+
+	return variables, nil
+}
+
 func (t *tmpl) parseTFVars(variableFile string) (variables map[string]interface{}, err error) {
 
 	c := &config.Config{}

template/template.go

@@ -16,6 +16,7 @@ type tmpl struct {
 }
 
 const (
+	jsonVarExtension      = ".json"
 	terraformVarExtension = ".tf"
 	yamlVarExtension      = ".yaml"
 	ymlVarExtension       = ".yml"