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Create better cloudformation templates with Clojure

Travis Build CII Best Practices


Crucible depends on clojure.spec, currently available in Clojure 1.9 alpha 10+ (breaking changes in spec around alpha 9)

Clojars Latest Version


(ns crucible.examples-test
  (:require [crucible.core :refer [template parameter resource output xref encode join]]
            [ :as ec2]))

(def simple (template "A simple sample template"
                      :my-vpc-cidr (parameter)
                      :my-vpc (ec2/vpc {::ec2/cidr-block (xref :my-vpc-cidr)})
                      :vpc (output (join "/" ["foo" (xref :my-vpc)]))))

repl> (clojure.pprint/pprint (encode simple))
{"AWSTemplateFormatVersion" "2010-09-09"
 "Description" "A simple sample template"
 "Parameters" {"MyVpcCidr" {"Type" "String"}}
 "Resources" {"MyVpc"
              {"Type" "AWS::EC2::VPC",
               "Properties" {"CidrBlock" {"Ref" "MyVpcCidr"}}}},
 "Outputs" {"Vpc" {"Value" {"Fn::Join" ["/" ["foo" {"Ref" "MyVpc"}]]}}}}

Alternative template construction function accepts map and string arguments for building a template from partials, for example:

(def simple (-> {:my-vpc-cidr (parameter)}
                (assoc :igw (ec2/internet-gateway {}))
                (assoc :my-vpc (ec2/vpc {::ec2/cidr-block (xref :my-vpc-cidr)}))
                (assoc :vpc (output (join "/" ["foo" (xref :my-vpc)])))
                (template "A simple sample template")))

Parameter Options

See crucible.parameters namespace, required as param in this example:

:my-vpc-cidr (parameter ::param/type ::param/number
                        ::param/description "A demonstration of parameter options"
                        ::param/allowed-values [1 2 3]
                        ::param/no-echo true)

Resource Policies

See crucible.policies namespace, required as policies in this example:

:my-vpc (ec2/vpc {::ec2/cidr-block (xref :my-vpc-cidr)}
                 (policies/deletion ::policies/retain)
                 (policies/depends-on :my-vpc-cidr))

Resource Types

Standard AWS resource types can be found as children of the namespace.

Examples of resource type usage can be found in the tests.

  • AWS::EC2::* partial coverage
  • AWS::ElasticLoadBalancingV2::*
  • AWS::ApiGateway::*
  • AWS::DynamoDB::Table
  • AWS::CloudWatch::Alarm
  • AWS::Lambda::Function
  • AWS::Lambda::EventSourceMapping
  • AWS::IAM::Role (basic support for Lambda applications)
  • AWS::ECR::Repository
  • AWS::S3::Bucket
  • AWS::CloudFormation::Stack
  • AWS::Kinesis::Stream
  • AWS::KinesisFirehose::DeliveryStream
  • AWS::Route53::RecordSet
  • AWS::SNS::Topic
  • AWS::SNS::TopicPolicy
  • AWS::SQS::Queue
  • AWS::Events::Rule
  • AWS::AutoScaling::AutoScalingGroup/LaunchConfiguration
  • Custom::* custom resources

Writing your own resource type

The easiest way is to use defresource and spec-or-ref from the crucible.resources namespace, eg.

  "Resources in AWS::EC2::*"
  (:require [crucible.resources :refer [spec-or-ref defresource] :as res]
            [clojure.spec :as s]))

;; spec-or-ref applies your spec if a literal value is given,
;; but also allows a parameter or function to be given instead of a literal.
(s/def ::cidr-block (spec-or-ref string?))

;; ::res/tags reuses the tags spec defined in the crucible/resources namespace
(s/def ::vpc (s/keys :req [::cidr-block]
                     :opt [::enable-dns-support

;; creates resource factory with type "AWS::EC2::VPC" 
;; and validates the data structure using the ::vpc spec
(defresource  vpc "AWS::EC2::VPC" ::vpc)

Pull requests to add or enhance resource types available in Crucible will be welcomed. If it's a standard AWS type please place it in the namespace and use defresource as it documents the resource type for you. At lest one test for the update would be great!

Testing a Resource Type

Although you can test a resource by testing the data structure and validity directly, testing the conversion from the crucible code to a map ready to encode as CloudFormation-valid JSON is good minimal coverage. The default clojure.test behaviour does not pretty-print the ex-data map on validation exceptions, which makes testing and debugging validation failures painful. A custom assertion crucible.assertion/resource= is available to ensure any failure map is pretty-printed. See the resource tests for examples.

Overriding JSON Keys

Crucible uses camel-snake-kebab's ->PascalCase function to convert Clojure map keys into JSON map keys. That takes care of most translations between Clojure-style :keyword-key and JSON/CloudFormation-style KeywordKey. To handle the occasional mistranslation, typically due to capitalisation, clojure.encoding.keys exposes a ->key multimethod, allowing overriding of the translation. For example, this problem occurs in AWS::CloudFormation::Stack, where a required key is TemplateURL. The following overrides the natural translation of :template-url to TemplateUrl.

  (:require [crucible.encoding.keys :refer [->key]]))

(defmethod ->key :template-url [_] "TemplateURL")

Note, these translations take place during the final JSON encoding step and do not see keyword namespacing.

Severless Application Model templates

The AWS Serverless Application Model (SAM) is supported by using SAM resources and the SAM encoder. All CloudFormation resources are valid SAM resources and they can be combined in the same template. The crucible.core/template function can be used to generate a template data structure for SAM resources.


AWS SAM supports global properties for functions and APIs. There is a two-arity version of build and encode in the SAM encoder that accepts a template and a globals object.


In the example below, the second argument to encoding.sam/build can be omitted if globals are not used.

(ns crucible.sam-example
  (:require [ :as k]
            [ :as f]
            [ :as es]
            [ :as es.k]
            [ :as g]
            [crucible.core :refer [template xref]]
            [crucible.encoding.serverless :as encoding.sam]))

(-> {:stream-processor (f/function
                        {::f/handler "index.handler"
                         ::f/runtime "nodejs6.10"
                         ::f/code-uri "src/"
                         ::f/events {:stream {::es/type "Kinesis"
                                              ::es.k/properties {::es.k/stream (xref :stream :arn)
                                                                 ::es.k/starting-position "TRIM_HORIZON"}}}})
     :stream (k/stream {::k/shard-count 1})}
    (template "A function that processes data from a Kinesis stream.")
    (encoding.sam/build (g/globals {::g/function {::f/memory-size 1024
                                                  ::f/timeout 15}})))

CLI Support

Basic CLI support, intended for use with Leiningen, is provided in the crucible.encoding.main/-main function. Running this function will reload the namespaces available in the project, then enumerate any vars that have a metadata tag provided by the crucible.core/template function. These vars are then encoded into CloudFormation templates and exported to the local filesystem. They can then be used directly or uploaded to S3 for use with CloudFormation.

Flag -h for help. Templates are exported to target/templates by default, override with -o output-dir. Namespaces are converted to filesystem locations by replacing . characters with / characters.

I create a templates directory within my project and then add it as a source-path and crucible as a dependency to the dev profile. Then I can work at the repl, write tests for my templates and use this tooling without having my template code or crucible mixed with my source code.

demo-project is an example of setting up a project with crucible templates defined alongside the code. The templates can see the code to verify any references they might have, but not the other way round. Run lein templates in the demo project to generate templates.

:aliases {"templates" ["run" "-m" crucible.encoding.main]} 
:profiles {:dev {:source-paths ["templates"]
                 :dependencies [[crucible "0.10.0-SNAPSHOT"]]}}

Helping Out

Any help appreciated! Happy to receive any issues and pull requests. See


Distributed under the Eclipse Public License

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