Serverless Template Plugin

This repository contains a very small Serverless plugin to enable re-use of small template files in larger CloudFormation templates.

This project is deliberately small, but things will be added as they're found to be useful. Rather than using a templating engine, we use short replacements to keep the CloudFormation template easy to understand.

Installation

This plugin is available via npm, so it can be installed as usual:

$ npm i --save-dev serverless-template-plugin

There's no need to include it in your production bundle, so make sure it's saved inside the development dependencies. Once installed, make sure to add it to your Serverless plugins list:

plugins:
  - serverless-template-plugin

The location within the plugin list shouldn't matter, as it's not reacting to any specific Serverless events.

Usage

The idea is to allow (minimal) templating via find/replace of values to avoid having to continuously redefine them inside templates. This is done by including a template file via the template(path):replacements syntax.

Replacements are separated by a spec in the form param1=1 param2=2 param3=3, which leaves your template fairly easy to understand. You can then access these replacements in your templated file via ${value}.

# Import file.yml and replace param1, param2 and param3
- template(./file.yml):param1=1 param2=2 param3=3}

# JSON is also supported in the same way as YAML
- template(./file.json):param1=1 param2=2 param3=3}

# If you want to use something that isn't a space
- template(./file.yml,&):param1=1&param2=2&param3=3}

# Whitespace is trimmed to allow better readability
- template(./file.yml,;):param1=1; param2=2; param3=3}

Just like the Serverless file directive, this plugin works with both yml and json files. This is determined by the file extension provided, so please make sure they match correctly.

Simple Example

A really basic example to demonstrate how this should be used is the idea of defining a database cluster with several instances. Typically this would require that you write the same CloudFormation block multiple times, but now it can be compressed:

# templates/database-instance.yml
Resources:
  DatabaseInstance${id}:
    Type: AWS::DocDB::DBInstance
    Properties:
    DBClusterIdentifier: !Ref DatabaseCluster
    DBInstanceClass: db.t4g.medium

# serverless.yml
resources:
  # Create the database cluster
  - Resources:
      DatabaseCluster:
        Type: AWS::DocDB::DBCluster
        Properties:
          DBClusterIdentifier: ${self:service}-${sls:stage}-db
          MasterUsername: guy
          MasterUserPassword: fieri

  # Create 5 database instances inside our cluster
  - ${template(templates/database-instance.yml):id=1}
  - ${template(templates/database-instance.yml):id=2}
  - ${template(templates/database-instance.yml):id=3}
  - ${template(templates/database-instance.yml):id=4}
  - ${template(templates/database-instance.yml):id=5}

As you can see, this cuts down on a fair chunk of templating. It also makes it much easier to keep things in sync; in cases where you might change the instance class, you now only have to change it in a single place.

Transformers

This plugin comes with some minimal string transformation to allow you to tweak the replacement. These transformers can be accessed via the syntax ${value:transfomer} and are optional. Pretty much all of these transformers are provided by lodash so you can check the docs there for further details on how they work. The current list of transformers is shown below:

Transformer	Description
${value:camel}	Converts the value to camelCase
${value:capitalize}	Converts the first letter to upper case and the rest to lower case
${value:escape}	Converts HTML characters to HTML entities
${value:escapeRegExp}	Converts RegExp special characters to their escaped form
${value:kebab}	Converts the value to kebab-case
${value:lower}	Converts the value to lower case
${value:lowerFirst}	Converts the first character in the value to lower case
${value:lowerWords}	Converts the value to lower case separated by spaces
${value:pascal}	Converts the value to PascalCase
${value:snake}	Converts the value to snake_case
${value:start}	Converts the value to Start Case
${value:unescape}	Converts HTML entities to their corresponding characters
${value:upper}	Converts the value to UPPER CASE
${value:upperFirst}	Converts the first character in the value to upper case
${value:upperWords}	Converts the value to UPPER CASE separated by spaces

These can be added easily, so feel free to file PRs for things which can be useful when creating templates.

Compatibility

This plugin uses some Serverless internals for parsing of inputs, so it's tied to the version of Serverless in use. I'm not sure of the exact range of compatibility so please file an issue if it appears to not work with your version of Serverless.