Lambda Warmer

Setup

Install Dependencies

The Lambda gets packaged and deployed using AWS SAM. Make sure to install the most recent version.
Install packages: npm i

Environmental Properties:

Create .env file with the following properties

SAM_S3_BUCKET = <bucket to hold SAM output>
AWS_REGION = <AWS region>
CF_STACK_NAME = <CloudFormation stack name>
AWS_PROFILE = <Optional: AWS credential profile>
WARMING_RATE = <Optional: Rate (in minutes) to warm functions; Default=5

Warming Schedule Configuration:

You can fully configure which lambdas you'd like to keep warm by creating a warming configuration file. Start by creating a file called warming_config.json. The configurations are defined in the following format:

[
    {
        "lambda": "<Lambda ARN or Name>",
        "concurrency": <Number>,
        "invokeArgs": <JSON Data>
    },
    { ... },
    { ... }
]

The following is an example of a configuration that warms two lambda functions. The first lambda is configured to keep 5 instances warm. The second lambda is configured to keep 3 instances warm, and also passes custom JSON data to the lambda when invoked:

[
  {
    "lambda": "arn:aws:lambda:us-east-1:1111111111:function:my-lambda-function",
    "concurrency": 5
  },
  {
    "lambda": "arn:aws:lambda:us-east-1:1111111111:function:my-other-lambda-function",
    "concurrency": 3,
    "invokeArgs": {
      "data": "This is some custom warming data to pass to our lambda",
      "extra": {
        "info": "It supports any valid JSON"
      }
    }
  }
]

Deployment

Deployment is easy. Run npm run deploy to create/update a CloudFront stack for the warming lambda. This automatically creates and starts the warming schedule

Handling Warming Calls From The Invoked Lambdas Side

When a Lambda is warmed, you usually don't want to run any of the function's logic. Doing so would be a waste of execution time, memory, and other resources. In order to handle this you can add catches to the beginning of your lambdas to look for invocations originating from the warmer.

In the warming configuration file you may have noticed that there is an optional invokeArgs property. Whatever JSON data you provide to this property will get passed to the lambda in it's event parameter.

If you did not specify a value for the invokeArgs property, then it will be set to { "warmer": true }. This then gets passed to the lambda's event parameter.

Here is an example of how you'd handle this in a Node Lambda Function:

exports.handler = (event, context, callback) => {
    if (event.warmer) {
        return callback();
    }
    ...
    <Core Function Logic>
}

jmrapp1 / Lambda-Warmer