Serverless WarmUp Plugin ♨

Keep your lambdas warm during winter.

Requirements:

Serverless v1.12.x or higher (Recommended v1.33.x or higher because of this).
AWS provider

How it works

WarmUp solves cold starts by creating a scheduled lambda that invokes all the selected service's lambdas in a configured time interval (default: 5 minutes) and forcing your containers to stay warm.

Installation

Install via npm in the root of your Serverless service:

npm install --save-dev serverless-plugin-warmup

Add the plugin to the plugins array in your Serverless serverless.yaml:

plugins:
  - serverless-plugin-warmup

Configuration

Most options are set under custom.warmup in the serverless.yaml file.

folderName (default _warmup)
cleanFolder (default true)
name (default ${service}-${stage}-warmup-plugin)
role (default to role in the provider)
tags (default to serverless default tags)
vpc (default to vpc in provider, can be set to false to deploy the warmup function outside of VPC)
memorySize (default 128)
events (default - schedule: rate(5 minutes), can be any Serverless event)
package (default package: { individually: true, exclude: ['**'], include: ['_warmup/**'] })
timeout (default 10 seconds)
environment (default to unset all package level environment variables. It can be use to set variables or to unset global variables by setting it to undefined. However, you should never have to change the default.)
prewarm (default false)

But there are some options which can also be set under custom.warmup to be applied to all lambdas to be warmed up or can be overridden on each individual lambda.

enabled (default false,an be a boolean, a stage or a list of stages.)
payload (default { "source": "serverless-plugin-warmup" }, payload will be stringified)
payloadRaw (default false)
concurrency (default 1)

custom:
  warmup:
    enabled: true # Whether to warm up functions by default or not
    folderName: '_warmup' # Name of the folder created for the generated warmup 
    cleanFolder: false
    memorySize: 256
    name: 'make-them-pop'
    role: myCustRole0
    tags:
      Project: foo
      Owner: bar 
    vpc: false
    events:
      - schedule: 'cron(0/5 8-17 ? * MON-FRI *)' # Run WarmUp every 5 minutes Mon-Fri between 8:00am and 5:55pm (UTC)
    package:
      individually: true
      exclude: # exclude additional binaries that are included at the serverless package level
        - ../**
        - ../../**
      include:
        - ./**
    timeout: 20
    prewarm: true # Run WarmUp immediately after a deploymentlambda
    payload: 
      source: my-custom-source
      other: 20
    payloadRaw: true # Won't JSON.stringify() the payload, may be necessary for Go/AppSync deployments
    concurrency: 5 # Warm up 5 concurrent instances
    
functions:
  myColdfunction:
    handler: 'myColdfunction.handler'
    events:
      - http:
          path: my-cold-function
          method: post
    warmup:
      enabled: false

  myLowConcurrencyFunction:
    handler: 'myLowConcurrencyFunction.handler'
    events:
      - http:
          path: my-low-concurrency-function
          method: post
    warmup:
      payload: different-source-only-for-this-lambda
      concurrency: 1
   
  myProductionOnlyFunction:
    handler: 'myProductionOnlyFunction.handler'
    events:
      - http:
          path: my-production-only-function
          method: post
    warmup:
      enabled: prod
      
   myDevAndStagingOnlyFunction:
    handler: 'myDevAndStagingOnlyFunction.handler'
    events:
      - http:
          path: my-dev-and-staging-only-function
          method: post
    warmup:
      enabled:
        - dev
        - staging

Options should be tweaked depending on:

Number of lambdas to warm up
Day cold periods
Desire to avoid cold lambdas after a deployment

Runtime Configuration

Concurrency can be modified post-deployment at runtime by using Lambda environment variables.
Two configuration options exist:

Globally set the concurrency of all lambdas on the stack (overriding the deployment configuration):
Set the environment variable WARMUP_CONCURRENCY
Individually set the concurrency per lambda
Set the environment variable WARMUP_CONCURRENCY_YOUR_FUNCTION_NAME. Must be all uppercase and hyphens (-) are replaced with underscores (_). If present, it overrides the global concurrency setting.

Legacy options

Over time some options have been removed form the pluging. For now, we keep backwards compatibility so they still work. However, they are listed here only to facilitate upgrading the pluging and we strongly recommend switching to the options defined above as soon as possible.

default Has been renamed to enabled
schedule schedule: rate(5 minutes) is equivalent to events: - schedule: rate(5 minutes).
source Has been renamed to payload
sourceRaw Has been renamed to payloadRaw

Permissions

WarmUp requires some permissions to be able to invoke your lambdas.

custom:
  warmup:
    folderName: '_warmup' # Name of the folder created for the generated warmup 
    cleanFolder: false
    memorySize: 256
    name: 'make-them-pop'
    role:  myCustRole0
    events:
      - schedule: 'cron(0/5 8-17 ? * MON-FRI *)' # Run WarmUp every 5 minutes Mon-Fri between 8:00am and 5:55pm (UTC)
    timeout: 20
    prewarm: true # Run WarmUp immediately after a deployment
    tags:
      Project: foo
      Owner: bar

.....

resources:
  Resources:
    myCustRole0:
      Type: AWS::IAM::Role
      Properties:
        Path: /my/cust/path/
        RoleName: MyCustRole0
        AssumeRolePolicyDocument:
          Version: '2017'
          Statement:
            - Effect: Allow
              Principal:
                Service:
                  - lambda.amazonaws.com
              Action: sts:AssumeRole
        Policies:
          - PolicyName: myPolicyName
            PolicyDocument:
              Version: '2017'
              Statement:
                - Effect: Allow # WarmUp lamda to send logs to CloudWatch
                  Action:
                    - logs:CreateLogGroup
                    - logs:CreateLogStream
                    - logs:PutLogEvents
                  Resource: 
                    - 'Fn::Join':
                      - ':'
                      -
                        - 'arn:aws:logs'
                        - Ref: 'AWS::Region'
                        - Ref: 'AWS::AccountId'
                        - 'log-group:/aws/lambda/*:*:*'
                - Effect: Allow # WarmUp lamda to manage ENIS (only needed if deploying to VPC, https://docs.aws.amazon.com/lambda/latest/dg/vpc.html)
                  Action:
                    - ec2:CreateNetworkInterface
                    - ec2:DescribeNetworkInterfaces
                    - ec2:DetachNetworkInterface
                    - ec2:DeleteNetworkInterface
                  Resource: "*"
                - Effect: 'Allow' # WarmUp lamda to invoke the functions to be warmed
                  Action:
                    - 'lambda:InvokeFunction'
                  Resource:
                  - Fn::Join:
                    - ':'
                    - - arn:aws:lambda
                      - Ref: AWS::Region
                      - Ref: AWS::AccountId
                      - function:${self:service}-${opt:stage, self:provider.stage}-*

The permissions can also be added to all lambdas using iamRoleStatements under provider (see https://serverless.com/framework/docs/providers/aws/guide/functions/#permissions):

provider:
  name: aws
  runtime: nodejs10.x
  iamRoleStatements:
    - Effect: 'Allow'
      Action:
        - 'lambda:InvokeFunction'
      Resource:
      - Fn::Join:
        - ':'
        - - arn:aws:lambda
          - Ref: AWS::Region
          - Ref: AWS::AccountId
          - function:${self:service}-${opt:stage, self:provider.stage}-*

If using pre-warm, the deployment user also needs a similar policy so it can run the WarmUp lambda.

On the function side

Lambdas invoked by WarmUp will have the event source serverless-plugin-warmup (unless otherwise specified using the payload option):

{
  "Event": {
    "source": "serverless-plugin-warmup"
  }
}

To minimize cost and avoid running your lambda unnecessarily, you should add an early return call before your lambda logic when that payload is received.

// Using the Promise style
module.exports.lambdaToWarm = async function(event, context) {
  /** Immediate response for WarmUp plugin */
  if (event.source === 'serverless-plugin-warmup') {
    console.log('WarmUp - Lambda is warm!');
    return 'Lambda is warm!';
  }

  ... add lambda logic after
}

// Using the Callback style
module.exports.lambdaToWarm = function(event, context, callback) {
  /** Immediate response for WarmUp plugin */
  if (event.source === 'serverless-plugin-warmup') {
    console.log('WarmUp - Lambda is warm!')
    return callback(null, 'Lambda is warm!')
  }

  ... add lambda logic after
}

// Using context.
// This could be useful if you are handling the raw input and output streams.
module.exports.lambdaToWarm = async function(event, context) {
  /** Immediate response for WarmUp plugin */
  if (context.custom.source === 'serverless-plugin-warmup') {
    console.log('WarmUp - Lambda is warm!');
    return 'Lambda is warm!';
  }

  ... add lambda logic after
}

If you're using the concurrency option you might want to add a slight delay before returning on warmup calls to ensure that your function doesn't return before all concurrent requests have been started:

module.exports.lambdaToWarm = async (event, context) => {
  if (event.source === 'serverless-plugin-warmup') {
    console.log('WarmUp - Lambda is warm!');
    /** Slightly delayed (25ms) response 
    	to ensure concurrent invocation */
    await new Promise(r => setTimeout(r, 25));
    return 'Lambda is warm!';
    
  }

  ... add lambda logic after
}

Deployment

WarmUp supports serverless deploy.

Packaging

WarmUp supports serverless package.

By default, the WarmUp function is packaged individually and it uses a folder named _warmup to store duiring the packaging process, which is deleted at the end of the process.

If you are doing your own package artifact you can set the cleanFolder option to false and include the _warmup folder in your custom artifact.

Gotchas

The WarmUp function use normal calls to the AWS SDK in order to keep your lambdas warm. By deafult, the WarmUp function is deployed outside of any VPC so it can reach AWS API. If you use the VPC option to deploy your WarmUp function to a VPC subnet it will need internet access. You can do it by using an Internet Gateway or a Network Address Translation (NAT) gateway.

Cost

You can check the Lambda pricing and CloudWatch pricing or can use the AWS Lambda Pricing Calculator to estimate the monthly cost

Example

If you want to warm 10 functions, each with memorySize = 1024 and duration = 10, using the default settings (and we ignore the free tier):

WarmUp: runs 8640 times per month = $0.18
10 warm lambdas: each invoked 8640 times per month = $14.4
Total = $14.58

CloudWatch costs are not in this example because they are very low.

Contribute

Help us making this plugin better and future proof.

Clone the code
Install the dependencies with npm install
Create a feature branch git checkout -b new_feature
Add your code and add tests if you implement a new feature
Validate your changes npm run lint and npm test (or npm run test-with-coverage)

License

This software is released under the MIT license. See the license file for more details.

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