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 one schedule event lambda that invokes all the service lambdas you select in a configured time interval (default: 5 minutes) or a specific time, forcing your containers to stay alive.

Setup

Installation

Install via npm in the root of your Serverless service:

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

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

plugins:
  - serverless-plugin-warmup

Global configuration

Add a warmup.enabled property to custom to enable/disable the warm up process by default for all the functions

Enable WarmUp in general:

custom:
  warmup:
    enabled: true

For a specific stage:

custom:
  warmup:
    enabled: production

For several stages:

custom:
  warmup:
    enabled: 
      - production
      - staging

Function-specific configuration

You can override the global enabled configuration on any function.

Enable WarmUp for a specific function

functions:
  hello:
    warmup:
      enabled: true

For a specific stage:

functions:
  hello:
    warmup:
      enabled: production

For several stages:

functions:
  hello:
    warmup:
      enabled:
        - production
        - staging

Do not warm-up a function if enabled is set to false:

custom:
 warmup:
   enabled: true

...

functions:
 hello:
   warmup:
     enabled: false

Other Options

Global options

• 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))
• timeout (default 10 seconds)
• prewarm (default false)

Options that can be overridden per function

• enabled (default false)
• payload (default { "source": "serverless-plugin-warmup" })
• 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)
    timeout: 20
    prewarm: true # Run WarmUp immediately after a deploymentlambda
    payload: 
      source: my-custom-source
      other: 20
    payloadRaw: true # Won't JSON.stringify() the source, may be necessary for Go/AppSync deployments
    concurrency: 5 # Warm up 5 concurrent instances

Options should be tweaked depending on:

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

Lambdas invoked by WarmUP will have event source serverless-plugin-warmup (unless otherwise specified above):

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

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 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: nodejs6.10
  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

Add an early callback call when the event source is serverless-plugin-warmup. You should do this early exit before running your code logic, it will save your execution duration and cost:

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
}

You can also check for the warmup event using the context variable. This could be useful if you are handling the raw input and output streams:

...

if(context.custom.source === 'serverless-plugin-warmup'){
  console.log('WarmUP - Lambda is warm!')
  return callback(null, 'Lambda is warm!')
}

...

If you're using the concurrency option you might consider adding a slight delay before returning when warming up to ensure your function doesn't return before all concurrent requests have been started:

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

  ... add lambda logic after
}

Deployment

Once everything is configured WarmUP will run on SLS deploy.

serverless deploy

Packaging

WarmUp also runs on SLS package.

If you are doing your own package artifact set the cleanFolder option to false and run

serverless package

This will allow you to extract the warmup NodeJS lambda file from the _warmup folder and add it in your custom artifact logic.

Gotchas

If you are deploying to a VPC, you need to use private subnets with a Network Address Translation (NAT) gateway (http://docs.aws.amazon.com/lambda/latest/dg/vpc.html). WarmUp requires this so it can call the other lambdas but this is applicable to any lambda that needs access to the public internet or to any other AWS service.

Cost

Lambda pricing here. CloudWatch pricing here. You can use AWS Lambda Pricing Calculator to check how much will cost you monthly.

Example

Free Tier not included + Default WarmUP options + 10 lambdas to warm, each with memorySize = 1024 and duration = 10:

• 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
• Lint with standard npm run lint

License

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