RFC: Environment variable based code injection

[ryanolee]

Intro:

Currently lambda@edge has no support for bundling environment variables along side code. It is commonly recommended when using lambda@edge to use custom headers on origin requests to pass these variables instead. This is often fairly finicky to set up and cannot be used from viewer-request events.

Proposal:

When generating the code bundles for the various lambda functions an option is given to inject environment variables into the process.env by adding a proper definition to the file so they can be set at runtime.

An example of how this might look can be seen below

functions:
  onViewerRequest:
    handler: src/handler.onViewerRequest
    memorySize: 128
    timeout: 5
    lambdaAtEdge:
      distribution: 'TestDistribution'
      eventType: 'viewer-request'
      pathPattern: '/preflight'
      injectEnv: true

a new (optional) option injectEnv would be added to the configuration for lambdaAtEdge. This would default to false and all existing installations of the library should remain uneffected.

When the injectEnv option is set to true the environment variables associated with the function would automatically be prepended (directly in code) to the package. This would effectively bypass the existing limitation.

Caveats

Given this deals with bundled code there are a few limitations to this approach:

• Injected env vars would be accessible in the S3 deployment bucket
• Env vars can only reliably be injected at a file level inside the package. Meaning files with multiple lambdas in a file would be
  fairly hard to reconcile especially when mixed with regular lambdas. In theory we could change the function entry point on a per
  file basis and make sure the bundle is properly split for each function so the ENV is properly isolated.
• Changing env vars would still require a redeploy to cloud formation. However env vars could still be changed and deployed to
  from CI environments

POC:

For now a fairly basic POC has been put together to demonstrate how this might work #77

[ryanolee]

Hi @jthomerson ,
I would be grateful if you could share your opinions on if you think a feature like this would be a reasonable addition to the existing plugin. 🚀
Thanks in advance!
-Ryan

[khenriksson]

+1 on this. I've recently had a need for using the environment variable in an edge function, this would be really helpful in that case.

[rodrigogs]

+1 on this. I tried the native Serverless method, but turns out that this plugin still much better.