goyder / sls-layers-bug

Repo to demonstrate bug in applying layers to lambdas.

Serverless Layers Bug

Layers can be utilised to deal with large dependencies. For example, this is utilised in the serverless-python-requirements plugin.

However,

• if an already-large lambda is updated to use a layer for dependencies,
• and the dependencies are more than half the size limit for code and dependencies (250 MB),

the deployment may fail as both the layer and the already-large lambda are both applied simultaneously, exceeding the size limit.

This is demonstrated in this repo and serverless stack.

Process to demonstrate issue

In each instance, the stack is being deployed via sls deploy --stage <stage_name>.

1. Update the provider.cfnRole option in the serverless.yml file.
2. Deploy tag v0.1 - Initial, layer-less deployment. This should deploy successfully, creating a single, large lambda with all dependencies bundled within it.
3. Deploy tag v0.2 - Large lambda with layer. This will not deploy due to size limitations, as the large layer is (presumably incorrectly) applied to the large lambda.
   • This is the bug being demonstrated.
4. Deploy tag v0.3 - Small lambda without layer. This will deploy successfully, creating a single, small, non-functional lambda without a layer attached, and a requirements layer.
   • This is a non-functional intermediate stage, required to deploy the layer without exceeding the file size limitation.
5. Deploy tag v0.4 - Small lambda with layer. This will deploy successfully, creating a single, small, functional lambda with a layer attached, and a requirements layer.
   • This is the desired end-state.