A harbour on your laptop

Lambda Cross-Account S3 Write - file sync · Open board

A small program with no desk of its own drops files onto someone else’s shelf. This lesson builds a smaller cousin of that mover on your own laptop, where no real shelf, account or bill exists.

Before you start. The rooms AWS runs for you gave you two rooms the harbour runs: a queue, a board where one program leaves a message until another takes it (hiding a taken message rather than deleting it), and Lambda, code AWS runs only while an event needs it. Lambda Cross-Account S3 Write - file sync showed a Lambda writing into S3, AWS’s storage, where each named storage area is a bucket, and Linux and the Shell: Processes, Files, Pipes, Permissions and Signals gave you the shell, the window where you type commands, and the container, an ordinary program that the machine walls off so it sees only its own files.

The idea in 30 seconds

LocalStack runs fake versions of AWS services as real little servers on your own machine, so code can be tested with no account and no bill.
An endpoint is the address a request is sent to; pointing it at LocalStack is the only change the same aws commands need.
A queue never calls anyone: an event source mapping is the setting that makes Lambda watch a queue and run your code with each batch of messages.
A cold start is the delay before a Lambda’s first run, spent fetching and starting its runtime; later runs skip it.
When a Lambda fails, the message is not deleted; it reappears after a pause and is tried again, and after a set number of failures a dead-letter queue catches it.
A local model checks your wiring and failure paths, not your permissions: here a role that never existed was accepted.

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