[Feature]: [Testing] Emulator should generate real-shaped durable execution ARNs (embed execution name) #686

What would you like?

Context

Reported downstream in aws/aws-sam-cli#9200: a user who starts a local durable execution with sam local invoke --durable-execution-name test cannot correlate that name with anything afterwards — sam local execution history/get take an ARN, and the emulator's ARN contains no trace of the name.

Current behavior

The emulator generates a placeholder ARN in Execution.new() (execution.py):

# make a nicer arn
# Pattern: arn:(aws[a-zA-Z-]*)?:lambda:...:durable-execution:...
return Execution(
    durable_execution_arn=str(uuid4()) + "/" + (input.invocation_id or str(uuid4())),
    ...
)

Two problems:

1. It doesn't match the real service ARN shape, so the execution name (and function name) appear nowhere in the identifier. Locally, DurableExecutionName is effectively write-only.
2. It conflates the second path segment with the invocation ID; the real ARN's last segment is a distinct execution ID.

(The comment's intended pattern is also stale — it predates the current public API shape.)

Expected behavior

Generate ARNs matching the public DurableExecutionArn pattern from the Lambda service model:

arn:{partition}:lambda:{region}:{account}:function:{name}:{qualifier}/durable-execution/{execution-name}/{execution-id}

Everything needed is already on StartDurableExecutionInput (account_id, function_name, function_qualifier, execution_name are required fields). Region can be fixed to us-east-1, consistent with the hardcoded FunctionArn in executor.py's GetDurableExecution response.

Possible Implementation

• Null/omitted execution name: callers (e.g. SAM CLI) may send "ExecutionName": null. Generate a name (uuid) when absent — matches cloud behavior and also fixes DurableExecutionName being null in GetDurableExecution responses.
• Execution ID: use a fresh uuid4(), not invocation_id.
• Routing is already safe: Route.from_string splits on / before URL-decoding, so a percent-encoded ARN stays one segment — the same mechanism that handles today's one-slash placeholder. botocore percent-encodes / in non-greedy URI params.
• Downstream (sam-cli) is already shape-agnostic after fix(tests): accept '/' in durable execution ARN regex aws-sam-cli#9038 and fix(local-lambda): accept documented Lambda DurableExecutionArn shape aws-sam-cli#9040 (\S+ ARN regex, Flask <path:> routes).
• Store/runner treat the ARN as an opaque key; only tests asserting the placeholder shape need updating.

Questions to resolve:

• Execution-name idempotency: mimic cloud idempotency behaviour — in the service, starting a durable execution with an ExecutionName that matches an existing execution for the same function is an idempotent start (the existing execution/ARN is returned rather than a new one created). The emulator currently creates a duplicate with a distinct ARN sharing the same name segment. Confirm the exact service semantics (idempotency window, retention, input-mismatch handling) and implement to match.
• Function name fidelity: SAM CLI hardcodes "FunctionName": "function" in its start payload, so local ARNs would read function:function:$LATEST/.... Decide whether to pair this with a sam-cli change to pass the real logical ID so the ARN is actually informative.

Blocked by

A new emulator image release is required for this to reach SAM CLI users (:latest on ECR is still v1.2.1, built 2026-05-20 from the pre-migration repo) — see #487.