[DRAFT] feat: support routing cookie

[daniel-sanche]

Proof of concept of the routing cookie feature. Grab routing cookie metadata from ffailed attempts, and pass it in with future attempts

TODO:

• feature flags?
• increment attempt counter?
• keep cookie value between operations?
• tests

[gemini-code-assist]

Summary of Changes

Hello @daniel-sanche, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces foundational support for a routing cookie feature. It establishes a mechanism to capture routing cookie metadata from failed operation attempts and subsequently inject this information into future retry attempts. This aims to improve routing efficiency and consistency across retries by leveraging server-provided routing hints.

Highlights

• Routing Cookie Data Model: Introduced a new routing_cookie field to the ActiveOperationMetric dataclass, allowing for the storage of routing cookie key-value pairs (strings or bytes) with a default empty dictionary.
• Routing Cookie Extraction: Modified the tracked_retry.py module to extract x-goog-cbt-cookie headers from RPC error metadata (both trailing and initial) and store them in the operation.routing_cookie for subsequent retry attempts.
• Routing Cookie Injection: Implemented logic in metrics_interceptor.py to iterate through the operation.routing_cookie and add each key-value pair as metadata to client_call_details before the continuation of unary-stream calls, ensuring routing cookies are propagated.

Changelog

• google/cloud/bigtable/data/_async/metrics_interceptor.py
  • Added a loop to iterate through operation.routing_cookie and add each key-value pair to client_call_details.metadata before invoking the continuation.
• google/cloud/bigtable/data/_metrics/data_model.py
  • Added routing_cookie: dict[str, str|bytes] = field(default_factory=dict) to the ActiveOperationMetric class to store routing cookie data.
• google/cloud/bigtable/data/_metrics/tracked_retry.py
  • Refactored metadata processing to first create a dictionary from RPC error metadata.
  • Added logic to identify and extract headers starting with "x-goog-cbt-cookie" from the metadata dictionary and assign them to operation.routing_cookie.

Activity

• This pull request is currently marked as a [DRAFT], indicating it's a work in progress or a proof of concept.
• The author has outlined TODO items, specifically questioning whether to increment an attempt counter and if the cookie value should persist between operations, suggesting further design considerations or implementation details are pending.

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[gemini-code-assist]

Code Review

This pull request introduces support for routing cookies as a proof-of-concept. The changes add a routing_cookie field to ActiveOperationMetric, extract cookie information from failed request metadata, and include it in subsequent requests.

My review has identified a couple of areas for improvement:

• The implementation for adding the cookie to request metadata in metrics_interceptor.py is not compatible with the synchronous client and doesn't follow gRPC interceptor best practices. I've suggested a fix.
• The logic for extracting the cookie from response metadata in tracked_retry.py could be refactored to avoid code duplication. I've recommended refactoring it into a shared helper function to improve maintainability and ensure consistency.

[gemini-code-assist]

The client_call_details object is immutable (it's a namedtuple). While metadata might be a mutable object in the async version, you should not modify it in place. The idiomatic way to add metadata in an interceptor is to create a new ClientCallDetails object with the updated metadata.

Furthermore, the add method does not exist on the metadata list in the synchronous version of the client, which will cause an AttributeError. The synchronous metadata is a list of tuples.

To fix this, you should create a copy of the metadata, append the new headers, and then create a new ClientCallDetails object using _replace. This new object will then be used in the continuation call.

Suggested change

	for k,v in operation.routing_cookie.items():
	client_call_details.metadata.add(k, v)
	metadata = list(client_call_details.metadata or []) + list(operation.routing_cookie.items())
	client_call_details = client_call_details._replace(metadata=metadata)

[gemini-code-assist]

This logic for processing metadata and extracting the routing cookie is also needed in _track_terminal_error. To avoid code duplication and ensure consistency, consider extracting this block into a helper function that can be called from both _track_retryable_error and _track_terminal_error.