[nikhilchowdary]

Add x-goog-is-transaction routing header in Python ndb SDK to differentiate transactional requests and non-transactional requests from each other. Having a routing header for transactions helps in routing grpc requests at GFE.

This commit attaches the x-goog-is-transaction routing header to all RPCs

Set to true for transactional RPCs
-- beginTransaction
-- rollback
-- transactional commit
-- transactional lookup
-- transactional runQuery
-- transactional runAggregationQuery.

Set to false for non transactional RPCs
-- allocateIds
-- reserveIds
-- non transactional commit
-- non transactional lookup
-- non transactional runQuery
-- non transactional runAggregationQuery.

cc: @MarkDuckworth

[gemini-code-assist]

Code Review

This pull request adds transaction detection to Datastore RPC requests by appending an x-goog-is-transaction metadata header. It introduces helper functions to determine if a request is transactional and updates various Datastore API calls and unit tests accordingly. The review feedback suggests optimizing the metadata generation in _add_routing_info by using itertools.chain to avoid copying overhead from multiple tuple concatenations.

[gemini-code-assist]

In performance-critical paths like Datastore RPC metadata generation, multiple tuple concatenations (e.g., result = result + (...)) introduce unnecessary copying overhead. Using itertools.chain to combine the metadata collections and converting to a tuple once at the end is much more efficient.

Suggested change

	result = tuple(metadata)
	if header_params:
	return tuple(metadata) + (
	result = result + (
	gapic_v1.routing_header.to_grpc_metadata(header_params),
	)
	return tuple(metadata)
	if is_transaction is None:
	is_transaction = _is_transaction_request(request)
	return result + (
	("x-goog-is-transaction", "true" if is_transaction else "false"),
	)
	additional = []
	if header_params:
	additional.append(
	gapic_v1.routing_header.to_grpc_metadata(header_params)
	)
	if is_transaction is None:
	is_transaction = _is_transaction_request(request)
	additional.append(
	("x-goog-is-transaction", "true" if is_transaction else "false")
	)
	import itertools
	return tuple(itertools.chain(metadata, additional))

References

1. In performance-critical paths, avoid combining collections by converting them to tuples and concatenating them (e.g., tuple(a) + tuple(b)), as this introduces copying overhead. Instead, use lazy iterators like itertools.chain to iterate over multiple collections efficiently.