grpc-streaming-test-author

Overview

Streaming RPCs are where gRPC clients and servers most often diverge from the proto contract - message ordering, completion signalling, cancellation, and partial-failure semantics are all testable surfaces. gRPC defines four streaming patterns - unary, server-streaming, client-streaming, bidirectional - per grpc.io/docs/what-is-grpc/core-concepts/; Step 1 classifies each RPC by pattern.

This skill walks through producing a comprehensive test suite from the proto file. It composes grpc-mock for the test harness and references/status-codes.md for status-code assertions; live-server (non-bufconn) patterns are in references/wire-level-testing.md.

When to use

• Adding tests for a new streaming RPC.
• Auditing an existing streaming-RPC test suite - is each pattern's required category covered?
• Investigating a streaming-RPC bug - minimal repro from the test matrix.
• PR review of changes to streaming RPCs (the proto-evolution rules on stream changes are subtle - per buf-cli-lint-breaking-build references/versioning-strategy.md).

Step 1 - Classify each RPC

Walk the .proto file and tabulate:

service Chat {
  rpc Send(Message) returns (Ack);                         // unary
  rpc Subscribe(SubReq) returns (stream Event);            // server streaming
  rpc Upload(stream Chunk) returns (UploadResult);         // client streaming
  rpc Conversation(stream Message) returns (stream Reply); // bidi
}

RPC	Pattern	Required test categories
Send	unary	success, every status code per references/status-codes.md
Subscribe	server-stream	success, ordering, server-side close after N messages, server-side mid-stream error, client-side cancel mid-stream, deadline-exceeded mid-stream
Upload	client-stream	success, server completes before client finishes, server-side error mid-upload, client-side cancel before send, empty stream
Conversation	bidi	success, ordering per direction, client closes send while still receiving, server closes send while still receiving, both close, deadline mid-conversation, error mid-conversation

Step 2 - Test categories per pattern

Unary (covered for completeness)

def test_send_returns_ok(stub):
    resp = stub.Send(Message(body="hi"))
    assert resp.ok

def test_send_returns_invalid_argument(stub):
    with pytest.raises(grpc.RpcError) as exc:
        stub.Send(Message(body=""))
    assert exc.value.code() == grpc.StatusCode.INVALID_ARGUMENT

Server-streaming

Ordering preservation - per gRPC docs: "gRPC guarantees message ordering within an individual RPC call."

def test_subscribe_preserves_ordering(stub_with_fake):
    stub_with_fake.fake_response_stream = [
        Event(seq=1), Event(seq=2), Event(seq=3),
    ]
    received = list(stub_with_fake.Subscribe(SubReq()))
    assert [e.seq for e in received] == [1, 2, 3]

Server closes after N messages - completion is the finalisation signal:

def test_subscribe_completes_after_finite_stream(stub_with_fake):
    stub_with_fake.fake_response_stream = [Event(seq=1), Event(seq=2)]
    events = list(stub_with_fake.Subscribe(SubReq()))
    # If list(...) terminates, the server-side close was signalled.
    assert len(events) == 2

Server-side error mid-stream:

def test_subscribe_propagates_error_mid_stream(stub_with_fake):
    stub_with_fake.fake_response_stream = [Event(seq=1)]
    stub_with_fake.fake_post_yield_status = grpc.StatusCode.INTERNAL
    events_iter = stub_with_fake.Subscribe(SubReq())
    # Receive first event OK
    next(events_iter)
    # Subsequent receive fails with INTERNAL per references/status-codes.md
    with pytest.raises(grpc.RpcError) as exc:
        next(events_iter)
    assert exc.value.code() == grpc.StatusCode.INTERNAL

Client cancel mid-stream:

def test_client_cancel_mid_stream(stub_with_fake):
    stub_with_fake.fake_response_stream = [Event(seq=i) for i in range(100)]
    events_iter = stub_with_fake.Subscribe(SubReq())
    next(events_iter)  # receive one
    events_iter.cancel()
    with pytest.raises(grpc.RpcError) as exc:
        next(events_iter)
    assert exc.value.code() == grpc.StatusCode.CANCELLED

Deadline-exceeded mid-stream:

def test_subscribe_deadline_exceeded(stub_with_slow_fake):
    events_iter = stub_with_slow_fake.Subscribe(SubReq(), timeout=0.1)
    with pytest.raises(grpc.RpcError) as exc:
        for _ in events_iter:
            pass
    assert exc.value.code() == grpc.StatusCode.DEADLINE_EXCEEDED

Client-streaming

Categories: success, server completes before client finishes (further Send yields io.EOF), server-side error mid-upload, client-side cancel before send, empty stream. Go bufconn skeletons: references/test-skeletons.md.

Bidirectional

The two streams operate independently, so test each direction plus a concurrent send/recv case. Categories: ordering per direction, client closes send while still receiving (CloseSend), server closes send while still receiving, both close, deadline mid-conversation, error mid-conversation. Go skeletons: references/test-skeletons.md.

Step 3 - Coverage matrix

For each streaming RPC, generate the matrix:

                              Send  Subscribe  Upload  Conversation
                              ----  ---------  ------  ------------
success                        X       X        X         X
ordering preserved             -       X        X         X (both)
N msgs then close              -       X        X         X
mid-stream server error        -       X        X         X
deadline exceeded              X       X        X         X
client cancel                  X       X        X         X
server cancel                  -       X        X         X
empty stream                   -       X        X         X
client half-close              -       -        -         X
server half-close              -       -        -         X

Empty cells in covered patterns = coverage gap. PR must justify or add a test.

Step 4 - Pick the test harness

Per grpc-mock:

Language	Harness
Go	bufconn in-process server + t.Cleanup
Python	pytest fixture with [::]:0 port + iterator-based stream API
JVM	InProcessServerBuilder + StreamObserver
Node	@grpc/grpc-js server with bindAsync("127.0.0.1:0") + call.write / call.end

Don't use mockgen / interface-mocks for streaming - they skip the marshalling + ordering guarantees that the streaming contract depends on.

Step 5 - Emit the test directory

tests/grpc-streaming/
  __init__.py
  conftest.py                   # shared fixtures (in-process server)
  test_subscribe.py             # server-streaming
  test_upload.py                # client-streaming
  test_conversation.py          # bidi
  test_send.py                  # unary (sibling)
  README.md                     # the coverage matrix

The README.md should document the matrix from Step 3 so reviewers and new contributors can see at a glance what is and isn't covered.

Streaming evolution - version-safety reminder

Per the versioning-strategy reference in buf-cli-lint-breaking-build, changing an RPC's streaming pattern is always breaking:

service Chat {
-  rpc Subscribe(SubReq) returns (Event);
+  rpc Subscribe(SubReq) returns (stream Event);
}

The wire format differs, so old clients won't parse the new response. Verify breaking-change detection via buf-cli-lint-breaking-build catches this with FILE or PACKAGE category.

Anti-patterns

Anti-pattern	Why it fails	Fix
Single happy-path test per streaming RPC	Misses ordering / cancellation / deadline categories	Use Step 2's per-pattern matrix
Test asserts on the last message only	Misses ordering bugs in earlier messages	Collect entire stream, assert on the full sequence
time.sleep to wait for server messages	Race-prone; flakes in CI	Use channel close / iterator exhaustion as completion signal
Cancellation test sleeps then cancels	Race: server may finish before cancel	Inject a controllable blocker in the fake server
No deadline-exceeded test	Production deadlines surface in mid-stream	Always include - per references/status-codes.md, DEADLINE_EXCEEDED is its own code
Mock at interface level for streaming	Skips marshalling + ordering	In-process server only for streaming
Bidi test where client and server are deterministic-interleaved	Misses race conditions inherent in "operate independently"	One test per direction + one test with concurrent send/recv
Don't CloseSend() in client-streaming tests	Server waits forever	Always close the send side explicitly

Limitations

• Wire-level fault injection. Partial-byte cutoffs and middlebox-induced disconnects aren't reachable through bufconn / InProcessServer. For these, use toxiproxy + a real server; live-server patterns in references/wire-level-testing.md.
• Backpressure semantics differ per stack. Go bufconn buffer size affects when Send blocks; Java StreamObserver is push-based; Python is iterator-pull. Tests are stack-specific.
• HTTP/2 flow control isn't exercised. In-process transports skip H2 framing; flow-control bugs need real transports.
• Doesn't test contract compliance. Mocks reflect what this test expects. Real-server contract tests are separate; see protobuf-compat-checking (in the qa-contract-testing plugin).
• Concurrent-client stress tests not in scope. For that see ghz-load (unary load) or hand-rolled multi-client streaming harnesses.

References

• gRPC core concepts (four streaming patterns): grpc.io/docs/what-is-grpc/core-concepts/.
• Status-code assertions: references/status-codes.md.
• Wire-level / live-server streaming patterns: references/wire-level-testing.md (+ references/status-codes-metadata-load.md).
• Test harness: grpc-mock.
• Proto evolution rules + breaking-change detection: buf-cli-lint-breaking-build (references/versioning-strategy.md).