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/ (opens in new window); 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 grpc-status-code-mapping-reference for status-code assertions.

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 protobuf-versioning-strategy-reference rules on stream changes are subtle).

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
}

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 grpc-status-code-mapping-reference
    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:

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 protobuf-versioning-strategy-reference, 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

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.
• 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/ (opens in new window).
• Status-code assertions: grpc-status-code-mapping-reference.
• Test harness: grpc-mock.
• Proto evolution rules: protobuf-versioning-strategy-reference.
• Breaking-change detection: buf-cli-lint-breaking-build.
• Sibling wire-level streaming patterns: grpc-streaming-tests.