---
name: grpc-streaming-test-author
description: "Workflow-driven skill that builds gRPC streaming-RPC test suites from a proto definition. Classifies each RPC by streaming pattern (unary, server-streaming, client-streaming, bidi), then for each streaming pattern emits the required test categories - ordering preservation, completion semantics (server closes after stream end, client closes by half-close), cancellation behaviour, deadline handling, and partial-stream-failure scenarios. Produces test skeletons for Go (bufconn + Send/Recv), Python (iterators), JVM (StreamObserver), Node (call.write/end). Use when adding tests for a new streaming RPC, or auditing an existing streaming-RPC suite for uncovered categories."
---

# 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.

Per
[grpc.io/docs/what-is-grpc/core-concepts/](https://grpc.io/docs/what-is-grpc/core-concepts/),
gRPC defines four streaming patterns:

| Pattern | Proto |
|---|---|
| Unary | `rpc M(R) returns (X);` |
| Server streaming | `rpc M(R) returns (stream X);` |
| Client streaming | `rpc M(stream R) returns (X);` |
| Bidirectional | `rpc M(stream R) returns (stream X);` |

This skill walks through producing a comprehensive test suite
from the proto file. It composes
[`grpc-mock`](../grpc-mock/SKILL.md) for the test harness and
[`grpc-status-code-mapping-reference`](../grpc-status-code-mapping-reference/SKILL.md)
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`](../protobuf-versioning-strategy-reference/SKILL.md)
  rules on `stream` changes are subtle).

## Step 1 - Classify each RPC

Walk the `.proto` file and tabulate:

```proto
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 [`grpc-status-code-mapping-reference`](../grpc-status-code-mapping-reference/SKILL.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)

```python
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."

```python
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:

```python
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**:

```python
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**:

```python
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**:

```python
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

**Success** - Go bufconn example:

```go
func TestUpload_Success(t *testing.T) {
    fake := &fakeUploader{accept: 3}
    client := setupClient(t, fake)

    stream, err := client.Upload(context.Background())
    if err != nil { t.Fatal(err) }

    chunks := []*pb.Chunk{{Data: []byte("a")}, {Data: []byte("b")}, {Data: []byte("c")}}
    for _, c := range chunks {
        if err := stream.Send(c); err != nil { t.Fatal(err) }
    }
    result, err := stream.CloseAndRecv()
    if err != nil { t.Fatal(err) }
    if result.Bytes != 3 { t.Fatalf("got %d, want 3", result.Bytes) }
}
```

**Server completes before client finishes** - per gRPC docs:
server response may arrive "typically but not necessarily after
it has received all the client's messages":

```go
func TestUpload_ServerCompletesEarly(t *testing.T) {
    fake := &fakeUploader{completeAfter: 1}
    client := setupClient(t, fake)

    stream, _ := client.Upload(context.Background())
    stream.Send(&pb.Chunk{Data: []byte("a")})

    // Sending more after server completes should yield io.EOF
    err := stream.Send(&pb.Chunk{Data: []byte("b")})
    if err != io.EOF {
        t.Fatalf("got %v, want io.EOF", err)
    }
    result, _ := stream.CloseAndRecv()
    if result.Bytes != 1 { t.Fatalf("got %d, want 1", result.Bytes) }
}
```

**Empty stream**:

```go
func TestUpload_EmptyStream(t *testing.T) {
    client := setupClient(t, &fakeUploader{})
    stream, _ := client.Upload(context.Background())
    result, err := stream.CloseAndRecv()
    if err != nil { t.Fatal(err) }
    if result.Bytes != 0 { t.Fatalf("got %d, want 0", result.Bytes) }
}
```

### Bidirectional

Per gRPC docs: "The two streams operate independently, so
clients and servers can read and write in whatever order they
like."

**Ordering per direction**:

```go
func TestConversation_Ordering(t *testing.T) {
    fake := &fakeChatter{
        clientMsgs: []*pb.Message{},
        replies: []*pb.Reply{{Seq: 1}, {Seq: 2}, {Seq: 3}},
    }
    client := setupClient(t, fake)
    stream, _ := client.Conversation(context.Background())

    // Client sends 3 messages
    for i := 0; i < 3; i++ {
        stream.Send(&pb.Message{Seq: int32(i)})
    }
    stream.CloseSend()

    // Server sends back 3 replies in order
    var got []int32
    for {
        r, err := stream.Recv()
        if err == io.EOF { break }
        if err != nil { t.Fatal(err) }
        got = append(got, r.Seq)
    }
    want := []int32{1, 2, 3}
    if !reflect.DeepEqual(got, want) {
        t.Fatalf("got %v, want %v", got, want)
    }
}
```

**Client closes send while still receiving**:

```go
func TestConversation_ClientHalfClose(t *testing.T) {
    // Server keeps sending after client CloseSend()
    fake := &fakeChatter{repliesAfterCloseSend: []*pb.Reply{{Seq: 99}}}
    client := setupClient(t, fake)
    stream, _ := client.Conversation(context.Background())
    stream.Send(&pb.Message{Seq: 0})
    stream.CloseSend()  // half-close: no more sends, still receiving

    r, err := stream.Recv()
    if err != nil { t.Fatal(err) }
    if r.Seq != 99 { t.Fatalf("got %d, want 99", r.Seq) }
}
```

**Cancellation from either side**:

```go
func TestConversation_ServerSideCancel(t *testing.T) {
    fake := &fakeChatter{cancelAfterMsg: 1}
    client := setupClient(t, fake)
    stream, _ := client.Conversation(context.Background())
    stream.Send(&pb.Message{Seq: 0})

    _, err := stream.Recv()
    st, _ := status.FromError(err)
    if st.Code() != codes.Cancelled {
        t.Fatalf("got %v, want Cancelled", st.Code())
    }
}
```

## 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`](../grpc-mock/SKILL.md):

| 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
[`protobuf-versioning-strategy-reference`](../protobuf-versioning-strategy-reference/SKILL.md),
changing an RPC's streaming pattern is **always breaking**:

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

The wire format differs: streaming uses HTTP/2's length-delimited
frames per message; unary uses one. Old clients won't parse the
new response. Verify breaking-change detection via
[`buf-cli-lint-breaking-build`](../buf-cli-lint-breaking-build/SKILL.md)
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 [`grpc-status-code-mapping-reference`](../grpc-status-code-mapping-reference/SKILL.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.
- **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`](../ghz-load/SKILL.md) (unary load) or hand-rolled
  multi-client streaming harnesses.

## References

- gRPC core concepts (four streaming patterns):
  [grpc.io/docs/what-is-grpc/core-concepts/](https://grpc.io/docs/what-is-grpc/core-concepts/).
- Status-code assertions:
  [`grpc-status-code-mapping-reference`](../grpc-status-code-mapping-reference/SKILL.md).
- Test harness:
  [`grpc-mock`](../grpc-mock/SKILL.md).
- Proto evolution rules:
  [`protobuf-versioning-strategy-reference`](../protobuf-versioning-strategy-reference/SKILL.md).
- Breaking-change detection:
  [`buf-cli-lint-breaking-build`](../buf-cli-lint-breaking-build/SKILL.md).
- Sibling wire-level streaming patterns:
  `grpc-streaming-tests`.