mxaccessgw/docs/WorkerFrameProtocol.md

Worker Frame Protocol

The gateway uses the worker frame protocol to move WorkerEnvelope protobuf messages over a bidirectional named pipe. The frame layer is deliberately small: it handles message boundaries, size limits, protobuf parsing, and envelope validation before higher-level worker client code routes commands, replies, events, and faults.

Frame Format

Each frame starts with a four-byte little-endian unsigned payload length, followed by the serialized WorkerEnvelope payload:

uint32 little-endian payload_length
payload_length bytes protobuf WorkerEnvelope

The reader rejects zero-length payloads and payloads larger than the configured maximum before allocating the payload buffer. The default maximum is the 16 MiB public gRPC cap plus a 64 KiB envelope-overhead reserve (16842752 bytes) so a maximally-sized accepted gRPC payload always fits one worker frame once wrapped in a WorkerEnvelope.

The gateway is the source of truth for this maximum: it conveys the negotiated value in the handshake as GatewayHello.max_frame_bytes, and the worker adopts it as its WorkerFrameProtocolOptions.MaxMessageBytes instead of a hard-coded default. A max_frame_bytes of 0 (an older gateway that never set the field) means "use the worker's built-in default". This keeps both ends framing to the same limit rather than depending on matched compile-time constants.

Every worker-to-gateway frame must serialize within this limit, control replies included, so reply builders truncate to fit rather than emit a frame the writer will reject. WorkerPipeSession pre-sizes a DrainEvents reply below the negotiated maximum (less a fixed envelope/reply-wrapper reserve) and reports the truncation in the reply's DiagnosticMessage; the caller contract is to repeat DrainEvents until it returns an empty reply. Should a reply still overshoot, MessageTooLarge at the reply-write seam is answered with a small InvalidRequest reply for that correlation, not with session teardown — no diagnostics command may kill a session.

An oversized event frame is the deliberate exception. Such an event is undeliverable end to end (the pipe maximum sits only the envelope-overhead reserve above the public gRPC cap), so the session faults: the worker logs the event's identity and sizes — never its value — writes a WorkerFault with category ProtocolViolation and command method EventDrain, then exits. Remediation is raising MxGateway:Worker:MaxMessageBytes for that workload.

A per-frame rejection does not consume an envelope sequence. The writer stamps a candidate sequence, runs the empty-payload and size checks against the stamped envelope, and commits the counter only immediately before the stream write, so the sequences observed on the wire stay contiguous across rejections and an operator reading a pipe capture never sees a phantom gap.

Envelope Validation

WorkerFrameReader and WorkerFrameWriter validate each envelope against the owning session before returning or writing it:

• protocol_version must match the configured worker protocol version,
• session_id must match the owning gateway session,
• the envelope must contain one typed body value.

Protocol violations throw WorkerFrameProtocolException with a WorkerFrameProtocolErrorCode so callers can distinguish malformed frames, oversized frames, protocol version mismatches, and session mismatches.

Write Scheduling And Sequencing

This section covers write scheduling (priority classes, enqueue-then-contend, flush coalescing) and sequencing (write-time stamping) together, because both are properties of the same single write lock.

WorkerFrameWriter is a two-class cooperative priority scheduler (WorkerFrameWritePriority.Control and .Event), not a strict per-kind priority order. A caller enqueues its frame into the control or event queue under a lock, then contends for a single write lock; whichever caller wins drains every frame queued at that moment, control frames first and each class in FIFO order, so a command reply, fault, heartbeat, or shutdown acknowledgement is never delayed behind a backlog of queued events. Priority only reorders which frame writes next — it does not affect the sequence value a frame receives (see below), so a caller cannot infer priority class from the wire sequence.

The envelope Sequence is stamped by the draining lock-holder at the actual moment of writing, not when the frame is enqueued, so the on-wire order and the stamped sequence always agree regardless of caller concurrency or priority reordering. Stamping uses peek-stamp-commit: a candidate sequence is assigned and the frame is validated (size, non-empty payload) against that stamped value, but the counter is committed only immediately before the stream write. A per-frame rejection therefore leaves the counter untouched — the next accepted frame reuses the candidate number, so the wire sequence stays contiguous across rejections and an operator reading a pipe capture never sees a phantom gap from a rejected frame.

Two failure shapes are distinguished during a drain pass:

• Per-frame rejection (InvalidEnvelope, MessageTooLarge, ProtocolVersionMismatch, SessionMismatch) is specific to the one frame that failed validation or sizing. Nothing was written for it, so it fails only that frame's completion and draining continues with the next queued frame.
• Stream failure (anything else — a broken pipe, an I/O error) means the underlying stream itself is no longer trustworthy. It fails the frame that triggered it, every frame already written this batch but not yet flushed, and every frame still queued, then stops draining entirely so no caller waits forever on a stream that will not recover.

Flushes are coalesced across a drained batch: each frame in the batch is written to the stream without an individual flush, then one FlushAsync runs after the whole batch, and only then does every successfully-written frame's completion resolve — so a caller's WriteAsync still does not complete until its bytes are both written and flushed, but a batch that happened to contain several queued frames pays one flush instead of one per frame. In practice this coalescing currently engages only when multiple frames are queued at the moment a lock-holder starts draining. The event drain loop (WorkerPipeSession.RunEventDrainLoopAsync) awaits each drained event's WriteAsync individually before writing the next, so today at most one event frame is queued per drain pass and each event still costs its own flush; a dedicated batch write entry point that submits a whole drained event batch under one lock acquisition is designed but not yet landed, so a burst of N events currently costs N flushes on the event hot path, not one.

Cancellation semantics for a WriteAsync call that is still waiting for the write lock when its token fires are not yet defined at this layer — pending a fix that will tombstone the queued frame so a cancelled call is guaranteed never to reach the wire. Until that lands, a cancelled caller may still see its frame written by whichever caller next holds the lock.

Verification

The frame protocol lives in ZB.MOM.WW.MxGateway.Worker.Ipc (WorkerFrameReader, WorkerFrameWriter, WorkerFrameProtocolOptions) and is covered by src/ZB.MOM.WW.MxGateway.Worker.Tests/Ipc/WorkerFrameProtocolTests.cs. The worker is an x86 process, so build and test it with -p:Platform=x86.

Run the focused tests after changing the frame protocol:

dotnet test src/ZB.MOM.WW.MxGateway.Worker.Tests/ZB.MOM.WW.MxGateway.Worker.Tests.csproj -p:Platform=x86 --filter WorkerFrameProtocolTests

Run the x86 worker build because the frame protocol is part of ZB.MOM.WW.MxGateway.Worker:

dotnet build src/ZB.MOM.WW.MxGateway.Worker/ZB.MOM.WW.MxGateway.Worker.csproj -p:Platform=x86

Related Documentation

• Gateway Process Detailed Design
• Protobuf Contracts