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SignalR Binary Protocol

AcBinaryHubProtocol (unsealed base) — custom IHubProtocol (name: "acbinary") replacing SignalR JSON+Base64 with AcBinarySerializer. General binary framing only — no consumer-specific logic.

AyCodeBinaryHubProtocol (derived) — project-specific consumer logic: SignalParams capture via OnArgumentRead, IsRawBytesData path, SignalDataType type resolution in ReadSingleArgument override.

Architecture (tag system, dispatch, request/response): SIGNALR.md Output writers (cached chunk, buffer states, chunk sizing): AyCode.Core/AyCode.Core/docs/BINARY/BINARY_WRITERS.md

Wire Format

[INT32 LE payload length] [1 byte message type] [message-specific fields]
Type Byte Fields
Invocation 1 nullable invocationId, target, arguments, streamIds, headers
StreamItem 2 invocationId, argument, headers
Completion 3 invocationId, nullable error, hasResult, optional result, headers
StreamInvocation 4 invocationId, target, arguments, streamIds, headers
CancelInvocation 5 invocationId, headers
Ping 6 (empty)
Close 7 nullable error, allowReconnect
Ack 8 sequenceId (INT64)
Sequence 9 sequenceId (INT64)

Argument framing: [INT32 LE arg length] [serialized bytes] — deferred deserialization (target parsed first → IInvocationBinder resolves types).

Strings: [VarUInt byte length] [UTF-8]. Nullable: 0=null, 1=value follows.

Headers: [VarUInt count] [key-value pairs...]. Count=0 → no headers.

Zero-Copy Write Pipeline

Writes directly to SignalR pipe's IBufferWriter<byte>, no intermediate byte[].

WriteMessage(HubMessage, IBufferWriter<byte> output)
├─ Reserve 4-byte outer length prefix
├─ BWO = BufferWriterBinaryOutput(output)  [standalone mode]
│  ├─ WriteByte(messageType)
│  ├─ WriteStringUtf8(invocationId, target)
│  ├─ WriteVarUInt(argCount)
│  ├─ Per argument:
│  │  ├─ byte[] (AcBinary) → raw length + bytes (no 0x44 wrapper — first byte is AcBinary FormatVersion 0x01, no VarUInt)
│  │  ├─ byte[] (other)    → tag (0x44) + raw bytes (no VarUInt — argLength implies size)
│  │  └─ object → FlushAndReset() → reserve INT32 arg prefix
│  │     → AcBinarySerializer.Serialize(value, output) → patch prefix
│  ├─ WriteStringArray(streamIds)
│  └─ WriteHeaders(headers)
├─ Patch outer length = BWO.Position + externalBytes
└─ BWO.Flush()

byte[] Write: isAcBinary Detection

When argument is byte[], the protocol checks if it's already AcBinary-serialized data:

var isAcBinary = byteArray.Length >= 2
    && byteArray[0] == AcBinarySerializerOptions.FormatVersion  // 1
    && (byteArray[1] & 0xF0) == BinaryTypeCode.HeaderFlagsBase; // 0x90
  • isAcBinary = true: [argLength=N][raw AcBinary bytes] — no 0x44 wrapper tag; first byte is the AcBinary FormatVersion 0x01 (the discriminator the receiver uses), reader deserializes directly
  • isAcBinary = false: [argLength=1+N][0x44 tag][raw bytes] — tag for type detection, no VarUInt (argLength implies size)

Dual BWO Pattern

Protocol and serializer each create own BufferWriterBinaryOutput on the same IBufferWriter. Sequential, never concurrent:

  1. Protocol BWO (standalone): framing — message type, strings, headers. Cached chunk, zero dispatch.
  2. FlushAndReset(): commits bytes to pipe, invalidates chunk.
  3. Serializer BWO (context mode): Serialize() creates internal BWO, acquires fresh chunk, writes, flushes.
  4. Protocol BWO re-acquires chunk on next write (via Grow).

Cost: one extra GetMemory per argument (nanoseconds). Benefit: zero-copy end-to-end, no intermediate byte[], no wrapper class.

Why two BWOs: serializer writes must live on BinarySerializationContext (sealed class) for JIT optimization — context owns its own BWO. See AyCode.Core/AyCode.Core/docs/BINARY/BINARY_WRITERS.md § "Why Writes Are on the Context".

Length Prefix Patching

var lengthSpan = output.GetSpan(4);
output.Advance(4);
// ... write payload ...
Unsafe.WriteUnaligned(ref lengthSpan[0], payloadLength);

Safe for PipeWriter — segments writable until FlushAsync.

GetMessageBytes caveat: ArrayBufferWriter initial capacity must include LengthPrefixSize to prevent resize after prefix reservation (stale span).

Read: Argument Deserialization

Base AcBinaryHubProtocol.ReadSingleArgument reads [INT32 argLength] [argBytes] from the pipe's ReadOnlySequence via SequenceReader<byte>:

ReadSingleArgument(SequenceReader, targetType):
  Read INT32 argLength
  if argLength == 0 → return null
  if argLength == 1 && first byte == 0 → return null  (null marker)

  argSlice = UnreadSequence.Slice(0, argLength)  — zero-copy reference
  Advance(argLength)

  1. byte[] fast-path:
     if first byte == BinaryTypeCode.ByteArray (0x44):
       return argSlice.Slice(1) as byte[]  — skip tag, rest is raw payload
       No VarUInt — argLength implies size

  2. Default: DeserializeFromSequence(argSlice, targetType, options)

AyCodeBinaryHubProtocol.ReadSingleArgument overrides with consumer-specific paths:

  (same argLength/null/slice logic as base)

  1. byte[] fast-path (same as base)

  2. IsRawBytesData path:
     if _currentSignalParams.IsRawBytesData == true:
       return SequenceToByteArray(argSlice)  — entire arg as raw byte[]
       Consumer (DataSource.PopulateMerge) handles deserialization

  3. Typed deserialization:
     if targetType == object && SignalDataType != null:
       resolve Type from SignalDataType (AssemblyQualifiedName)
     DeserializeFromSequence(argSlice, resolvedType, options)

SignalParams Capture

Base AcBinaryHubProtocol.ReadArguments calls OnArgumentRead(value, index) after each argument. AyCodeBinaryHubProtocol overrides this hook to capture SignalParams (arg[2]) for type-aware deserialization of subsequent args (arg[3] = data). Thread-safe: SignalR processes messages sequentially per connection.

SequenceToByteArray

Zero-copy when possible: if single-segment and backing array matches exactly → return the array directly. Otherwise ReadOnlySequence.ToArray().

SequenceBinaryInput (Multi-Segment Deserialization)

struct SequenceBinaryInput : IBinaryInputBase — reads ReadOnlySequence<byte> without linearizing. Lazy iteration via ReadOnlySequence.TryGet — zero ctor alloc, no pre-extracted segment array.

Context _buffer points directly to current segment's backing byte[] (zero-copy). Cross-boundary reads (value straddling segments) copy only affected bytes into ArrayPool-rented scratch. After scratch read, _afterCrossBoundary flag restores context to next segment's backing array.

Typical overhead — 225KB payload, 4096-byte segments: ~224.5KB zero-copy + ~500 bytes scratch at ~55 boundaries. Scratch is rented once (lazy, first boundary) and reused; Release() returns it to ArrayPool.

Known issues: AyCode.Core/AyCode.Core/docs/BINARY/BINARY_ISSUES.md

Configuration

Hub protocol settings via AcBinaryHubProtocolOptions (mutable class). Pass to ctor directly, or configure via DI in Program.cs (services.Configure<AcBinaryHubProtocolOptions>(opts => …)).

Property Default Purpose
SerializerOptions AcBinarySerializerOptions.Default Binary serializer options (also usable standalone via ToBinary/BinaryTo).
ProtocolMode Bytes Wire format and pipeline strategy — see BinaryProtocolMode below.
BufferSize 4096 Per-chunk size. 4 KB aligns with Kestrel's slab. Max 65535 (UINT16).
FlushPolicy Coalesced AsyncSegment flush strategy — see trade-off below.
FlushTimeout 10 s Per-flush wait limit. Timeout.InfiniteTimeSpan = disabled.
Name "acbinary" SignalR handshake protocol name. Client and server must match.
Logger null Optional ILogger; injected from DI when registered.

Inner AcBinarySerializerOptions defaults relevant for SignalR: UseGeneratedCode=true (hybrid source-gen + reflection), UseStringInterning=All, InitialBufferCapacity=16384.

FlushPolicy (AsyncSegment-only)

Value Peak memory Pro Con
FlushPolicy.PerChunk ~chunk × 1 Strictly bounded peak memory regardless of consumer speed; guaranteed end-to-end zero-copy. No producer/flush parallelism — wall-clock = sum of (serialize + flush) per chunk.
FlushPolicy.DoubleBuffered ~chunk × 2 Maximum producer/flush parallelism with bounded memory — wall-clock = max(serialize, flush) × N_chunks. Recommended for balanced streaming. Slow consumer propagates back as server-thread blocking at next Grow (bounded by FlushTimeout).
FlushPolicy.Coalesced (default) ~64 KB per window Batches chunks into ~64 KB flush windows: while a flush is in-flight, new chunks accumulate; when the window fills, one batched FlushAsync covers the whole window. Major throughput win — a 9.5 MB payload at 4 KB chunks fires ~150 flushes (one per window) instead of ~2 300 per-chunk flushes. Especially impactful on transports with non-trivial FlushAsync overhead (network sockets, Kestrel WebSocket, kernel TCP). Per-window peak memory ~64 KB; under heavy backpressure a chunk may fall back to an owned (copied) buffer, losing zero-copy for that chunk.

FlushTimeout rationale (10 s default)

  • AsyncSegment chunks are ≤ 65 KB (UINT16). Even GPRS-class links (~60 Kbit/s) transfer 65 KB in ~9 s — so any flush exceeding 10 s indicates a genuinely stuck consumer.
  • Pro: fast failure detection; server thread never blocks indefinitely.
  • Con: an unusually slow but otherwise healthy consumer will be disconnected — tune up for satellite / throttled links.
  • Complementary to SignalR's connection-level timeouts (ClientTimeoutInterval, KeepAliveInterval). Set FlushTimeout < ClientTimeoutInterval so this per-operation guard fires first.
  • Set to Timeout.InfiniteTimeSpan to fully disable (legacy behavior).

BinaryProtocolMode

enum BinaryProtocolMode — constructor parameter for AcBinaryHubProtocol, selects serialization + transport strategy:

Value Serialize Deserialize (non-WASM) Pro / Con
Bytes (default) ArrayBinaryOutput → byte[] → write to pipe as raw blob ArrayBinaryInput (single contiguous buffer via MemoryMarshal.TryGetArray zero-copy / pool-rent). Pro: simplest, fastest per-call, WASM-safe on both sides. Con: no zero-copy write, no pipeline overlap.
Segment BufferWriterBinaryOutput → directly to PipeWriter, chunk-by-chunk, single Flush at end Same as Bytes (unified ArrayBinaryInput receive path — _protocolMode affects send only). Pro: zero-copy write, WASM-safe. Con: no pipeline overlap — receiver must wait for full payload before deser starts.
AsyncSegment AsyncPipeWriterOutput → self-describing chunked framing [201][UINT16 size][data] per chunk + [202] end marker (or [203] CHUNK_ABORT on mid-stream serialize failure — fault isolation, see below), per-chunk FlushAsync with timeout-bounded sync-await AsyncPipeReaderInput (growing contiguous byte[] with sliding-window cycle); background Task.Run deserializes while chunks arrive. WASM: synchronous deser on CHUNK_END. Pro: zero-copy write + pipeline parallelism (ser / network / deser overlap); per-message fault isolation via CHUNK_ABORT. Con: send-side not WASM-compatible (see below); slow consumer propagates as server-thread blocking (bounded by FlushTimeout). Max chunk: 65535 bytes.

In AsyncSegment mode, WriteMessage dispatches to WriteMessageChunked which sends: (1) CHUNK_START — standard SignalR framing [INT32 len][200][original message with INT32 -1 for streamed arg], (2) N x CHUNK_DATA + final CHUNK_END — [201][UINT16 size][data] per chunk + [202] end marker, all emitted by AsyncPipeWriterOutput in framed mode (zero-copy via PipeWriter.Advance with 3-byte header reservation; protocol layer no longer writes its own [202] or extra FlushAsync). The receiver's TryParseChunkData accumulates into an AsyncPipeReaderInput (multiMessage:false — protocol parses [201]/[202] framing externally and feeds raw data via Feed); on non-WASM platforms a background Task.Run deserializes in parallel via AsyncPipeReaderInputAdapter, on WASM the deserializer runs synchronously on CHUNK_END over the already-buffered data.

Fault isolation (CHUNK_ABORT, [203]): if the streamed-arg serialize throws after CHUNK_START has been sent (e.g. a property-getter NRE on the data graph), WriteMessageChunked's catch emits a single [203] byte on the host PipeWriter instead of letting the exception propagate. The receiver's TryParseChunkData recognises [203] as a first-class abort: tears down the chunk-state safely (mirrors the [202] Complete → DeserTask observe → Dispose ordering), then surfaces the failure to the awaiting caller as a synthesised CompletionMessage.WithError(invocationId, ...) — the caller's task faults with an actionable error instead of waiting for a transport-level timeout. Blast radius = one message; the SignalR transport connection stays alive and other in-flight invocations on the same WebSocket are unaffected. If the abort marker itself can't be emitted (transport-level fault), behaviour falls back to the baseline (exception propagates, SignalR aborts the connection). Any other unexpected byte in chunk mode (not [201]/[202]/[203]) is a protocol-invariant violation: TryParseChunkData tears down safely and throws System.IO.InvalidDataException rather than guessing. Edge case: a mid-header serialize failure (post-[201][UINT16=N] commit, pre-data) is currently caught by the protocol-violation path; the robust graceful-abort fix is tracked in ../../../AyCode.Core/docs/BINARY/BINARY_ASYNCPIPE_TODO.md#accore-bin-t-z6n2.

In Bytes and Segment mode, the standard WriteMessage path is used.

WebAssembly compatibility

Send and receive paths handle WASM (OperatingSystem.IsBrowser()) asymmetrically — send strictly bound to _protocolMode; receive adapts to wire format, falls back synchronously when platform can't support the optimal strategy.

  • Send path: AsyncSegment is not supported on WebAssembly. AcBinaryHubProtocolOptions.Validate() throws PlatformNotSupportedException if IsBrowser && ProtocolMode == AsyncSegment (the AsyncPipeWriterOutput.SyncAwaitFlush sync-over-async pattern would block the single UI thread). WASM clients must use Bytes or Segment.
  • Receive path: works on WASM with any server-side mode (including AsyncSegment → chunked wire). TryParseChunkData detects the platform at runtime:
    • Non-browser: first CHUNK_DATA spawns a background Task.Run over an AsyncPipeReaderInput (pipeline parallelism — serialize / network / deserialize overlap). CHUNK_END awaits the task's result.
    • Browser: the background task is skipped. Chunks accumulate in AsyncPipeReaderInput; on CHUNK_END the buffer is Complete()d and the deserializer runs synchronously on the current thread via the streaming overload AcBinaryDeserializer.Deserialize(AsyncPipeReaderInput, Type, opts). After Complete(), TryAdvanceSegment sees _completed=true and never calls ManualResetEventSlim.Wait() (which throws PlatformNotSupportedException on WASM).

Consequence: mixed topology (desktop server AsyncSegment + WASM client Bytes) works without negotiation or protocol-name variation — client converts incoming chunked wire to its synchronous processing model.

Future: enabling true pipeline streaming on WASM via WasmEnableThreads (Atomics-backed ManualResetEventSlim.Wait()) is documented in SIGNALR_BINARY_PROTOCOL_WASMTHREADING.md — doc-only, not implemented; revisit at .NET 10 stable / .NET 11.

Registration in Program.cs

Server

builder.Services.AddSignalR(hubOptions =>
    {
        hubOptions.EnableDetailedErrors = true;
        hubOptions.MaximumReceiveMessageSize = 30_000_000;
        hubOptions.KeepAliveInterval = TimeSpan.FromSeconds(60);
        hubOptions.ClientTimeoutInterval = TimeSpan.FromSeconds(180);
        hubOptions.StatefulReconnectBufferSize = 30_000_000;
    })
    .AddAcBinaryProtocol(opts =>
    {
        opts.ProtocolMode = BinaryProtocolMode.AsyncSegment;
        // opts.FlushTimeout = TimeSpan.FromSeconds(10);  // default
    });

Client — HubConnectionBuilder as a DI transient

The consumer (e.g. a class derived from AcSignalRClientBase) receives the builder via DI:

services.AddTransient<HubConnectionBuilder>(sp =>
{
    var logger = sp.GetRequiredService<MyLoggerClient>();
    var hubUrl = $"{Config.BaseUrl}/{Config.HubName}";

    var builder = new HubConnectionBuilder()
        .WithUrl(hubUrl, HttpTransportType.WebSockets, options =>
        {
            options.TransportMaxBufferSize = 30_000_000;
            options.ApplicationMaxBufferSize = 30_000_000;
            options.CloseTimeout = TimeSpan.FromSeconds(10);
            options.SkipNegotiation = true;
        })
        .ConfigureLogging(logging =>
        {
            logging.SetMinimumLevel(LogLevel.Information);
            logging.AddAcLogger(_ => logger);
        })
        .WithAutomaticReconnect()
        .WithStatefulReconnect()
        .WithKeepAliveInterval(TimeSpan.FromSeconds(60))
        .WithServerTimeout(TimeSpan.FromSeconds(180));

    builder.AddAcBinaryProtocol(opts =>
    {
        // Desktop / server / native: AsyncSegment for pipeline parallelism.
        // WebAssembly: must be Bytes or Segment (Validate throws on AsyncSegment).
        opts.ProtocolMode = OperatingSystem.IsBrowser()
            ? BinaryProtocolMode.Segment
            : BinaryProtocolMode.AsyncSegment;
    });

    return builder;
});

services.AddSingleton<MySignalRClient>();   // derived from AcSignalRClientBase

Note: AcSignalRClientBase is HubConnectionBuilder-injected and calls only Build() + dispatch wiring internally. All transport/protocol configuration lives in Program.cs — visible, overridable per environment, and identical on both ends of the wire.

Related ADRs

  • adr/0001-acbinary-decorator-feature-stack-design.md — AcBinaryHubProtocol optional feature stack — decorator-based composition design (Status: Proposed). Umbrella ADR for optional decorator-based feature stack (encryption, compression with MinSize, OpenTelemetry tracing, HMAC signing). NuGet-competitiveness TODO entries (ACCORE-SBP-T-H7M5 / N9F3 / J5W8 / B3K6 in SIGNALR_BINARY_PROTOCOL_TODO.md) resolve under this umbrella. Leaf ADRs (0002-0005) for per-feature design + threat model.
  • AyCode.Core/docs/adr/0003-acbinary-streaming-receive-architecture.md — AcBinary streaming receive — AsyncPipeReaderInput unified primitive (Status: Accepted (2026-05-03), partially executed — Steps 4 & 5 NamedPipe / FileStream helpers dropped). Repo-level cross-cutting ADR. The receive-side migration delivered: SegmentBufferReader + SegmentBufferReaderInput consolidated into a single AsyncPipeReaderInput class; SignalR's TryParseChunkData migrated (ACCORE-SBP-T-G7T2). The unified AsyncSegment chunked wire format ([INT32 length][200 CHUNK_START][201][UINT16 size][data][202 CHUNK_END]) documented in this README's "Wire Format" / "BinaryProtocolMode" sections is preserved verbatim. Transport-agnostic helpers (NamedPipe / FileStream wrappers — Steps 4 & 5) were dropped during execution: the framework stays consumer-implements-transport (only PipeWriter / PipeReader primitives exposed); see ADR-0003's "Status" section for the rationale.

Source: AyCode.Services/SignalRs/AcBinaryHubProtocol.cs (base), AyCode.Services/SignalRs/AyCodeBinaryHubProtocol.cs (consumer logic), AyCode.Services/SignalRs/BinaryProtocolMode.cs (enum), AyCode.Services/SignalRs/AcBinaryHubProtocolOptions.cs (options), AyCode.Services/SignalRs/AcSignalRProtocolExtensions.cs (DI extensions)