sequenceDiagram
    participant C as FIX Client Simulator
    participant G as Gateway FIX callback
    participant W as Gateway bounded worker
    participant P as Protobuf mapper / gRPC stub
    participant O as OMS
    C->>G: FIX 4.4 NewOrderSingle (35=D)
    G->>G: MessageCracker -> FixOrderMapper -> IncomingOrder
    G->>W: Atomic admission with original SessionID
    Note over G,W: Callback returns without waiting for OMS; full queue logs NOT_SUBMITTED
    W->>P: FixToProtobufMapper.map(order)
    P->>O: gRPC SubmitOrder(SubmitOrderRequest), per-call deadline
    O->>O: Validate; atomic in-memory identity lookup / business decision
    alt Definitive accepted or rejected decision
        O-->>P: SubmitOrderResponse(order_id, decision, reject_reason)
        P-->>W: OrderSubmissionResult
        W->>W: Create and track ExecutionReport with stable ExecID
        alt Original session logged on; local engine accepts
            W-->>C: FIX ExecutionReport (35=8), NEW or REJECTED
            C->>C: Correlate SessionID + ClOrdID; detect repeated ExecID
        else Original session absent or send fails
            W->>W: DELIVERY_PENDING / DELIVERY_FAILED; no reconnect drain
        end
    else Invalid request
        O-->>P: gRPC INVALID_ARGUMENT
        P-->>W: INVALID_REQUEST; log without ExecutionReport
    else Deadline, unavailable transport, or unusable response
        P->>P: Client observes failure or cannot establish a decision
        Note over P,O: OMS may already have stored an acceptance before the deadline
        P-->>W: UNKNOWN_OUTCOME
        W->>W: Log correlation; no ExecutionReport; no application retry
    end
    Note over C,O: FIX resend of persisted reports preserves ExecID; business state is volatile