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What does “send” actually mean in distributed computing? It depends on the layer. A send call can return after a local queue accepts data, after a broker accepts it, or after a remote service confirms receipt. None of those events necessarily means the receiving application finished its work. If the send call returned, whether the other service got the message depends on the API’s acknowledgment contract.
“Send” has several possible completion points
Distributed systems separate request submission, delivery, processing, and confirmation. TU Delft’s distributed-systems course describes three synchronization points: submission acknowledged by the communication environment; dispatch acknowledged as delivered for execution; and full processing at the destination. An application may add a later business acknowledgment to report that the requested work succeeded.
- Local submission: The calling API accepts the request or places it in a local queue. This confirms something about the caller’s process, not necessarily the network or recipient.
- Transport or broker acceptance: A transport endpoint or messaging broker accepts data. This still does not establish that the receiving application has acted on it.
- Receiver delivery: The message is handed to the destination for execution. The application may not yet have completed its work.
- Application processing: The destination finishes the requested operation.
- Business acknowledgment: The destination sends an application-level confirmation of the outcome.
When interpreting a return value, future, or acknowledgment, identify which of these milestones it represents. “Synchronous” and “asynchronous” usually describe whether the caller waits; they do not, by themselves, define how far the message traveled or what the recipient completed. AWS defines synchronous communication as a request where the workload blocks while waiting for a response, but the precise response and delivery behavior still depend on the API and service.
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| Example | What completion can mean | What it does not establish |
|---|---|---|
| TCP SEND | The TCP endpoint may queue the data and return a local acknowledgment. RFC 9293 says SENDs that cannot be serviced immediately are queued in first-come, first-served order, and notes that SENDs “might return immediate local acknowledgment” before the distant TCP endpoint acknowledges the segment. | It does not prove that the remote application received, parsed, or processed an application message. TCP provides an ordered byte stream, not application message boundaries; the PUSH flag is not a record delimiter. |
| Azure Service Bus send operation | The operation completes when the broker’s acceptance result arrives. This is confirmation of broker acceptance, not a receiver’s processing result. | It does not show that a downstream consumer received or completed the message. |
| Akka actor tell | Akka 2.10.2 documents at-most-once delivery: a message is delivered once or not at all. Direct sends from one sender to one recipient are ordered relative to each other. | A successful tell does not confirm that the actor processed the message or that business work succeeded. Akka says the meaningful way for a sender to know an interaction succeeded is a business-level acknowledgment from the receiver. |
These examples are deliberately scoped to their documented APIs and versions. They are not interchangeable definitions of “send.” For Azure Service Bus, Microsoft distinguishes send completion from receive settlement: a broker acceptance result completes the send operation, while a receiver settles its message separately.
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Delivery guarantees, ordering, and persistence are separate questions
A system’s delivery mode answers whether messages may be lost or duplicated under its stated conditions; it does not necessarily say whether application work succeeded. At-most-once means delivery happens once or not at all, as in Akka’s documented baseline. Other services may offer different delivery behavior, so check the specific service’s contract rather than inferring reliability from the word “send.”
Ordering also has a scope. Akka 2.10.2 documents ordering for direct sends between a particular sender-recipient pair; traffic from different senders can interleave. AWS cautions that messaging order is not guaranteed unless a FIFO option is used, and the details remain service-specific. A local ordering guarantee does not imply one global order across producers, partitions, or consumers.
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Persistence is another independent property. A local queue, transport buffer, or broker may accept data without persisting it durably. For a brokered system, verify whether acceptance means storage, what failures that storage survives, and how long the message remains available. An acknowledgment is only as strong as the event the service defines it to represent.
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Why a timeout and retry can create duplicates
Suppose a receiver processes a request, but its acknowledgment is lost. The sender sees a timeout and cannot tell whether the receiver never got the request or completed the work before the connection failed. Retrying can therefore cause the same operation to run twice. AWS explicitly recommends handling duplicate messages with idempotency.
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- Give each logical operation a stable idempotency key when the receiving API supports one.
- Where appropriate, record processed message identifiers so a receiver can detect and suppress repeats.
- Make retry policies bounded and observable: define which failures are retried, how long to retry, and where exhausted messages go.
- Separate transport acknowledgment from the application’s result. If the sender needs proof of completed work, require an explicit business-level response.
Idempotency and deduplication are implementation patterns, not guarantees automatically provided by a send call. Their correctness depends on where identifiers are stored and how the operation handles concurrent or repeated requests.
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- Find the return condition: Does the call return after local acceptance, network transmission, broker acceptance, receiver delivery, or an application response?
- Check whether the caller blocks: A synchronous wait may still end at a limited milestone; an asynchronous future may resolve on broker acceptance rather than processing.
- Check storage and delivery semantics: Determine whether an intermediary persists messages and whether the documented behavior is at-most-once, at-least-once, or otherwise scoped.
- Check ordering boundaries: Identify whether ordering applies per connection, sender-recipient pair, queue, partition, or FIFO group, and whether multiple senders can interleave.
- Assign responsibility for retries and duplicates: Establish which component retries, how retry limits work, and how the receiver makes repeated work safe.
- Define the confirmation the application needs: If success means completed business work, make that an explicit application-level acknowledgment rather than assuming transport delivery proves it.
The central question is not simply “Did send succeed?” It is “Which system acknowledged which milestone?” Answering that precisely prevents a local return, transport acknowledgment, or broker acceptance from being mistaken for completed work.
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Sources: IETF RFC 9293, Transmission Control Protocol; AWS Well-Architected Framework, REL04-BP01; TU Delft OpenCourseWare, Naming and Communication in Distributed Systems; Akka 2.10.2, Message Delivery Reliability; Microsoft Learn, Message Transfers, Locks, and Settlement.
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