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A free scan shows the junk files, broken settings and background clutter dragging Windows down - then fixes them in one click.Free scan · Windows 10 & 11What does “send” actually mean in distributed computing? It depends on the API and the layer doing the work. A send call returning may mean only that the local program accepted the request; it does not necessarily mean a broker stored the message, a remote service received it, or that service finished processing it. To know what happened, identify the exact event the API acknowledges.
What “send” can mean: a ladder of milestones
A distributed send is better understood as a sequence of possible milestones than as one universal event. Different APIs return or acknowledge at different points:
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- Local submission: the calling process hands a request to its communication library or operating system. The call may return while the request is queued locally.
- Transport or broker acceptance: a network transport or message broker accepts data or a message. This still does not establish that the receiving application has acted on it.
- Receiver delivery: the communication system reports that the message was delivered to the receiving side for execution or handling.
- Application processing: the receiver finishes the relevant application work.
- Business acknowledgment: the receiver sends an explicit response confirming the outcome the sender cares about, such as a completed transaction.
TU Delft describes submission, dispatch/delivery, and full processing as distinct synchronization points. A given API may expose one, several, or none of them directly; read its contract rather than inferring a milestone from the word “send.” TU Delft OpenCourseWare
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If the send call returned, did the other service get my message?
Not necessarily. The answer depends on what the return value or completed future represents. It might report local queuing, transport progress, broker acceptance, or a later acknowledgment. Even a remote transport acknowledgment does not prove that the receiving application completed its work.
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TCP SEND: local acceptance is not remote delivery
TCP exposes a byte stream, not application messages with preserved boundaries. RFC 9293 says SEND requests that cannot be serviced immediately may be queued, and notes that a SEND can return an immediate local acknowledgment before the distant TCP endpoint acknowledges the segment. The TCP PUSH flag indicates a prompt-transmission intent; it is not an application record delimiter. A successful local TCP write therefore does not mean the peer application received or processed one complete message. RFC 9293, §3.9.1.2
Azure Service Bus: broker acceptance is not receiver processing
Azure Service Bus send operations complete when the broker’s acceptance result arrives. That is a broker-side milestone, not confirmation that a downstream consumer processed the message. On receipt, Receive-and-Delete settles a message as it is transferred, so a failed transfer can lose it. Peek-Lock instead lets the receiver settle the message explicitly after processing. These choices move the acknowledgment point and affect what can happen when a consumer fails. Microsoft Learn: Message transfers, locks, and settlement
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Actor tell: delivery does not imply successful work
In Akka 2.10.2, the documented delivery guarantee is at-most-once: a message is delivered once or not at all. Direct sends preserve ordering for a particular sender-recipient pair, but messages from different senders may interleave. Akka also distinguishes message delivery from successful application interaction: a sender needs a business-level acknowledgment to know that the receiver’s work succeeded. These are Akka-specific documented guarantees, not universal properties of actor systems. Akka 2.10.2: Message Delivery Reliability
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1Scan for outdated or missing drivers - takes under a minute2Clear out junk files and repair common Windows errors3Fix the driver behind crashes, sound loss and screen glitchesSynchronous versus asynchronous describes waiting, not the guarantee
“Synchronous” and “asynchronous” commonly describe whether the caller waits for a response, not how far a message has progressed. AWS defines synchronous communication as a workload sending a request to a dependency and blocking while it waits for a response. An asynchronous send may let the caller continue sooner, but that fact alone says nothing about persistence, retries, delivery guarantees, or receiver completion. AWS Well-Architected Framework, REL04-BP01
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When comparing APIs, ask what event completes the call, whether the caller blocks, whether an intermediary persists the message, what delivery behavior applies, what ordering scope is promised, who retries, whether duplicates are possible, whether receiver processing is acknowledged, and how timeouts or backpressure are handled. “Reliable” is too vague unless those behaviors are specified.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Why a retry can duplicate work
Suppose a sender times out while waiting for an acknowledgment. The receiver may never have received the message—or it may have completed the work and its acknowledgment may have been lost. The sender cannot distinguish those cases from the timeout alone. Retrying may therefore deliver the request again even when the first attempt succeeded.
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AWS warns that network failure or a missing acknowledgment can produce duplicate messages and recommends designing handlers to cope with them through idempotency. An idempotent operation can be repeated without creating an unintended second effect. Common implementation patterns include a stable idempotency key or receiver-side deduplication, but “send” does not provide either automatically. Set retry limits and make retries observable so repeated failures do not become an unbounded stream of duplicate work. AWS Well-Architected Framework, REL04-BP01
Ordering is limited to the scope the system promises
Do not infer global ordering from a statement that messages are ordered. Akka 2.10.2 documents ordering for direct messages sent by one sender to one recipient; messages from separate senders can interleave. AWS likewise cautions that messaging order is not guaranteed unless a FIFO option is used, and the exact behavior depends on the service. If order matters, verify the relevant sender, recipient, queue or partition scope—and what happens when retries occur—against the specific API’s documentation. Akka 2.10.2 documentation · AWS Well-Architected Framework
Quick Recap
A checklist for interpreting a send API
- Find the precise event represented by a successful return, resolved future, or acknowledgment.
- Distinguish local queuing, transport acknowledgment, broker acceptance, receiver delivery, and application-level success.
- Check whether the message is persisted, and for how long or under what conditions.
- Read the delivery and ordering guarantees for the specific API, service, and version.
- Determine who retries after timeouts, how many attempts are allowed, and how duplicate effects are prevented.
- Confirm whether the receiver explicitly acknowledges completed business work, rather than merely accepting a message.
- Check how the system handles timeouts and backpressure when queues or recipients cannot keep up.
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