At-least-once delivery

Q: How deeply does C# Concurrency: Asynchronous and multithreaded programming cover at-least-once delivery?

Explains removal timing, crash windows, delivery trade-offs, and poison-message loops without detailing queue-provider implementations.

As explained in C# Concurrency: Asynchronous and multithreaded programming

At-least-once delivery is a message-delivery guarantee that permits duplicates but ensures a message is delivered one or more times.

At-least-once delivery removes a message only after processing finishes. A crash between processing and removal can therefore cause the same message to be processed again. section 8.2.3

Choose the guarantee by weighing duplicate work against lost data:

Prefer at-least-once delivery when losing data is worse than processing an item twice. section 8.2.3
Expect a poison message to create a restart-and-crash loop unless processing failures are monitored and handled. section 8.2.3 section 8.3
Use a persistent queue when work must survive a program crash; available options include a database, RabbitMQ, AWS SQS, and Azure Storage Queues. section 8.3

Mail merge after a lost confirmation

If a mail server connection fails after sending but before confirmation, the sender cannot know whether the message arrived. Retrying risks a duplicate; skipping risks losing the message, so exactly-once delivery is usually unavailable. section 8.2.3

How C# Concurrency: Asynchronous and multithreaded programming covers it

Cover of C# Concurrency: Asynchronous and multithreaded programming

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Also explained in

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Kafka Streams in Action, Second EditionAt-least-once delivery is a processing guarantee that prevents loss of records but permits a record to be processed more than once.
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Akka in Action, Second EditionAt-least-once delivery is a processing guarantee in which records may be delivered or processed more than once because retries can repeat work.
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Apache Kafka in ActionAt-least-once delivery is a message delivery guarantee that prevents message loss but permits duplicate delivery.

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Related concepts

at-most-once deliverycontrasts with · At-most-once delivery is a messaging guarantee that each message is delivered no more than once, so messages may be lost but are not duplicated.
message brokerproduced by · A message broker is intermediary software that receives, routes, and delivers messages between communicating producers and consumers.
Consumer.committableSourceproduced by · Consumer.committableSource is an Alpakka Kafka source that emits records together with committable offset information for at-least-once processing.
retryproduced by · A retry is a subsequent attempt to perform an operation after an earlier attempt fails or does not complete successfully.
idempotencyaddresses · Idempotency is the property of an operation whereby repeating it has the same effective result or externally observable effect as performing it once.

This explainer was generated by Manning's knowledge graph from the text of C# Concurrency: Asynchronous and multithreaded programming. The section links point to the parts of the book it draws on.