Different systems will provide different messaging solutions, understanding these can help you understand how your messages will be given to downstream services.

At-most-once delivery

• Delivery means message will be delivered once
• If it fails it can be lost

At-least-once delivery

• Message may duplicate to consumer.
• Multiple attempts can be made to deliver message.
• Important to have idempotent consumers
• Multiple attempts can be made to deliver the message to the target until one succeeds. This means messages can be duplicated but not lost.

Exactly-once delivery

• Message is given to the target exactly once
• Message cannot be lost of duplicated

Extra resources

• At-least-once delivery - Short blog post to help.
• Exactly-once delivery - Short blog post to help.