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| title: Tutorials - Exact-Once Processing |
| top-nav-group: messaging |
| top-nav-pos: 4 |
| top-nav-title: Exact-Once Processing |
| layout: default |
| --- |
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| .. contents:: Messaging Tutorial - Exact-Once Processing |
| |
| Exact-Once Processing |
| ===================== |
| |
| Applications typically choose between `at-least-once` and `exactly-once` processing semantics. |
| `At-least-once` processing guarantees that the application will process all the log records, |
| however when the application resumes after failure, previously processed records may be re-processed |
| if they have not been acknowledged. `Exactly once` processing is a stricter guarantee where applications |
| must see the effect of processing each record exactly once. `Exactly once` semantics can be achieved |
| by maintaining reader positions together with the application state and atomically updating both the |
| reader position and the effects of the corresponding log records. |
| |
| This tutorial_ shows how to do `exact-once` processing. |
| |
| .. _tutorial: https://github.com/apache/distributedlog/blob/master/distributedlog-tutorials/distributedlog-messaging/src/main/java/org/apache/distributedlog/messaging/StreamTransformer.java |
