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| <div class="col-md-8 col-md-offset-2"> |
| <div class="contents topic" id="messaging-tutorial-at-least-once-processing"> |
| <p class="topic-title first">Messaging Tutorial - At-least-once Processing</p> |
| <ul class="auto-toc simple"> |
| <li><a class="reference internal" href="#at-least-once-processing" id="id1">1 At-least-once Processing</a><ul class="auto-toc"> |
| <li><a class="reference internal" href="#how-to-track-reading-positions" id="id2">1.1 How to track reading positions</a><ul class="auto-toc"> |
| <li><a class="reference internal" href="#open-the-offset-store" id="id3">1.1.1 Open the offset store</a></li> |
| <li><a class="reference internal" href="#read-the-reader-read-position" id="id4">1.1.2 Read the reader read position</a></li> |
| <li><a class="reference internal" href="#read-records" id="id5">1.1.3 Read records</a></li> |
| <li><a class="reference internal" href="#track-read-position" id="id6">1.1.4 Track read position</a></li> |
| <li><a class="reference internal" href="#record-read-position" id="id7">1.1.5 Record read position</a></li> |
| </ul> |
| </li> |
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| </div> |
| <div class="section" id="at-least-once-processing"> |
| <h2><a class="toc-backref" href="#id1">1 At-least-once Processing</a></h2> |
| <p>Applications typically choose between <cite>at-least-once</cite> and <cite>exactly-once</cite> processing semantics. |
| <cite>At-least-once</cite> 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. With at least once processing guarantees the application can store |
| reader positions in an external store and update it periodically. Upon restart the application will |
| reprocess messages since the last updated reader position.</p> |
| <p>This tutorial shows how to do <cite>at-least-once</cite> processing by using a <cite>offset-store</cite> to track the reading positions.</p> |
| <div class="section" id="how-to-track-reading-positions"> |
| <h3><a class="toc-backref" href="#id2">1.1 How to track reading positions</a></h3> |
| <p>Applications typically choose an external storage (e.g key/value storage) or another log stream to store their |
| read positions. In this example, we used a local key/value store - <cite>LevelDB</cite> to store the read positions.</p> |
| <div class="section" id="open-the-offset-store"> |
| <h4><a class="toc-backref" href="#id3">1.1.1 Open the offset store</a></h4> |
| <pre class="literal-block"> |
| String offsetStoreFile = ...; |
| Options options = new Options(); |
| options.createIfMissing(true); |
| DB offsetDB = factory.open(new File(offsetStoreFile), options); |
| </pre> |
| </div> |
| <div class="section" id="read-the-reader-read-position"> |
| <h4><a class="toc-backref" href="#id4">1.1.2 Read the reader read position</a></h4> |
| <p>Read the reader read position from the offset store.</p> |
| <pre class="literal-block"> |
| byte[] offset = offsetDB.get(readerId.getBytes(UTF_8)); |
| DLSN dlsn; |
| if (null == offset) { |
| dlsn = DLSN.InitialDLSN; |
| } else { |
| dlsn = DLSN.deserializeBytes(offset); |
| } |
| </pre> |
| </div> |
| <div class="section" id="read-records"> |
| <h4><a class="toc-backref" href="#id5">1.1.3 Read records</a></h4> |
| <p>Start read from the read position that recorded in offset store.</p> |
| <pre class="literal-block"> |
| final AsyncLogReader reader = FutureUtils.result(dlm.openAsyncLogReader(dlsn)); |
| </pre> |
| </div> |
| <div class="section" id="track-read-position"> |
| <h4><a class="toc-backref" href="#id6">1.1.4 Track read position</a></h4> |
| <p>Track the last read position while reading using <cite>AtomicReference</cite>.</p> |
| <pre class="literal-block"> |
| final AtomicReference<DLSN> lastReadDLSN = new AtomicReference<DLSN>(null); |
| reader.readNext().addEventListener(new FutureEventListener<LogRecordWithDLSN>() { |
| ... |
| |
| @Override |
| public void onSuccess(LogRecordWithDLSN record) { |
| lastReadDLSN.set(record.getDlsn()); |
| // process the record |
| ... |
| // read next record |
| reader.readNext().addEventListener(this); |
| } |
| }); |
| </pre> |
| </div> |
| <div class="section" id="record-read-position"> |
| <h4><a class="toc-backref" href="#id7">1.1.5 Record read position</a></h4> |
| <p>Periodically update the last read position to the offset store.</p> |
| <pre class="literal-block"> |
| final ScheduledExecutorService executorService = |
| Executors.newSingleThreadScheduledExecutor(); |
| executorService.scheduleAtFixedRate(new Runnable() { |
| @Override |
| public void run() { |
| if (null != lastDLSN.get()) { |
| offsetDB.put(readerId.getBytes(UTF_8), lastDLSN.get().serializeBytes()); |
| } |
| } |
| }, 10, 10, TimeUnit.SECONDS); |
| </pre> |
| <p>Check <a class="reference external" href="https://github.com/apache/distributedlog/blob/master/distributedlog-tutorials/distributedlog-messaging/src/main/java/org/apache/distributedlog/messaging/ReaderWithOffsets.java">distributedlog-tutorials/distributedlog-messaging/src/main/java/org/apache/distributedlog/messaging/ReaderWithOffsets</a> for more details.</p> |
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