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| <div class="section" id="at-least-once-processing"> |
| <h1>At-least-once Processing<a class="headerlink" href="#at-least-once-processing" title="Permalink to this headline">¶</a></h1> |
| <p>Applications typically choose between <tt class="docutils literal"><span class="pre">at-least-once</span></tt> and |
| <tt class="docutils literal"><span class="pre">exactly-once</span></tt> processing semantics. <tt class="docutils literal"><span class="pre">At-least-once</span></tt> 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 <tt class="docutils literal"><span class="pre">at-least-once</span></tt> processing by using a |
| <tt class="docutils literal"><span class="pre">offset-store</span></tt> to track the reading positions.</p> |
| <div class="section" id="how-to-track-reading-positions"> |
| <h2>How to track reading positions<a class="headerlink" href="#how-to-track-reading-positions" title="Permalink to this headline">¶</a></h2> |
| <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 - <tt class="docutils literal"><span class="pre">LevelDB</span></tt> to store the read |
| positions.</p> |
| <ul> |
| <li><p class="first">Open the offset store.</p> |
| <div class="highlight-python"><pre>String offsetStoreFile = ...; |
| Options options = new Options(); |
| options.createIfMissing(true); |
| DB offsetDB = factory.open(new File(offsetStoreFile), options);</pre> |
| <div style='display:none;' class='raw-code'><pre>String offsetStoreFile = ...; |
| Options options = new Options(); |
| options.createIfMissing(true); |
| DB offsetDB = factory.open(new File(offsetStoreFile), options);</pre> |
| </div></div> |
| </li> |
| <li><p class="first">Read the reader read position from the offset store.</p> |
| <div class="highlight-python"><pre>byte[] offset = offsetDB.get(readerId.getBytes(UTF_8)); |
| DLSN dlsn; |
| if (null == offset) { |
| dlsn = DLSN.InitialDLSN; |
| } else { |
| dlsn = DLSN.deserializeBytes(offset); |
| }</pre> |
| <div style='display:none;' class='raw-code'><pre>byte[] offset = offsetDB.get(readerId.getBytes(UTF_8)); |
| DLSN dlsn; |
| if (null == offset) { |
| dlsn = DLSN.InitialDLSN; |
| } else { |
| dlsn = DLSN.deserializeBytes(offset); |
| }</pre> |
| </div></div> |
| </li> |
| <li><p class="first">Start read from the read position that recorded in offset store.</p> |
| <div class="highlight-python"><pre>final AsyncLogReader reader = FutureUtils.result(dlm.openAsyncLogReader(dlsn));</pre> |
| <div style='display:none;' class='raw-code'><pre>final AsyncLogReader reader = FutureUtils.result(dlm.openAsyncLogReader(dlsn));</pre> |
| </div></div> |
| </li> |
| <li><p class="first">Track the last read position while reading using <tt class="docutils literal"><span class="pre">AtomicReference</span></tt>.</p> |
| <div class="highlight-python"><pre>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 style='display:none;' class='raw-code'><pre>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> |
| </li> |
| <li><p class="first">Periodically update the last read position to the offset store.</p> |
| <div class="highlight-python"><pre>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> |
| <div style='display:none;' class='raw-code'><pre>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> |
| </div></div> |
| </li> |
| </ul> |
| <p>Check |
| <tt class="docutils literal"><span class="pre">distributedlog-tutorials/distributedlog-messaging/src/main/java/com/twitter/distributedlog/messaging/ReaderWithOffsets</span></tt> |
| for more details.</p> |
| </div> |
| </div> |
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| <li class="toctree-l2"><a class="reference internal" href="../design/main.html#logsegment-lifecycle">LogSegment Lifecycle</a></li> |
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| xmlhttp.withCredentials = true; |
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| right: -280px; |
| z-index: 1; |
| } |
| #slidebox .close{ |
| margin-top: -5px; |
| opacity: 0.5; |
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| #slidebox .close:hover{ |
| opacity: 0.7; |
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| <script type="text/javascript"> |
| $(function() { |
| // $(window).scroll(function(){ |
| // var distanceTop = $('#last').offset().top - $(window).height(); |
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| // if ($(window).scrollTop() > distanceTop) |
| // $('#slidebox').animate({'right':'3px'},300); |
| // else |
| // $('#slidebox').stop(true).animate({'right':'-280px'},100); |
| // }); |
| |
| // $('#slidebox .close').bind('click',function(){ |
| // $(this).parent().remove(); |
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| $("#rateYo").rateYo({ |
| normalFill: "#A0A0A0", |
| halfStar: true, |
| rating: (Cookies.get('docbird.rating.distributedlog.distributedlog') || 0.0) |
| }).on("rateyo.set", function (e, data) { |
| var event_data = { |
| comment: '', // see todo note below |
| rating: data.rating, |
| page: window.location.href |
| }; |
| Cookies.get('docbird.rating.distributedlog.distributedlog', data.rating) |
| $.post('/event/distributedlog:distributedlog/rating', event_data) |
| // xmlhttp=new XMLHttpRequest(); |
| // xmlhttp.withCredentials = true; |
| // var event_data = { |
| // comment: '', // see todo note below |
| // rating: data.rating, |
| // page: window.location.href |
| // }; |
| // xmlhttp.open("GET", "/event/distributedlog/rating?" + $.param(event_data), false); |
| // xmlhttp.setRequestHeader("Content-type", "application/x-www-form-urlencoded"); |
| // // todo: implement comment form in rating slide out, |
| // // and instead of hooking this event, include a submit button, |
| // // and read the rating with rating() method |
| |
| // // alert("send: " + $.param(event_data)); |
| // xmlhttp.send(); |
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