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| <li><a class="reference internal" href="#id1" id="id2">Best Practices</a><ul> |
| <li><a class="reference internal" href="#write-records-using-fat-client-or-thin-client" id="id3">Write records using Fat Client or Thin Client</a></li> |
| <li><a class="reference internal" href="#how-to-position-reader-by-time" id="id4">How to position reader by time</a></li> |
| <li><a class="reference internal" href="#how-to-seal-a-stream" id="id5">How to seal a stream</a></li> |
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| <h2><a class="toc-backref" href="#id2">Best Practices</a></h2> |
| <div class="section" id="write-records-using-fat-client-or-thin-client"> |
| <h3><a class="toc-backref" href="#id3">Write records using Fat Client or Thin Client</a></h3> |
| <p><cite>Fat Client</cite> is the writer in distributedlog core library which talks to ZooKeeper and BookKeeper directly, |
| while <cite>Thin Client</cite> is the write proxy client which talks to write proxy service.</p> |
| <p>It is strongly recommended that writing records via <cite>Write Proxy</cite> service rather than using core library directly. |
| Because using <cite>Thin Client</cite> has following benefits:</p> |
| <ul class="simple"> |
| <li><cite>Thin Client</cite> is purely thrift RPC based client. It doesn't talk to zookeeper and bookkeeper directly and less complicated.</li> |
| <li><cite>Write Proxy</cite> manages ownerships of log writers. <cite>Thin Client</cite> doesn't have to deal with ownerships of log writers.</li> |
| <li><cite>Thin Client</cite> is more upgrade-friendly than <cite>Fat Client</cite>.</li> |
| </ul> |
| <p>The only exception to use distributedlog core library directly is when you application requires:</p> |
| <ul class="simple"> |
| <li>Write Ordering. <cite>Write Ordering</cite> means all the writes issued by the writer should be written in a strict order |
| in the log. <cite>Write Proxy</cite> service could only guarantee <cite>Read Ordering</cite>. <cite>Read Ordering</cite> means the write proxies will write |
| the write requests in their receiving order and gurantee the data seen by all the readers in same order.</li> |
| <li>Ownership Management. If the application already has any kind of ownership management, like <cite>master-slave</cite>, it makes more |
| sense that it uses distributedlog core library directly.</li> |
| </ul> |
| </div> |
| <div class="section" id="how-to-position-reader-by-time"> |
| <h3><a class="toc-backref" href="#id4">How to position reader by time</a></h3> |
| <p>Sometimes, application wants to read data by time, like read data from 2 hours ago. This could be done by positioning |
| the reader using <cite>Transaction ID</cite>, if the <cite>Transaction ID</cite> is the timestamp (All the streams produced by <cite>Write Proxy</cite> use |
| timestamp as <cite>Transaction ID</cite>).</p> |
| <pre class="literal-block"> |
| DistributedLogManager dlm = ...; |
| |
| long timestamp = System.currentTimeMillis(); |
| long startTxId = timestamp - TimeUnit.MILLISECONDS.convert(2, TimeUnit.HOURS); |
| AsyncLogReader reader = Await.result(dlm.openAsyncLogReader(startTxId)); |
| ... |
| </pre> |
| </div> |
| <div class="section" id="how-to-seal-a-stream"> |
| <h3><a class="toc-backref" href="#id5">How to seal a stream</a></h3> |
| <p>Typically, DistributedLog is used as endless streams. In some use cases, application wants to <cite>seal</cite> the stream. So writers |
| can't write more data into the log stream and readers could receive notifications about the stream has been sealed.</p> |
| <p>Write could seal a log stream as below:</p> |
| <pre class="literal-block"> |
| DistributedLogManager dlm = ...; |
| |
| LogWriter writer = dlm.startLogSegmentNonPartitioned; |
| // writer writes bunch of records |
| ... |
| |
| // writer seals the stream |
| writer.markEndOfStream(); |
| </pre> |
| <p>Reader could detect a stream has been sealed as below:</p> |
| <pre class="literal-block"> |
| DistributedLogManager dlm = ...; |
| |
| LogReader reader = dlm.getInputStream(1L); |
| LogRecord record; |
| try { |
| while ((record = reader.readNext(false)) != null) { |
| // process the record |
| ... |
| } |
| } catch (EndOfStreamException eos) { |
| // the stream has been sealed |
| ... |
| } |
| </pre> |
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