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| <div class="contents topic" id="distributedlog-overview"> |
| <p class="topic-title first">DistributedLog Overview</p> |
| <ul class="simple"> |
| <li><a class="reference internal" href="#introduction" id="id1">Introduction</a><ul> |
| <li><a class="reference internal" href="#logs" id="id2">Logs</a><ul> |
| <li><a class="reference internal" href="#log-records" id="id3">Log Records</a></li> |
| <li><a class="reference internal" href="#log-segments" id="id4">Log Segments</a></li> |
| <li><a class="reference internal" href="#namespaces" id="id5">Namespaces</a></li> |
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| <li><a class="reference internal" href="#writers" id="id6">Writers</a></li> |
| <li><a class="reference internal" href="#readers" id="id7">Readers</a></li> |
| <li><a class="reference internal" href="#fan-in-and-fan-out" id="id8">Fan-in and Fan-out</a></li> |
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| <div class="section" id="introduction"> |
| <h2><a class="toc-backref" href="#id1">Introduction</a></h2> |
| <p>DistributedLog (DL) is a high performance replicated log service. |
| It offers durability, replication and strong consistency, which provides a fundamental building block |
| for building reliable distributed systems, e.g replicated-state-machines, general pub/sub systems, |
| distributed databases, distributed queues and etc.</p> |
| <p>DistributedLog maintains sequences of records in categories called <em>Logs</em> (aka <em>Log Streams</em>). |
| The processes that write records to a DL log are <em>writers</em>, while the processes that read |
| from logs and process the records are <em>readers</em>.</p> |
| <div class="figure align-center"> |
| <img alt="../images/softwarestack.png" src="../images/softwarestack.png" /> |
| <p class="caption">Figure 1. DistributedLog Software Stack</p> |
| </div> |
| <div class="section" id="logs"> |
| <h3><a class="toc-backref" href="#id2">Logs</a></h3> |
| <p>A <strong>log</strong> is an ordered, immutable sequence of <em>log records</em>.</p> |
| <div class="figure align-center"> |
| <img alt="../images/datamodel.png" src="../images/datamodel.png" /> |
| <p class="caption">Figure 2. Anatomy of a log stream</p> |
| </div> |
| <div class="section" id="log-records"> |
| <h4><a class="toc-backref" href="#id3">Log Records</a></h4> |
| <p>Each <strong>log record</strong> is a sequence of bytes. |
| <strong>Log records</strong> are written sequentially into a <em>log stream</em>, and will be assigned with |
| a unique sequence number <em>called</em> <strong>DLSN</strong> (DistributedLog Sequence Number). Besides <em>DLSN</em>, |
| applications could assign its own sequence number while constructing log records. The |
| application defined sequence number is called <strong>TransactionID</strong> (<em>txid</em>). Either <em>DLSN</em> |
| or <em>TransactionID</em> could be used for positioning readers to start reading from a specific |
| <em>log record</em>.</p> |
| </div> |
| <div class="section" id="log-segments"> |
| <h4><a class="toc-backref" href="#id4">Log Segments</a></h4> |
| <p>A <strong>log</strong> is broken down into <em>segments</em>, which each log segment contains its subset of |
| records. <strong>Log segments</strong> are distributed and stored in a log segment store (e.g Apache BookKeeper). |
| DistributedLog rolls the log segments based on configured rolling policy - either a configurable |
| period of time (e.g. every 2 hours) or a configurable maximum size (e.g. every 128MB). |
| So the data of logs will be divided into equal-sized <em>log segments</em> and distributed evenly |
| across log segment storage nodes. It allows the log to scale beyond a size that will fit on |
| a single server and also spread read traffic among the cluster.</p> |
| <p>The data of logs will either be kept forever until application <em>explicitly</em> truncates or be retained |
| for a configurable period of time. <strong>Explicit Truncation</strong> is useful for building replicated |
| state machines such as distributed databases. They usually require strong controls over when |
| the data could be truncated. <strong>Time-based Retention</strong> is useful for real-time analytics. They only |
| care about the data within a period of time.</p> |
| </div> |
| <div class="section" id="namespaces"> |
| <h4><a class="toc-backref" href="#id5">Namespaces</a></h4> |
| <p>The <em>log streams</em> belong to same organization are usually categorized and managed under |
| a <strong>namespace</strong>. A DL <strong>namespace</strong> is basically for applications to locate where the |
| <em>log streams</em> are. Applications could <em>create</em> and <em>delete</em> streams under a namespace, |
| and also be able to <em>truncate</em> a stream to given sequence number (either <em>DLSN</em> or <em>TransactionID</em>).</p> |
| </div> |
| </div> |
| <div class="section" id="writers"> |
| <h3><a class="toc-backref" href="#id6">Writers</a></h3> |
| <p>Writers write data into the logs of their choice. All the records are |
| appended into the logs in order. The sequencing is done by the writer, |
| which means there is only one active writer for a log at a given time. |
| DL guarantees correctness when two writers attempt writing to |
| to a same log when network partition happens - via fencing mechanism |
| in log segment store.</p> |
| <p>The log writers are served and managed in a service tier called <em>Write Proxy</em>. |
| The <em>Write Proxy</em> is used for accepting fan-in writes from large number |
| of clients. Details on <strong>Fan-in and Fan-out</strong> can be found further into this doc.</p> |
| </div> |
| <div class="section" id="readers"> |
| <h3><a class="toc-backref" href="#id7">Readers</a></h3> |
| <p>Readers read records from the logs of their choice, starting from a provided |
| position. The provided position could be either <em>DLSN</em> or <em>TransactionID</em>. |
| The readers will read records in strict order from the logs. Different readers |
| could read records starting from different positions in a same log.</p> |
| <p>Unlike other pub/sub systems, DistributedLog doesn't record/manage readers' positions. |
| It leaves the tracking responsibility to applications, as different applications |
| might have different requirements on tracking and coordinating positions. It is hard |
| to get it right with a single approach. For example, distributed databases might store |
| the reader positions along with SSTables, so they would resume applying transactions |
| from the positions stored in SSTables. Tracking reader positions could easily be done |
| in application level using various stores (e.g. ZooKeeper, file system, or key/value stores).</p> |
| <p>The log records could be cached in a service tier called <em>Read Proxy</em>, to serve |
| a large number of readers.</p> |
| </div> |
| <div class="section" id="fan-in-and-fan-out"> |
| <h3><a class="toc-backref" href="#id8">Fan-in and Fan-out</a></h3> |
| <p>The core of DistributedLog supports single-writer, multiple-readers semantics. The service layer |
| built on top of the <em>DistributedLog Core</em> to support large scale of number of writers and readers. |
| The service layer includes <strong>Write Proxy</strong> and <strong>Read Proxy</strong>. <strong>Write Proxy</strong> manages |
| the writers of logs and fail over them when machines are failed. It allows supporting |
| which don't care about the log ownership by aggregating writes from many sources (aka <em>Fan-in</em>). |
| <strong>Read Proxy</strong> optimize reader path by caching log records in cases where hundreds or |
| thousands of readers are consuming a same log stream.</p> |
| </div> |
| <div class="section" id="guarantees"> |
| <h3><a class="toc-backref" href="#id9">Guarantees</a></h3> |
| <p>At a high level, DistributedLog gives the following guarantees:</p> |
| <ul class="simple"> |
| <li>Records written by a writer to a log will be appended in the order they are written. That is, if a record <em>R1</em> is written by same writer as a record <em>R2</em>, <em>R1</em> will have a smaller sequence number than <em>R2</em>.</li> |
| <li>Readers see records in the same order they were written to the log.</li> |
| <li>All records are persisted on disk before acknowledgments, to gurantee durability.</li> |
| <li>For a log with replication factor of N, DistributedLog tolerates up to N-1 server failures without losing any records.</li> |
| </ul> |
| <p>More details on these guarantees are given in the [design section](<a class="reference external" href="http://bookkeeper.apache.org/distributedlog/docs/0.4.0-incubating/user_guide/design/main.html">http://bookkeeper.apache.org/distributedlog/docs/0.4.0-incubating/user_guide/design/main.html</a>).</p> |
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