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     <h1 id="项目背景">项目背景</h1>
     <h1 id="设计目标">设计目标</h1>

 <ul>
   <li>高可用：系统必须是高可用的。即使部分服务器宕机，Pegasus集群也能在极短时间（秒级）内恢复服务，尽量减少对用户的影响，要求服务可靠性达到99.99%以上。</li>
   <li>高性能：系统能够提供高性能的读写服务，P99延迟需要在毫秒级别。</li>
   <li>强一致：系统对用户提供强一致性的语义，使用户在编写业务逻辑时更容易。</li>
   <li>易伸缩：系统能够很方便地扩容和缩容，以应对业务吞吐负载的变化。</li>
   <li>易使用：系统给用户提供简单易用的库和接口，方便用户使用。</li>
 </ul>

 <h1 id="实现方案">实现方案</h1>

 <p>在设计Pegasus时，我们在目标、实现难度、开发效率等方面做一些权衡。总的来说，包括这几个方面：</p>
 <ul>
   <li>开发语言：基于性能考虑，我们选择了C++。</li>
   <li>数据模型：采用简单的Key-Value数据模型。这既简化了开发，也能满足大部分业务需求。进一步地，我们将Key拆分为了HashKey和SortKey两级，加强了其表达能力。</li>
   <li>数据分布：采用固定Hash分布。相比Range分布和一致性Hash分布，固定Hash分布实现更简单，数据倾斜和可伸缩性可以通过合理设计Hash键、预设更多的数据分片等措施来解决。我们也支持<a href="https://pegasus.apache.org/zh/administration/partition-split">Partition Split</a>功能来扩展分片数量。</li>
   <li>存储介质：建议选择SSD（固态硬盘）。SSD的性能和成本都介于内存和HDD（机械硬盘）之间，从业务需求和成本综合考虑，选择SSD是比较合适的。</li>
   <li>本地存储引擎：选择<a href="https://github.com/facebook/rocksdb">RocksDB</a>。RocksDB在LevelDB基础上做了很多优化，能充分利用SSD的IOPS性能和多核服务器的计算性能。</li>
   <li>一致性协议：选择<a href="https://www.microsoft.com/en-us/research/publication/pacifica-replication-in-log-based-distributed-storage-systems/">PacificA</a>。相比<a href="https://raft.github.io/">Raft</a>，PacificA协议具有其自身的特点和优势。</li>
   <li>故障检测：和HBase不同，Pegasus没有使用Zookeeper来进行故障检测，而是在MetaServer和ReplicaServer之间实现了基于租约的故障检测机制。</li>
 </ul>

 <h1 id="与hbase比较">与HBase比较</h1>

 <p>Pegasus系统的最初目的就是弥补HBase的不足，这里从用户使用角度比较一下两者的区别：</p>
 <ul>
   <li>数据模型：HBase是表格模型，采用Range分片；Pegasus是Key-Value模型，采用Hash分片。</li>
   <li>接口：HBase的API接口功能虽然很丰富，但是使用也更复杂；Pegasus的接口简单，对用户更友好。</li>
   <li>可靠性：由于架构和实现的原因（如Pegasus采用的本地存储、故障检测、使用C++语言实现等），Pegasus的可靠性通常优于HBase。</li>
   <li>性能：由于分层架构，HBase的读写性能不是太好，P99延迟通常在几十甚至几百毫秒，而且GC问题会带来毛刺问题；Pegasus的P99可以在几毫秒，满足敏感在线业务的需求。</li>
 </ul>

 <h1 id="与redis比较">与Redis比较</h1>

 <p>如果仅从读写延迟和单机吞吐比较，Redis显然是优于Pegasus的。但如果从读写延迟、可用性、伸缩性、成本等方面综合比较，Pegasus也是有其自身的优势的。</p>

 <p>与Redis进行比较的主要区别如下：</p>
 <ul>
   <li>数据模型：两者都是Key-Value模型，但是Pegasus支持(HashKey + SortKey)的二级键。</li>
   <li>接口：Redis的接口更丰富，支持List、Set、Map等容器特性；Pegasus的接口相对简单，功能更单一。</li>
   <li>读写延迟：Redis性能比Pegasus好。</li>
   <li>伸缩性：Pegasus伸缩性更好，可以很方便地增减机器节点，并支持自动的负载均衡；Redis的分布式方案在增减机器的时候比较麻烦。</li>
   <li>可靠性：Pegasus数据总是持久化的，系统架构保证其较高的数据完整性；Redis在机器宕机后需要较长时间恢复，可用性不够好，还可能丢掉最后一段时间的数据。</li>
   <li>成本：Pegasus使用SSD存储全量数据，而Redis需要使用内存来存储全量数据，Pegasus成本更低。</li>
 </ul>

 <h1 id="综合比较">综合比较</h1>

 <p>业务在系统选型时，通常遇到这些问题：</p>
 <ul>
   <li>HBase虽然可用性高也易伸缩，但是性能不够好。</li>
   <li>Redis虽然性能好，但是需要大量内存，带来更昂贵的硬件成本。如果数据量太大，采用<a href="https://redis.io/topics/cluster-tutorial">Redis Cluster</a>方案，在机器宕机故障情况下的可用性又不够。</li>
   <li>HBase+Redis的方案，即使用HBase做底层存储，使用Redis做上层缓存。该方案的缺点是：涉及两个系统，用户的读写逻辑会比较复杂；同时写两个系统，容易出现一致性问题；一份数据要同时存储在HBase和Redis中，成本比较高；Redis机器宕机后造成部分缓存丢失，此时从HBase读取的性能又明显降低。</li>
 </ul>

 <p>Pegasus综合了HBase和Redis的优点，它既保证高的可靠性，又具有好的伸缩性，还具有良好的性能。</p>

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	体验 Onebox 集群
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	<div class="column is-half">



	<div class="content">
	<h1 id="项目背景">项目背景</h1>
	<h1 id="设计目标">设计目标</h1>

	<ul>
	<li>高可用：系统必须是高可用的。即使部分服务器宕机，Pegasus集群也能在极短时间（秒级）内恢复服务，尽量减少对用户的影响，要求服务可靠性达到99.99%以上。</li>
	<li>高性能：系统能够提供高性能的读写服务，P99延迟需要在毫秒级别。</li>
	<li>强一致：系统对用户提供强一致性的语义，使用户在编写业务逻辑时更容易。</li>
	<li>易伸缩：系统能够很方便地扩容和缩容，以应对业务吞吐负载的变化。</li>
	<li>易使用：系统给用户提供简单易用的库和接口，方便用户使用。</li>
	</ul>

	<h1 id="实现方案">实现方案</h1>

	<p>在设计Pegasus时，我们在目标、实现难度、开发效率等方面做一些权衡。总的来说，包括这几个方面：</p>
	<ul>
	<li>开发语言：基于性能考虑，我们选择了C++。</li>
	<li>数据模型：采用简单的Key-Value数据模型。这既简化了开发，也能满足大部分业务需求。进一步地，我们将Key拆分为了HashKey和SortKey两级，加强了其表达能力。</li>
	<li>数据分布：采用固定Hash分布。相比Range分布和一致性Hash分布，固定Hash分布实现更简单，数据倾斜和可伸缩性可以通过合理设计Hash键、预设更多的数据分片等措施来解决。我们也支持<a href="https://pegasus.apache.org/zh/administration/partition-split">Partition Split</a>功能来扩展分片数量。</li>
	<li>存储介质：建议选择SSD（固态硬盘）。SSD的性能和成本都介于内存和HDD（机械硬盘）之间，从业务需求和成本综合考虑，选择SSD是比较合适的。</li>
	<li>本地存储引擎：选择<a href="https://github.com/facebook/rocksdb">RocksDB</a>。RocksDB在LevelDB基础上做了很多优化，能充分利用SSD的IOPS性能和多核服务器的计算性能。</li>
	<li>一致性协议：选择<a href="https://www.microsoft.com/en-us/research/publication/pacifica-replication-in-log-based-distributed-storage-systems/">PacificA</a>。相比<a href="https://raft.github.io/">Raft</a>，PacificA协议具有其自身的特点和优势。</li>
	<li>故障检测：和HBase不同，Pegasus没有使用Zookeeper来进行故障检测，而是在MetaServer和ReplicaServer之间实现了基于租约的故障检测机制。</li>
	</ul>

	<h1 id="与hbase比较">与HBase比较</h1>

	<p>Pegasus系统的最初目的就是弥补HBase的不足，这里从用户使用角度比较一下两者的区别：</p>
	<ul>
	<li>数据模型：HBase是表格模型，采用Range分片；Pegasus是Key-Value模型，采用Hash分片。</li>
	<li>接口：HBase的API接口功能虽然很丰富，但是使用也更复杂；Pegasus的接口简单，对用户更友好。</li>
	<li>可靠性：由于架构和实现的原因（如Pegasus采用的本地存储、故障检测、使用C++语言实现等），Pegasus的可靠性通常优于HBase。</li>
	<li>性能：由于分层架构，HBase的读写性能不是太好，P99延迟通常在几十甚至几百毫秒，而且GC问题会带来毛刺问题；Pegasus的P99可以在几毫秒，满足敏感在线业务的需求。</li>
	</ul>

	<h1 id="与redis比较">与Redis比较</h1>

	<p>如果仅从读写延迟和单机吞吐比较，Redis显然是优于Pegasus的。但如果从读写延迟、可用性、伸缩性、成本等方面综合比较，Pegasus也是有其自身的优势的。</p>

	<p>与Redis进行比较的主要区别如下：</p>
	<ul>
	<li>数据模型：两者都是Key-Value模型，但是Pegasus支持(HashKey + SortKey)的二级键。</li>
	<li>接口：Redis的接口更丰富，支持List、Set、Map等容器特性；Pegasus的接口相对简单，功能更单一。</li>
	<li>读写延迟：Redis性能比Pegasus好。</li>
	<li>伸缩性：Pegasus伸缩性更好，可以很方便地增减机器节点，并支持自动的负载均衡；Redis的分布式方案在增减机器的时候比较麻烦。</li>
	<li>可靠性：Pegasus数据总是持久化的，系统架构保证其较高的数据完整性；Redis在机器宕机后需要较长时间恢复，可用性不够好，还可能丢掉最后一段时间的数据。</li>
	<li>成本：Pegasus使用SSD存储全量数据，而Redis需要使用内存来存储全量数据，Pegasus成本更低。</li>
	</ul>

	<h1 id="综合比较">综合比较</h1>

	<p>业务在系统选型时，通常遇到这些问题：</p>
	<ul>
	<li>HBase虽然可用性高也易伸缩，但是性能不够好。</li>
	<li>Redis虽然性能好，但是需要大量内存，带来更昂贵的硬件成本。如果数据量太大，采用<a href="https://redis.io/topics/cluster-tutorial">Redis Cluster</a>方案，在机器宕机故障情况下的可用性又不够。</li>
	<li>HBase+Redis的方案，即使用HBase做底层存储，使用Redis做上层缓存。该方案的缺点是：涉及两个系统，用户的读写逻辑会比较复杂；同时写两个系统，容易出现一致性问题；一份数据要同时存储在HBase和Redis中，成本比较高；Redis机器宕机后造成部分缓存丢失，此时从HBase读取的性能又明显降低。</li>
	</ul>

	<p>Pegasus综合了HBase和Redis的优点，它既保证高的可靠性，又具有好的伸缩性，还具有良好的性能。</p>

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