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     <h1 id="Data Model">Data Model</h1>
     <h2 id="introduction">Introduction</h2>

 <p>The data model of Pegasus is a simple Key-Value model, it does not support complex schemas. However, to enhance its expressive power, Key is split into <strong>HashKey</strong> and <strong>SortKey</strong>, namely composite key (<code class="language-plaintext highlighter-rouge">[HashKey, SortKey] -&gt;Value</code>), which is similar to <a href="https://aws.amazon.com/dynamodb/">DynamoDB</a>’s <a href="http://docs.aws.amazon.com/amazondynamodb/latest/developerguide/howitworks.corecomponents.html#howitworks.corecomponents.primarykey">composite primary key</a>.</p>

 <h3 id="hashkey">HashKey</h3>

 <p>Byte string. Similar to the partition key in DynamoDB, HashKey is used to calculate which partition (a.k.a. shard) the data belongs to. Pegasus uses a specific hash function to calculate the hash value for a HashKey, and then modulo the number of partitions to obtain the <strong>Partition ID</strong> for the data. Therefore, data with the same HashKey is always stored in the same partition.</p>

 <blockquote>
   <p>Note:
 On the C++ client side, the HashKey length limit is 64KB.
 On the Java client side, if <a href="https://github.com/apache/incubator-pegasus/blob/v2.5.0/java-client/src/main/java/org/apache/pegasus/client/ClientOptions.java#L360C12-L360C12">WriteLimiter</a> is enabled, then the limit is 1KB.
 On the server side, since Pegasus 2.0.0, if <code class="language-plaintext highlighter-rouge">[replication]max_allowed_write_size</code> is set as non-zero, limit the size of the entire request packet to this value, defaulting to 1MB.</p>
 </blockquote>

 <h3 id="sortkey">SortKey</h3>

 <p>Byte string. Similar to the sort key in DynamoDB, SortKey is used for sorting data within a partition. In fact, when storing data internally in RocksDB, we concatenate HashKey and SortKey as the keys of RocksDB.</p>
 <blockquote>
   <p>Note:
 On the C++ client side, there is no limit to the length of SortKey.
 On the Java client side, if <a href="https://github.com/apache/incubator-pegasus/blob/v2.5.0/java-client/src/main/java/org/apache/pegasus/client/ClientOptions.java#L360C12-L360C12">WriteLimiter</a> is enabled, then the limit is 1KB.
 On the server side, since Pegasus 2.0.0, if <code class="language-plaintext highlighter-rouge">[replication]max_allowed_write_size</code> is set as non-zero, limit the size of the entire request packet to this value, defaulting to 1MB.</p>
 </blockquote>

 <h3 id="value">Value</h3>

 <p>Byte string.</p>
 <blockquote>
   <p>Note:
 On the C++ client side, there is no limit to the length of the Value.
 On the Java client side, if <a href="https://github.com/apache/incubator-pegasus/blob/v2.5.0/java-client/src/main/java/org/apache/pegasus/client/ClientOptions.java#L360C12-L360C12">WriteLimiter</a> is enabled, then the limit is 400KB.
 On the server side, since Pegasus 2.0.0, if <code class="language-plaintext highlighter-rouge">[replication]max_allowed_write_size</code> is set as non-zero, limit the size of the entire request packet to this value, defaulting to 1MB.</p>
 </blockquote>

 <p><img src="/assets/images/pegasus-data-model.png" alt="pegasus-data-model" class="img-responsive docs-image" /></p>

 <h2 id="pegasus-vs-hbase">Pegasus vs. HBase</h2>

 <p>Although Pegasus is not as semantically rich as HBase’s tabular model, it can still meet most applications’ needs, thanks to its HashKey+SortKey combination key design.
 For example, users can treat HashKey as a row key and SortKey as an attribute name or column name, so that multiple data of the same HashKey can be viewed as one row, which can also express the concept of row in HBase.
 Taking this into consideration, Pegasus not only provides the <code class="language-plaintext highlighter-rouge">get</code>/<code class="language-plaintext highlighter-rouge">set</code>/<code class="language-plaintext highlighter-rouge">del</code> interface for accessing individual data, but also provides the <code class="language-plaintext highlighter-rouge">multi_get</code>/<code class="language-plaintext highlighter-rouge">multi_set</code>/<code class="language-plaintext highlighter-rouge">multi_del</code> interfaces for accessing batch data in the same HashKey, and these interfaces provide single line atomic semantics, making it convenient for users to use.</p>

 <p><img src="/assets/images/pegasus-data-model-sample.png" alt="pegasus-data-model" class="img-responsive docs-image" /></p>

 <h2 id="pegasus-vs-redis">Pegasus vs. Redis</h2>

 <p>Although Pegasus does not support rich data structures such as <code class="language-plaintext highlighter-rouge">List</code>/<code class="language-plaintext highlighter-rouge">Set</code>/<code class="language-plaintext highlighter-rouge">Hash</code> like Redis, users can still use Pegasus to implement similar semantics.
 For example, users can equate HashKey with Redis’ <code class="language-plaintext highlighter-rouge">key</code> and use SortKey as the <code class="language-plaintext highlighter-rouge">field</code> of Hash (or <code class="language-plaintext highlighter-rouge">member</code> of Set) to implement Hash in Redis.</p>

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   <li><a href="#Data Model">Data Model</a>
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       <li><a href="#introduction">Introduction</a>
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           <li><a href="#hashkey">HashKey</a></li>
           <li><a href="#sortkey">SortKey</a></li>
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	<h1 id="Data Model">Data Model</h1>
	<h2 id="introduction">Introduction</h2>

	<p>The data model of Pegasus is a simple Key-Value model, it does not support complex schemas. However, to enhance its expressive power, Key is split into <strong>HashKey</strong> and <strong>SortKey</strong>, namely composite key (<code class="language-plaintext highlighter-rouge">[HashKey, SortKey] ->Value</code>), which is similar to <a href="https://aws.amazon.com/dynamodb/">DynamoDB</a>’s <a href="http://docs.aws.amazon.com/amazondynamodb/latest/developerguide/howitworks.corecomponents.html#howitworks.corecomponents.primarykey">composite primary key</a>.</p>

	<h3 id="hashkey">HashKey</h3>

	<p>Byte string. Similar to the partition key in DynamoDB, HashKey is used to calculate which partition (a.k.a. shard) the data belongs to. Pegasus uses a specific hash function to calculate the hash value for a HashKey, and then modulo the number of partitions to obtain the <strong>Partition ID</strong> for the data. Therefore, data with the same HashKey is always stored in the same partition.</p>

	<blockquote>
	<p>Note:
	On the C++ client side, the HashKey length limit is 64KB.
	On the Java client side, if <a href="https://github.com/apache/incubator-pegasus/blob/v2.5.0/java-client/src/main/java/org/apache/pegasus/client/ClientOptions.java#L360C12-L360C12">WriteLimiter</a> is enabled, then the limit is 1KB.
	On the server side, since Pegasus 2.0.0, if <code class="language-plaintext highlighter-rouge">[replication]max_allowed_write_size</code> is set as non-zero, limit the size of the entire request packet to this value, defaulting to 1MB.</p>
	</blockquote>

	<h3 id="sortkey">SortKey</h3>

	<p>Byte string. Similar to the sort key in DynamoDB, SortKey is used for sorting data within a partition. In fact, when storing data internally in RocksDB, we concatenate HashKey and SortKey as the keys of RocksDB.</p>
	<blockquote>
	<p>Note:
	On the C++ client side, there is no limit to the length of SortKey.
	On the Java client side, if <a href="https://github.com/apache/incubator-pegasus/blob/v2.5.0/java-client/src/main/java/org/apache/pegasus/client/ClientOptions.java#L360C12-L360C12">WriteLimiter</a> is enabled, then the limit is 1KB.
	On the server side, since Pegasus 2.0.0, if <code class="language-plaintext highlighter-rouge">[replication]max_allowed_write_size</code> is set as non-zero, limit the size of the entire request packet to this value, defaulting to 1MB.</p>
	</blockquote>

	<h3 id="value">Value</h3>

	<p>Byte string.</p>
	<blockquote>
	<p>Note:
	On the C++ client side, there is no limit to the length of the Value.
	On the Java client side, if <a href="https://github.com/apache/incubator-pegasus/blob/v2.5.0/java-client/src/main/java/org/apache/pegasus/client/ClientOptions.java#L360C12-L360C12">WriteLimiter</a> is enabled, then the limit is 400KB.
	On the server side, since Pegasus 2.0.0, if <code class="language-plaintext highlighter-rouge">[replication]max_allowed_write_size</code> is set as non-zero, limit the size of the entire request packet to this value, defaulting to 1MB.</p>
	</blockquote>

	<p><img src="/assets/images/pegasus-data-model.png" alt="pegasus-data-model" class="img-responsive docs-image" /></p>

	<h2 id="pegasus-vs-hbase">Pegasus vs. HBase</h2>

	<p>Although Pegasus is not as semantically rich as HBase’s tabular model, it can still meet most applications’ needs, thanks to its HashKey+SortKey combination key design.
	For example, users can treat HashKey as a row key and SortKey as an attribute name or column name, so that multiple data of the same HashKey can be viewed as one row, which can also express the concept of row in HBase.
	Taking this into consideration, Pegasus not only provides the <code class="language-plaintext highlighter-rouge">get</code>/<code class="language-plaintext highlighter-rouge">set</code>/<code class="language-plaintext highlighter-rouge">del</code> interface for accessing individual data, but also provides the <code class="language-plaintext highlighter-rouge">multi_get</code>/<code class="language-plaintext highlighter-rouge">multi_set</code>/<code class="language-plaintext highlighter-rouge">multi_del</code> interfaces for accessing batch data in the same HashKey, and these interfaces provide single line atomic semantics, making it convenient for users to use.</p>

	<p><img src="/assets/images/pegasus-data-model-sample.png" alt="pegasus-data-model" class="img-responsive docs-image" /></p>

	<h2 id="pegasus-vs-redis">Pegasus vs. Redis</h2>

	<p>Although Pegasus does not support rich data structures such as <code class="language-plaintext highlighter-rouge">List</code>/<code class="language-plaintext highlighter-rouge">Set</code>/<code class="language-plaintext highlighter-rouge">Hash</code> like Redis, users can still use Pegasus to implement similar semantics.
	For example, users can equate HashKey with Redis’ <code class="language-plaintext highlighter-rouge">key</code> and use SortKey as the <code class="language-plaintext highlighter-rouge">field</code> of Hash (or <code class="language-plaintext highlighter-rouge">member</code> of Set) to implement Hash in Redis.</p>

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	Table of contents
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	<ul class="menu-list">
	<li><a href="#Data Model">Data Model</a>
	<ul>
	<li><a href="#introduction">Introduction</a>
	<ul>
	<li><a href="#hashkey">HashKey</a></li>
	<li><a href="#sortkey">SortKey</a></li>
	<li><a href="#value">Value</a></li>
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	<li><a href="#pegasus-vs-hbase">Pegasus vs. HBase</a></li>
	<li><a href="#pegasus-vs-redis">Pegasus vs. Redis</a></li>
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