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| <section id="reading-writing-ipc-formats"> |
| <h1>Reading/Writing IPC formats<a class="headerlink" href="#reading-writing-ipc-formats" title="Link to this heading">¶</a></h1> |
| <p>Arrow defines two types of binary formats for serializing record batches:</p> |
| <ul class="simple"> |
| <li><p><strong>Streaming format</strong>: for sending an arbitrary number of record |
| batches. The format must be processed from start to end, and does not support |
| random access</p></li> |
| <li><p><strong>File or Random Access format</strong>: for serializing a fixed number of record |
| batches. It supports random access, and thus is very useful when used with |
| memory maps</p></li> |
| </ul> |
| <section id="writing-and-reading-streaming-format"> |
| <h2>Writing and Reading Streaming Format<a class="headerlink" href="#writing-and-reading-streaming-format" title="Link to this heading">¶</a></h2> |
| <p>First, let’s populate a <code class="xref py py-class docutils literal notranslate"><span class="pre">VectorSchemaRoot</span></code> with a small batch of records</p> |
| <div class="highlight-Java notranslate"><div class="highlight"><pre><span></span><span class="n">BitVector</span><span class="w"> </span><span class="n">bitVector</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">new</span><span class="w"> </span><span class="n">BitVector</span><span class="p">(</span><span class="s">"boolean"</span><span class="p">,</span><span class="w"> </span><span class="n">allocator</span><span class="p">);</span> |
| <span class="n">VarCharVector</span><span class="w"> </span><span class="n">varCharVector</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">new</span><span class="w"> </span><span class="n">VarCharVector</span><span class="p">(</span><span class="s">"varchar"</span><span class="p">,</span><span class="w"> </span><span class="n">allocator</span><span class="p">);</span> |
| <span class="k">for</span><span class="w"> </span><span class="p">(</span><span class="kt">int</span><span class="w"> </span><span class="n">i</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span><span class="w"> </span><span class="n">i</span><span class="w"> </span><span class="o"><</span><span class="w"> </span><span class="mi">10</span><span class="p">;</span><span class="w"> </span><span class="n">i</span><span class="o">++</span><span class="p">)</span><span class="w"> </span><span class="p">{</span> |
| <span class="w"> </span><span class="n">bitVector</span><span class="p">.</span><span class="na">setSafe</span><span class="p">(</span><span class="n">i</span><span class="p">,</span><span class="w"> </span><span class="n">i</span><span class="w"> </span><span class="o">%</span><span class="w"> </span><span class="mi">2</span><span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="mi">0</span><span class="w"> </span><span class="o">?</span><span class="w"> </span><span class="mi">0</span><span class="w"> </span><span class="p">:</span><span class="w"> </span><span class="mi">1</span><span class="p">);</span> |
| <span class="w"> </span><span class="n">varCharVector</span><span class="p">.</span><span class="na">setSafe</span><span class="p">(</span><span class="n">i</span><span class="p">,</span><span class="w"> </span><span class="p">(</span><span class="s">"test"</span><span class="w"> </span><span class="o">+</span><span class="w"> </span><span class="n">i</span><span class="p">).</span><span class="na">getBytes</span><span class="p">(</span><span class="n">StandardCharsets</span><span class="p">.</span><span class="na">UTF_8</span><span class="p">));</span> |
| <span class="p">}</span> |
| <span class="n">bitVector</span><span class="p">.</span><span class="na">setValueCount</span><span class="p">(</span><span class="mi">10</span><span class="p">);</span> |
| <span class="n">varCharVector</span><span class="p">.</span><span class="na">setValueCount</span><span class="p">(</span><span class="mi">10</span><span class="p">);</span> |
| |
| <span class="n">List</span><span class="o"><</span><span class="n">Field</span><span class="o">></span><span class="w"> </span><span class="n">fields</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">Arrays</span><span class="p">.</span><span class="na">asList</span><span class="p">(</span><span class="n">bitVector</span><span class="p">.</span><span class="na">getField</span><span class="p">(),</span><span class="w"> </span><span class="n">varCharVector</span><span class="p">.</span><span class="na">getField</span><span class="p">());</span> |
| <span class="n">List</span><span class="o"><</span><span class="n">FieldVector</span><span class="o">></span><span class="w"> </span><span class="n">vectors</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">Arrays</span><span class="p">.</span><span class="na">asList</span><span class="p">(</span><span class="n">bitVector</span><span class="p">,</span><span class="w"> </span><span class="n">varCharVector</span><span class="p">);</span> |
| <span class="n">VectorSchemaRoot</span><span class="w"> </span><span class="n">root</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">new</span><span class="w"> </span><span class="n">VectorSchemaRoot</span><span class="p">(</span><span class="n">fields</span><span class="p">,</span><span class="w"> </span><span class="n">vectors</span><span class="p">);</span> |
| </pre></div> |
| </div> |
| <p>Now, we can begin writing a stream containing some number of these batches. For this we use <code class="xref py py-class docutils literal notranslate"><span class="pre">ArrowStreamWriter</span></code> |
| (DictionaryProvider used for any vectors that are dictionary encoded is optional and can be null))</p> |
| <div class="highlight-Java notranslate"><div class="highlight"><pre><span></span><span class="k">try</span><span class="w"> </span><span class="p">(</span> |
| <span class="w"> </span><span class="n">ByteArrayOutputStream</span><span class="w"> </span><span class="n">out</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">new</span><span class="w"> </span><span class="n">ByteArrayOutputStream</span><span class="p">();</span> |
| <span class="w"> </span><span class="n">ArrowStreamWriter</span><span class="w"> </span><span class="n">writer</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">new</span><span class="w"> </span><span class="n">ArrowStreamWriter</span><span class="p">(</span><span class="n">root</span><span class="p">,</span><span class="w"> </span><span class="cm">/*DictionaryProvider=*/</span><span class="kc">null</span><span class="p">,</span><span class="w"> </span><span class="n">Channels</span><span class="p">.</span><span class="na">newChannel</span><span class="p">(</span><span class="n">out</span><span class="p">));</span> |
| <span class="p">)</span><span class="w"> </span><span class="p">{</span> |
| <span class="w"> </span><span class="c1">// ... do write into the ArrowStreamWriter</span> |
| <span class="p">}</span> |
| </pre></div> |
| </div> |
| <p>Here we used an in-memory stream, but this could have been a socket or some other IO stream. Then we can do</p> |
| <div class="highlight-Java notranslate"><div class="highlight"><pre><span></span><span class="n">writer</span><span class="p">.</span><span class="na">start</span><span class="p">();</span> |
| <span class="c1">// write the first batch</span> |
| <span class="n">writer</span><span class="p">.</span><span class="na">writeBatch</span><span class="p">();</span> |
| |
| <span class="c1">// write another four batches.</span> |
| <span class="k">for</span><span class="w"> </span><span class="p">(</span><span class="kt">int</span><span class="w"> </span><span class="n">i</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span><span class="w"> </span><span class="n">i</span><span class="w"> </span><span class="o"><</span><span class="w"> </span><span class="mi">4</span><span class="p">;</span><span class="w"> </span><span class="n">i</span><span class="o">++</span><span class="p">)</span><span class="w"> </span><span class="p">{</span> |
| <span class="w"> </span><span class="c1">// populate VectorSchemaRoot data and write the second batch</span> |
| <span class="w"> </span><span class="n">BitVector</span><span class="w"> </span><span class="n">childVector1</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="p">(</span><span class="n">BitVector</span><span class="p">)</span><span class="n">root</span><span class="p">.</span><span class="na">getVector</span><span class="p">(</span><span class="mi">0</span><span class="p">);</span> |
| <span class="w"> </span><span class="n">VarCharVector</span><span class="w"> </span><span class="n">childVector2</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="p">(</span><span class="n">VarCharVector</span><span class="p">)</span><span class="n">root</span><span class="p">.</span><span class="na">getVector</span><span class="p">(</span><span class="mi">1</span><span class="p">);</span> |
| <span class="w"> </span><span class="n">childVector1</span><span class="p">.</span><span class="na">reset</span><span class="p">();</span> |
| <span class="w"> </span><span class="n">childVector2</span><span class="p">.</span><span class="na">reset</span><span class="p">();</span> |
| <span class="w"> </span><span class="c1">// ... do some populate work here, could be different for each batch</span> |
| <span class="w"> </span><span class="n">writer</span><span class="p">.</span><span class="na">writeBatch</span><span class="p">();</span> |
| <span class="p">}</span> |
| |
| <span class="n">writer</span><span class="p">.</span><span class="na">end</span><span class="p">();</span> |
| </pre></div> |
| </div> |
| <p>Note that, since the <code class="xref py py-class docutils literal notranslate"><span class="pre">VectorSchemaRoot</span></code> in the writer is a container that can hold batches, batches flow through |
| <code class="xref py py-class docutils literal notranslate"><span class="pre">VectorSchemaRoot</span></code> as part of a pipeline, so we need to populate data before <code class="docutils literal notranslate"><span class="pre">writeBatch</span></code>, so that later batches |
| could overwrite previous ones.</p> |
| <p>Now the <code class="xref py py-class docutils literal notranslate"><span class="pre">ByteArrayOutputStream</span></code> contains the complete stream which contains 5 record batches. |
| We can read such a stream with <code class="xref py py-class docutils literal notranslate"><span class="pre">ArrowStreamReader</span></code>. Note that the <code class="xref py py-class docutils literal notranslate"><span class="pre">VectorSchemaRoot</span></code> within the reader |
| will be loaded with new values on every call to <code class="xref py py-class docutils literal notranslate"><span class="pre">loadNextBatch()</span></code></p> |
| <div class="highlight-Java notranslate"><div class="highlight"><pre><span></span><span class="k">try</span><span class="w"> </span><span class="p">(</span><span class="n">ArrowStreamReader</span><span class="w"> </span><span class="n">reader</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">new</span><span class="w"> </span><span class="n">ArrowStreamReader</span><span class="p">(</span><span class="k">new</span><span class="w"> </span><span class="n">ByteArrayInputStream</span><span class="p">(</span><span class="n">out</span><span class="p">.</span><span class="na">toByteArray</span><span class="p">()),</span><span class="w"> </span><span class="n">allocator</span><span class="p">))</span><span class="w"> </span><span class="p">{</span> |
| <span class="w"> </span><span class="c1">// This will be loaded with new values on every call to loadNextBatch</span> |
| <span class="w"> </span><span class="n">VectorSchemaRoot</span><span class="w"> </span><span class="n">readRoot</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">reader</span><span class="p">.</span><span class="na">getVectorSchemaRoot</span><span class="p">();</span> |
| <span class="w"> </span><span class="n">Schema</span><span class="w"> </span><span class="n">schema</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">readRoot</span><span class="p">.</span><span class="na">getSchema</span><span class="p">();</span> |
| <span class="w"> </span><span class="k">for</span><span class="w"> </span><span class="p">(</span><span class="kt">int</span><span class="w"> </span><span class="n">i</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span><span class="w"> </span><span class="n">i</span><span class="w"> </span><span class="o"><</span><span class="w"> </span><span class="mi">5</span><span class="p">;</span><span class="w"> </span><span class="n">i</span><span class="o">++</span><span class="p">)</span><span class="w"> </span><span class="p">{</span> |
| <span class="w"> </span><span class="n">reader</span><span class="p">.</span><span class="na">loadNextBatch</span><span class="p">();</span> |
| <span class="w"> </span><span class="c1">// ... do something with readRoot</span> |
| <span class="w"> </span><span class="p">}</span> |
| <span class="p">}</span> |
| </pre></div> |
| </div> |
| <p>Here we also give a simple example with dictionary encoded vectors</p> |
| <div class="highlight-Java notranslate"><div class="highlight"><pre><span></span><span class="c1">// create provider</span> |
| <span class="n">DictionaryProvider</span><span class="p">.</span><span class="na">MapDictionaryProvider</span><span class="w"> </span><span class="n">provider</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">new</span><span class="w"> </span><span class="n">DictionaryProvider</span><span class="p">.</span><span class="na">MapDictionaryProvider</span><span class="p">();</span> |
| |
| <span class="k">try</span><span class="w"> </span><span class="p">(</span> |
| <span class="w"> </span><span class="kd">final</span><span class="w"> </span><span class="n">VarCharVector</span><span class="w"> </span><span class="n">dictVector</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">new</span><span class="w"> </span><span class="n">VarCharVector</span><span class="p">(</span><span class="s">"dict"</span><span class="p">,</span><span class="w"> </span><span class="n">allocator</span><span class="p">);</span> |
| <span class="w"> </span><span class="kd">final</span><span class="w"> </span><span class="n">VarCharVector</span><span class="w"> </span><span class="n">vector</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">new</span><span class="w"> </span><span class="n">VarCharVector</span><span class="p">(</span><span class="s">"vector"</span><span class="p">,</span><span class="w"> </span><span class="n">allocator</span><span class="p">);</span> |
| <span class="p">)</span><span class="w"> </span><span class="p">{</span> |
| <span class="w"> </span><span class="c1">// create dictionary vector</span> |
| <span class="w"> </span><span class="n">dictVector</span><span class="p">.</span><span class="na">allocateNewSafe</span><span class="p">();</span> |
| <span class="w"> </span><span class="n">dictVector</span><span class="p">.</span><span class="na">setSafe</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span><span class="w"> </span><span class="s">"aa"</span><span class="p">.</span><span class="na">getBytes</span><span class="p">());</span> |
| <span class="w"> </span><span class="n">dictVector</span><span class="p">.</span><span class="na">setSafe</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="s">"bb"</span><span class="p">.</span><span class="na">getBytes</span><span class="p">());</span> |
| <span class="w"> </span><span class="n">dictVector</span><span class="p">.</span><span class="na">setSafe</span><span class="p">(</span><span class="mi">2</span><span class="p">,</span><span class="w"> </span><span class="s">"cc"</span><span class="p">.</span><span class="na">getBytes</span><span class="p">());</span> |
| <span class="w"> </span><span class="n">dictVector</span><span class="p">.</span><span class="na">setValueCount</span><span class="p">(</span><span class="mi">3</span><span class="p">);</span> |
| |
| <span class="w"> </span><span class="c1">// create dictionary</span> |
| <span class="w"> </span><span class="n">Dictionary</span><span class="w"> </span><span class="n">dictionary</span><span class="w"> </span><span class="o">=</span> |
| <span class="w"> </span><span class="k">new</span><span class="w"> </span><span class="n">Dictionary</span><span class="p">(</span><span class="n">dictVector</span><span class="p">,</span><span class="w"> </span><span class="k">new</span><span class="w"> </span><span class="n">DictionaryEncoding</span><span class="p">(</span><span class="mi">1L</span><span class="p">,</span><span class="w"> </span><span class="kc">false</span><span class="p">,</span><span class="w"> </span><span class="cm">/*indexType=*/</span><span class="kc">null</span><span class="p">));</span> |
| <span class="w"> </span><span class="n">provider</span><span class="p">.</span><span class="na">put</span><span class="p">(</span><span class="n">dictionary</span><span class="p">);</span> |
| |
| <span class="w"> </span><span class="c1">// create original data vector</span> |
| <span class="w"> </span><span class="n">vector</span><span class="p">.</span><span class="na">allocateNewSafe</span><span class="p">();</span> |
| <span class="w"> </span><span class="n">vector</span><span class="p">.</span><span class="na">setSafe</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span><span class="w"> </span><span class="s">"bb"</span><span class="p">.</span><span class="na">getBytes</span><span class="p">());</span> |
| <span class="w"> </span><span class="n">vector</span><span class="p">.</span><span class="na">setSafe</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="s">"bb"</span><span class="p">.</span><span class="na">getBytes</span><span class="p">());</span> |
| <span class="w"> </span><span class="n">vector</span><span class="p">.</span><span class="na">setSafe</span><span class="p">(</span><span class="mi">2</span><span class="p">,</span><span class="w"> </span><span class="s">"cc"</span><span class="p">.</span><span class="na">getBytes</span><span class="p">());</span> |
| <span class="w"> </span><span class="n">vector</span><span class="p">.</span><span class="na">setSafe</span><span class="p">(</span><span class="mi">3</span><span class="p">,</span><span class="w"> </span><span class="s">"aa"</span><span class="p">.</span><span class="na">getBytes</span><span class="p">());</span> |
| <span class="w"> </span><span class="n">vector</span><span class="p">.</span><span class="na">setValueCount</span><span class="p">(</span><span class="mi">4</span><span class="p">);</span> |
| |
| <span class="w"> </span><span class="c1">// get the encoded vector</span> |
| <span class="w"> </span><span class="n">IntVector</span><span class="w"> </span><span class="n">encodedVector</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="p">(</span><span class="n">IntVector</span><span class="p">)</span><span class="w"> </span><span class="n">DictionaryEncoder</span><span class="p">.</span><span class="na">encode</span><span class="p">(</span><span class="n">vector</span><span class="p">,</span><span class="w"> </span><span class="n">dictionary</span><span class="p">);</span> |
| |
| <span class="w"> </span><span class="n">ByteArrayOutputStream</span><span class="w"> </span><span class="n">out</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">new</span><span class="w"> </span><span class="n">ByteArrayOutputStream</span><span class="p">();</span> |
| |
| <span class="w"> </span><span class="c1">// create VectorSchemaRoot</span> |
| <span class="w"> </span><span class="n">List</span><span class="o"><</span><span class="n">Field</span><span class="o">></span><span class="w"> </span><span class="n">fields</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">Arrays</span><span class="p">.</span><span class="na">asList</span><span class="p">(</span><span class="n">encodedVector</span><span class="p">.</span><span class="na">getField</span><span class="p">());</span> |
| <span class="w"> </span><span class="n">List</span><span class="o"><</span><span class="n">FieldVector</span><span class="o">></span><span class="w"> </span><span class="n">vectors</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">Arrays</span><span class="p">.</span><span class="na">asList</span><span class="p">(</span><span class="n">encodedVector</span><span class="p">);</span> |
| <span class="w"> </span><span class="k">try</span><span class="w"> </span><span class="p">(</span><span class="n">VectorSchemaRoot</span><span class="w"> </span><span class="n">root</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">new</span><span class="w"> </span><span class="n">VectorSchemaRoot</span><span class="p">(</span><span class="n">fields</span><span class="p">,</span><span class="w"> </span><span class="n">vectors</span><span class="p">))</span><span class="w"> </span><span class="p">{</span> |
| |
| <span class="w"> </span><span class="c1">// write data</span> |
| <span class="w"> </span><span class="n">ArrowStreamWriter</span><span class="w"> </span><span class="n">writer</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">new</span><span class="w"> </span><span class="n">ArrowStreamWriter</span><span class="p">(</span><span class="n">root</span><span class="p">,</span><span class="w"> </span><span class="n">provider</span><span class="p">,</span><span class="w"> </span><span class="n">Channels</span><span class="p">.</span><span class="na">newChannel</span><span class="p">(</span><span class="n">out</span><span class="p">));</span> |
| <span class="w"> </span><span class="n">writer</span><span class="p">.</span><span class="na">start</span><span class="p">();</span> |
| <span class="w"> </span><span class="n">writer</span><span class="p">.</span><span class="na">writeBatch</span><span class="p">();</span> |
| <span class="w"> </span><span class="n">writer</span><span class="p">.</span><span class="na">end</span><span class="p">();</span> |
| <span class="w"> </span><span class="p">}</span> |
| |
| <span class="w"> </span><span class="c1">// read data</span> |
| <span class="w"> </span><span class="k">try</span><span class="w"> </span><span class="p">(</span><span class="n">ArrowStreamReader</span><span class="w"> </span><span class="n">reader</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">new</span><span class="w"> </span><span class="n">ArrowStreamReader</span><span class="p">(</span><span class="k">new</span><span class="w"> </span><span class="n">ByteArrayInputStream</span><span class="p">(</span><span class="n">out</span><span class="p">.</span><span class="na">toByteArray</span><span class="p">()),</span><span class="w"> </span><span class="n">allocator</span><span class="p">))</span><span class="w"> </span><span class="p">{</span> |
| <span class="w"> </span><span class="n">reader</span><span class="p">.</span><span class="na">loadNextBatch</span><span class="p">();</span> |
| <span class="w"> </span><span class="n">VectorSchemaRoot</span><span class="w"> </span><span class="n">readRoot</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">reader</span><span class="p">.</span><span class="na">getVectorSchemaRoot</span><span class="p">();</span> |
| <span class="w"> </span><span class="c1">// get the encoded vector</span> |
| <span class="w"> </span><span class="n">IntVector</span><span class="w"> </span><span class="n">intVector</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="p">(</span><span class="n">IntVector</span><span class="p">)</span><span class="w"> </span><span class="n">readRoot</span><span class="p">.</span><span class="na">getVector</span><span class="p">(</span><span class="mi">0</span><span class="p">);</span> |
| |
| <span class="w"> </span><span class="c1">// get dictionaries and decode the vector</span> |
| <span class="w"> </span><span class="n">Map</span><span class="o"><</span><span class="n">Long</span><span class="p">,</span><span class="w"> </span><span class="n">Dictionary</span><span class="o">></span><span class="w"> </span><span class="n">dictionaryMap</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">reader</span><span class="p">.</span><span class="na">getDictionaryVectors</span><span class="p">();</span> |
| <span class="w"> </span><span class="kt">long</span><span class="w"> </span><span class="n">dictionaryId</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">intVector</span><span class="p">.</span><span class="na">getField</span><span class="p">().</span><span class="na">getDictionary</span><span class="p">().</span><span class="na">getId</span><span class="p">();</span> |
| <span class="w"> </span><span class="k">try</span><span class="w"> </span><span class="p">(</span><span class="n">VarCharVector</span><span class="w"> </span><span class="n">varCharVector</span><span class="w"> </span><span class="o">=</span> |
| <span class="w"> </span><span class="p">(</span><span class="n">VarCharVector</span><span class="p">)</span><span class="w"> </span><span class="n">DictionaryEncoder</span><span class="p">.</span><span class="na">decode</span><span class="p">(</span><span class="n">intVector</span><span class="p">,</span><span class="w"> </span><span class="n">dictionaryMap</span><span class="p">.</span><span class="na">get</span><span class="p">(</span><span class="n">dictionaryId</span><span class="p">)))</span><span class="w"> </span><span class="p">{</span> |
| <span class="w"> </span><span class="c1">// ... use decoded vector</span> |
| <span class="w"> </span><span class="p">}</span> |
| <span class="w"> </span><span class="p">}</span> |
| <span class="p">}</span> |
| </pre></div> |
| </div> |
| </section> |
| <section id="writing-and-reading-random-access-files"> |
| <h2>Writing and Reading Random Access Files<a class="headerlink" href="#writing-and-reading-random-access-files" title="Link to this heading">¶</a></h2> |
| <p>The <code class="xref py py-class docutils literal notranslate"><span class="pre">ArrowFileWriter</span></code> has the same API as <code class="xref py py-class docutils literal notranslate"><span class="pre">ArrowStreamWriter</span></code></p> |
| <div class="highlight-Java notranslate"><div class="highlight"><pre><span></span><span class="k">try</span><span class="w"> </span><span class="p">(</span> |
| <span class="w"> </span><span class="n">ByteArrayOutputStream</span><span class="w"> </span><span class="n">out</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">new</span><span class="w"> </span><span class="n">ByteArrayOutputStream</span><span class="p">();</span> |
| <span class="w"> </span><span class="n">ArrowFileWriter</span><span class="w"> </span><span class="n">writer</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">new</span><span class="w"> </span><span class="n">ArrowFileWriter</span><span class="p">(</span><span class="n">root</span><span class="p">,</span><span class="w"> </span><span class="cm">/*DictionaryProvider=*/</span><span class="kc">null</span><span class="p">,</span><span class="w"> </span><span class="n">Channels</span><span class="p">.</span><span class="na">newChannel</span><span class="p">(</span><span class="n">out</span><span class="p">));</span> |
| <span class="p">)</span><span class="w"> </span><span class="p">{</span> |
| <span class="w"> </span><span class="n">writer</span><span class="p">.</span><span class="na">start</span><span class="p">();</span> |
| <span class="w"> </span><span class="c1">// write the first batch</span> |
| <span class="w"> </span><span class="n">writer</span><span class="p">.</span><span class="na">writeBatch</span><span class="p">();</span> |
| <span class="w"> </span><span class="c1">// write another four batches.</span> |
| <span class="w"> </span><span class="k">for</span><span class="w"> </span><span class="p">(</span><span class="kt">int</span><span class="w"> </span><span class="n">i</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span><span class="w"> </span><span class="n">i</span><span class="w"> </span><span class="o"><</span><span class="w"> </span><span class="mi">4</span><span class="p">;</span><span class="w"> </span><span class="n">i</span><span class="o">++</span><span class="p">)</span><span class="w"> </span><span class="p">{</span> |
| <span class="w"> </span><span class="c1">// ... do populate work</span> |
| <span class="w"> </span><span class="n">writer</span><span class="p">.</span><span class="na">writeBatch</span><span class="p">();</span> |
| <span class="w"> </span><span class="p">}</span> |
| <span class="w"> </span><span class="n">writer</span><span class="p">.</span><span class="na">end</span><span class="p">();</span> |
| <span class="p">}</span> |
| </pre></div> |
| </div> |
| <p>The difference between <code class="xref py py-class docutils literal notranslate"><span class="pre">ArrowFileReader</span></code> and <code class="xref py py-class docutils literal notranslate"><span class="pre">ArrowStreamReader</span></code> is that the input source |
| must have a <code class="docutils literal notranslate"><span class="pre">seek</span></code> method for random access. Because we have access to the entire payload, we know the |
| number of record batches in the file, and can read any at random</p> |
| <div class="highlight-Java notranslate"><div class="highlight"><pre><span></span><span class="k">try</span><span class="w"> </span><span class="p">(</span><span class="n">ArrowFileReader</span><span class="w"> </span><span class="n">reader</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">new</span><span class="w"> </span><span class="n">ArrowFileReader</span><span class="p">(</span> |
| <span class="w"> </span><span class="k">new</span><span class="w"> </span><span class="n">ByteArrayReadableSeekableByteChannel</span><span class="p">(</span><span class="n">out</span><span class="p">.</span><span class="na">toByteArray</span><span class="p">()),</span><span class="w"> </span><span class="n">allocator</span><span class="p">))</span><span class="w"> </span><span class="p">{</span> |
| |
| <span class="w"> </span><span class="c1">// read the 4-th batch</span> |
| <span class="w"> </span><span class="n">ArrowBlock</span><span class="w"> </span><span class="n">block</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">reader</span><span class="p">.</span><span class="na">getRecordBlocks</span><span class="p">().</span><span class="na">get</span><span class="p">(</span><span class="mi">3</span><span class="p">);</span> |
| <span class="w"> </span><span class="n">reader</span><span class="p">.</span><span class="na">loadRecordBatch</span><span class="p">(</span><span class="n">block</span><span class="p">);</span> |
| <span class="w"> </span><span class="n">VectorSchemaRoot</span><span class="w"> </span><span class="n">readBatch</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">reader</span><span class="p">.</span><span class="na">getVectorSchemaRoot</span><span class="p">();</span> |
| <span class="p">}</span> |
| </pre></div> |
| </div> |
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