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| <section id="parquet-metadata-cache"> |
| <h1>Parquet Metadata Cache<a class="headerlink" href="#parquet-metadata-cache" title="Link to this heading">#</a></h1> |
| <section id="overview"> |
| <h2>Overview<a class="headerlink" href="#overview" title="Link to this heading">#</a></h2> |
| <p>Paimon C++ can cache serialized Parquet metadata footer bytes for Parquet data files. |
| The cache is used by <code class="docutils literal notranslate"><span class="pre">ParquetReaderBuilder</span></code> before opening the Arrow Parquet |
| reader. On a cache miss, Paimon C++ loads the Parquet file metadata, serializes |
| it as a complete metadata footer, and stores those bytes in the public |
| <code class="docutils literal notranslate"><span class="pre">Cache</span></code> abstraction. On a cache hit, Paimon C++ parses the cached footer bytes into |
| <code class="docutils literal notranslate"><span class="pre">parquet::FileMetaData</span></code> and passes the metadata to the Parquet reader.</p> |
| <p>The cache stores serialized metadata footer bytes instead of caching a |
| <code class="docutils literal notranslate"><span class="pre">parquet::FileMetaData</span></code> instance. This keeps the cache value compact and |
| similar to manifest cache values: the cache weight follows the actual cached |
| bytes, while the Parquet library still owns metadata parsing and validation.</p> |
| <p>This optimization is useful when the same Parquet files are opened repeatedly |
| in the same process, for example repeated <code class="docutils literal notranslate"><span class="pre">get</span></code> or <code class="docutils literal notranslate"><span class="pre">scan</span></code> requests over the |
| same snapshot. On a cache hit, the read path avoids reading the Parquet footer |
| bytes from the filesystem again. Paimon C++ still parses the cached footer bytes |
| into <code class="docutils literal notranslate"><span class="pre">parquet::FileMetaData</span></code> for each reader open. Data pages, page indexes, |
| and column chunks are still read from the file as usual.</p> |
| </section> |
| <section id="configuration"> |
| <h2>Configuration<a class="headerlink" href="#configuration" title="Link to this heading">#</a></h2> |
| <p>Parquet metadata caching is disabled by default. Embedding applications that |
| need it can provide a custom <code class="docutils literal notranslate"><span class="pre">Cache</span></code> implementation and inject it through |
| <code class="docutils literal notranslate"><span class="pre">ScanContextBuilder</span></code> or <code class="docutils literal notranslate"><span class="pre">ReadContextBuilder</span></code>. Parquet reader builders |
| receive the cache from the read context and create cache keys with |
| <code class="docutils literal notranslate"><span class="pre">CacheKind::DATA_FILE_FOOTER</span></code> internally.</p> |
| <p>The cache key represents the file footer and is created from the file URI with |
| position <code class="docutils literal notranslate"><span class="pre">-1</span></code> and length <code class="docutils literal notranslate"><span class="pre">-1</span></code>. Callers do not need to construct this key |
| directly; they only need to route <code class="docutils literal notranslate"><span class="pre">CacheKind::DATA_FILE_FOOTER</span></code> entries to an |
| appropriate cache backend.</p> |
| <p>Example:</p> |
| <div class="highlight-cpp notranslate"><div class="highlight"><pre><span></span><span class="k">class</span><span class="w"> </span><span class="nc">RoutingCache</span><span class="w"> </span><span class="o">:</span><span class="w"> </span><span class="k">public</span><span class="w"> </span><span class="n">paimon</span><span class="o">::</span><span class="n">Cache</span><span class="w"> </span><span class="p">{</span> |
| <span class="w"> </span><span class="k">public</span><span class="o">:</span> |
| <span class="w"> </span><span class="n">RoutingCache</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">shared_ptr</span><span class="o"><</span><span class="n">paimon</span><span class="o">::</span><span class="n">Cache</span><span class="o">></span><span class="w"> </span><span class="n">default_cache</span><span class="p">,</span> |
| <span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">shared_ptr</span><span class="o"><</span><span class="n">paimon</span><span class="o">::</span><span class="n">Cache</span><span class="o">></span><span class="w"> </span><span class="n">parquet_metadata_cache</span><span class="p">)</span> |
| <span class="w"> </span><span class="o">:</span><span class="w"> </span><span class="n">default_cache_</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">move</span><span class="p">(</span><span class="n">default_cache</span><span class="p">)),</span> |
| <span class="w"> </span><span class="n">parquet_metadata_cache_</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">move</span><span class="p">(</span><span class="n">parquet_metadata_cache</span><span class="p">))</span><span class="w"> </span><span class="p">{}</span> |
| |
| <span class="w"> </span><span class="n">paimon</span><span class="o">::</span><span class="n">Result</span><span class="o"><</span><span class="n">std</span><span class="o">::</span><span class="n">shared_ptr</span><span class="o"><</span><span class="n">paimon</span><span class="o">::</span><span class="n">CacheValue</span><span class="o">>></span><span class="w"> </span><span class="n">Get</span><span class="p">(</span> |
| <span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">shared_ptr</span><span class="o"><</span><span class="n">paimon</span><span class="o">::</span><span class="n">CacheKey</span><span class="o">>&</span><span class="w"> </span><span class="n">key</span><span class="p">,</span> |
| <span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">function</span><span class="o"><</span><span class="n">paimon</span><span class="o">::</span><span class="n">Result</span><span class="o"><</span><span class="n">std</span><span class="o">::</span><span class="n">shared_ptr</span><span class="o"><</span><span class="n">paimon</span><span class="o">::</span><span class="n">CacheValue</span><span class="o">>></span><span class="p">(</span> |
| <span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">shared_ptr</span><span class="o"><</span><span class="n">paimon</span><span class="o">::</span><span class="n">CacheKey</span><span class="o">>&</span><span class="p">)</span><span class="o">></span><span class="w"> </span><span class="n">supplier</span><span class="p">)</span><span class="w"> </span><span class="k">override</span><span class="w"> </span><span class="p">{</span> |
| <span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">Select</span><span class="p">(</span><span class="n">key</span><span class="p">)</span><span class="o">-></span><span class="n">Get</span><span class="p">(</span><span class="n">key</span><span class="p">,</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">move</span><span class="p">(</span><span class="n">supplier</span><span class="p">));</span> |
| <span class="w"> </span><span class="p">}</span> |
| |
| <span class="w"> </span><span class="c1">// Put(), Invalidate(), InvalidateAll(), and Size() route in the same way.</span> |
| |
| <span class="w"> </span><span class="k">private</span><span class="o">:</span> |
| <span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">shared_ptr</span><span class="o"><</span><span class="n">paimon</span><span class="o">::</span><span class="n">Cache</span><span class="o">></span><span class="w"> </span><span class="n">Select</span><span class="p">(</span> |
| <span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">shared_ptr</span><span class="o"><</span><span class="n">paimon</span><span class="o">::</span><span class="n">CacheKey</span><span class="o">>&</span><span class="w"> </span><span class="n">key</span><span class="p">)</span><span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="p">{</span> |
| <span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">key</span><span class="w"> </span><span class="o">&&</span><span class="w"> </span><span class="n">key</span><span class="o">-></span><span class="n">GetKind</span><span class="p">()</span><span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="n">paimon</span><span class="o">::</span><span class="n">CacheKind</span><span class="o">::</span><span class="n">DATA_FILE_FOOTER</span> |
| <span class="w"> </span><span class="o">?</span><span class="w"> </span><span class="n">parquet_metadata_cache_</span> |
| <span class="w"> </span><span class="o">:</span><span class="w"> </span><span class="n">default_cache_</span><span class="p">;</span> |
| <span class="w"> </span><span class="p">}</span> |
| |
| <span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">shared_ptr</span><span class="o"><</span><span class="n">paimon</span><span class="o">::</span><span class="n">Cache</span><span class="o">></span><span class="w"> </span><span class="n">default_cache_</span><span class="p">;</span> |
| <span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">shared_ptr</span><span class="o"><</span><span class="n">paimon</span><span class="o">::</span><span class="n">Cache</span><span class="o">></span><span class="w"> </span><span class="n">parquet_metadata_cache_</span><span class="p">;</span> |
| <span class="p">};</span> |
| |
| <span class="k">auto</span><span class="w"> </span><span class="n">cache</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">make_shared</span><span class="o"><</span><span class="n">RoutingCache</span><span class="o">></span><span class="p">(</span> |
| <span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">make_shared</span><span class="o"><</span><span class="n">MyDefaultCache</span><span class="o">></span><span class="p">(),</span> |
| <span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">make_shared</span><span class="o"><</span><span class="n">MyParquetMetadataCache</span><span class="o">></span><span class="p">());</span> |
| |
| <span class="n">paimon</span><span class="o">::</span><span class="n">ScanContextBuilder</span><span class="w"> </span><span class="nf">scan_builder</span><span class="p">(</span><span class="n">table_path</span><span class="p">);</span> |
| <span class="n">scan_builder</span><span class="p">.</span><span class="n">WithCache</span><span class="p">(</span><span class="n">cache</span><span class="p">);</span> |
| |
| <span class="n">paimon</span><span class="o">::</span><span class="n">ReadContextBuilder</span><span class="w"> </span><span class="nf">read_builder</span><span class="p">(</span><span class="n">table_path</span><span class="p">);</span> |
| <span class="n">read_builder</span><span class="p">.</span><span class="n">WithCache</span><span class="p">(</span><span class="n">cache</span><span class="p">);</span> |
| </pre></div> |
| </div> |
| <p>Passing <code class="docutils literal notranslate"><span class="pre">nullptr</span></code> or omitting <code class="docutils literal notranslate"><span class="pre">WithCache()</span></code> leaves Parquet metadata caching |
| disabled. If a file URI cannot be obtained, Paimon C++ also bypasses the cache |
| and opens the Parquet file normally.</p> |
| </section> |
| <section id="future-optimizations"> |
| <h2>Future Optimizations<a class="headerlink" href="#future-optimizations" title="Link to this heading">#</a></h2> |
| <ul class="simple"> |
| <li><p>Add hit, miss, bypass, and eviction metrics for Parquet metadata cache.</p></li> |
| <li><p>Add single-flight loading for high-concurrency misses on the same Parquet |
| file.</p></li> |
| <li><p>Evaluate sharing cached metadata footer bytes with page-index prefetch logic when |
| those read paths can use the same cache abstraction.</p></li> |
| </ul> |
| </section> |
| </section> |
| |
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