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| <section id="extension-codecs"> |
| <span id="id1"></span><h1>Extension codecs<a class="headerlink" href="#extension-codecs" title="Link to this heading">#</a></h1> |
| <p>A codec is what lets your objects survive being serialized into a plan and |
| rebuilt somewhere else — another process, or another program. If your library |
| contributes table providers, functions, or execution plan nodes and those plans |
| have to leave the process, you need one.</p> |
| <p>Codecs are contributed to a session either through an |
| <a class="reference internal" href="bundles.html#extension-bundles"><span class="std std-ref">extension bundle</span></a>, which is the recommended route, |
| or one at a time through |
| <code class="xref py py-meth docutils literal notranslate"><span class="pre">with_logical_extension_codec()</span></code> and |
| <code class="xref py py-meth docutils literal notranslate"><span class="pre">with_physical_extension_codec()</span></code>.</p> |
| <section id="codecs-compose"> |
| <h2>Codecs compose<a class="headerlink" href="#codecs-compose" title="Link to this heading">#</a></h2> |
| <p>Each call to <code class="docutils literal notranslate"><span class="pre">with_logical_extension_codec</span></code> or |
| <code class="docutils literal notranslate"><span class="pre">with_physical_extension_codec</span></code> <strong>appends</strong> the codec to the session’s codec |
| chain rather than replacing prior codecs. One session can therefore carry |
| codecs from several independent libraries at once.</p> |
| <p><strong>Nothing is asked of the codec itself.</strong> Implement <code class="docutils literal notranslate"><span class="pre">LogicalExtensionCodec</span></code> or |
| <code class="docutils literal notranslate"><span class="pre">PhysicalExtensionCodec</span></code> exactly as you would for a session that installs only |
| yours. When your codec writes bytes into a serialized plan, datafusion-python |
| records which codec wrote them, and strips that record off again before handing |
| the bytes back. So your codec receives, byte for byte, the payload it wrote, |
| and is never offered a payload another codec wrote.</p> |
| <p>A codec that also ships to hosts which dispatch differently may still want its |
| own guard against foreign payloads. Keeping one is fine; it is simply not |
| needed for the datafusion-python path.</p> |
| </section> |
| <section id="encode-metadata-not-a-handle-to-a-live-object"> |
| <span id="extension-codec-durable-metadata"></span><h2>Encode metadata, not a handle to a live object<a class="headerlink" href="#encode-metadata-not-a-handle-to-a-live-object" title="Link to this heading">#</a></h2> |
| <p>Your payload has to be enough to rebuild the object somewhere your process is |
| not. Write the metadata a fresh instance can be constructed from — a path, a |
| connection string, a schema, the options the object was created with.</p> |
| <p>The example codecs in this repository do not do this, and it is worth knowing |
| before copying them. They keep a process-local <code class="docutils literal notranslate"><span class="pre">HashMap</span></code> of live providers and |
| encode an integer token into it: encoding inserts, decoding removes. That makes |
| Rust type identity observable across three separately loaded libraries in one |
| test, which is what the examples exist to show. It also means a decode consumes |
| its token, so the same bytes cannot be decoded twice, one encoded plan cannot |
| fan out to several readers, and a plan that never reaches a decoder keeps its |
| provider alive for the life of the process. A real codec has none of those |
| properties because it does not park the object anywhere.</p> |
| </section> |
| <section id="codec-ids"> |
| <span id="extension-codec-ids"></span><h2>Codec ids<a class="headerlink" href="#codec-ids" title="Link to this heading">#</a></h2> |
| <p>That record is the codec’s <strong>id</strong>: a short string stored inside the plan, |
| naming the codec that wrote each payload. Because plans are decoded in another |
| process — or another program — the id has to name the same codec there as it |
| did where the plan was written.</p> |
| <p>Ids are assigned for you. A codec’s id is normally its exporting class’s import |
| path, such as <code class="docutils literal notranslate"><span class="pre">my_library.Codec</span></code>, which is what you will see in |
| <code class="xref py py-meth docutils literal notranslate"><span class="pre">logical_extension_codec_ids()</span></code> and in decode |
| errors. You choose one yourself in three cases:</p> |
| <ul> |
| <li><p><strong>Two instances of one class.</strong> Both get the same id, so the second install |
| raises <code class="docutils literal notranslate"><span class="pre">ValueError</span></code>. Pass <code class="docutils literal notranslate"><span class="pre">codec_id=</span></code> to tell them apart.</p></li> |
| <li><p><strong>A bare <code class="docutils literal notranslate"><span class="pre">PyCapsule</span></code>.</strong> A capsule has no class to take a name from, so |
| installing one through <code class="docutils literal notranslate"><span class="pre">with_logical_extension_codec</span></code> or |
| <code class="docutils literal notranslate"><span class="pre">with_physical_extension_codec</span></code> gives it an id private to the session that |
| installed it; plans it encodes fail with a clear error on any other session |
| rather than being decoded by the wrong codec. Pass <code class="docutils literal notranslate"><span class="pre">codec_id=</span></code> if those plans |
| have to cross sessions.</p> |
| <p><code class="xref py py-meth docutils literal notranslate"><span class="pre">with_extensions()</span></code> takes no <code class="docutils literal notranslate"><span class="pre">codec_id=</span></code>, |
| so it refuses a bare capsule outright and tells you to wrap it. See |
| <a class="reference internal" href="bundles.html#extension-bundles-codecs-are-objects"><span class="std std-ref">Codecs are objects, not capsules</span></a>.</p> |
| </li> |
| <li><p><strong>A class you intend to rename.</strong> The id follows the class name, so renaming |
| stops older plans from decoding. Declare <code class="docutils literal notranslate"><span class="pre">__datafusion_codec_id__</span></code> on the |
| exporting object to pin an id that survives the rename.</p></li> |
| </ul> |
| <p><code class="xref py py-meth docutils literal notranslate"><span class="pre">logical_extension_codec_ids()</span></code> and its |
| physical counterpart list the ids installed on a session, which is also what a |
| decode failure names.</p> |
| <p>Installing one context’s codec stack on another session composes the two |
| sessions rather than copying codecs out of one: the imported codecs resolve |
| their task context against the original and stop working when it is dropped — |
| see <a class="reference internal" href="../contributor-guide/ffi-internals.html#ffi-internals-one-arc"><span class="std std-ref">One session, one Arc<SessionContext></span></a>. Pass the context itself rather than the |
| capsule it exports, so its codecs get an id that other sessions can decode.</p> |
| </section> |
| <section id="functions-whose-name-is-the-whole-encoding"> |
| <h2>Functions whose name is the whole encoding<a class="headerlink" href="#functions-whose-name-is-the-whole-encoding" title="Link to this heading">#</a></h2> |
| <p>A codec may own functions that need no payload at all, where the name is the |
| whole encoding: <code class="docutils literal notranslate"><span class="pre">try_encode_udf</span></code> writes nothing and <code class="docutils literal notranslate"><span class="pre">try_decode_udf</span></code> rebuilds |
| the function from <code class="docutils literal notranslate"><span class="pre">name</span></code>. That is supported and needs no id, because an <code class="docutils literal notranslate"><span class="pre">Ok</span></code> |
| with an empty buffer is read as “no opinion” and passes the object to the next |
| codec. <code class="docutils literal notranslate"><span class="pre">NameOnlyUdfCodec</span></code> in <a class="reference external" href="https://github.com/apache/datafusion-python/tree/main/examples/datafusion-ffi-example"><code class="docutils literal notranslate"><span class="pre">datafusion-ffi-example</span></code></a> is the worked case. |
| Anything no installed codec claims falls through to |
| <code class="docutils literal notranslate"><span class="pre">Default{Logical,Physical}ExtensionCodec</span></code>.</p> |
| <p>This is the one case where your decoder is consulted about something you may |
| not own, because an empty payload has no id to route on. <code class="docutils literal notranslate"><span class="pre">try_decode_udf</span></code> and |
| its aggregate and window siblings can therefore be called with an empty <code class="docutils literal notranslate"><span class="pre">buf</span></code> |
| and a <code class="docutils literal notranslate"><span class="pre">name</span></code> belonging to another library. Decide from <code class="docutils literal notranslate"><span class="pre">name</span></code> and return an |
| error if it is not yours; do not assume <code class="docutils literal notranslate"><span class="pre">buf</span></code> is non-empty.</p> |
| <div class="admonition note"> |
| <p class="admonition-title">Note</p> |
| <p>The framing itself — how an id is stored alongside a payload and routed back, |
| and the two cases that stay unframed — is internal to datafusion-python and |
| documented in <code class="docutils literal notranslate"><span class="pre">crates/core/src/codec.rs</span></code> for anyone changing it.</p> |
| </div> |
| <p>The current FFI logical codec supports providers and UDFs but not arbitrary |
| custom <code class="docutils literal notranslate"><span class="pre">LogicalPlan::Extension</span></code> nodes. See both example READMEs for the |
| supported flow and local build commands.</p> |
| </section> |
| <section id="when-codec-order-matters"> |
| <span id="extension-codec-order"></span><h2>When codec order matters<a class="headerlink" href="#when-codec-order-matters" title="Link to this heading">#</a></h2> |
| <p>Decoding is never order-dependent: a payload names its codec by id and the |
| chain dispatches straight to it. Encoding walks the chain in install order and |
| stops at the first codec that claims the node. Most of the time that is |
| invisible, because libraries claim disjoint things — one owns its table |
| providers, another its UDFs, a third its own execution plan nodes.</p> |
| <p>It stops being invisible when a codec claims <em>broadly</em>. A node that came from |
| another library arrives as an opaque <code class="docutils literal notranslate"><span class="pre">ForeignExecutionPlan</span></code>, and a codec that |
| claims any of those will take nodes it does not own from any library installed |
| after it. The query still succeeds. What changes is which library wrote the |
| bytes — so a plan that has to decode in another process now needs whichever |
| library happened to win, not the one whose node it is. |
| <code class="docutils literal notranslate"><span class="pre">MyPhysicalExtensionCodec</span></code> in <a class="reference external" href="https://github.com/apache/datafusion-python/tree/main/examples/datafusion-ffi-example"><code class="docutils literal notranslate"><span class="pre">datafusion-ffi-example</span></code></a> claims this way, and |
| the query-planner example’s test suite pins the consequence.</p> |
| <p>Two rules of thumb:</p> |
| <ul class="simple"> |
| <li><p><strong>Writing a codec, claim narrowly.</strong> Downcast to your own types. Claiming a |
| broad category makes your library order-sensitive for everyone downstream of |
| it.</p></li> |
| <li><p><strong>Shipping plans out of the process, verify.</strong> Do not assume your node |
| reached your codec just because both are installed. Round-trip a plan through |
| <a class="reference internal" href="../autoapi/datafusion/index.html#datafusion.ExecutionPlan.to_bytes" title="datafusion.ExecutionPlan.to_bytes"><code class="xref py py-meth docutils literal notranslate"><span class="pre">ExecutionPlan.to_bytes</span></code></a> / |
| <a class="reference internal" href="../autoapi/datafusion/index.html#datafusion.ExecutionPlan.from_bytes" title="datafusion.ExecutionPlan.from_bytes"><code class="xref py py-meth docutils literal notranslate"><span class="pre">from_bytes()</span></code></a> in a test and assert your |
| codec did the work.</p></li> |
| </ul> |
| </section> |
| <section id="a-codec-decodes-against-the-session-running-the-query"> |
| <span id="extension-codec-decode-session"></span><h2>A codec decodes against the session running the query<a class="headerlink" href="#a-codec-decodes-against-the-session-running-the-query" title="Link to this heading">#</a></h2> |
| <p>Because the task-context provider comes from the host — see |
| <a class="reference internal" href="capsule-protocol.html#extension-getter-argument"><span class="std std-ref">What your getter receives</span></a> — a decode callback running inside an |
| extension library resolves names against the session running the query. A |
| function registered with <code class="docutils literal notranslate"><span class="pre">ctx.register_udf(...)</span></code> is visible to a foreign codec |
| decoding a node that references it by name, and the handle is live rather than |
| a snapshot, so a registration made after the codec is installed is visible too.</p> |
| <p>This is covered in |
| <code class="docutils literal notranslate"><span class="pre">examples/datafusion-ffi-query-planner-example/python/tests/_test_three_library_query_planner.py</span></code>, |
| where the example codecs take a <code class="docutils literal notranslate"><span class="pre">require_udf_on_decode</span></code> name and resolve it out |
| of the task context they are handed.</p> |
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