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| <div id="searchbox"></div> |
| <article class="bd-article"> |
| |
| <section id="dissociated-ipc-protocol"> |
| <span id="dissociated-ipc"></span><h1>Dissociated IPC Protocol<a class="headerlink" href="#dissociated-ipc-protocol" title="Link to this heading">#</a></h1> |
| <div class="admonition warning"> |
| <p class="admonition-title">Warning</p> |
| <p>Experimental: The Dissociated IPC Protocol is experimental in its current |
| form. Based on feedback and usage the protocol definition may change until |
| it is fully standardized.</p> |
| </div> |
| <section id="rationale"> |
| <h2>Rationale<a class="headerlink" href="#rationale" title="Link to this heading">#</a></h2> |
| <p>The <a class="reference internal" href="Columnar.html#format-ipc"><span class="std std-ref">Arrow IPC format</span></a> describes a protocol for transferring |
| Arrow data as a stream of record batches. This protocol expects a continuous |
| stream of bytes divided into discrete messages (using a length prefix and |
| continuation indicator). Each discrete message consists of two portions:</p> |
| <ul class="simple"> |
| <li><p>A <a class="reference external" href="http://github.com/google/flatbuffers">Flatbuffers</a> header message</p></li> |
| <li><p>A series of bytes consisting of the flattened and packed body buffers (some |
| message types, like Schema messages, do not have this section) |
| - This is referred to as the <em>message body</em> in the IPC format spec.</p></li> |
| </ul> |
| <p>For most cases, the existing IPC format as it currently exists is sufficiently efficient:</p> |
| <ul class="simple"> |
| <li><p>Receiving data in the IPC format allows zero-copy utilization of the body |
| buffer bytes, no deserialization is required to form Arrow Arrays</p></li> |
| <li><p>An IPC file format can be memory-mapped because it is location agnostic |
| and the bytes of the file are exactly what is expected in memory.</p></li> |
| </ul> |
| <p>However, there are use cases that aren’t handled by this:</p> |
| <ul class="simple"> |
| <li><p>Constructing the IPC record batch message requires allocating a contiguous |
| chunk of bytes and copying all of the data buffers into it, packed together |
| back-to-back. This pessimizes the common case of wrapping existing, directly |
| consumable data into an IPC message.</p></li> |
| <li><p>Even if Arrow data is located in a memory accessible across process boundaries |
| or transports (such as UCX), there is no standard way to specify that shared |
| location to consumers which could take advantage of it.</p></li> |
| <li><p>Arrow data located on a non-CPU device (such as a GPU) cannot be sent using |
| Arrow IPC without having to copy the data back to the host device or copying |
| the Flatbuffers metadata bytes into device memory.</p> |
| <ul> |
| <li><p>By the same token, receiving IPC messages into device memory would require |
| performing a copy of the Flatbuffers metadata back to the host CPU device. This |
| is due to the fact that the IPC stream interleaves data and metadata across a |
| single stream.</p></li> |
| </ul> |
| </li> |
| </ul> |
| <p>This protocol attempts to solve these use cases in an efficient manner.</p> |
| <section id="goals"> |
| <h3>Goals<a class="headerlink" href="#goals" title="Link to this heading">#</a></h3> |
| <ul class="simple"> |
| <li><p>Define a generic protocol for passing Arrow IPC data, not tied to any particular |
| transport, that also allows for utilizing non-CPU device memory, shared memory, and |
| newer “high performance” transports such as <a class="reference external" href="https://openucx.org/">UCX</a> or <a class="reference external" href="https://ofiwg.github.io/libfabric/">libfabric</a>.</p> |
| <ul> |
| <li><p>This allows for the data in the body to be kept on non-CPU devices (like GPUs) |
| without expensive device-to-host copies.</p></li> |
| </ul> |
| </li> |
| <li><p>Allow for using <a class="reference internal" href="Flight.html#flight-rpc"><span class="std std-ref">Flight RPC</span></a> purely for control flow by separating |
| the stream of IPC metadata from IPC body bytes</p></li> |
| </ul> |
| </section> |
| <section id="definitions"> |
| <h3>Definitions<a class="headerlink" href="#definitions" title="Link to this heading">#</a></h3> |
| <dl class="glossary"> |
| <dt id="term-IPC-Metadata">IPC Metadata<a class="headerlink" href="#term-IPC-Metadata" title="Link to this term">#</a></dt><dd><p>The Flatbuffers message bytes that encompass the header of an Arrow IPC message</p> |
| </dd> |
| <dt id="term-Tag">Tag<a class="headerlink" href="#term-Tag" title="Link to this term">#</a></dt><dd><p>A little-endian <code class="docutils literal notranslate"><span class="pre">uint64</span></code> value used for flow control and used in determining |
| how to interpret the body of a message. Specific bits can be masked to allow |
| identifying messages by only a portion of the tag, leaving the rest of the bits |
| to be used for control flow or other message metadata. Some transports, such as |
| UCX, have built-in support for such tag values and will provide them in CPU |
| memory regardless of whether or not the body of the message may reside on a |
| non-CPU device.</p> |
| </dd> |
| <dt id="term-Sequence-Number">Sequence Number<a class="headerlink" href="#term-Sequence-Number" title="Link to this term">#</a></dt><dd><p>A little-endian, 4-byte unsigned integer starting at 0 for a stream, indicating |
| the sequence order of messages. It is also used to identify specific messages to |
| tie the IPC metadata header to its corresponding body since the metadata and body |
| can be sent across separate pipes/streams/transports.</p> |
| <p>If a sequence number reaches <code class="docutils literal notranslate"><span class="pre">UINT32_MAX</span></code>, it should be allowed to roll over as |
| it is unlikely there would be enough unprocessed messages waiting to be processed |
| that would cause an overlap of sequence numbers.</p> |
| <p>The sequence number serves two purposes: To identify corresponding metadata and |
| tagged body data messages and to ensure we do not rely on messages having to arrive |
| in order. A client should use the sequence number to correctly order messages as |
| they arrive for processing.</p> |
| </dd> |
| </dl> |
| </section> |
| </section> |
| <section id="the-protocol"> |
| <h2>The Protocol<a class="headerlink" href="#the-protocol" title="Link to this heading">#</a></h2> |
| <p>A reference example implementation utilizing <a class="reference external" href="https://docs.rapids.ai/api">libcudf</a> and <a class="reference external" href="https://openucx.org/">UCX</a> can be found in the |
| <a class="reference external" href="https://github.com/apache/arrow-experiments/tree/main/dissociated-ipc">arrow-experiments repo</a>.</p> |
| <section id="requirements"> |
| <h3>Requirements<a class="headerlink" href="#requirements" title="Link to this heading">#</a></h3> |
| <p>A transport implementing this protocol <strong>MUST</strong> provide two pieces of functionality:</p> |
| <ul class="simple"> |
| <li><p>Message sending</p> |
| <ul> |
| <li><p>Delimited messages (like gRPC) as opposed to non-delimited streams (like plain TCP |
| without further framing).</p></li> |
| <li><p>Alternatively, a framing mechanism like the <a class="reference internal" href="Columnar.html#ipc-message-format"><span class="std std-ref">encapsulated message format</span></a> |
| for the IPC protocol can be used while leaving out the body bytes.</p></li> |
| </ul> |
| </li> |
| <li><p>Tagged message sending</p> |
| <ul> |
| <li><p>Sending a message that has an attached little-endian, unsigned 64-bit integral tag |
| for control flow. A tag like this allows control flow to operate on a message whose body |
| is on a non-CPU device without requiring the message itself to get copied off of the device.</p></li> |
| </ul> |
| </li> |
| </ul> |
| </section> |
| <section id="uri-specification"> |
| <h3>URI Specification<a class="headerlink" href="#uri-specification" title="Link to this heading">#</a></h3> |
| <p>When providing a URI to a consumer to contact for use with this protocol (such as via |
| the <a class="reference internal" href="Flight.html#flight-location-uris"><span class="std std-ref">Location URI for Flight</span></a>), the URI should specify a scheme |
| like <em>ucx:</em> or <em>fabric:</em>, that is easily identifiable. In addition, the URI should |
| encode the following URI query parameters:</p> |
| <div class="admonition note"> |
| <p class="admonition-title">Note</p> |
| <p>As this protocol matures, this document will get updated with commonly recognized |
| transport schemes that get used with it.</p> |
| </div> |
| <ul class="simple"> |
| <li><p><code class="docutils literal notranslate"><span class="pre">want_data</span></code> - <strong>REQUIRED</strong> - uint64 integer value</p> |
| <ul> |
| <li><p>This value should be used to tag an initial message to the server to initiate a |
| data transfer. The body of the initiating message should be an opaque binary identifier |
| of the data stream being requested (like the <code class="docutils literal notranslate"><span class="pre">Ticket</span></code> in the Flight RPC protocol)</p></li> |
| </ul> |
| </li> |
| <li><p><code class="docutils literal notranslate"><span class="pre">free_data</span></code> - <strong>OPTIONAL</strong> - uint64 integer value</p> |
| <ul> |
| <li><p>If the server might send messages using offsets / addresses for remote memory accessing |
| or shared memory locations, the URI should include this parameter. This value is used to |
| tag messages sent from the client to the data server, containing specific offsets / addresses |
| which were provided that are no longer required by the client (i.e. any operations that |
| directly reference those memory locations, such as copying the remote data into local memory, |
| have been completed).</p></li> |
| </ul> |
| </li> |
| <li><p><code class="docutils literal notranslate"><span class="pre">remote_handle</span></code> - <strong>OPTIONAL</strong> - base64-encoded string</p> |
| <ul> |
| <li><p>When working with shared memory or remote memory, this value indicates any required |
| handle or identifier that is necessary for accessing the memory.</p> |
| <ul> |
| <li><p>Using UCX, this would be an <em>rkey</em> value</p></li> |
| <li><p>With CUDA IPC, this would be the value of the base GPU pointer or memory handle, |
| and subsequent addresses would be offsets from this base pointer.</p></li> |
| </ul> |
| </li> |
| </ul> |
| </li> |
| </ul> |
| </section> |
| <section id="handling-of-backpressure"> |
| <h3>Handling of Backpressure<a class="headerlink" href="#handling-of-backpressure" title="Link to this heading">#</a></h3> |
| <p><em>Currently</em> this proposal does not specify any way to manage the backpressure of |
| messages to throttle for memory and bandwidth reasons. For now, this will be |
| <strong>transport-defined</strong> rather than lock into something sub-optimal.</p> |
| <p>As usage among different transports and libraries grows, common patterns will emerge |
| that will allow for a generic, but efficient, way to handle backpressure across |
| different use cases.</p> |
| <div class="admonition note"> |
| <p class="admonition-title">Note</p> |
| <p>While the protocol itself is transport agnostic, the current usage and examples |
| only have been tested using UCX and libfabric transports so far, but that’s all.</p> |
| </div> |
| </section> |
| </section> |
| <section id="protocol-description"> |
| <h2>Protocol Description<a class="headerlink" href="#protocol-description" title="Link to this heading">#</a></h2> |
| <p>There are two possibilities that can occur:</p> |
| <ol class="arabic simple"> |
| <li><p>The streams of metadata and body data are sent across separate connections</p></li> |
| </ol> |
| <object data="../_images/mermaid-8abfde41766e4e63ac35f7f099c36aab16139fbc.svg" type="image/svg+xml"> |
| <p class="warning">%% Licensed to the Apache Software Foundation (ASF) under one |
| %% or more contributor license agreements. See the NOTICE file |
| %% distributed with this work for additional information |
| %% regarding copyright ownership. The ASF licenses this file |
| %% to you under the Apache License, Version 2.0 (the |
| %% "License"); you may not use this file except in compliance |
| %% with the License. You may obtain a copy of the License at |
| %% |
| %% http://www.apache.org/licenses/LICENSE-2.0 |
| %% |
| %% Unless required by applicable law or agreed to in writing, |
| %% software distributed under the License is distributed on an |
| %% "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY |
| %% KIND, either express or implied. See the License for the |
| %% specific language governing permissions and limitations |
| %% under the License. |
| |
| sequenceDiagram |
| participant D as Data Stream |
| participant C as Client |
| participant M as Metadata Stream |
| |
| activate C |
| C-->>+M: TaggedMessage(server.want_data, bytes=ID_of_desired_data) |
| C-->>+D: TaggedMessage(server.want_data, bytes=ID_of_desired_data) |
| M-->>C: Message(bytes([1]) + le_bytes(sequence_number) + schema_metadata) |
| loop each batch |
| par |
| M-->>C: Message(bytes([1]) + le_bytes(sequence_number) + batch_metadata) |
| and |
| alt |
| D-->>C: TaggedMessage((bytes[0] << 55) | le_bytes(sequence_number),<br/>bytes=batch_data) |
| else |
| D-->>C: TaggedMessage((bytes[1] << 55) | le_bytes(sequence_number),<br/>bytes=uint64_pairs) |
| end |
| end |
| end |
| M-->>C: Message(bytes([0]) + le_bytes(sequence_number)) |
| deactivate M |
| loop |
| C-->>D: TaggedMessage(server.free_data, bytes=uint64_list) |
| end |
| deactivate D |
| deactivate C</p></object> |
| <ol class="arabic simple" start="2"> |
| <li><p>The streams of metadata and body data are sent simultaneously across the |
| same connection</p></li> |
| </ol> |
| <object data="../_images/mermaid-e962a4be5befa97b0166c4cae51c8201f88d2bb5.svg" type="image/svg+xml"> |
| <p class="warning">%% Licensed to the Apache Software Foundation (ASF) under one |
| %% or more contributor license agreements. See the NOTICE file |
| %% distributed with this work for additional information |
| %% regarding copyright ownership. The ASF licenses this file |
| %% to you under the Apache License, Version 2.0 (the |
| %% "License"); you may not use this file except in compliance |
| %% with the License. You may obtain a copy of the License at |
| %% |
| %% http://www.apache.org/licenses/LICENSE-2.0 |
| %% |
| %% Unless required by applicable law or agreed to in writing, |
| %% software distributed under the License is distributed on an |
| %% "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY |
| %% KIND, either express or implied. See the License for the |
| %% specific language governing permissions and limitations |
| %% under the License. |
| |
| sequenceDiagram |
| participant C as Client |
| participant S as Server |
| activate C |
| C-->>+S: TaggedMessage(server.want_data, bytes=ID_of_desired_data) |
| S-->>C: Message(bytes([1]) + le_bytes(sequence_number) + schema_metadata) |
| par |
| loop each chunk |
| S-->>C: Message(bytes([1]) + le_bytes(sequence_number) + batch_metadata) |
| end |
| S-->>C: Message(bytes([0]) + le_bytes(sequence_number)) |
| and |
| loop each chunk |
| alt |
| S-->>C: TaggedMessage((bytes[0] << 55) | le_bytes(sequence_number),<br/>bytes=batch_data) |
| else |
| S-->>C: TaggedMessage((bytes[1] << 55) | le_bytes(sequence_number),<br/>bytes=uint64_pairs) |
| end |
| end |
| end |
| |
| loop |
| C-->>S: TaggedMessage(server.free_data, bytes=uint64_list) |
| end |
| deactivate S |
| deactivate C</p></object> |
| <section id="server-sequence"> |
| <h3>Server Sequence<a class="headerlink" href="#server-sequence" title="Link to this heading">#</a></h3> |
| <p>There can be either a single server handling both the IPC Metadata stream and the |
| Body data streams, or separate servers for handling the IPC Metadata and the body |
| data. This allows for streaming of data across either a single transport pipe or |
| two pipes if desired.</p> |
| <section id="metadata-stream-sequence"> |
| <h4>Metadata Stream Sequence<a class="headerlink" href="#metadata-stream-sequence" title="Link to this heading">#</a></h4> |
| <p>The standing state of the server is waiting for a <strong>tagged</strong> message with a specific |
| <code class="docutils literal notranslate"><span class="pre"><want_data></span></code> tag value to initiate a transfer. This <code class="docutils literal notranslate"><span class="pre"><want_data></span></code> value is defined |
| by the server and propagated to any clients via the URI they are provided. This protocol |
| does not prescribe any particular value so that it will not interfere with any other |
| existing protocols that rely on tag values. The body of that message will contain an |
| opaque, binary identifier to indicate a particular dataset / data stream to send.</p> |
| <div class="admonition note"> |
| <p class="admonition-title">Note</p> |
| <p>For instance, the <strong>ticket</strong> that was passed with a <em>FlightInfo</em> message would be |
| the body of this message. Because it is opaque, it can be anything the server wants |
| to use. The URI and identifier do not need to be given to the client via Flight RPC, |
| but could come across from any transport or protocol desired.</p> |
| </div> |
| <p>Upon receiving a <code class="docutils literal notranslate"><span class="pre"><want_data></span></code> request, the server <em>should</em> respond by sending a stream |
| of messages consisting of the following:</p> |
| <object data="../_images/mermaid-73154fdcb1d4fb160464a776ec1b8cdf50794f49.svg" type="image/svg+xml"> |
| <p class="warning">block-beta |
| columns 8 |
| |
| block:P["\n\n\n\nPrefix"]:5 |
| T["Message type\nByte 0"] |
| S["Sequence number\nBytes 1-4"] |
| end |
| H["Flatbuffer bytes\nRest of the message"]:3</p></object> |
| <ul class="simple"> |
| <li><p>A 5-byte prefix</p> |
| <ul> |
| <li><p>The first byte of the message indicates the type of message, currently there are only |
| two allowed message types (more types may get added in the future):</p> |
| <ol class="arabic simple" start="0"> |
| <li><p>End of Stream</p></li> |
| <li><p>Flatbuffers IPC Metadata Message</p></li> |
| </ol> |
| </li> |
| <li><p>the next 4-bytes are a little-endian, unsigned 32-bit integer indicating the sequence number of |
| the message. The first message in the stream (<strong>MUST</strong> always be a schema message) <strong>MUST</strong> |
| have a sequence number of <code class="docutils literal notranslate"><span class="pre">0</span></code>. Each subsequent message <strong>MUST</strong> increment the number by |
| <code class="docutils literal notranslate"><span class="pre">1</span></code>.</p></li> |
| </ul> |
| </li> |
| <li><p>The full Flatbuffers bytes of an Arrow IPC header</p></li> |
| </ul> |
| <p>As defined in the Arrow IPC format, each metadata message can represent a chunk of data or |
| dictionaries for use by the stream of data.</p> |
| <p>After sending the last metadata message, the server <strong>MUST</strong> indicate the end of the stream |
| by sending a message consisting of <strong>exactly</strong> 5 bytes:</p> |
| <ul class="simple"> |
| <li><p>The first byte is <code class="docutils literal notranslate"><span class="pre">0</span></code>, indicating an <strong>End of Stream</strong> message</p></li> |
| <li><p>The last 4 bytes are the sequence number (4-byte, unsigned integer in little-endian byte order)</p></li> |
| </ul> |
| </section> |
| <section id="data-stream-sequence"> |
| <h4>Data Stream Sequence<a class="headerlink" href="#data-stream-sequence" title="Link to this heading">#</a></h4> |
| <p>If a single server is handling both the data and metadata streams, then the data messages |
| <strong>should</strong> begin being sent to the client in parallel with the metadata messages. Otherwise, |
| as with the metadata sequence, the standing state of the server is to wait for a <strong>tagged</strong> |
| message with the <code class="docutils literal notranslate"><span class="pre"><want_data></span></code> tag value, whose body indicates the dataset / data stream |
| to send to the client.</p> |
| <p>For each IPC message in the stream of data, a <strong>tagged</strong> message <strong>MUST</strong> be sent on the data |
| stream if that message has a body (i.e. a Record Batch or Dictionary message). The |
| <a class="reference internal" href="#term-Tag"><span class="xref std std-term">tag</span></a> for each message should be structured as follows:</p> |
| <object data="../_images/mermaid-4a15cc28d43fb7ee5221b8108ca361508723820b.svg" type="image/svg+xml"> |
| <p class="warning">block-beta |
| columns 8 |
| |
| S["Sequence number\nBytes 0-3"]:4 |
| U["Unused (Reserved)\nBytes 4-6"]:3 |
| T["Message type\nByte 7"]:1</p></object> |
| <ul class="simple"> |
| <li><p>The <em>least significant</em> 4-bytes (bits 0 - 31) of the tag should be the unsigned 32-bit, little-endian sequence |
| number of the message.</p></li> |
| <li><p>The <em>most significant</em> byte (bits 56 - 63) of the tag indicates the message body <strong>type</strong> as an 8-bit |
| unsigned integer. Currently only two message types are specified, but more can be added as |
| needed to expand the protocol:</p> |
| <ol class="arabic simple" start="0"> |
| <li><p>The body contains the raw body buffer bytes as a packed buffer (i.e. the standard IPC |
| format body bytes)</p></li> |
| <li><p>The body contains a series of unsigned, little-endian 64-bit integer pairs to represent |
| either shared or remote memory, schematically structured as</p> |
| <ul> |
| <li><p>The first two integers (e.g. the first 16 bytes) represent the <em>total</em> size (in bytes) |
| of all buffers and the number of buffers in this message (and thus the number of following |
| pairs of <code class="docutils literal notranslate"><span class="pre">uint64</span></code>)</p></li> |
| <li><p>Each subsequent pair of <code class="docutils literal notranslate"><span class="pre">uint64</span></code> values are an address / offset followed the length of |
| that particular buffer.</p></li> |
| </ul> |
| </li> |
| </ol> |
| </li> |
| <li><p>All unspecified bits (bits 32 - 55) of the tag are <em>reserved</em> for future use by potential updates |
| to this protocol. For now they <strong>MUST</strong> be 0.</p></li> |
| </ul> |
| <div class="admonition note"> |
| <p class="admonition-title">Note</p> |
| <p>Any shared/remote memory addresses that are sent across <strong>MUST</strong> be kept alive by the server |
| until a corresponding tagged <code class="docutils literal notranslate"><span class="pre"><free_data></span></code> message is received. If the client disconnects |
| before sending any <code class="docutils literal notranslate"><span class="pre"><free_data></span></code> messages, it can be assumed to be safe to clean up the memory |
| if desired by the server.</p> |
| </div> |
| <p>After sending the last tagged IPC body message, the server should maintain the connection and wait |
| for tagged <code class="docutils literal notranslate"><span class="pre"><free_data></span></code> messages. The structure of these <code class="docutils literal notranslate"><span class="pre"><free_data></span></code> messages is simple: |
| one or more unsigned, little-endian 64-bit integers which indicate the addresses/offsets that can |
| be freed.</p> |
| <p>Once there are no more outstanding addresses to be freed, the work for this stream is complete.</p> |
| </section> |
| </section> |
| <section id="client-sequence"> |
| <h3>Client Sequence<a class="headerlink" href="#client-sequence" title="Link to this heading">#</a></h3> |
| <p>A client for this protocol needs to concurrently handle both the data and metadata streams of |
| messages which may either both come from the same server or different servers. Below is a flowchart |
| showing how a client might handle the metadata and data streams:</p> |
| <object data="../_images/mermaid-8ff51316a5bfe716c8346df112ea33beaa5228f4.svg" type="image/svg+xml"> |
| <p class="warning">%% Licensed to the Apache Software Foundation (ASF) under one |
| %% or more contributor license agreements. See the NOTICE file |
| %% distributed with this work for additional information |
| %% regarding copyright ownership. The ASF licenses this file |
| %% to you under the Apache License, Version 2.0 (the |
| %% "License"); you may not use this file except in compliance |
| %% with the License. You may obtain a copy of the License at |
| |
| %% http://www.apache.org/licenses/LICENSE-2.0 |
| |
| %% Unless required by applicable law or agreed to in writing, |
| %% software distributed under the License is distributed on an |
| %% "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY |
| %% KIND, either express or implied. See the License for the |
| %% specific language governing permissions and limitations |
| %% under the License. |
| |
| graph LR |
| client((Client))-->c1{{Send #60;want_data#gt; Msg}} |
| subgraph meta [Meta Message] |
| direction LR |
| m1[/Msg Type #40;byte 0#41;<br/>Seq Num #40;bytes 1-5#41;/]-- type 1 -->m2[[Process IPC Header]] |
| m2-- IPC has body -->m3[Get Corresponding<br/>Tagged Msg] |
| m2-- Schema Msg -->m4[/Store Schema/] |
| m1-- type 0 -->e[Indicate End of Stream] |
| end |
| subgraph data [Data Stream] |
| direction LR |
| d1[Request Msg<br/>for Seq Num]-->d2{Most Significant<br/>Byte} |
| d2-- 0 -->d3[Construct from<br/>Metadata and Body] |
| d2-- 1 -->d4[Get shared/remote<br/>buffers] |
| d4 -->d5[Construct from<br/>Metadata and buffers] |
| d3 & d5 -->e2[Output Batch] |
| end |
| |
| client -- recv untagged msg --> meta |
| client -- get tagged msg --> data</p></object> |
| <ol class="arabic simple"> |
| <li><p>First the client sends a tagged message using the <code class="docutils literal notranslate"><span class="pre"><want_data></span></code> value it was provided in the |
| URI as the tag, and the opaque ID as the body.</p> |
| <ul class="simple"> |
| <li><p>If the metadata and data servers are separate, then a <code class="docutils literal notranslate"><span class="pre"><want_data></span></code> message needs to be sent |
| separately to each.</p></li> |
| <li><p>In either scenario, the metadata and data streams can be processed concurrently and/or asynchronously |
| depending on the nature of the transports.</p></li> |
| </ul> |
| </li> |
| <li><p>For each <strong>untagged</strong> message the client receives in the metadata stream:</p> |
| <ul class="simple"> |
| <li><p>The first byte of the message indicates whether it is an <em>End of Stream</em> message (value <code class="docutils literal notranslate"><span class="pre">0</span></code>) |
| or a metadata message (value <code class="docutils literal notranslate"><span class="pre">1</span></code>).</p></li> |
| <li><p>The next 4 bytes are the sequence number of the message, an unsigned 32-bit integer in |
| little-endian byte order.</p></li> |
| <li><p>If it is <strong>not</strong> an <em>End of Stream</em> message, the remaining bytes are the IPC Flatbuffer bytes which |
| can be interpreted as normal.</p> |
| <ul> |
| <li><p>If the message has a body (i.e. Record Batch or Dictionary message) then the client should retrieve |
| a tagged message from the Data Stream using the same sequence number.</p></li> |
| </ul> |
| </li> |
| <li><p>If it <strong>is</strong> an <em>End of Stream</em> message, then it is safe to close the metadata connection if there are |
| no gaps in the sequence numbers received.</p></li> |
| </ul> |
| </li> |
| <li><p>When a metadata message that requires a body is received, the tag mask of <code class="docutils literal notranslate"><span class="pre">0x00000000FFFFFFFF</span></code> <strong>should</strong> |
| be used alongside the sequence number to match the message regardless of the higher bytes (e.g. we only |
| care about matching the lower 4 bytes to the sequence number)</p> |
| <ul class="simple"> |
| <li><p>Once received, the Most Significant Byte’s value determines how the client processes the body data:</p> |
| <ul> |
| <li><p>If the most significant byte is 0: Then the body of the message is the raw IPC packed body buffers |
| allowing it to easily be processed with the corresponding metadata header bytes.</p></li> |
| <li><p>If the most significant byte is 1: The body of the message will consist of a series of pairs of |
| unsigned, 64-bit integers in little-endian byte order.</p> |
| <ul> |
| <li><p>The first two integers represent <em>1)</em> the total size of all the body buffers together to allow |
| for easy allocation if an intermediate buffer is needed and <em>2)</em> the number of buffers being sent (<code class="docutils literal notranslate"><span class="pre">nbuf</span></code>).</p></li> |
| <li><p>The rest of the message will be <code class="docutils literal notranslate"><span class="pre">nbuf</span></code> pairs of integers, one for each buffer. Each pair is |
| <em>1)</em> the address / offset of the buffer and <em>2)</em> the length of that buffer. Memory can then be retrieved |
| via shared or remote memory routines based on the underlying transport. These addresses / offsets <strong>MUST</strong> |
| be retained so they can be sent back in <code class="docutils literal notranslate"><span class="pre"><free_data></span></code> messages later, indicating to the server that |
| the client no longer needs the shared memory.</p></li> |
| </ul> |
| </li> |
| </ul> |
| </li> |
| </ul> |
| </li> |
| <li><p>Once an <em>End of Stream</em> message is received, the client should process any remaining un-processed |
| IPC metadata messages.</p></li> |
| <li><p>After individual memory addresses / offsets are able to be freed by the remote server (in the case where |
| it has sent these rather than the full body bytes), the client should send corresponding <code class="docutils literal notranslate"><span class="pre"><free_data></span></code> messages |
| to the server.</p> |
| <ul class="simple"> |
| <li><p>A single <code class="docutils literal notranslate"><span class="pre"><free_data></span></code> message consists of an arbitrary number of unsigned 64-bit integer values, representing |
| the addresses / offsets which can be freed. The reason for it being an <em>arbitrary number</em> is to allow a client |
| to choose whether to send multiple messages to free multiple addresses or to coalesce multiple addresses into |
| fewer messages to be freed (thus making the protocol less “chatty” if desired)</p></li> |
| </ul> |
| </li> |
| </ol> |
| </section> |
| </section> |
| <section id="continuing-development"> |
| <h2>Continuing Development<a class="headerlink" href="#continuing-development" title="Link to this heading">#</a></h2> |
| <p>If you decide to try this protocol in your own environments and system, we’d love feedback and to learn about |
| your use case. As this is an <strong>experimental</strong> protocol currently, we need real-world usage in order to facilitate |
| improving it and finding the right generalizations to standardize on across transports.</p> |
| <p>Please chime in using the Arrow Developers Mailing list: <a class="reference external" href="https://arrow.apache.org/community/#mailing-lists">https://arrow.apache.org/community/#mailing-lists</a></p> |
| </section> |
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