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<div class="title">gRPC (Core) Compression Cookbook </div>  </div>
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<div class="textblock"><h2>Introduction</h2>
<p>This document describes compression as implemented by the gRPC C core. See <a class="el" href="compression_8md.html">the</a>full compression specification" for details.</p>
<h3>Intended Audience</h3>
<p>Wrapped languages developers, for the purposes of supporting compression by interacting with the C core.</p>
<h2>Criteria for GA readiness</h2>
<ol type="1">
<li>Be able to set compression at <a href="#per-channel-settings">channel</a>, <a href="#per-call-settings">call</a> and <a href="#per-message-settings">message</a> level. In principle this API should be based on <em>compression levels</em> as opposed to algorithms. See the discussion <a href="#level-vs-algorithms">below</a>.</li>
</ol>
<ol type="1">
<li>Have unit tests covering <a href="https://github.com/grpc/grpc/blob/master/doc/compression.md#test-cases">the cases from the spec</a>.</li>
</ol>
<ol type="1">
<li>Interop tests implemented and passing on Jenkins. The two relevant interop test cases are <a href="https://github.com/grpc/grpc/blob/master/doc/interop-test-descriptions.md#large_compressed_unary">large_compressed_unary</a> and <a href="https://github.com/grpc/grpc/blob/master/doc/interop-test-descriptions.md#server_compressed_streaming">server_compressed_streaming</a>.</li>
</ol>
<h2>Summary Flowcharts</h2>
<p>The following flowcharts depict the evolution of a message, both <em>incoming</em> and <em>outgoing</em>, irrespective of the client/server character of the call. Aspects still not symmetric between clients and servers (e.g. the <a href="https://github.com/grpc/grpc/blob/master/doc/compression.md#compression-levels-and-algorithms">use of compression levels</a>) are explicitly marked. The in-detail textual description for the different scenarios is described in subsequent sections.</p>
<h2>Incoming Messages</h2>
<div class="image">
<img src="images/compression_cookbook_incoming.png"  alt="image"/>
</div>
<h2>Outgoing Messages</h2>
<div class="image">
<img src="images/compression_cookbook_outgoing.png"  alt="image"/>
</div>
<h2>Levels vs Algorithms</h2>
<p>As mentioned in <a href="https://github.com/grpc/grpc/blob/master/doc/compression.md#compression-levels-and-algorithms">the relevant discussion on the spec document</a>, compression <em>levels</em> are the primary mechanism for compression selection <em>at the server side</em>. In the future, it'll also be at the client side. The use of levels abstracts away the intricacies of selecting a concrete algorithm supported by a peer, on top of removing the burden of choice from the developer. As of this writing (Q2 2016), clients can only specify compression <em>algorithms</em>. Clients will support levels as soon as an automatic retry/negotiation mechanism is in place.</p>
<h2>Per Channel Settings</h2>
<p>Compression may be configured at channel creation. This is a convenience to avoid having to repeatedly configure compression for every call. Note that any compression setting on individual <a href="#per-call-settings">calls</a> or <a href="#per-message-settings">messages</a> overrides channel settings.</p>
<p>The following aspects can be configured at channel-creation time via channel arguments:</p>
<h4>Disable Compression <em>Algorithms</em></h4>
<p>Use the channel argument key <code>GRPC_COMPRESSION_CHANNEL_ENABLED_ALGORITHMS_BITSET</code> (from <a href="https://github.com/grpc/grpc/blob/master/include/grpc/impl/codegen/compression_types.h">`grpc/impl/codegen/compression_types.h`</a>), takes a 32 bit bitset value. A set bit means the algorithm with that enum value according to <code>grpc_compression_algorithm</code> is <em>enabled</em>. For example, <code>GRPC_COMPRESS_GZIP</code> currently has a numeric value of 2. To enable/disable GZIP for a channel, one would set/clear the 3rd LSB (eg, 0b100 = 0x4). Note that setting/clearing 0th position, that corresponding to <code>GRPC_COMPRESS_NONE</code>, has no effect, as no-compression (a.k.a. <em>identity</em>) is always supported. Incoming messages compressed (ie, encoded) with a disabled algorithm will result in the call being closed with <code>GRPC_STATUS_UNIMPLEMENTED</code>.</p>
<h4>Default Compression <em>Level</em></h4>
<p>**(currently, Q2 2016, only applicable for server side channels. It's ignored for clients.)** Use the channel argument key <code>GRPC_COMPRESSION_CHANNEL_DEFAULT_LEVEL</code> (from <a href="https://github.com/grpc/grpc/blob/master/include/grpc/impl/codegen/compression_types.h">`grpc/impl/codegen/compression_types.h`</a>), valued by an integer corresponding to a value from the <code>grpc_compression_level</code> enum.</p>
<h4>Default Compression <em>Algorithm</em></h4>
<p>Use the channel argument key <code>GRPC_COMPRESSION_CHANNEL_DEFAULT_ALGORITHM</code> (from <a href="https://github.com/grpc/grpc/blob/master/include/grpc/impl/codegen/compression_types.h">`grpc/impl/codegen/compression_types.h`</a>), valued by an integer corresponding to a value from the <code>grpc_compression_level</code> enum.</p>
<h2>Per Call Settings</h2>
<h3>Compression <b>Level</b> in Call Responses</h3>
<p>The server requests a compression level via initial metadata. The <code>send_initial_metadata</code> <code><a class="el" href="structgrpc__op.html" title="Operation data: one field for each op type (except SEND_CLOSE_FROM_CLIENT which has no arguments) ...">grpc_op</a></code> contains a <code>maybe_compression_level</code> field with two fields, <code>is_set</code> and <code>compression_level</code>. The former must be set when actively choosing a level to disambiguate the default value of zero (no compression) from the proactive selection of no compression.</p>
<p>The core will receive the request for the compression level and automatically choose a compression algorithm based on its knowledge about the peer (communicated by the client via the <code>grpc-accept-encoding</code> header. Note that the absence of this header means no compression is supported by the client/peer).</p>
<h3>Compression <b>Algorithm</b> in Call Responses</h3>
<p><b>Server should avoid setting the compression algorithm directly</b>. Prefer setting compression levels unless there's a <em>very</em> compelling reason to choose specific algorithms (benchmarking, testing).</p>
<p>Selection of concrete compression algorithms is performed by adding a <code>(GRPC_COMPRESS_REQUEST_ALGORITHM_KEY, &lt;algorithm-name&gt;)</code> key-value pair to the initial metadata, where <code>GRPC_COMPRESS_REQUEST_ALGORITHM_KEY</code> is defined in <a href="https://github.com/grpc/grpc/blob/master/include/grpc/impl/codegen/compression_types.h">`grpc/impl/codegen/compression_types.h`</a>), and <code>&lt;algorithm-name&gt;</code> is the human readable name of the algorithm as given in https://github.com/grpc/grpc/blob/master/doc/PROTOCOL-HTTP2.md "the HTTP2 spec" for <code>Message-Encoding</code> (e.g. gzip, identity, etc.). See <a href="https://github.com/grpc/grpc/blob/master/src/core/lib/compression/compression.c">`grpc_compression_algorithm_name`</a> for the mapping between the <code>grpc_compression_algorithm</code> enum values and their textual representation.</p>
<h2>Per Message Settings</h2>
<p>To disable compression for a specific message, the <code>flags</code> field of <code><a class="el" href="structgrpc__op.html" title="Operation data: one field for each op type (except SEND_CLOSE_FROM_CLIENT which has no arguments) ...">grpc_op</a></code> instances of type <code>GRPC_OP_SEND_MESSAGE</code> must have its <code>GRPC_WRITE_NO_COMPRESS</code> bit set. Refer to <a href="https://github.com/grpc/grpc/blob/master/include/grpc/impl/codegen/compression_types.h">`grpc/impl/codegen/compression_types.h`</a>), </p>
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