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