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008: Use server preference for compression priority

This RFC proposes amending the Connect protocol to have servers define the preference for a compression algorithm among the supported set as opposed to the client. This allows improving compression in scenarios where the client’s ordering cannot be changed, like the browser, and aligns with the commonly accepted behavior of servers like Envoy and NGINX.

The proposed diff to the protocol specification is in PR 322. This document focuses on the rationale for the change and the anticipated end user impact.

Server preference for compression

Section titled “Server preference for compression”

The compression algorithm to use for a request is determined by two factors

  • The compression methods supported by a server, configured by the user
  • The list of compressions in the client’s Accept-Encoding header, e.g., gzip, br, zstd

Currently, it is the order in the Accept-Encoding header that determines which algorithm to prefer, i.e., if all methods are supported by the server, gzip is always used for the above header.

We propose changing so servers are configured with an ordered list of compression methods, and the first matching method present in the Accept-Encoding header is selected. For the above header, if the server is configured with gzip, br, zstd, gzip is selected; if it is configured with br, zstd, gzip, br is selected.

Because common browsers always send gzip, deflate, br, zstd, it is currently impossible for connect-web users to use the more modern and efficient br and zstd algorithms. Moving preference to the server will allow preferring the more efficient algorithms, finally unlocking them for connect-web. Servers like Envoy and NGINX behave in the same way for the same reason.

While br and zstd have come a long way and commonly have similar CPU and RAM usage to gzip now, it can’t be confirmed all workloads will see no regression from a change from gzip to e.g., br. In an abundance of caution, this RFC is considered a semver breaking change. It is planned to be introduced to Connect-Go in v2, Connect-Py before v1, and Connect-ES in v3.