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upstream_filters.md

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Converting a downstream HTTP filter to be a dual filter

Before coverting a filter to be an upstream HTTP filter you should do some basic functionality analysis. Make sure

  • The filter does not use any downstream-only functionality, accessed via downstreamCallbacks() such as clearing cached routes, or performing internal redirects.
  • Either the filter does not sendLocalReply, or you test and document how sendLocalReply code paths will play with hedging / retries (cut off the hedge attempt, and local-reply failures won't trigger retries)
  • Either the filter does not access streamInfo in a non-const way, or you test and document how the filter interacts with hedging and retries. Note that for hedging, a single downstream StreamInfo is accessible in parallel to both instances of the upstream HTTP filter instance, so it must be resiliant to parallel access.
  • Any code accessing the downstream connection checks to make sure it is present. The downstream connection will not be available for mirrored/shadowed requests.

Once you've done this, you're ready to convert. An example converted filter is the Envoy Buffer Filter

Assuming your filter inherits from FactoryBase:

  • Change from inheriting from FactoryBase to inheriting from DualFactoryBase
  • This changes the type of createFilterFactoryFromProto. Update accordingly. Please note if you use the init manager or a stats context you must get them from DualInfo rather than from the server factory context or xDS reloads will not work correctly and may leak memory.
  • Add using UpstreamMyFilterFactory = MyFilterFactory; in your config.h file and REGISTER_FACTORY(UpstreamMyFilterFactory, Server::Configuration::UpstreamHttpFilterConfigFactory); to your config.cc file.
  • If your filter is listed in source/extensions/extensions_metadata.yaml add envoy.filters.http.upstream to the filter category.

Your filter should now be available as an upstream HTTP filter.

An example PR for conversion is 23071