HTTP filters

Much like the network level filter stack, Envoy supports an HTTP level filter stack within the connection manager. Filters can be written that operate on HTTP level messages without knowledge of the underlying physical protocol (HTTP/1.1, HTTP/2, etc.) or multiplexing capabilities. There are three types of HTTP level filters:

  • Decoder: Decoder filters are invoked when the connection manager is decoding parts of the request stream (headers, body, and trailers).

  • Encoder: Encoder filters are invoked when the connection manager is about to encode parts of the response stream (headers, body, and trailers).

  • Decoder/Encoder: Decoder/Encoder filters are invoked both when the connection manager is decoding parts of the request stream and when the connection manager is about to encode parts of the response stream.

The API for HTTP level filters allows the filters to operate without knowledge of the underlying protocol. Like network level filters, HTTP filters can stop and continue iteration to subsequent filters. This allows for more complex scenarios such as health check handling, calling a rate limiting service, buffering, routing, generating statistics for application traffic such as DynamoDB, etc. HTTP level filters can also share state (static and dynamic) among themselves within the context of a single request stream. Refer to data sharing between filters for more details. Envoy already includes several HTTP level filters that are documented in this architecture overview as well as the configuration reference.

Filter ordering

Filter ordering in the http_filters field matters. If filters are configured in the following order (and assuming all three filters are decoder/encoder filters):

  1. http_filters:
  2.   - A
  3.   - B
  4.   # The last configured filter has to be a terminal filter, as determined by the
  5.   # NamedHttpFilterConfigFactory::isTerminalFilterByProto(config, context) function. This is most likely the router
  6.   # filter.
  7.   - C

The connection manager will invoke decoder filters in the order: A, B, C. On the other hand, the connection manager will invoke encoder filters in the reverse order: C, B, A.

Conditional Filter Configuration

There is some support for having the filter configuration used change based on the incoming request. See the composite filter for details on how to configure a match tree that can resolve filter configuration to use for a given request.

Filter route mutation

During HTTP filter chain processing, when decodeHeaders() is invoked by a filter, the connection manager performs route resolution and sets a cached route pointing to an upstream cluster.

Filters have the capability to directly mutate this cached route after route resolution, via the setRoute callback and DelegatingRoute mechanism. A filter may create a derived/child class of DelegatingRoute to override specific methods (for example, the route’s timeout value or the route entry’s cluster name) while preserving the rest of the properties/behavior of the base route that the DelegatingRoute wraps around. Then, setRoute can be invoked to manually set the cached route to this DelegatingRoute instance. An example of such a derived class can be found in ExampleDerivedDelegatingRoute.

If no other filters in the chain modify the cached route selection (for example, a common operation that filters do is clearRouteCache(), and setRoute will not survive that), this route selection makes it way to the router filter which finalizes the upstream cluster that the request will get forwarded to.