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 in the http_filters field matters. If filters are configured in the following order (and assuming all three filters are decoder/encoder filters):
# The last configured filter has to be a terminal filter, as determined by the
# NamedHttpFilterConfigFactory::isTerminalFilter() function. This is most likely the router
The connection manager will invoke decoder filters in the order:
On the other hand, the connection manager will invoke encoder filters in the reverse