Matching API

Attention

The matching API is alpha and is currently under active development. Capabilities will be expanded over time and the configuration structures are likely to change.

Envoy makes use of a matching API to allow the various subsystems to express actions that should be performed based on incoming data.

The matching API is designed as a tree structure to allow for sublinear matching algorithms for better performance than the linear list matching as seen in Envoy’s HTTP routing. It makes heavy use of extension points to make it easy to extend to different inputs based on protocol or environment data as well as custom sublinear matchers and direct matchers.

Filter Integration

Within supported environments (currently only HTTP filters), a wrapper proto can be used to instantiate a matching filter associated with the wrapped structure:

static_resources:
  listeners:
  - address:
      socket_address:
        address: 0.0.0.0
        port_value: 443
    listener_filters:
    filter_chains:
    - filters:
      - name: envoy.filters.network.http_connection_manager
        typed_config:
          "@type": type.googleapis.com/envoy.extensions.filters.network.http_connection_manager.v3.HttpConnectionManager
          stat_prefix: ingress_http
          http_filters:
          - name: with-matcher
            typed_config:
              "@type": type.googleapis.com/envoy.extensions.common.matching.v3.ExtensionWithMatcher
              extension_config:
                name: envoy.filters.http.fault
                typed_config:
                  "@type": type.googleapis.com/envoy.extensions.filters.http.fault.v3.HTTPFault
                  abort:
                    http_status: 503
                    percentage:
                      numerator: 0
                      denominator: HUNDRED
                  delay:
                    fixed_delay: 3s
                    percentage:
                      numerator: 0
                      denominator: HUNDRED
              xds_matcher:
                matcher_tree:
                  input:
                    name: request-headers
                    typed_config:
                      "@type": type.googleapis.com/envoy.type.matcher.v3.HttpRequestHeaderMatchInput
                      header_name: some-header
                  exact_match_map:
                    # Note this additional indirection; this is a workaround for Protobuf oneof limitations.
                    map:
                      some_value_to_match_on:  # This is the header value we're trying to match against.
                        action:
                          name: skip
                          typed_config:
                            "@type": type.googleapis.com/envoy.extensions.filters.common.matcher.action.v3.SkipFilter
          - name: envoy.filters.http.router
          route_config:
            virtual_hosts:
            - name: default
              domains: ["*"]
              routes:
              - match: {prefix: "/"}
                route:
                  cluster: service_foo
  clusters:
  - name: service_foo
    load_assignment:
      cluster_name: some_service
      endpoints:
      - lb_endpoints:
        - endpoint:
            address:
              socket_address:
                address: 127.0.0.1
                port_value: 8080

The above example wraps a HTTP filter (the HTTPFault filter) in an ExtensionWithMatcher, allowing us to define a match tree to be evaluated in conjunction with evaluation of the wrapped filter. Prior to data being made available to the filter, it will be provided to the match tree, which will then attempt to evaluate the matching rules with the provided data, triggering an action if match evaluation results in an action.

In the above example, we are specifying that we want to match on the incoming request header some-header by setting the input to HttpRequestHeaderMatchInput and configuring the header key to use. Using the value contained by this header, the provided exact_match_map specifies which values we care about: we’ve configured a single value (some_value_to_match_on) to match against. As a result, this config means that if we receive a request which contains some-header: some_value_to_match_on as a header, the SkipFilter action will be resolved (causing the associated HTTP filter to be skipped). If no such header is present, no action will be resolved and the filter will be applied as usual.

static_resources:
  listeners:
  - address:
      socket_address:
        address: 0.0.0.0
        port_value: 443
    listener_filters:
    filter_chains:
    - filters:
      - name: envoy.filters.network.http_connection_manager
        typed_config:
          "@type": type.googleapis.com/envoy.extensions.filters.network.http_connection_manager.v3.HttpConnectionManager
          stat_prefix: ingress_http
          http_filters:
          - name: with-matcher
            typed_config:
              "@type": type.googleapis.com/envoy.extensions.common.matching.v3.ExtensionWithMatcher
              extension_config:
                name: envoy.filters.http.fault
                typed_config:
                  "@type": type.googleapis.com/envoy.extensions.filters.http.fault.v3.HTTPFault
                  abort:
                    http_status: 503
                    percentage:
                      numerator: 0
                      denominator: HUNDRED
                  delay:
                    fixed_delay: 3s
                    percentage:
                      numerator: 0
                      denominator: HUNDRED
              xds_matcher:
                # The top level matcher is a matcher tree which conceptually selects one of several subtrees.
                matcher_tree:
                  input:
                    name: request-headers
                    typed_config:
                      "@type": type.googleapis.com/envoy.type.matcher.v3.HttpRequestHeaderMatchInput
                      header_name: some-header
                  exact_match_map:
                    # Note this additional indirection; this is a workaround for Protobuf oneof limitations.
                    map:
                      some_value_to_match_on:  # This is the header value we're trying to match against.
                        # The OnMatch resulting on matching with this branch of the exact matcher is another matcher, allowing for recursive matching.
                        matcher:
                          # The inner matcher is a matcher list, which attempts to match a list of predicates.
                          matcher_list:
                            matchers:
                            - predicate:
                                or_matcher:
                                  predicate:
                                  - single_predicate:
                                      input:
                                        name: request-headers
                                        typed_config:
                                          "@type": type.googleapis.com/envoy.type.matcher.v3.HttpRequestHeaderMatchInput
                                          header_name: second-header
                                      value_match:
                                        exact: foo
                                  - single_predicate:
                                      input:
                                        name: request-headers
                                        typed_config:
                                          "@type": type.googleapis.com/envoy.type.matcher.v3.HttpRequestHeaderMatchInput
                                          header_name: second-header
                                      value_match:
                                        exact: bar
                              on_match:
                                action:
                                  name: skip
                                  typed_config:
                                    "@type": type.googleapis.com/envoy.extensions.filters.common.matcher.action.v3.SkipFilter
          - name: envoy.filters.http.router
          route_config:
            virtual_hosts:
            - name: default
              domains: ["*"]
              routes:
              - match: {prefix: "/"}
                route:
                  cluster: service_foo
  clusters:
  - name: service_foo
    load_assignment:
      cluster_name: some_service
      endpoints:
      - lb_endpoints:
        - endpoint:
            address:
              socket_address:
                address: 127.0.0.1
                port_value: 8080

Above is a slightly more complicated example which combines a top level tree matcher with a linear matcher. While the tree matchers provide very efficient matching, they are not very expressive. The list matcher can be used to provide a much richer matching API, and can be combined with the tree matcher in an arbitrary order. The example describes the following match logic: skip the filter if some-header: skip_filter is present and second-header is set to either foo or bar.

HTTP Filter Iteration Impact

The above example only demonstrates matching on request headers, which ends up being the simplest case due to it happening before the associated filter receives any data. Matching on other HTTP input sources is supported (e.g. response headers), but some discussion is warranted on how this works at a filter level.

Currently the match evaluation for HTTP filters does not impact control flow at all: if insufficient data is available to perform the match, callbacks will be sent to the associated filter as normal. Once sufficient data is available to match an action, this is provided to the filter. A consequence of this is that if the filter wishes to gate some behavior on a match result, it has to manage stopping the iteration on its own.

When it comes to actions such as SkipFilter, this means that if the skip condition is based on anything but the request headers, the filter might get partially applied, which might result in surprising behavior. An example of this would be to have a matching tree that attempts to skip the gRPC-Web filter based on response headers: clients assume that if they send a gRPC-Web request to Envoy, the filter will transform that into a gRPC request before proxying it upstream, then back into a gRPC-Web response on the encoding path. By skipping the filter based on response headers, the forward transformation will happen (the upstream receives a gRPC request), but the response is never converted back to gRPC-Web. As a result, the client will receive an invalid response back from Envoy. If the skip action was instead resolved on trailers, the same gRPC-Web filter would consume all the data but never write it back out (as this happens when it sees the trailers), resulting in a gRPC-Web response with an empty body.

HTTP Routing Integration

The matching API can be used with HTTP routing, by specifying a match tree as part of the virtual host and specifying a Route as the resulting action. See examples in the above sections for how the match tree can be configured.

Match Tree Validation

As the match tree structure is very flexible, some filters might need to impose additional restrictions on what kind of match trees can be used. This system is somewhat inflexible at the moment, only supporting limiting the input sources to a specific set. For example, a filter might specify that it only works with request headers: in this case a match tree that attempts to match on request trailers or response headers will fail during configuration load, reporting back which data input was invalid.

This is done for example to limit the issues talked about in the above section or to help users understand in what context a match tree can be used for a specific filter. Due to the limitations of the validation framework at the current time, it is not used for all filters.

For HTTP filters, the restrictions are specified by the filter implementation, so consult the individual filter documentation to understand whether there are restrictions in place.

For example, in the example below, the match tree could not be used with a filter that restricts the the match tree to only use HttpRequestHeaderMatchInput.

static_resources:
  listeners:
  - address:
      socket_address:
        address: 0.0.0.0
        port_value: 443
    listener_filters:
    filter_chains:
    - filters:
      - name: envoy.filters.network.http_connection_manager
        typed_config:
          "@type": type.googleapis.com/envoy.extensions.filters.network.http_connection_manager.v3.HttpConnectionManager
          stat_prefix: ingress_http
          http_filters:
          - name: with-matcher
            typed_config:
              "@type": type.googleapis.com/envoy.extensions.common.matching.v3.ExtensionWithMatcher
              extension_config:
                name: envoy.filters.http.fault
                typed_config:
                  "@type": type.googleapis.com/envoy.extensions.filters.http.fault.v3.HTTPFault
                  abort:
                    http_status: 503
                    percentage:
                      numerator: 0
                      denominator: HUNDRED
                  delay:
                    fixed_delay: 3s
                    percentage:
                      numerator: 0
                      denominator: HUNDRED
              xds_matcher:
                matcher_list:
                  matchers:
                  - predicate:
                      or_matcher:
                        predicate:
                        - single_predicate:
                            input:
                              name: request-headers
                              typed_config:
                                "@type": type.googleapis.com/envoy.type.matcher.v3.HttpRequestHeaderMatchInput
                                header_name: request-header
                            value_match:
                              exact: foo
                        - single_predicate:
                            input:
                              name: request-headers
                              typed_config:
                                "@type": type.googleapis.com/envoy.type.matcher.v3.HttpResponseHeaderMatchInput
                                header_name: response-header
                            value_match:
                              exact: bar
                    on_match:
                      action:
                        name: skip
                        typed_config:
                          "@type": type.googleapis.com/envoy.extensions.filters.common.matcher.action.v3.SkipFilter
          - name: envoy.filters.http.router
          route_config:
            virtual_hosts:
            - name: default
              domains: ["*"]
              routes:
              - match: {prefix: "/"}
                route:
                  cluster: service_foo
  clusters:
  - name: service_foo
    load_assignment:
      cluster_name: some_service
      endpoints:
      - lb_endpoints:
        - endpoint:
            address:
              socket_address:
                address: 127.0.0.1
                port_value: 8080