Envoy Header-To-Metadata Filter

  • v3 API reference

  • This filter should be configured with the name envoy.filters.http.header_to_metadata.

This filter is configured with rules that will be matched against requests. Each rule has either a header and can be triggered either when the header is present or missing.

When a rule is triggered, dynamic metadata will be added based on the configuration of the rule. If the header is present, it’s value is extracted and used along with the specified key as metadata. If the header is missing, on missing case is triggered and the value specified is used for adding metadata.

The metadata can then be used for load balancing decisions, consumed from logs, etc.

A typical use case for this filter is to dynamically match requests with load balancer subsets. For this, a given header’s value would be extracted and attached to the request as dynamic metadata which would then be used to match a subset of endpoints.

Example

A sample filter configuration to route traffic to endpoints based on the presence or absence of a version header could be:

thrift_filters:
  - name: envoy.filters.thrift.header_to_metadata
    typed_config:
      "@type": type.googleapis.com/envoy.extensions.filters.thrift.header_to_metadata.v3.HeaderToMetadata
      request_rules:
        - header: x-version
          on_header_present:
            metadata_namespace: envoy.lb
            key: version
            type: STRING
          on_header_missing:
            metadata_namespace: envoy.lb
            key: default
            value: 'true'
            type: STRING
          remove: false

A corresponding upstream cluster configuration could be:

clusters:
  - name: versioned-cluster
    type: EDS
    lb_policy: ROUND_ROBIN
    lb_subset_config:
      fallback_policy: ANY_ENDPOINT
      subset_selectors:
        - keys:
            - default
        - keys:
            - version

This would then allow requests with the x-version header set to be matched against endpoints with the corresponding version. Whereas requests with that header missing would be matched with the default endpoints.

If the header’s value needs to be transformed before it’s added to the request as dynamic metadata, this filter supports regex matching and substitution:

thrift_filters:
  - name: envoy.filters.thrift.header_to_metadata
    typed_config:
      "@type": type.googleapis.com/envoy.extensions.filters.thrift.header_to_metadata.v3.HeaderToMetadata
      request_rules:
        - header: ":path"
          on_header_present:
            metadata_namespace: envoy.lb
            key: cluster
            regex_value_rewrite:
              pattern:
                google_re2: {}
                regex: "^/(cluster[\\d\\w-]+)/?.*$"
              substitution: "\\1"

Statistics

Currently, this filter generates no statistics.