Dynamic modules route specifier

Overview

The DynamicModuleRouteSpecifier configuration specifies a route specifier backed by a dynamic module. For each request the module keeps the route that route matching resolved, refines it, replaces it with one of the route templates it declares, or drops it.

The module is invoked while the route is being resolved, and again whenever the route is recomputed, so it must be able to reach a decision from the request and the stream info alone. The call is synchronous and cannot be time boxed, so a module must not block or perform I/O.

Route templates

A module cannot build a route out of thin air. Instead it selects one of the route_templates the specifier declares. Each template is an ordinary Route that is built and validated once, when the specifier is configured, so an invalid template is rejected at configuration load rather than on the request path. A template inherits from the virtual host and the route configuration the specifier is configured on, exactly like a configured route does.

When the module selects a template, Envoy evaluates it against the request like a configured route. The match must hold, and the action is resolved the usual way, including weighted_clusters and cluster specifier plugins. A match that does not hold is a failure, because the rewrites of the action are only correct for a request the match accepts.

Decisions

The module returns one of five decisions:

• PassThrough uses the route the specifier was given, unchanged.

• Override uses that route with the properties the module recorded applied on top.

• SelectTemplate uses the selected template, with the recorded properties applied on top.

• NoRoute uses no route, so the request is handled as if nothing had matched.

• Error reports that the module could not decide.

A decision Envoy cannot honor is handled by the configured failure_policy, which either passes the request through to the route table or drops the route. The SDK reports an error when the module panics, so a panic is handled by the same policy rather than crashing Envoy.

Shadowing a routing change

A module receives the route that route matching resolved through its context, so it can shadow a routing change on its own without any support from Envoy. In a dry run the module computes the route it would ask for, compares it against the resolved route, counts the outcome on its own metrics, and returns PassThrough so that routing stays unchanged. Once the counts give confidence, the same module returns Override or SelectTemplate to apply the decision. The route_specifier_shadow.rs test module under test/extensions/dynamic_modules/test_data/rust shows both runs, comparing the cluster name.

Route overrides

The properties that are built from other extensions, such as the retry policy and the request mirroring policies, are declared as route_overrides. Each override is built and validated once when the specifier is configured, and the module selects one by override_id. An override that replaces no property is rejected, so a module can rely on a declared override changing something.

Notes

• The request headers are read-only. A recorded path, authority or header mutation is applied through the header transforms of the route the decision produces, so that Envoy applies it at the right point of the request lifetime.

• get_cluster_host_count reports whether a cluster is routable from the current worker and returns host counts at a priority level. It uses getThreadLocalCluster(), so it can return false even when the cluster is configured but not yet warmed on the worker.

• Custom counters, gauges and histograms can be defined during configuration and recorded during resolution, and are emitted under the metrics_namespace prefix of DynamicModuleConfig.

Statistics

The specifier emits statistics rooted at <metrics_namespace>.route_specifier.<stat_prefix>., where stat_prefix is the stat_prefix of the specifier, sharing the metrics_namespace of the module-defined metrics above.

Name	Type	Description
decision_pass_through	Counter	Requests for which the module kept the resolved route.
decision_override	Counter	Requests for which the module refined the resolved route.
decision_select_template	Counter	Requests for which the module selected a route template.
decision_no_route	Counter	Requests for which the module dropped the route.
decision_error	Counter	Requests for which the module could not decide.
runtime_skipped	Counter	Requests outside runtime_fraction.
failure_module_error	Counter	Decisions not honored because the module reported an error.
failure_template_not_selected	Counter	Decisions not honored because no known template was selected.
failure_template_match_failed	Counter	Decisions not honored because the match of the selected template did not hold.
failure_override_without_route	Counter	Decisions not honored because there was no route to refine.
failure_override_on_non_route_entry	Counter	Decisions not honored because route entry properties were recorded for a direct response.
failure_route_metadata	Counter	Decisions not honored because a typed metadata factory rejected the recorded metadata.
on_route_duration	Histogram	Time in microseconds the module spent deciding.
specifier_duration	Histogram	Time in microseconds the specifier spent on a request.

Configuration

• This extension should be configured with the type URL type.googleapis.com/envoy.extensions.router.route_specifiers.dynamic_modules.v3.DynamicModuleRouteSpecifier.

• v3 API reference

Attention

Dynamic modules run in-process with the same privileges as Envoy. Only load modules you trust. This extension is currently under active development. Capabilities and ABI are expected to evolve.

Configuration example

route_config:
  virtual_hosts:
  - name: default
    domains: ["*"]
    route_specifiers:
    - name: envoy.router.route_specifiers.dynamic_modules
      typed_config:
        "@type": type.googleapis.com/envoy.extensions.router.route_specifiers.dynamic_modules.v3.DynamicModuleRouteSpecifier
        dynamic_module_config:
          name: my_route_specifier
          do_not_close: true
        specifier_name: my_specifier_impl
        stat_prefix: my_specifier
        failure_policy: PASS_THROUGH
        route_templates:
        - template_id: canary
          route:
            match:
              prefix: "/"
            route:
              cluster: canary_service
              timeout: 5s
    routes:
    - match:
        prefix: "/"
      route:
        cluster: web_service