Router

• v2 API reference

The router consumes and sets various HTTP headers both on the egress/request path as well as on the ingress/response path. They are documented in this section.

• x-envoy-retry-on
• x-envoy-upstream-alt-stat-name
• x-envoy-upstream-rq-timeout-alt-response
• x-envoy-immediate-health-check-fail
• x-envoy-decorator-operation

If a is in place, Envoy will default to retrying one time unless explicitly specified. The number of retries can be explicitly set in the route retry config or by using this header. If a is not configured and x-envoy-retry-on or headers are not specified, Envoy will not retry a failed request.

A few notes on how Envoy does retries:

• The route timeout (set via x-envoy-upstream-rq-timeout-ms or the ) includes all retries. Thus if the request timeout is set to 3s, and the first request attempt takes 2.7s, the retry (including backoff) has .3s to complete. This is by design to avoid an exponential retry/timeout explosion.
• Envoy uses a fully jittered exponential backoff algorithm for retries with a base time of 25ms. The first retry will be delayed randomly between 0-24ms, the 2nd between 0-74ms, the 3rd between 0-174ms and so on.
• If max retries is set both by header as well as in the route configuration, the maximum value is taken when determining the max retries to use for the request.

x-envoy-retry-on

Setting this header on egress requests will cause Envoy to attempt to retry failed requests (number of retries defaults to 1 and can be controlled by x-envoy-max-retries header or the ). The value to which the x-envoy-retry-on header is set indicates the retry policy. One or more policies can be specified using a ‘,’ delimited list. The supported policies are:

5xx

Envoy will attempt a retry if the upstream server responds with any 5xx response code, or does not respond at all (disconnect/reset/read timeout). (Includes connect-failure and refused-stream)

• NOTE: Envoy will not retry when a request exceeds x-envoy-upstream-rq-timeout-ms (resulting in a 504 error code). Use if you want to retry when individual attempts take too long. x-envoy-upstream-rq-timeout-ms is an outer time limit for a request, including any retries that take place.

gateway-error

This policy is similar to the 5xx policy but will only retry requests that result in a 502, 503, or 504.

connect-failure

Envoy will attempt a retry if a request is failed because of a connection failure to the upstream server (connect timeout, etc.). (Included in 5xx)

• NOTE: A connection failure/timeout is a the TCP level, not the request level. This does not include upstream request timeouts specified via or via route configuration.

retriable-4xx

Envoy will attempt a retry if the upstream server responds with a retriable 4xx response code. Currently, the only response code in this category is 409.

• NOTE: Be careful turning on this retry type. There are certain cases where a 409 can indicate that an optimistic locking revision needs to be updated. Thus, the caller should not retry and needs to read then attempt another write. If a retry happens in this type of case it will always fail with another 409.

refused-stream

Envoy will attempt a retry if the upstream server resets the stream with a REFUSED_STREAM error code. This reset type indicates that a request is safe to retry. (Included in 5xx)

The number of retries can be controlled via the header or via the route configuration.

Note that retry policies can also be applied at the .

By default, Envoy will not perform retries unless you’ve configured them per above.

x-envoy-retry-grpc-on

cancelled

Envoy will attempt a retry if the gRPC status code in the response headers is “cancelled” (1)

deadline-exceeded

Envoy will attempt a retry if the gRPC status code in the response headers is “deadline-exceeded” (4)

resource-exhausted

Envoy will attempt a retry if the gRPC status code in the response headers is “resource-exhausted” (8)

unavailable

Envoy will attempt a retry if the gRPC status code in the response headers is “unavailable” (14)

As with the x-envoy-retry-grpc-on header, the number of retries can be controlled via the x-envoy-max-retries header

Note that retry policies can also be applied at the .

By default, Envoy will not perform retries unless you’ve configured them per above.

x-envoy-upstream-alt-stat-name

Setting this header on egress requests will cause Envoy to emit upstream response code/timing statistics to a dual stat tree. This can be useful for application level categories that Envoy doesn’t know about. The output tree is documented here.

This should not be confused with which is specified while defining the cluster and when provided specifies an alternative name for the cluster at the root of the statistic tree.

x-envoy-upstream-canary

If an upstream host sets this header, the router will use it to generate canary specific statistics. The output tree is documented here.

Setting this header on egress requests will cause Envoy to set a 204 response code (instead of 504) in the event of a request timeout. The actual value of the header is ignored; only its presence is considered. See also .

x-envoy-upstream-rq-timeout-ms

Setting this header on egress requests will cause Envoy to override the route configuration. The timeout must be specified in millisecond units. See also .

x-envoy-upstream-rq-per-try-timeout-ms

Setting this header on egress requests will cause Envoy to set a per try timeout on routed requests. This timeout must be <= the global route timeout (see x-envoy-upstream-rq-timeout-ms) or it is ignored. This allows a caller to set a tight per try timeout to allow for retries while maintaining a reasonable overall timeout.

x-envoy-immediate-health-check-fail

x-envoy-overloaded

If this header is set by upstream, Envoy will not retry. Currently the value of the header is not looked at, only its presence.

If this header is present on ingress requests, its value will override any locally defined operation (span) name on the server span generated by the tracing mechanism. Similarly, if this header is present on an egress response, its value will override any locally defined operation (span) name on the client span.

The router sets various HTTP headers both on the egress/request path as well as on the ingress/response path. They are documented in this section.

• x-envoy-upstream-service-time

x-envoy-expected-rq-timeout-ms

This is the time in milliseconds the router expects the request to be completed. Envoy sets this header so that the upstream host receiving the request can make decisions based on the request timeout, e.g., early exit. This is set on internal requests and is either taken from the x-envoy-upstream-rq-timeout-ms header or the , in that order.

x-envoy-upstream-service-time

Contains the time in milliseconds spent by the upstream host processing the request. This is useful if the client wants to determine service time compared to network latency. This header is set on responses.

x-envoy-original-path

If the route utilizes prefix_rewrite, Envoy will put the original path header in this header. This can be useful for logging and debugging.

x-envoy-overloaded

Envoy will set this header on the downstream response if a request was dropped due to either or upstream circuit breaking.

The router outputs many statistics in the cluster namespace (depending on the cluster specified in the chosen route). See for more information.

The router filter outputs statistics in the http.. namespace. The stat prefix comes from the owning HTTP connection manager.

Virtual cluster statistics are output in the vhost..vcluster.. namespace and include the following statistics:

The router filter supports the following runtime settings:

upstream.base_retry_backoff_ms

Base exponential retry back off time. See for more information. Defaults to 25ms.

upstream.maintenance_mode.

% of requests that will result in an immediate 503 response. This overrides any routing behavior for requests that would have been destined for . This can be used for load shedding, failure injection, etc. Defaults to disabled.

upstream.use_retry

% of requests that are eligible for retry. This configuration is checked before any other retry configuration and can be used to fully disable retries across all Envoys if needed.