# Rate limit¶

The HTTP rate limit filter will call the rate limit service when the request’s route or virtual host has one or more rate limit configurations that match the filter stage setting. The route can optionally include the virtual host rate limit configurations. More than one configuration can apply to a request. Each configuration results in a descriptor being sent to the rate limit service.

If the rate limit service is called, and the response for any of the descriptors is over limit, a 429 response is returned. The rate limit filter also sets the x-envoy-ratelimited header.

If there is an error in calling rate limit service or rate limit service returns an error and failure_mode_deny is set to true, a 500 response is returned.

## Composing Actions¶

Each rate limit action on the route or virtual host populates a descriptor entry. A vector of descriptor entries compose a descriptor. To create more complex rate limit descriptors, actions can be composed in any order. The descriptor will be populated in the order the actions are specified in the configuration.

### Example 1¶

For example, to generate the following descriptor:

("generic_key", "some_value")
("source_cluster", "from_cluster")


The configuration would be:

actions:
- {source_cluster: {}}
- {generic_key: {descriptor_value: some_value}}


### Example 2¶

If an action doesn’t append a descriptor entry, no descriptor is generated for the configuration.

For the following configuration:

actions:
- {source_cluster: {}}
- {generic_key: {descriptor_value: some_value}}


If a request did not set x-forwarded-for, no descriptor is generated.

If a request sets x-forwarded-for, the the following descriptor is generated:

("generic_key", "some_value")
("source_cluster", "from_cluster")


## Statistics¶

The rate limit filter outputs statistics in the cluster.<route target cluster>.ratelimit. namespace. 429 responses are emitted to the normal cluster dynamic HTTP statistics.

Name Type Description
ok Counter Total under limit responses from the rate limit service
error Counter Total errors contacting the rate limit service
over_limit Counter total over limit responses from the rate limit service
failure_mode_allowed Counter Total requests that were error(s) but were allowed through because of failure_mode_deny set to false.

## Runtime¶

The HTTP rate limit filter supports the following runtime settings:

ratelimit.http_filter_enabled
% of requests that will call the rate limit service. Defaults to 100.
ratelimit.http_filter_enforcing
% of requests that will call the rate limit service and enforce the decision. Defaults to 100. This can be used to test what would happen before fully enforcing the outcome.
ratelimit.<route_key>.http_filter_enabled
% of requests that will call the rate limit service for a given route_key specified in the rate limit configuration. Defaults to 100.