Circuit breaking

• Cluster maximum connections: The maximum number of connections that Envoy will establish to all hosts in an upstream cluster. In practice this is only applicable to HTTP/1.1 clusters since HTTP/2 uses a single connection to each host. If this circuit breaker overflows the upstream_cx_overflow counter for the cluster will increment.

• Cluster maximum requests: The maximum number of requests that can be outstanding to all hosts in a cluster at any given time. In practice this is applicable to HTTP/2 clusters since HTTP/1.1 clusters are governed by the maximum connections circuit breaker. If this circuit breaker overflows the counter for the cluster will increment.

• Cluster maximum concurrent connection pools: The maximum number of connection pools that can be concurrently instantiated. Some features, such as the Original Src Listener Filter, can create an unbounded number of connection pools. When a cluster has exhausted its concurrent connection pools, it will attempt to reclaim an idle one. If it cannot, then the circuit breaker will overflow. This differs from in that connection pools never time out, whereas connections typically will. Connections automatically clean up; connection pools do not. Note that in order for a connection pool to function it needs at least one upstream connection, so this value should likely be no greater than Cluster maximum connections. If this circuit breaker overflows the counter for the cluster will increment.

Note that circuit breaking will cause the x-envoy-overloaded header to be set by the router filter in the case of HTTP requests.