Circuit breaking

Cluster maximum connections: The maximum number of connections that Envoy will establish to all hosts in an upstream cluster. If this circuit breaker overflows the upstream_cx_overflow counter for the cluster will increment. All connections, whether active or draining, count against this limit. Even if this circuit breaker has overflowed, Envoy will ensure that a host selected by cluster load balancing has at least one connection allocated. This has the implication that the count for a cluster may be higher than the cluster maximum connection circuit breaker, with an upper bound of . This bound applies to the sum of connections across all workers threads. See connection pooling for details on how many connection pools a cluster may have.
Cluster maximum concurrent connection pools: The maximum number of connection pools that can be concurrently instantiated. Some features, such as the , 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 Cluster maximum connections 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 . If this circuit breaker overflows the upstream_cx_pool_overflow counter for the cluster will increment.

Workers threads share circuit breaker limits, i.e. if the active connection threshold is 500, worker thread 1 has 498 connections active, then worker thread 2 can only allocate 2 more connections. Since the implementation is eventually consistent, races between threads may allow limits to be potentially exceeded.

Circuit breakers are enabled by default and have modest default values, e.g. 1024 connections per cluster. To disable circuit breakers, set the to the highest allowed values.