Connection pooling
For HTTP traffic, Envoy supports abstract connection pools that are layered on top of the underlying wire protocol (HTTP/1.1, HTTP/2, HTTP/3). The utilizing filter code does not need to be aware of whether the underlying protocol supports true multiplexing or not. In practice the underlying implementations have the following high level properties:
HTTP/1.1
The HTTP/1.1 connection pool acquires connections as needed to an upstream host (up to the circuit breaking limit). Requests are bound to connections as they become available, either because a connection is done processing a previous request or because a new connection is ready to receive its first request. The HTTP/1.1 connection pool does not make use of pipelining so that only a single downstream request must be reset if the upstream connection is severed.
HTTP/2
The HTTP/2 connection pool multiplexes multiple requests over a single connection, up to the limits imposed by max concurrent streams and max requests per connection. The HTTP/2 connection pool establishes as many connections as are needed to serve requests. With no limits, this will be only a single connection. If a GOAWAY frame is received or if the connection reaches the maximum requests per connection limit, the connection pool will drain the affected connection. Once a connection reaches its maximum concurrent stream limit, it will be marked as busy until a stream is available. New connections are established anytime there is a pending request without a connection that can be dispatched to (up to circuit breaker limits for connections). HTTP/2 is the preferred communication protocol when Envoy is operating as a reverse proxy, as connections rarely, if ever, get severed(因为连接很少（如果有的话）被切断).
Automatic protocol selection
For Envoy acting as a forward proxy, the preferred configuration is the AutoHttpConfig , configued via http_protocol_options. By default it will use TCP and ALPN to select the best available protocol of HTTP/2 and HTTP/1.1.
Number of connection pools
Each host in each cluster will have one or more connection pools. If the cluster has a single explicit protocol configured, then the host may have only a single connection pool. However, if the cluster supports multiple upstream protocols, then unless it is using ALPN, one connection pool per protocol may be allocated. Separate connection pools are also allocated for each of the following features:
Each worker thread maintains its own connection pools for each cluster, so if an Envoy has two threads and a cluster with both HTTP/1 and HTTP/2 support, there will be at least 4 connection pools.
Health checking interactions
If Envoy is configured for either active or passive health checking, all connection pool connections will be closed on behalf of a host that transitions from an available state to an unavailable state. If the host reenters the load balancing rotation it will create fresh connections which will maximize the chance of working around a bad flow (due to ECMP route or something else).
