Akka-http client: measure response latency with host connection pool

I couldn’t find any way to measure request latency using a host connection pool. I could measure time between dispatching request into pool and receiving response from it, but then I end up with additional time spent on pool operations (establishing connections) logged as request latency.

This is the definition of my pool:

val connFlow = Http(actorSystem)
    .newHostConnectionPoolHttps[Int](host = "foo.com", settings = settings)

I don’t see any way to get the info from the request, response or pool objects. Any help is much appreciated.

Hi @jalc,

that’s not currently possible indeed. If we would measure such a value it would still of some questionable value as processing a request can contain those steps:

  • (maybe) look up the hostname on DNS
  • (maybe) establish a TCP connection
  • (maybe) establish a TLS connection
  • serialize the request
  • send data to the OS on the client
  • send data over the network
  • dynamically adjust transmission speeds between hosts
  • receive data on the server
  • deserialize request on the server
  • actually process the request
  • serialize response
  • … (everything in the other direction)

From the view of the client, most of those steps are opaque, so measuring complete end-to-end latencies seems more useful.

If you are particularly concerned about connection creation overhead, you can use akka.http.host-connection-pool.min-connections to let the pool pre-connect to the host. Afterwards, the connections should be “somewhat hot”. Even this won’t be perfect as a TCP connection doesn’t necessarily ramp up connection windows until enough actual data has moved back and forth (especially on high-latency, high-throughput connections).

That doesn’t mean that we might add some simple metrics like this to the pool at some point.


While measuring complete end-to-end latencies is useful, metrics and actually hooks for applications to trace connection related events are quite useful as well. Some other http clients, provide such capabilities:

• OkHttp has EventListener.
• Async Http Client has AsyncHandler.