Up to release 1.0/1.1-M7 the spray-io module provided a low-level network I/O layer for directly connecting Akka actors to asynchronous Java NIO sockets. Since then the spray and Akka teams have joined forces to build upon the work in spray and come up with an extended and improved implementation, which lives directly in Akka as of Akka 2.2.
Over time more and more things that were previously provided by spray-io (e.g. the pipelining infrastructure and the SSL/TLS support) have found their way, in an improved form, from the spray codebase into the Akka codebase, so that spray’s own IO module will cease to exist in the near future.
In release 1.2 spray-io only contains a few remnants of the earlier infrastructure, which haven’t been completely upgraded to the Akka 2.2 I/O layer yet. So, usually there should be no reason to depend on spray-io from your own applications anymore.
All documentation for the new I/O layer can be found in the docs to Akka 2.2, namely: