I might be influenced by the german software company I currently work at, but for those cases I really like FMC (Fundamental Modeling Concepts), which is -- said oversimplified -- a more abstract version of modeling (than e.g. UML) and is mainly used by software architects.
The point why I recommend that here, is that FMC already has the a concept of agents and thus can be easily used for actor diagrams. It has connectors for something called channels, which can be unidirectional and thus are good to visualize message passing and simple connectors for data access (read, read/write, and write; e.g. database access). I used it for some modelings of a actor-based Akka systems and it worked quite well for me.
If it's not FMC, I'd also use data flow diagrams as will recommended.