State Watch

Notify clients when specific values change on the server


Clients are interested in changes to the specific values on the server. It's difficult for clients to structure their logic if they need to poll the server continuously to look for changes. If clients open too many connections to the server for watching changes, it can overwhelm the server.


Allow clients to register their interest with the server for specific state changes. The server notifies the interested clients when state changes happen. The client maintains a Single-Socket Channel with the server. The server sends state change notifications on this channel. Clients might be interested in multiple values, but maintaining a connection per watch can overwhelm the server. So clients can use a Request Pipeline.

for more details go to Chapter 27 of the online ebook at

This pattern is part of Patterns of Distributed Systems

23 November 2023