Martin Fowler

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 Request Pipeline.

Considering a simple key value store example used in Consistent Core: a client can be interested when a value changes for a particular key or a key is removed. There are two parts to the implementation, a client side implementation and a server side implementation.

Client side implementation

The client accepts the key and the function to be invoked when it gets watch events from the server. The client stores the function object for later invocation. It then sends the request to register the watch to the

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