Ceph: Designing and Implementing Scalable Storage Systems

Example of the librados application that uses watchers and notifiers

The following librados application is written in C and shows how to use the watch or notify functionality in RADOS. Ceph enables a client to create a watcher on an object and receive notifications from a completely separate client connected to the same cluster.

The watcher functionality is implemented via callback functions. When you call the librados function to create the watcher, two of the arguments are for callback functions, one is for what to do when a notification is received and another is for what to do if the watcher loses contact or encounters an error with the object. These callback functions then contain the code you want to run when a notification or error ...