Sirius is a library for distributing and coordinating data updates amongst a cluster of nodes. It handles building an absolute ordering for updates that arrive in the cluster, ensuring that cluster nodes eventually receive all updates, and persisting the updates on each node. These updates are generally used to build in-memory data structures on each node, allowing applications using Sirius to have direct access to native data structures representing up-to-date data. Sirius does not, however, build these data structures itself -- instead, the client application supplies a callback handler, which allows developers using Sirius to build whatever structures are most appropriate for their application.
Said another way: Sirius enables a cluster of nodes to keep developer-controlled in-memory data structures eventually consistent, allowing I/O-free access to shared information.