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What's Sirius

Sirius is a distributed in memory datastore that uses the Paxos protocol set to perform it's distributed messaging and updates.

Sirius is intended to provide a means to replicate in-memory state consistently across a cluster. This is accomplished by establishing a total ordering of all updates to the state stored by Sirius and distributing these updates across all nodes in the cluster. Updates are not applied until their turn comes. When applying an update a serialized version is also stored, ahead of its application, in order to allow the state to be recovered in the event of a restart.

When Sirius receives a request for an update it first decides an ordering using a variant of multi-paxos derived from the algorithm described in Paxos Made Moderately Complex. Once the Paxos algorithm arrives at a decision for the ordering it distributes this decision to the cluster. When nodes receive a new decision they notify the requesting client of the completion of ordering and queue it for application. As updates may arrive out of order, they are queued in memory until their time has come to be applied. An update is applied once all prior updates have been applied. Updates may be received from the result of a Paxos round, from local storage when recovering a node’s state, or from another node in the cluster when catching up in the event of missed updates.

Before applying any update to the in-memory state it is first written to some persistent storage, then applied to memory asynchronously. It is important to note that this persistent storage must supply the means for retrieving stored events in the order dictated on writing. The state maintained by Sirius can be queried by submitting a request similar to that for updates. The main difference is that this request is executed locally and does not need to be distributed.