Abstract: Methods and apparatuses, including computer program products, are described for providing unified communication services for communication sessions between client devices. A plurality of globally-distributed computing device clusters are coupled to each other over a communication network. Each cluster is configured to receive, from a client device, a transaction request associated with a communication session, including a presence event. Each cluster comprises a SIP server configured to process the transaction request, an event queue configured to extract the presence event from the transaction request for storage in the event queue, and a session state handler configured to retrieve the presence event from the event queue and notify an XMPP server of the presence event. The SIP server, XMPP server, and session state handler each stores and retrieves data associated with the communication session in a database shard at the cluster. The shards of each cluster collectively comprise a single logical database.