1. A computerized system for providing unified communication services for communication sessions between two or more client devices, the system comprising: a plurality of globally-distributed computing device clusters coupled to each other over a communication network, each cluster configured to: receive, from a client device, a transaction request associated with a communication session, the transaction request including a presence event; wherein each cluster comprises: a Session Initiation Protocol (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 Extensible Messaging and Presence Protocol (XMPP) server of the presence event; wherein 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; wherein the database shards of each cluster collectively comprise a single logical database.