The Session Initiation Protocol (SIP) server can be comprised of an engine tier and a state tier distributed on a cluster network environment. The engine tier can send, receive and process various messages. The state tier can maintain in-memory state data associated with various SIP sessions. Various applications can be running on the engine tier. A new version of an application can be deployed alongside the old version, simultaneously running on the SIP server. Incoming messages for new calls can be directed by the SIP server to the new version of the application. Incoming messages for previously established calls can be directed to the old version of the application. Once the old version is finished processing calls, it can be undeployed.