|Getting Started with GemFire XD / Understanding the GemFire XD Distributed System|
A peer member (a server or peer client process) announces itself to the distributed system using one of two mechanisms.
Locator (TCP/IP). A locator service maintains a registry of all peer members in the distributed system at any given moment. A locator is typically started as a separate process (with redundancy), but you can also embed a locator in any peer member, such as a GemFire XD server. The locator opens a TCP port through which all new members connect to get initial membership information.
UDP/IP Multicast. Members can optionally use a multicast address to broadcast their presence and receive membership notification information.
Configuring Discovery Mechanisms provides more information.