Publisher
The publisher is a required server in all clusters, and as shown in Figure 8-1, there can be only one publisher per cluster. This server is the first to be installed and provides the database services to all other subscribers in the cluster. The publisher server is the only server that has full read and write access to the configuration database.
On larger systems with more than 1250 users, Cisco recommends a dedicated publisher to prevent administrative operations from affecting the telephony services. A dedicated publisher does not provide call processing or TFTP services running on the server. Instead, other subscriber servers within the cluster provide these services.
The choice of hardware platform for the publisher should be based on the desired scale and performance of the cluster. Cisco recommends that the publisher have the same server performance capability as the call processing subscribers. Ideally the publisher should also be a high-availability server to minimize the impact of a hardware failure.
Subscriber
During installation of the Unified CM software, you can define two types of servers, publisher and subscriber. These terms are used to define the database relationship during installation. When the software is installed initially, only the database and network services are enabled. All subscriber nodes subscribe to the publisher to obtain a copy of the database information. However, in order to reduce initialization time for the Unified CM cluster, all subscriber servers in the cluster attempt to use their local copy of the database when initializing. This reduces the overall initialization time for a Unified CM cluster. All subscriber nodes rely on change notification from the publisher or other subscriber nodes in order to keep their local copy of the database updated.
As shown in Figure 8-1, multiple subscriber nodes can be members of the same cluster. Subscriber nodes include Unified CM call processing subscriber nodes, TFTP subscriber nodes, and media resource subscriber nodes that provide functions such as conferencing and music on hold (MoH).
You can read more from the SRND here..Its an excellent document
http://www.cisco.com/en/US/docs/voice_ip_comm/cucm/srnd/8x/callpros.html#wp1148528
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