## Servers

Nearly any server with sufficient capacity can be configured as Sun Ray server so long as it runs a supported version of the Solaris operating system or one of the supported flavors of Linux. Early Sun Ray implementations relied on dedicated interconnects, using physically dedicated Ethernet networks, to connect the DTUs with the servers. DTUs can now also be deployed on existing Local Area Network (LAN) infrastructure, eliminating the requirement for a dedicated interconnect.

To ensure uninterrupted service, several Sun Ray servers can be configured as a failover group. Servers in a group authenticate (or learn to trust) one another by using a common group signature, a key used to sign messages sent between servers in the group. The group signature must be configured, using the `utgroupsig` command, to be identical on each server.

A failover group consists of a primary server and one or more secondary servers, configured with the `utreplica` command. Each Sun Ray server hosts its own local Sun Ray data store. However, only read access is permitted on the local data stores. Any data changes (write access) are first written on the primary server and later replicated on the secondary servers' Sun Ray data stores.

Much like the `utgstatus` command, the Servers tab lists all servers in the failover group. Click on the server names to display additional information, such as the number of connected DTUs, the number of user sessions, or the current server mode.

Two server modes are supported:

- Online: The server participates in the normal session creation process controlled by the load balancing algorithm in the failover group.

- Offline: The server does not participate in load balancing any more (the load balancing algorithm does not select this server for new sessions), although sessions can still be created on it, either explicitly, through the use of the `utswitch` or `utselect` command, or implicitly, if all other servers are down.

In addition, LAN support can be enabled or disabled separately with the `utadm` command: If enabled, the Sun Ray server accepts DTU connections from shared subnets.

Note – You do not need to enable LAN support if you have configured the server to use dedicated interconnects.

Some configuration changes do not take effect until the Sun Ray services on each server are restarted. If a change requires Sun Ray services to be restarted, a notification message appears in the upper right corner of the Admin GUI. Click on the provided link to switch to the Servers tab and to perform a Warm Restart or a Cold Restart:

- Warm Restart: Existing Sun Ray sessions are preserved. Use this option if a minor configuration change was made. With minor changes it is not necessary to terminate existing sessions.

- Cold Restart: All existing Sun Ray sessions are terminated. Use this option if a significant change has been made.

Note – To perform a group-wide restart, select all servers in the servers table (either individually or with the Select All button) before clicking the Warm Restart or Cold Restart button.