For environments where it may be important to provide backup host connections to the primary connections in use by workstation emulator sessions, Communications Server provides you with options that enable configuration of host connections that are activated in response to failure conditions. The options you can use for this depend on the level of backup you want to provide:
These options are explained in the following sections.
In Communications Server, you can configure host links to activate automatically if a critical server fails. Configured connections to a host can continue to function by activating alternate connections on a backup server. This function is known as hot standby.
For the user of a workstation emulator session with a host machine whose initial connection fails due to a Communications Server failure, hot standby enables a subsequent attempt to reestablish the connection with the host machine, automatically routing through an alternate Communications Server. The initial server providing the connection is known as the critical server. The alternate server providing a connection to the host machine after failure of the critical server is known as the backup server. The backup server provides connections that are automatically started in response to detection of a critical server failure and licensing charges related to critical server host connectivity are automatically managed on the backup server.
A server can be both a critical server and a backup server at the same time. Servers can back each other up.
Hot standby enables backup server support for LAN based IP or IPX emulator sessions. Hot standby does not provide backup server support for the SNA environment.
In Figure 34, the backup server is configured with a complete backup connection for HOSTCON1, which provides the primary host connectivity on the critical server. The dotted lines between the backup server and the workstation emulator clients are not active while the critical server is in operation.
Figure 34. A Critical Server with One Backup Server
In Figure 35, each of the backup servers are configured with a partial backup connection for HOSTCON1, which provides the primary host connectivity on the critical server. The combination of the two backup servers providing a complete backup for HOSTCON1. The dotted lines between the backup servers and the workstation emulator clients are not active while the critical server is in operation.
Figure 35. A Critical Server with Two Backup Servers
If the critical server fails, recovery occurs as follows:
|Note:||When the critical server is active again, the backup server connections activated for it are not automatically deactivated. You must deactivate the backup server connections manually.|
You can not install the same Communications Server licenses on more than one server running on the same network. Install Communications Server on multiple servers by purchasing an additional Communications Server package for each additional server.
Before configuring your Communications Server for hot standby, decide whether to run Communications Server as both a production server and a backup server, or as a dedicated backup server.
A production server is a Communications Server that takes care of your day-to-day communication requirements. This environment configures Communications Server with host connections that are activated outside of a critical server failure scenario. Activation and use of such connections requires purchasing of adequate licenses for them. You can not install the same Communications Server licenses on more than one server. To install Communications Server for use as a production server on multiple systems, you must purchase an additional Communications Server package for each additional server installation.
When you use a Communications Server as both a production server and a backup server, there are no special license considerations. You install Communications Server and the licenses you purchased for use of Communications Server as a production server.
The role of Communications Server as a backup server requires no additional licensing beyond the base licensing purchased for the critical servers it is configured to monitor. You do not need any special license considerations on the backup server. When the backup server detects a failure of a critical server, the backup server automatically activates the connections configured for the critical server and adjusts the backup server's licenses based on those in effect for the critical server. Communications Server manages hot standby licensing without your intervention.
However, if the connections configured for the critical server have already been activated outside of a critical server failure scenario, the backup server's adjustment for the critical server's licenses is not performed and separate licensing on the backup server is required.
A dedicated backup server does not function as a production server and does not activate connections outside of a critical server failure scenario. No connections can be activated on it at startup or activated manually. Its function is to monitor one or more critical servers.
A dedicated backup server requires no additional licensing beyond the base licensing purchased for the critical servers it is configured to monitor. Communications Server manages hot standby licensing without your intervention.
Scenarios for using hot standby involve normal Communications Server functional scenarios for support of LAN based emulator clients over IP or IPX, with some specific hot standby operations.
To configure a hot standby environment, there are two basic dependencies:
Some emulator clients may not provide for alternate routing. Planning for a hot standby environment requires careful evaluation of emulator software and the type of connectivity used between the client and the server. The following are some choices for alternate routing:
IBM Personal Communications support for TN3270E, TN5250, API, or QEL/MU emulator clients provide options for alternate routing through one or more of the hot rollover, LU pools, or multiple emulator sessions options. Some vendor emulator clients provide options for alternate routing.
From the SNA Node Configuration advanced panel, use the Configure Hot Standby Critical Servers configuration option to configure hot standby on the backup server.
You do not need to perform any hot standby configuration on the critical server, but the backup server requires hot standby configuration. Using the SNA Node Configuration advanced option, configure the backup server with critical server definitions. A critical server definition specifies the name of the critical server monitored by the backup server and the names of the connections activated when the critical server fails. Multiple critical server definitions can be configured, enabling a backup server to monitor and provide backup for multiple critical servers. After configuring critical server definitions on the backup server, restart the backup server node for the changes to be put into effect. For detailed information on configuring hot standby, see the online help for SNA Node Configuration.
Follow these steps to configure a hot standby environment:
You must deactivate of the backup server host connections manually. Manual deactivation of the backup server connections should be performed during off hours. You should notify users of workstation emulator sessions with the backup server prior to the deactivation of the connections.
Because hot standby is limited to environments supporting LAN based IP or IPX workstation emulator sessions, some environments can not use hot standby for backup connectivity. Many considerations for planning a backup environment without hot standby are similar to backup planning using hot standby. Follow these steps to configure a backup environment without using hot standby:
You can not use hot standby for backup connectivity in a single server, because hot standby only applies to environments with backup connections between servers. However, some considerations for planning a backup environment within a single server are similar to backup planning using hot standby. The differences are summarized by the following steps for backup connections in a single server: