Communications Server

Network Administration Guide


Chapter 12. Planning for Backup Host Connections

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.


Planning for Backup Connections Between Servers Using Hot Standby

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


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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


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If the critical server fails, recovery occurs as follows:

  1. The backup server, the host, and the workstation emulator software on the workstation detect the failure at approximately the same time.
  2. The connections named in the backup server's critical server configuration are automatically activated and their connectivity to the host machine is established.
  3. The workstation establishes a new session with the backup server that has just activated its critical server connections.
  4. The workstation contacts the host machine and displays a logon prompt for the user.
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.

Using a Production or Dedicated Backup Server for Hot Standby

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.

Using a Production 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.

Using a Dedicated Server

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.

Planning for the Complete Hot Standby Environment

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.

Configuring Hot Standby on the Backup Server

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.

Configuration of a Hot Standby Environment

Follow these steps to configure a hot standby environment:

  1. Configure the Critical Server: The critical server is configured without any hot standby related configuration. Configure the host connectivity to match the alternate routing option used with the emulator clients.
  2. Configure the Backup Server: Backup server configuration involves two key concepts:
    1. Configuration of host connectivity on the backup server is similar to the critical server, except that host connections are configured for manual initiation.
    2. Critical server configuration on the backup server to define the host connections started when a critical server failure is detected.
  3. Configure the emulator clients: The workstation emulator clients are configured to match an alternate routing option to the connectivity being supplied by the servers.
  4. Start the Critical Server and Backup Server nodes: Start the nodes normally. SNA Node Operations can verify the hot standby status for the critical and backup servers.
  5. Critical Server failure: When the critical server fails, recovery occurs as follows:
    1. The backup server, the host, and the workstation emulator software on the client detect the failure at approximately the same time.
    2. The connections named in the critical server configuration on the backup server are activated and connectivity to the host machine is established.
    3. The workstation emulator client establishes a new session with the backup server. The backup server routes sessions to the host machine through the connection activated on the backup server when the critical server failed.
    4. The workstation contacts the host machine and displays a logon prompt for the user.
  6. Critical Server back online: When the critical server is active again, new workstation emulator sessions use the critical server host connectivity. Existing workstation emulator sessions routed through the backup server continue operation until the sessions are closed.

    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.


Planning for Backup Connections Between Servers without Using Hot Standby

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:

  1. On the backup server, install a separately purchased Communications Server package with adequate licenses to provide for backup host connections to the primary server.
  2. On the backup server, configure host connections used as backups to activate on demand.
  3. Configure the workstation emulator clients to provide automatic routing to the resources on the backup server when the primary server fails. However, routing should be restricted to access the backup server only if a failure of the primary server is detected.
  4. Both nodes are started, but only the primary node with its active upstream connections initially supply connectivity to the host.
  5. When the primary server fails, the workstation emulator clients attempt subsequent connections through the backup server. Host connections on the backup server are activated.
  6. The duplicate licensing installed on the backup server provides availability of the backup server to clients previously routed through the primary server.
  7. When the primary server is active again, connections on the backup server are deactivated manually.

Planning for Backup Connections in a Single Server

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:

  1. On the server acting as the gateway to the host, configure primary and backup connections that supply LUs to the same LU pool, but with the backup connection configured to activate on demand.
  2. Configure the workstation emulator clients to connect through the LU pool.
  3. When the primary connection fails, or the LUs in the pool are exhausted, an attempt to connect to the pool initiates activation of the backup connection.
  4. When the primary server is active again, connections on the backup server are deactivated manually.


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