SNA Server load balancing and hot backup

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This article was previously published under Q128244

SUMMARY

This article describes how Microsoft SNA Server clients perform load balancing and hot backups across SNA Servers.

When an SNA session is opened on an SNA Server client machine, the underlying SNA client software randomly chooses a server to try. In effect, load balancing and hot backups are initiated by the client. However, this requires two or more SNA Servers to be configured to support the same LU resources (3270, LUA pool, or APPC LU aliases spanning two or more servers).

Note SNA Server does not support load balancing across connections defined on a single server.

Load balancing and hot backups are supported regardless of the SNA Server client-server LAN interface used (Named pipes, IPX/SPX, TCP/IP sockets, Banyan IP, or AppleTalk).

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MORE INFORMATION

How SNA Server clients load balance and hot backup to SNA servers

The SnaBase service on each SNA Server broadcasts a list of SNA Server services running on the server to all other SNA Servers in the subdomain. The SnaBase service listens for these broadcasts and dynamically maintains a list of SNA Server services in the subdomain.

Note The transports used and frequency of SNA Server broadcasts are controlled at the Server Broadcasts dialog box, found by choosing Server Broadcasts from the Options menu of the SNA Server Administrator for SNA Server 2.x or by choosing the Server Broadcasts tab in the SNA Subdomain Properties page in SNA Server Manager for version 3.0. This configures the subdomain.
The SNA client process (SnaBase service on Windows NT, WNAP.exe on Windows 3.x, SNABASE.exe on MS-DOS, and COMNAP.exe on OS/2) opens a sponsor connection to the SnaBase service on an SNA Server in the subdomain. This sponsor connection remains active while the SNA client process is running. When the SNA client process first starts, the client receives a list of all SNA Servers in the subdomain. Afterward, only server changes are sent (as SNA Servers are started and stopped in the subdomain).

Note The transports that an SNA Server supports for client connections is configured in the Protocol dialog box in SNA Server Setup. This is configured on a server by server basis.
When an SNA application requests a session (3270, LUA, APPC, or CPIC, and so forth), the underlying SNA client sends the request to a randomly selected server. If the server supports the LU being requested and the LU resource is available, the session is established through the server.

Note Besides opening sessions to SNA Servers in the sponsor subdomain, an "otherservers" parameter can be configured on the client to specify additional servers that the client should try to open sessions with. More information about the otherservers parameter is included below.

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If a second or subsequent client session is opened, the client first tries the servers that it already has a session with to conserve LAN resources on the client.

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Configuring fault tolerance for 3270 sessions

Group similar types of 3270 display LUs across servers and connections into a 3270 LU pool using the LU Pools dialog box in SNA Server Administrator. Only LUs of a given type can be pooled together. Secondary connections can be configured as "On demand" to only start when needed.

Note SNA Server does not support pooling of 3270 Printer LUs. Specific Printer LUs must be preassigned to a user in order for print jobs to be routed to a known printer destination. For example:

In the Servers and Connections window in SNA Administrator (on SNA Server 2.x) or under the Connection's folder in SNA Server Manager(SNA Server 3.0):
```
      SERVER1                         SERVER2
         TOKEN1  connection               TOKEN2 connection
              SRV1LU02 (3270 LU)              SRV2LU02  (3270 LU)
              SRV1LU03 (3270 LU)              SRV2LU03  (3270 LU)
              SRV1LU04 (3270 LU)              SRV2LU04  (3270 LU)

      In the LU Pools window in SNA Server Administrator or in the LU Pools
      folder in SNA Server Manager:

         Pool: 3270POOL
         LU Names: SRV1LU02
                   SRV1LU03
                   SRV2LU02
                   SRV2LU03
					
```
Assign one or more instances of the 3270 pool to an SNA Server user or group, using either the Users and Groups window in SNA Server Administrator or the Configured Users folder in SNA Server Manager. For example:
```
User: Everyone
         LU or LU Pool Session #1: 3270POOL

      User: DOMAIN\Domain Users
         LU or LU Pool Session #1: 3270POOL
         LU or LU Pool Session #2: 3270POOL
```
Note In this example, one pooled session has been granted to the Everyone group and two pooled sessions have been granted to users from the Domain Users group.
When an SNA client opens a 3270 pooled session, a session is established through a server which supports an available instance of the LU pool. For example, in the Session / Session Configuration dialog box in the 3270 applet, configure the pool like this:
LU or Pool Name: 3270POOL
and choose Session/Connect.

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The above example assumes different host LUs are used for hot backup. To configure SNA Server for 3270 hot backup to the same LU:

Configure two connections to the host, where each connection is emulating the same PU definition. One of the connections should be configured for "On Server Startup activation, with the other connection set for "On Demand" (only one connection can be active at a time). For example:
```
SERVER1                              SERVER2
         TOKEN1 connection                    TOKEN2 connection
         (Activation: On Server Startup)      (Activation: On Demand)
            SRV1LU02 (3270 LU)                    SRV2LU02 (3270 LU)	
```
Define an LU pool for each LU local address spanning each connection. For example:
```
Pool: LU2POOL
      LU Names: SRV1LU02
                SRV2LU02					
```
Assign an instance of the LU2POOL to a specific user. Only one user will be able to access one LU from the pool at any one time (since the pool effectively contains the same host LU).

When the user opens "LU2POOL" using a 3270 emulator, the user will be routed to the server with the active connection. If this server is stopped or unavailable, the on-demand connection on the backup server will be activated automatically and the user will gain access to the same 3270 LU from the backup server.

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Configuring fault tolerance for APPC/5250 sessions

The following section describes how to configure SNA Server to support fault tolerance of LU6.2 sessions for APPC and CPIC (or 5250) applications which "invoke" LU6.2 conversations. An APPC application invokes a conversation by calling [MC_]ALLOCATE. A CPIC application invokes a conversation by calling CMALLC. A 5250 emulator always invokes the LU6.2 conversation.

If the APPC or CPIC application is "invoked" by the remote system (in other words, if the APPC application calls RECEIVE_ALLOCATE, or CPIC application calls CMACCP), then the remote system determines the LU6.2 session (and thus, the connection and SNA Server) that a conversation start request (in other words, the IBM SNA FMH-5 attach message) is sent over. If a host system is invoking the conversation, the VTAM "ALSLIST" parameter can be used to control the LU over which the attach request is sent. See the IBM VTAM documentation for more information about this parameter.

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Configuring fault tolerance for APPC, CPIC or 5250 invoking programs. Configure redundant Local/Remote LU Aliases across SNA Servers in the SNA Server Administrator or Manager. For example:

Within Servers and Connections window in SNA Administrator or within
      SNA Server Manager:

      SERVER1
             Local APPC LU alias = SNASERV
             Local APPC LU network name = APPN, LU Name = SERVER1

         TOKEN1  connection
             Remote APPC LU alias = AS400
             Remote APPC LU network name = APPN  LU Name = AS400
             (partnered with SNASERV using the QPCSUPP mode)

Note SNA Server 3.0 introduces the use of dynamic APPC partnering. Therefore, an administrator configures remote LUs, but does not need to partner them with local LUs. SNA Server 3.0 will automatically partner the LUs when needed. SNA Server 3.0 also supports the dynamic creation of Remote APPC LUs, if the checkbox for "Dynamic Remote APPC LU definition" is checked on the connection dialog. If an SNA Server receives a client allocate request for a Remote LU not defined on the server, and dynamic Remote LU definition is enabled, this causes SNA Server to automatically create the Remote APPC LU requested by the application. If dynamic creation is selected for connections owned by one server and not others in the same subdomain, this may tend to cause client sessions to use this server rather than others.

SERVER2
             Local APPC LU alias = SNASERV
             Local APPC LU network name = APPN, LU Name = SERVER2

         TOKEN2  connection
             Remote APPC LU alias = AS400
             Remote APPC LU network name = APPN  LU Name = AS400
             (partnered with SNASERV using the QPCSUPP mode)

SERVER1 Local APPC LU (SNASERV): Check the "Member of default outgoing Local APPC LU pool" checkbox
SERVER2 Local APPC LU (SNASERV): Check the "Member of default outgoing Local APPC LU pool" checkbox

Note While the Local APPC LU alias must be the same to support fault tolerance across SNA Servers, the LU Name must be unique so that unique AS/400 Device Names are requested for each user. Otherwise, users will be requesting the same device name, causing their 5250 emulator to hang while waiting for the device to become available.

In the APPC application (such as the 5250 emulator), open a session using the Local and Remote LU aliases supported by the servers. For example, using the 5250 applet, open the Session / Session Configuration dialog box and enter:

Local LU alias = (leave blank) Partner LU alias = AS400

Note

For load balancing to work across SNA Servers, the following conditions must be true:
- The Local APPC LU alias must not be specified by the APPC application in its TP_STARTED request.
- The "default" Local LU alias must be left blank in the user or group record in SNA Server Admin/SNA Server Manager.
- The LocalLU parameter must not be set in the SNA client configuration for the transaction program. The LocalLU parameter is described in the SNA Server APPC and CPIC Programmer's Guide, chapter 2.
If the Local LU alias is defaulted using any one of these options, then the SNA Server APPC interface may not choose an SNA Server with an available LU6.2 session, causing an APPC/CPIC conversation startup request to "hang" until a session becomes available. Relying on the "default outgoing Local APPC LU pool" checkbox on the Local APPC LU is recommended for best LU6.2 load balancing and hot backup.

When APPC LU aliases are defaulted on a user/group record, these LU aliases are used if no LU aliases are provided by the APPC application. Other ways of defaulting APPC LUs are described in Chapter 6 of the "SNA Server 2.1 Admin Guide" or in Part 4 of the SNA Server 3.0 Administration Guide (<snaroot>\system\admingd.hlp).
The Local LU alias can be specified using any method above and load balancing will work normally if the LUWIDSUP parameter is set to NO in the SNA client software, as described in Appendix C of the SNA Server Reference Guide. This parameter causes the SNA Server APPC interface to deactivate support for the logical unit of work identifier (luw_id) for APPC programs. This causes the SNA client APPC interface to always defer its location of an SNA Server until the application issues an [MC_]ALLOCATE or CMALLC request.

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The "Otherservers" feature

The SNA Server client software supports an "otherservers" configuration entry, as described in Appendix C of the SNA Server Reference Guide. This entry allows the client to try connecting to a list of SNA Servers when opening an SNA session, in addition to the SNA Servers running in the subdomain to which the client has a sponsor connection.

The SNA Server client will always retrieve its user/group (3270 and APPC alias) configuration entries from the subdomain which the client has an established sponsor connection. The SNA Server client has no way of retrieving the user/group record configured on servers outside of this subdomain (unless the SNA client is reconfigured to connect to a different sponsor server).

In order to open a session through an "otherserver" (located outside of the client subdomain), these servers must be configured to support the same LU resource names (that is, 3270 pool names and APPC LU alias names) as the primary subdomain.

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Troubleshooting suggestions

If users do not appear to be load balancing across SNA Servers

Using SNA Administrator or SNA Manager, make sure the SNA Servers are running, the connections supporting the pooled LUs (or APPC LUs) are active and the LUs are available. The 3270 LU status should appear as "Available". The APPC LU status is viewable in SNA Server Admin by zooming on the Local LU and choosing "Status". The APPC LU session limit should be non-zero. If it is zero, this indicates an APPC LU session activation problem where SNA Server Data Link Control traces and SNA Server configuration file (COM.CFG) are needed to diagnose the cause. If the active session count is the same as the LU session limit, this means all available sessions are in use on this APPC LU/LU pair.

Note SNA Server 3.0 does not include a way of checking the status of APPC LUs since it does not do CNOS negotiation until a session is requested. SNA Server 2.x performs CNOS negotiation by default when a connection with APPC LUs was activated. The DISPLAY.EXE program, included with SNA Server 3.0, can be used to determine the negotiated session limits on all APPC LU/LU/mode partnerships for any SNA Server in a subdomain.

If the servers are operating normally, check the Windows NT application event log (using Event Viewer) to see if the SNA Server service or connections were recently restarted. If so, only recent users have established sessions to this server.
For 3270, make sure the users opening sessions against the servers have been assigned an instance of an LU pool which spans the servers evenly. Note that once a user has a session against a server, additional client sessions will be opened to the same server first.
Make sure the user has LAN access to open a session against the servers in the subdomain or to the "otherservers". This is especially true if a user is unable to get a session through an "otherserver" which resides outside of the subdomain.
Check the Windows NT system event log for any indication of Server service resource shortages. If so, this could indicate LAN connection problems for SNA clients connecting over a named pipe connection.

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