The information in this article applies to:
- Microsoft SNA Server for Windows NT, versions 2.0, 2.1, 2.11, 2.11
SP1, and 3.0
SUMMARY
This article describes how 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 doesn’t 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).
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,
etc.), 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.
- 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.
Configuring Fault Tolerance For 3270 Sessions
- Group similar types of 3270 display LU's across servers and connections
into a 3270 LU pool using the LU Pools dialog box in SNA Server
Administrator. Only LU's of a given type can be pooled together.
Secondary connections can be configured as "On demand" to only start
when needed.
NOTE: SNA Server doesn't support pooling of 3270 Printer LU's. Specific
Printer LU's 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) SRV2LU03 (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.
The above example assumes different host LU's are used for hot backup. To
configure SNA Server for 3270 hot backup to the same 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.
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.
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
LU's, 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 #1: For load balancing to work across SNA Servers, the Local APPC
LU alias (1) must not be specified by the APPC application in it's
TP_STARTED request, (2) the "default" Local LU alias must be left
blank in the user or group record in SNA Server Admin/SNA Server Manager
and (3) 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 LU's 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).
NOTE #2: 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
it's location of an SNA Server until the application issues an
[MC_]ALLOCATE or CMALLC request.
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 it's 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 (i.e. 3270 pool names and APPC LU alias names) as the
primary subdomain.
Troubleshooting Suggestions
If users don't 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 LU's (or APPC LU's) are
active and the LU's 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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