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Q128244: INFO: SNA Server Load Balancing and Hot Backup |
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Article: Q128244
Product(s): Microsoft SNA Server
Version(s): 2.0,2.1,2.11,2.11 SP1,3.0,4.0
Operating System(s):
Keyword(s): kbnetwork kbsna211sp1
Last Modified: 12-JUN-2001
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The information in this article applies to:
- Microsoft SNA Server, versions 2.0, 2.1, 2.11, 2.11 SP1, 3.0, 4.0
- Microsoft Host Integration Server 2000
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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, 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.
- 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
---------------------------------------------
1. 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[ASCII 146]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
2. 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.
3. 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:
- 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.
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
1. 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.
1. 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 " (without the
quotation marks) 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[ASCII 148]
(<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 (that is, 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.
REFERENCES
==========
For additional information, click the article number below to view the article
in the Microsoft Knowledge Base:
Q266275 Host Integration Server 2000 Client LU6.2 Load Balancing Across
Servers
Additional query words: prodsna backup snafaq
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Keywords : kbnetwork kbsna211sp1
Technology : kbAudDeveloper kbSNAServSearch kbHostIntegServ2000 kbSNAServ300 kbSNAServ200 kbSNAServ211 kbSNAServ400 kbSNAServ210 kbSNAServ211SP1
Version : :2.0,2.1,2.11,2.11 SP1,3.0,4.0
Issue type : kbinfo
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