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Chapter 2 87
Introduction to SNAplus2
SNAplus2 Components
In a domain with multiple SNAplus2 servers, one server holds the
master copy of the SNAplus2 domain configuration file. This server is
known as the master server. You can define other servers on the LAN to
be backup servers. The domain configuration file is copied to backup
servers—either when they are started, or when the master copy is
changed—so that all backup servers hold a copy of the latest
information.
In general, you should define at least one backup server in addition to
the master server. Any remaining servers can be defined as additional
backup servers, or they can be left as peer servers. A peer server obtains
domain configuration information from the master server as required,
but cannot act as a backup server.
If the master server fails, the first backup server on the list of servers
defined for the domain takes over as the master. The domain
configuration file on this server is used as the master copy, and is copied
to other servers as necessary. When the master server is restarted, it
receives a copy of the domain configuration from the backup server
currently acting as master, and then takes over as the master.
If at any time the master server and all backup servers are inactive, a
node on a peer server can still operate, and you can still change the
node's configuration. However, you cannot access the domain
configuration file, and therefore cannot access the configuration of
domain resources (as opposed to node resources). This means that you
cannot start the 3270 emulation program, start the RJE programs, or
allocate CPI-C conversations using symbolic destination names defined
in the configuration file.
NOTE If the LAN is split by a network failure into two noncommunicating
domains, each containing one or more backup servers, SNAplus2 cannot
maintain a consistent configuration of domain resources across the LAN.
In this situation, each domain has an acting master server, each tracking
changes made to the domain configuration file in its own domain but
unaware of any changes made in the other domain. When the LAN
connection is re-established, the domain configuration file from the
original master server becomes the domain configuration file across the
LAN, and any domain resource files on other servers are overwritten. (If
the master is inactive at this point, the domain configuration file from
the highest backup server available in either of the two domains is used.)
Because changes to a domain configuration file are not necessarily