HP Archive Backup System for OpenVMS Guide to Operations

11.1.1.4 Failover of the DB Server
11.1.1.5 Role of the DB server
Architecture
11.1 The Server Process
Once a MDMS server loses contact to the DB server it starts to search for a new DB server using
its own search list in MDMS$DATABASE_SERVER. The server tries the whole search list three
times. The search for the DB server finally ends with either:
a. the node became the DB server itself
b. the node found another DB server
c. the request failed with MDMS_NODBACC (“no access to database server”)
Once a new DB server has been established, all nodes start to forward requests to this server.
The DB server receives all user requests in an MDMS Domain. It coordinates all activities and
accesses the MDMS database files. The DB server uses a write through cache to access the database.
All database files are RMS index-sequential files and their key layout is defined by “.FDL”
(File Definition Language) files in MDMS$SYSTEM.
Most user requests can be executed entirely on the DB server. In some cases the DB server has to
send remote requests to other servers in the domain. For example remote load volume requests
or remote scheduling requests.
11.1.2 Server Communications
An MDMS server can establish three types of listeners:
• The Mailbox Listener
• The DECnet Listener
• The TCP/IP Listener
The Mailbox Listener is always enabled. The server receives user request through its mailbox
described by logical MDMS$MAILBOX. Each user process has its own mailbox to receive the
response from the server.
The DECnet Listener is enabled during server startup if DECnet is available on this node indicated
by the existence of logical name SYS$NODE or logical SYS$NODE_FULL_NAME.
Once the server had access to the database and DECNET is not defined in its TRANSPORT setting
the server shuts down the DECnet Listener.
The TCPIP Listener is enabled during server startup if TCP/IP is available on this node indicated
by the existence of logical names {UCX|TCPI}$INET_HOST and
{UCX|TCPI}$INET_DOMAIN. Once the server had access to the database and TCPIP is not
defined in its TRANSPORT setting the server shuts down the TCPIP Listener.
Startup and shutdown of the listeners is logged in the MDMS server logfile. Also the “MDMS
SHOW SERVER” display shows the current servers network names at the top and its current
TRANSPORT setting which reflects the active network listeners.
Even though a DB server has received a request via DECnet it could use TCPIP to request a
remote operation (e.g. load volume) at a third node. It all depends on the TRANSPORT setting
of the individual nodes.
Architecture 11–3