Kbase 15937: What does the Progress Watchdog do?
| Autor |
Progress Software Corporation - Progress |
| Acesso |
Público |
| Publicação |
5/10/1998 |
|
What does the Progress Watchdog do?
What does the Progress Watchdog do?
PURPOSE:
------------------------------------------------------------
To explain what the Progress Watchdog does to protect the
database.
WHAT HAPPENS:
------------------------------------------------------------
The Progress Watchdog is a background process for shared
memory systems (such as UNIX, VMS and NT, not DOS/Windows),
its purpose is to protect the database from
corruption and database hang.
The Progress Watchdog runs all the time:
If you do not run the PROWDOG (Progress Watchdog startup script)
as a separate process, the BROKER handles all the Watchdog
functions.
The Watchdog process wakes up every 30 seconds and performs
its function.
Watchdog works in two (2) areas to protect the database and
to resolve conflicts -> Shared Memory and .lk file
Shared Memory
------------------------------------------------------------
In Shared Memory the Watchdog looks for missing processes.
If a system process has disappeared from the system, but
Progress did not get notified (this can happen by a process
being killed with kill -9 or by signal trapping at the system
level):
1) Watchdog senses the missing process (by checking the user
table in shared memory and comparing it to the system
processes) it marks the user table and goes back to sleep
If the user table shows the process missing 3 times (or
60 to 90 seconds) watchdog disconnects the user from the
database.
2) Watchdog backs out all transactions for the missing
process.
3) Watchdog releases all locks held by the missing process.
4) If the process is killed BY A SIGNAL WE CAN'T TRAP (like
kill -9) and has a lock on shared memory, watchdog will
bring down the broker to preserve the db integrity. If
the process is killed by a signal we CAN TRAP (like -1)
then the user is logged out normally not affecting
the status of the db.
dbname.lk file
------------------------------------------------------------
The watchdog uses the 'dbname.lk' file to protect the data-
base from having multiple processes writing to it.
(NOTE: the dbname.lk file does not exsist in VMS. All
locking is handled by the VMS operating system)
Watchdog uses the 3 items in the dbname.lk file which are
1) Single user or Multiuser start up mode of db.
2) Process ID of process that started the database.
3) Hostname where the process is running.
(All processes use the dbname.lk file to determine if the
database is currently in use.)
Watchdog, using the dbname.lk file,
1) checks to see if the BROKER is still alive
(ONLY the Watchdog can do this!).
2) verifies the dbname.lk file exists.
3) verifies the dbname.lk file is valid.
If any of these checks fail, watchdog will protect the
database by initiating an abnormal shutdown.
Benefits to starting the watchdog as a separate process:
1. It frees up the broker to do other things.
2. It can check for the disappearance of the broker
process itself.
rve-8/2/96
Progress Software Technical Support Note # 15937