Why a virtual db module?
Until now all modules requiring db support had only one (real) db_url
connection and it was bad if that connection went down.
Here is where virtual db module comes in:
it provides to other modules a (virtual) db_url that maps
multiple (real) db_urls to real dbs.
The real connections are handled transparently to the module using the
virtual url.
Now if the current real connection goes down the virtual db_url will
just switch to a new one transparently to the module, giving uninterrupted db
access.
This mode is called failover and can be used for write and read
operations.
The virtual url could just as well use all the real connections at the
same time .
Ex: when an insert operation takes place it will insert in all the real
dbs.
This mode is called parallel and makes sence for write
operations(insert, update, delete).
Yet another mode would be to use the real dbs in a round-robin fashion
maybe for load-balancing. This could be used to spread queries, inserts across
multiple db servers.
see:
http://www.opensips.org/html/docs/modules/devel/db_virtual.html#id226415
What if i don't want the module to use a real db at runtime?
There is MI support to get the state information for all real dbs (per
virtual url).
The module also provides a MI function to set the
state(enabled/disabled) for each real db (per virtual url).
see:
http://www.opensips.org/html/docs/modules/devel/db_virtual.html#id228157
What happens when a connection fails?
If a real connection fails it will be marked globally and closed.
The module that uses it will not spent time trying to reconnect and
will not block waiting for timeout.
A separate timer process will probe the real url. If is succeeds
connecting, the connection will be marked globally and considered for
reconnection by each process that uses it.
see:
http://www.opensips.org/html/docs/modules/devel/db_virtual.html#id227167
So:
The virtual db module provides means to:
create virtual urls
populate them with many real db urls
get state info about real connection
enable or disable their use
reconnect failed connections
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