Forum OpenACS Development: Re: Not handled by server cluster procs?
Actually, Mnesia probably does support the the necessary concurrency, as well as other neat featuers. But I haven't really looked into it, it's all in Erlang, and AOLserver would need to talk to it via either SNMP or CORBA. Seems strange, but it looks like Erlang support client-side but not server-side ODBC. Erlang can use ODBC to talk to some external SQL database (Oracle or whatever), but a Tcl or C client cannot use ODBC to talk to Erang's Mnesia RDBMS. Ick.
Andrew, I have a module already working for this type of application. Essentially is is an abstraction layer which looks much like the ns_db sp_* api, and has many of the same goals of the OpenACS db_* api, although more general, and much simplified. Right now I have it working with odbc and postgresql drivers, these are called providers. I just finished removing all the ns_db calls into callbacks, so you can easily create a new provider with your own callbacks in this case.
The mutex/condition variables are used to protect a pool of connections/handles so you don't need to reconnect for each request. Also, you can reuse a connection for the life of your ns_conn without re-using the mutex/condition. It has a built in explicit transaction start/commit/rollback set of callbacks as well which could be used to issue special commands, such as locking a mutex on the remote server. That might be a little tricky in the case of a misbehaving or crashed client. Also transactions are handled without the need for catch/rollback. You could use these if you want, but when you start a transaction, the initializing routine registers a callback (once per ns_conn) to rollback any outstanding and uncommitted transactions upon connection/scheduled proc close, using ns_atclose. Ns_atclose also returns the token to the pool and signals any waiting threads that a token is now available. All this is created automatically when you create a new datasource and add a list of connection handles. Your callback code would need to translate this handle into an actual connection to use, for instance the callback code for odbc/postgresql is simply: 'return [ns_db gethandle $pool]'.
As far as maintaining the connections, I haven't yet looked at burrow. But I remember setting up something like this when I wrote a jabber module. It would vigorously maintain a connection with the jabber server.
Too bad we don't yet have udp in AOLserver. The changes to ns_sock are almost zero, but the problem is you need configuration changes so that a particular sock knows if it is udp or tcp. But the application doesn't need to know anything. With udp you could efficiently support server push. It might be worth writing a simple udp echo server. Whenever a change is made to an NV, the change is echoed back (to all connected nodes), which record the change. Using udp would probably complicate some things on both ends. How do you know when the server is down? Also you would probably need a timeout on every NV, where it would become stale and need to be rechecked. But this should be perfect for session management, with the session manager sitting on your database server and providing simple answers that might initially be stored in a database, but live in memory so multiple servers can track a single user session.