I have a process that reads raw data and writes this to a database every few seconds.
What is the best way to tell if the database has been written to? I know that Oracle and MS-SQL can use triggers or something to communicate with other services, but I was hoping there would be a technique that would work with more types of SQL databases (SQL lite, MySQL, PostGRES).
It depends on what you wish to do. If it is something external to the database that needs to be kicked off then a simple poll of the database would do the trick, otherwise a db specific trigger is probably best.
Your question is lacking specifics needed for a good answer but I'll give it a try. Triggers are good for targeting tables but if you are interested in system-wide writes then you'll need a better method that is easier to maintain. For system-wide writes I'd investigate methods that detect changes in the transaction log. Unfortunately, each vendor implements this part differently, so one method that works for all vendors is not likely. That is, a method that works within the database server is unlikely. But there may be more elegant ways outside of the server at the OS level. For instance, if the transaction log is a file on disk then a simple script of some sort that detects changes in the file would indicate the DB was written to.
Keep in mind you have asked only to detect a db write. If you need to know what type of write it was then you'll need to get into the transaction log to see what is there. And that will definitely be vendor specific.
If you want to be database independant, polling can work. It's not very efficient or elegant. It also works if you are cursed to using a database that doesn't support triggers. A workaround that we've used in the past is to use a script that is timed (say via cron) to do a select MAX(primary_key_id) from saidTable. I am assuming that your primary key is an a sequential integer and is indexed.
And then compare that to the value you obtained the last time you ran the script. If they match, tell the script to exit or sleep. If not, do your thing.
There are other issues with this approach (ie: backlogs if the script takes too long, or concurrency issues, etc.). And of course performance can become an issue too!