Provides a default listener when no listener is set

[tdwu]

Provides a default listener when no listener is set. And add parameters to know the exact event source.
Sometimes we need to manage and monitor the database as a server and adjust the database parameters when it happens.

For example, deleteFilesOnDisconnect is automatically set to true on the target node when the cluster is created. This can easily trigger the deletion of data files when the database is closed, resulting in the loss of data files and tables. We can correct the parameters through the listener

[sonatype-lift]

THREAD_SAFETY_VIOLATION: Read/Write race. Non-private method Database.setEventListenerClass(...) reads without synchronization from this.eventListener. Potentially races with write in method Database.removeSession(...).
Reporting because another access to the same memory occurs on a background thread, although this access may not.

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[sonatype-lift]

⚠️ 222 God Classes were detected by Lift in this project. Visit the Lift web console for more details.

[katzyn]

Sorry, I think we cannot accept this pull request due to following reasons:

1. org.h2.api.DatabaseEventListener is a part of API and we cannot change it in incompatible way, at least without a major release (3.0 or something like it).
2. org.h2.engine.Database is not a part of API and applications should never do anything with it directly. It also may not be referenced from org.h2.api package.
3. Presence of event listener changes behaviour of database even if it does nothing in its methods, so if someone will set a default, it can break some applications.

For example, deleteFilesOnDisconnect is automatically set to true on the target node when the cluster is created.

Clustering code in H2 was written a long time ago, and, unfortunately, it is implemented in a simple and naive way with significant design flaws. Here you're trying to work around some problem in it, but it will be far better to improve the implementation itself. Please, describe your problem in a new issue, maybe there are better options for its resolution.

[tdwu]

Thank you very much for your quick feedback. I can understand what you said. It's necessary for the long-term development of the product and should not affect other people who are using it.

I have a point. On the server side, it provides EventListener class that also runs on the server, but cannot be configured by itself.
Instead, it is set when the client connects, which looks kind of strange. For most servers, it is necessary to configure by themself. (As a server, it is important you can handle somethings by yourself)
So, Can I recreate a PR to add a new feature only to set the default listener by the server? Or can you help implement this feature in your plan?

[grandinj]

You could start the database server from code e.g.
https://h2database.com/html/tutorial.html#using_server
and provide settings like the database event listener there.

Or we could extend
Server.createTcpServe
to pass down an instance of a class that implements DatabaseEventListener.
Since the Server class is part of the public API.

Then your clients can just connect to the server using a URL without any parameters.