Why is class loading not functioning as expected?

tmugandani

I have a singleton in my Axon Ivy application which I instantiate using the Holder-Pattern (private static final MyClass INSTANCE = new MyClass()).
This singleton creates a connection to an external system. When the external system was down I lost the connection. After the external was fixed I stopped
my application and restarted it again but I didn't get an instance of the Singleton (no connection). I restarted the engine twice and still I couldn't get
an instance/connection. I then redeployed the application and only then did I get a connection to the external system.
How does class loading function in Axon Ivy? Or what could be the possible reasons for the above problems?
Axon Ivy 10.0.32

Louis Müller

Hello @tmugandani

Not sure if I get you correct. Do you load your connection in the constructor of the singleton? Maybe share a simple example for illustrating your concrete problem would help.

I'm testing with 10.0.36. In my singleton example in the screenshot below, the singleton is instantiated the first time callit/start.ivp is run.

activate/deactivate application (in engine cockpit) + process call -> does not lead to new instance, same instance as before is used when running the calllit.start.ivp. That's questionable. I think the singleton should be instantiated here.
restart engine + process call -> leads to new instance
redeploy project + process call -> leads to new instance

tmugandani

Louis Müller
Here a snippet of the singleton:

Louis Müller

where and when do you call "getInstance()"? Engine restart should trigger connectCluster() in case you call getInstance().

tmugandani

Louis Müller
It is called for example by a dashboard widget when a user logs in and also in some interactive dialogs.

Louis Müller

Does my first post help you understand when the singleton is instantiated (e.g., after restarting the engine) and when it isn't? Otherwise, please let me know what you expect.

tmugandani

Louis Müller
Yes I understood your first post. In my case the singleton was not instatiated on engine restart nor after stopping and restarting the application. Only after redployment was it instantiated.

Louis Müller

tmugandani

Singleton is not instantiated after the app is stopped and restarted -> is as expected.

After the server is restarted, the singleton should be instantiated if CouchebaseConnector.getInstance() is called.
Are there any log entries indicating a runtime exception in connectCluster()? To get more insights, I would add Ivy.log().info(...) to getInstance() and to the constructor before connectCluster() is called.

mhoffmann

Not related to this topic itself, but Couchbase has a native JDBC connector. Unless you really need to ensure that only one instance is alive at any given time, it's probably a better solution to use the common JDBC pooling Axon Ivy supports.

Since Couchbase is not available by default, you'll have to create an OSGi plugin for it.

This might be a better approach than writing all the management yourself.