In #9414 I switched the default behavior regarding offline sessions preloading. An offline session is lazy loaded from a database when requested. The previous behavior where offline sessions were preloaded from the database caused problems with slow server startup in deployments with many offline sessions.

Required changes

To keep Keycloak working with lazy loading of offline sessions I needed to change several things that are described below.

Search offline sessions by broker session ID

We can obtain stream of offline sessions filtered by userId, brokerUserId or brokerSessionId in current code base. However, the PersistentUserSessionEntity has only userId as a property (column) from those three fields. The rest is encoded as JSON in data property (column). That means, we need to figure out how to search PersistentUserSessionEntity by those two fields.

Search by brokerUserId can be accomplished in two steps:

1. Try to obtain a UserModel by searchable attributes UserModel.IDP_ALIAS and UserModel.IDP_USER_ID
2. Search offline user sessions by the obtained UserModel

Search by brokerSessionId is more problematic though. There is no searchable brokerSessionId field/attribute in UserModel. Looking at usages from the current code base, it's used only from the back channel logout. So here is how I changed the behavior. We can first leverage brokerUserId from a logout token to obtain offline user sessions and then use brokerSessionId from the token to filter down obtained offline user sessions which will be then revoked.

Enable preloading in crossDC tests

One of the CrossDC tests in the base testsuite tests whether the offline sessions were correctly preloaded and then distributed to another DC. This test requires the preloading to be enabled. However, I only managed to turn on preloading globally for all CrossDC tests.
The scenario where the offline session preloading is turned off in CrossDC environment is covered by model tests, where I added couple of tests.

Queries for offline sessions by delegated clients from external storage

Queries by clients Id for clients delegated from external storage didn't work because we put "external" as a client Id for PersistentClientSessionEntity. Therefore I needed to add some logic to JpaUserSessionPersisterProvider and introduce new queries that search by externalClientId and clientStorageProvider.

@mposolda @stianst I would be interested in your opinions on this. Feedback from others is also welcomed. Thanks!