document the need for local sessions within the readonly mode

[Hinterwaeldlers]

Starting with commit c47ef90 the readonly mode requires local sessions to be activated.
This commit appends the related information to the docu.

In addition the impossibility to a client downgrade is noted

[eolivelli]

interesting.

we should probably set localSessionsEnabled=true
it is no more a new feature

[eolivelli]

what happens if you do not enable them ?

[Hinterwaeldlers]

what happens if you do not enable them ?

The server is not serving any requests

I consider changing the behaviour within the code. But haven't found a sweet spot to activate the local session handling.

A different solution would be to change the behaviour of the session validation:

zookeeper/zookeeper-server/src/main/java/org/apache/zookeeper/server/quorum/ReadOnlyZooKeeperServer.java

Line 122 in c74658d

protected void validateSession(ServerCnxn cnxn, long sessionId) throws IOException {

In addition I am not sure, if Local-Sessions should be activated in regular mode or if there might be a scenario, where this leads to an unexpected behaviour

[maoling]

@Hinterwaeldlers
Thanks for your contribution.