Clustered environments Javers will incorrectly version records.

[duane-staehli]

Issue:
User on node 1 inserts a record (version 1, initial snapshot is inserted).
User on node 2 updates the record (version 2, update is inserted).
User on node 3 updates the record (version 2, update is inserted). Note this is incorrect is should be version 3.

JV_SNAPSHOT table example.

snapshot_pk	type	version
5292412	UPDATE	2
5292604	UPDATE	2
5292701	INITIAL	1

What is should look like:

snapshot_pk	type	version
5292412	UPDATE	3
5292604	UPDATE	2
5292701	INITIAL	1

Problem Code (I think):
https://github.com/javers/javers/blob/master/javers-persistence-sql/src/main/java/org/javers/repository/sql/finders/CdoSnapshotFinder.java#L104

The logic seems to be select the max pk and then find the version number from that.

I think here you are assuming the numbers are always representative of insert order. But there is many cases where this would not be true. In this case since the first node that did the insert was ahead in snapshot_pk, subsequent inserts are always ignored for selection as the latest version.

[bartoszwalacik]

Seems that in clustered environments Javers should not use the Sequence Allocation trick, see https://github.com/javers/javers/blob/master/javers-persistence-sql/src/main/java/org/javers/repository/sql/session/Sequence.java#L12

Without the Sequence Allocation trick, primary keys ordering would be guaranteed by the database sequence.

Sequence nextval call should be inlined in the insert into jv_snapshot statement as it is not used anywhere as a foreign key.

[bartoszwalacik]

fixed in 5.7.1

[duane-staehli]

Hi Bartosz,

Just some general questions about this fix.

It appears that you were multiplying the sequence number by a hundred as some sort of caching mechanism.

e.g.
The current value of our sequence in the DB for JV_SNAPSHOT_PK_SEQ is 52930.
But the values I'm seeing in JV_SNAPSHOT (for one record, this was updated across four nodes):
SNAPSHOT_PK's = 5292702, 5292417, 5292605, 5292416, 5292413, 5292412, 5292604, 5292701

Meaning I need to promote the JV_SNAPSHOT_PK_SEQ to SNAPSHOT_PK + 1?

Are there any other Sequences I need to promote? Your change appears to be to very generic code, so I assume you changed all the sequences to use this behaviour?

While I manage the JV tables/sequences (I've disabled schema management), I didn't see code in the commit that would automatically promote other sequences in the managed cased.
You may also want to put a release note to indicate that people who are managing the schema need to update their sequences if they don't let Javers manage the sequence.

Thanks,

Duane

[bartoszwalacik]

Looks like the bigger problem is in this line in Sequence.java

currentValue = SEQUENCE_ALLOCATION_SIZE * currentSequenceValue ;

So when SEQUENCE_ALLOCATION trick is discontinued, db sequences should be multiplied by 100. I need to check it. For now, version 5.7.1 has to be withdrawn.

[bartoszwalacik]

In the new fix, sequence.nextval is multipilied by 100. Sequences will not have to be incremented manually.