Major Description
Why?
CassandraThreadIdGuessingAlgorithm tables occupies a non neglictible amount of space.
Out of a 20 GB database I have in one of my production platform:
Table: threadlookuptable SSTable count: 4 Space used (total): 360 263 739 Table: threadtable SSTable count: 8 Space used (total): 1 050 590 715
Which is non neglictible.
The goal here would be to reduce the space used in database by thread allocation.
Other concerns
Storing subjects as is linked to usernames is likely problematic in terms of privacy.
How ?
Thread Guessing Algorithm do not need raw values to operate but works with hashs as demonstrated by https://issues.apache.org/jira/browse/JAMES-3937.
Impact ?
As threads are partitionned by users risk of collision is extremly low and false posotives might only result in incorrect thread grouping, making this use case none sensitive to hash collisions. Use of non cryptographic hash methods is thus acceptable.
We expect a significant space reduction.
Migration: We will just create a new table and drop the old one. THis will cause a discontinuity in thread allocation: 2 threads instead of one. This seems acceptable and preferable to a complex migration in our eyes.
