René Cordier created JAMES-3586:
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Summary: CL one option for the Cassandra blob store
Key: JAMES-3586
URL: https://issues.apache.org/jira/browse/JAMES-3586
Project: James Server
Issue Type: Improvement
Reporter: René Cordier
Fix For: 3.7.0
h2. Context
Some users are storing all message content in Cassandra and thus stores huge
amount of data.
We would like to reduce the performance costs to read this large amount of data.
Blobs being immutable, we have a guaranty that:
* If we read something the value is up-to date
* If we fail at reading something we have a guaranty the content had not been
replicated yet. A second read with a higher consistency level will read the
data (and consistency piggy backed on consistency levels will heal the data)
Cassandra being very efficient at replicating things (think hinted handoff,
direct asynchronous replication), we can expect that data is correctly
duplicated before reads are attempted.
h2. Decision
Via a configuration option, allow optimizing blob access.
* If enabled, perform a first read at CL one and fallback if needed by
performing a second read at the regular CL
* If disabled, only a read at the regular CL will be attempted
A metric should be implemented to track the CL one hit rate, allowing an
effective review of the effectiveness of this solution.
h2. Consequences
In a multiDC setup with RF=3 DC=2 this implies a factor 4 in IO reduction
across the cluster, lowering a lot the read pressure on the Cassandra BlobStore.
h2. Work to be conducted
Add a configuration option in cassandra.properties:
{code:bash}
# Experimental configuration option. Defaults to false.
# Enabling it resutls in reading strictly immutable (not deleted, not updated)
data at CL ONE. If the data is missing,
# we can be sure that the data had not been replicated yet, a second read is
performed with a higher consistency level.
# This option still offer the same level of consistency (thanks to strict
immutability) but might result in higher resource usage in case of mis-behaving
replication.
# Metrics can be used to mesure the efficiency of this.
optimistic.consistency.level.enabled=false
{code}
Also apply it for messagev3 and attachmentv2 tables to benefit from it.
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