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From "Aleksey Yeschenko (JIRA)" <>
Subject [jira] [Commented] (CASSANDRA-6504) counters++
Date Thu, 16 Jan 2014 15:01:38 GMT


Aleksey Yeschenko commented on CASSANDRA-6504:

with the nits addressed. The caching issue is not an issues since we only serialize the cache
keys themselves, though.

> counters++
> ----------
>                 Key: CASSANDRA-6504
>                 URL:
>             Project: Cassandra
>          Issue Type: Improvement
>            Reporter: Aleksey Yeschenko
>            Assignee: Aleksey Yeschenko
>             Fix For: 2.1
> Continuing CASSANDRA-4775 here.
> We are changing counter write path to explicitly lock-read-modify-unlock-replicate, thus
getting rid of the previously used 'local' (deltas) and 'remote' shards distinction. Unfortunately,
we can't simply start using 'remote' shards exclusively, since shard merge rules prioritize
the 'local' shards. Which is why we are introducing the third shard type - 'global', the only
shard type to be used in 2.1+.
> The updated merge rules are going to look like this:
> global + global = keep the shard with the highest logical clock
> global + local or remote = keep the global one
> local + local = sum counts (and logical clock)
> local + remote = keep the local one
> remote + remote = keep the shard with highest logical clock
> This is required for backward compatibility with pre-2.1 counters. To make 2.0-2.1 live
upgrade possible, 'global' shard merge logic will have to be back ported to 2.0. 2.0 will
not produce them, but will be able to understand the global shards coming from the 2.1 nodes
during the live upgrade. See CASSANDRA-6505.
> Other changes introduced in this issue:
> 1. replicate_on_write is gone. From now on we only avoid replication at RF 1.
> 2. REPLICATE_ON_WRITE stage is gone
> 3. counter mutations are running in their own COUNTER_MUTATION stage now
> 4. counter mutations have a separate counter_write_request_timeout setting
> 5. mergeAndRemoveOldShards() code is gone, for now, until/unless a better solution is
> 6. we only replicate the fresh global shard now, not the complete (potentially quite
large) counter context
> 7. to help with concurrency and reduce lock contention, we cache node's global shards
in a new counter cache ({cf id, partition key, cell name} -> {count, clock}). The cache
is only used by counter writes, to help with 'hot' counters being simultaneously updated.
> Improvements to be handled by separate JIRA issues:
> 1. Split counter context into separate cells - one shard per cell. See CASSANDRA-6506.
This goes into either 2.1 or 3.0.
> Potential improvements still being debated:
> 1. Coalesce the mutations in COUNTER_MUTATION stage if they share the same partition
key, and apply them together, to improve the locking situation when updating different counter
cells in one partition. See CASSANDRA-6508. Will to into 2.1 or 3.0, if deemed beneficial.

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