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From "Aleksey Yeschenko (JIRA)" <>
Subject [jira] [Created] (CASSANDRA-13691) Fix incorrect [2.1 <— 3.0] serialization of counter cells with pre-2.1 local shards
Date Fri, 14 Jul 2017 02:05:00 GMT
Aleksey Yeschenko created CASSANDRA-13691:

             Summary: Fix incorrect [2.1 <— 3.0] serialization of counter cells with
pre-2.1 local shards
                 Key: CASSANDRA-13691
             Project: Cassandra
          Issue Type: Bug
          Components: Coordination
            Reporter: Aleksey Yeschenko
            Assignee: Aleksey Yeschenko
             Fix For: 3.0.x, 3.11.x

We stopped generating local shards in C* 2.1, after CASSANDRA-6504 (Counters 2.0). But it’s
still possible to have counter cell values
around, remaining from 2.0 times, on 2.1, 3.0, 3.11, and even trunk nodes, if they’ve never
been overwritten.

In 2.1, we used two classes for two kinds of counter columns:
{{CounterCell}} class to store counters - internally as collections of {{CounterContext}}
blobs, encoding collections of (host id, count, clock) tuples
{{CounterUpdateCell}} class to represent unapplied increments - essentially a single long
value; this class was never written to commit log, memtables, or sstables, and was only used
inside {{Mutation}} object graph - in memory, and marshalled over network in cases when counter
write coordinator and counter write leader were different nodes
3.0 got rid of {{CounterCell}} and {{CounterUpdateCell}}, among other {{Cell}} classes. In
order to represent these unapplied increments - equivalents of 2.1 {{CounterUpdateCell}} -
in 3.0 we encode them as regular counter columns, with a ‘special’ {{CounterContext}}
value. I.e. a counter context with a single local shard. We do that so that we can reuse local
shard reconcile logic (summing up) to seamlessly support counters with same names collapsing
to single increments in batches. See {{UpdateParameters.addCounter()}} method comments [here|]
for details. It also assumes that nothing else can generate a counter with local shards.

It works fine in pure 3.0 clusters, and in mixed 2.1/3.0 clusters, assuming that there are
no counters with legacy local shards remaining from 2.0 era. It breaks down badly if there

{{LegacyLayout.serializeAsLegacyPartition()}} and consequently {{LegacyCell.isCounterUpdate()}}
- classes responsible for serializing and deserialising in 2.1 format for compatibility -
use the following logic to tell if a cell of {{COUNTER}} kind is a regular final counter or
an unapplied increment:

private boolean isCounterUpdate()
    // See UpdateParameters.addCounter() for more details on this
    return isCounter() && CounterContext.instance().isLocal(value);

{{CounterContext.isLocal()}} method here looks at the first shard of the collection of tuples
and returns true if it’s a local one.

This method would correctly identify a cell generated by {{UpdateParameters.addCounter()}}
as a counter update and serialize it correctly as a 2.1 {{CounterUpdateCell}}. However, it
would also incorrectly flag any regular counter cell that just so happens to have a local
shard as the first tuple of the counter context as a counter update. If a 2.1 node as a coordinator
of a read requests fetches such a value from a 3.0 node, during a rolling upgrade, instead
of the expected {{CounterCell}} object it will receive a {{CounterUpdateCell}}, breaking all
the things. In the best case scenario it will cause an assert in {{AbstractCell.reconcileCounter()}}
to be raised.

To fix the problem we must find an unambiguous way, without false positives or false negatives,
to represent and identify unapplied counter updates on 3.0 side. 

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