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From Benoit Chesneau <bchesn...@gmail.com>
Subject distributed counters
Date Wed, 29 Jan 2014 13:48:10 GMT
Hi all,

I wonder if someone already thought to implement distributed counters using
couchdb? Or more exactly counters that follow these requirements:

- works over data replicated between different machine
- beeing updated with the last value even if the machine became offline.
- if a counter is {in/de}cremented on a replication the counter value on
the network should be updated with this increment on the next replication
-

I saw different solutions but I am none of them are really convincing:

1. At the application, have a script taking a doc value and incrementing it
then saving the doc with the new value. It doesn't really work with many
machines. A lot of concurrent updates can happen and it is a good way to
introduce a race condition.

2. Update the counter on one place, only when the machine is online.

3. Take one document maintaining a counter state.

ie we could have a tree maintaining decrement/increments so we could find
the conflicts

Only this doc would be updated and replicated. But we have still a possible
race condition

4. Put all the increments as standalone docs. And associate them to a
counter id. All the documents would be retrieved using a views:

    emit(counterid, val)

And reduce them. I am seeing one problem with that: the value of the
counter is different on each machines.  I am actually not sure it's a
problem.


5. making a combination of 3 and 4.  A instant T a concensus document is
created with will collect the counter value on X machines using this
concensus doc will create an update tree or an LWW-Set or smth like it if
we follow the CRDT. once this doc has been created on each machines choosen
it is replicated.

I am not exactly sure about the details needed for 5 right now...


Anyway any feedback is welcome, let me know.

- benoit

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