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
