-Distributed STM with Zookeeper 3.4.x possible?
Martin Kou 2012-05-09, 20:48
It seems to me that with the multi-update operation in Zookeeper 3.4.x, it
should be possible to do a form of distributed STM?
The idea I have is this...
- For every zNode that I'll set or delete, I perform a get first, s.t. I
have its last version number.
- This means when I commit the transaction, I can add the version number
to the set and delete ops. The transaction would fail if any of the nodes
have been changed after the get operation.
- If I assume all other operations I do are also atomic transactions, then
create should also safe:
- Assume my commit fails because of the create, then it's because another
atomic transaction had created the node before me
- Assume my create succeeds, then it's either because nothing happened
during my transaction or other atomic transactions have created and then
deleted the node
So a typical transaction would look like this:
- Get all involved nodes for their version numbers
- Queue up create, set, delete operations into a multi-op array
- Commit the multi-update atomically
Problem is.. I have no way to read the values atomically.
If I simply issue a sequence of get() and get_children() requests - then
it's possible for a atomic commit to happen between my get requests, and
thus it's possible for me to get inconsistent states.
The solution I have in mind is to issue the get() and get_children()
requests with watches - if I don't see a watch notification between the
first get and the final get, then the data I get should correspond to a
single transaction (the transaction may be one that happened after the
first get, however - but that's still correct). If I got a watch
notification during my get requests, then I know my data is inconsistent
and I should try again. This should work but it seems quite clumsy to me.