Thanks a lot for your inputs, Ted.
On Thu, Jun 15, 2023 at 2:52 PM Ted Dunning <ted.dunn...@gmail.com> wrote:
> Breaking a semantic invariant is a dangerous solution here.
Totally agree. We should not break a semantic invariant if there is one.
What's the semantic invariant here and how ZK is supposed to behave in the
overflow case?
> It would be better to use a longer integer.
>
Yes, I thought about this too. Longer integer will overflow too, so the
issue of not generating unique numbers will still exist.
Best,
Li
>
>
>
> On Thu, Jun 15, 2023, 13:35 Li Wang <li4w...@gmail.com> wrote:
>
> > Thanks for the response, Enrico.
> >
> > Please see comments below.
> >
> > On Thu, Jun 15, 2023 at 5:47 AM Enrico Olivelli <eolive...@gmail.com>
> > wrote:
> >
> > > Li,
> > > thanks for reporting your problem.
> > >
> > > Most likely you have found a bug.
> > >
> > > I have one question, related to your use case,
> > > is the problem that the numbers are not "unique" or that the number is
> > > not monotonically increasing ?
> > >
> >
> > Technically speaking, monotonically increasing means either always
> > increasing or *remaining constant. *With tha*t, * the problem is only
> > the numbers are not "unique'. In this case, the parent cversion
> > remains 2147483647
> > after reaching Integer.MAX_VALUE, not unique any more.
> >
> > >
> > > Do you have 2147483647 concurrent sessions and you found that two
> > > sessions got the same sequenceId ?
> > > or are you storing the sequenceId somewhere and you use it as a
> > > globally unique id, not only among the connected sessions but also
> > > among all the sessions that are ever connected to the cluster ?
> > >
> >
> > In our use case, we create *persistent* *sequential* nodes. We store the
> > sequence id in the client application and use it as a globally unique id.
> > Currently Zookeeper guarantees the following non-overflow case but not
> > after overflow.
> >
> > 1. Monotonically increasing
> > 2. Uniqueness
> > 3. Sequentially increase by 1
> >
> > For customers who have an overflow use case and can handle the sequence
> > number cycling in a circular fashion, how about having a simple patch
> > to support it and handle the overflow case better? The change is adding
> a
> > condition to allow the wraparound when it flows to negative. We can also
> > have a property to control whether to add the additional condition if
> > needed.
> >
> > New
> > ===
> > if (parentCVersion > currentParentCVersion
> > *|| parentCVersion == Integer.MIN_VALUE &&
> > currentParentCVersion == Integer.MAX_VALUE) *{
> > parent.stat.setCversion(parentCVersion);
> > parent.stat.setPzxid(zxid);
> > }
> >
> > Current
> > =====
> > if (parentCVersion > parent.stat.getCversion()) {
> > parent.stat.setCversion(parentCVersion);
> > parent.stat.setPzxid(zxid);
> > }
> >
> > Please let me know what you think. Any input or feedback would be
> > appreciated.
> >
> > Thanks,
> >
> > Li
> >
> >
> > > Enrico
> > >
> > > Il giorno ven 9 giu 2023 alle ore 21:10 Li Wang <li4w...@gmail.com> ha
> > > scritto:
> > > >
> > > > Hello,
> > > >
> > > > We are running 3.7.1 in production and running into an "issue" that
> the
> > > > names of sequence nodes are not unique after the counter hits the max
> > int
> > > > (i.e 2147483647) and overflows. I would like to start a thread to
> > > discuss
> > > > the following
> > > >
> > > > 1. Is this a bug or "expected" behavior?
> > > > 2. Is ZK supposed to support the overflow scenario and need to make
> > sure
> > > > the name is unique when overflow happens?
> > > >
> > > > The name is not unique after hitting the max int value because of we
> > > > have the following in zk code base:
> > > >
> > > > 1. The cversion of parent znode is used to build the child name in
> > > > PrepRequestProcessor
> > > >
> > > > int parentCVersion = parentRecord.stat.getCversion();
> > > > if (createMode.isSequential()) {
> > > > path = path + String.format(Locale.ENGLISH, "%010d",
> > > > parentCVersion);
> > > > }
> > > >
> > > >
> > > >
> > >
> >
> https://github.com/apache/zookeeper/blob/master/zookeeper-server/src/main/
> > > > java/org/apache/zookeeper/server/PrepRequestProcessor.java#L668-L671
> > > >
> > > >
> > > > 2. The parent znode is read from either zks.outstandingChangesForPath
> > map
> > > > or zk database/datatree.
> > > >
> > > > lastChange = zks.outstandingChangesForPath.get(path);
> > > > if (lastChange == null) {
> > > > DataNode n = zks.getZKDatabase().getNode(path);
> > > >
> > > >
> > > >
> > >
> >
> https://github.com/apache/zookeeper/blob/master/zookeeper-server/src/main/java/org/apache/zookeeper/server/PrepRequestProcessor.java#L168-L170
> > > >
> > > >
> > > >
> > > > 3. The cversion of the parent node in outstandingChangesForPath map
> is
> > > > always updated but not in zk database as we added the following code
> > in
> > > 3.6
> > > >
> > > > if (parentCVersion > parent.stat.getCversion()) {
> > > > parent.stat.setCversion(parentCVersion);
> > > > parent.stat.setPzxid(zxid);
> > > > }
> > > >
> > > >
> > >
> >
> https://github.com/apache/zookeeper/blob/master/zookeeper-server/src/main/java/org/apache/zookeeper/server/DataTree.java#L477-L480
> > > >
> > > > https://issues.apache.org/jira/browse/ZOOKEEPER-3249
> > > >
> > > >
> > > > When overflow happens, the new parentCversion is changed to
> > -2147483648.
> > > > It's updated in the outstandingChangesForPath map. It's not updated
> in
> > > > DataTree and the value stays as 2147483647 because -2147483648 is
> less
> > > > than 2147483647, so the cVerson is inconsistent in ZK.
> > > >
> > > > Due to the inconsistent cVersion, when the next request comes in
> after
> > > > overflow, the sequence number is non-deterministic and not unique
> > > depending
> > > > on where the parent node is read from. It can be 2147483647 if the
> > > > parent node is read from DataTree or -2147483648, -2147483647 and so
> > on
> > > if
> > > > it's from the outstandingChangesForPath map.
> > > >
> > > > We have the following doc about unique naming but no info on
> > "expected"
> > > > behavior after overflow.
> > > >
> > > > Sequence Nodes -- Unique Naming
> > > >
> > > >
> > > > When creating a znode you can also request that ZooKeeper append a
> > > > monotonically increasing counter to the end of path. This counter is
> > > unique
> > > > to the parent znode. The counter has a format of %010d -- that is 10
> > > digits
> > > > with 0 (zero) padding (the counter is formatted in this way to
> simplify
> > > > sorting), i.e. "0000000001". See Queue Recipe for an example use of
> > this
> > > > feature. Note: the counter used to store the next sequence number is
> a
> > > > signed int (4bytes) maintained by the parent node, the counter will
> > > > overflow when incremented beyond 2147483647 (resulting in a name
> > > > "-2147483648").
> > > >
> > > >
> > > >
> > > > Please let me know if you have any comments or inputs.
> > > >
> > > >
> > > > Thanks,
> > > >
> > > >
> > > > Li
> > >
> >
>
