Hi Enrico, Thanks for the great suggestions.
On Fri, Jun 16, 2023 at 3:43 AM Enrico Olivelli <[email protected]> wrote: > Li, > > Il giorno ven 16 giu 2023 alle ore 11:43 Li Wang <[email protected]> ha > scritto: > > > > Thanks for the comments and inputs, Ted, Josef and Kezhu. The discussions > > are great. > > > > > The invariant is that the value should be increasing except in > > failure modes. You found a somewhat surprising failure mode. > > Does this mean the value is supposed to be always increasing, never > > decreasing or remaining constant, otherwise it's a failure mode and we > need > > to fix it? > > > > > Please compute how long it would take for a 64-bit counter to overflow > > if incremented a million times per second. (hint, half a million years). > > Haha, it would take way too long (> 0.3M years) for a 64-bit counter to > > overflow, so we don't need to worry about it. > > > > > It would be better to use a longer integer. > > > 1. Increasing to a 64-bit counter certainly solves the problem, but > > this might require conversion of zk data when the current counter is > stored > > as 32-bit > > > I support this, but it demands massive work and probably relates to > > a long term goal[6]. The "int" fact of "cversion" is exposed both > > in API(Stat) and storage(StatPersisted). > > I agree. Using a 64-bit counter is a better way but the scope of the > change > > is big. If we have a consensus on this is the way to go, maybe we can > scope > > it out and break it into sub-tasks so we can have more people contribute > to > > it if there is a need. > > > > > I wrote a test[1] and found that overflow of cversion results > > in NODEEXISTS[2]. I think this is better than what the doc[3] says. > > > There is no wrap-around on the client side. > > NODEEXISTS is thrown because the same prefix is used in the test. If you > > use different prefixes, you will get the wrap-around, as the child path > is > > different even if the sequence is the same. > > > > > I think you could take a look at ZOOKEEPER-4332[4]. ZooKeeper does not > > work well with massive children > > Yes, to work around that, we only keep the most recent x number of > children > > and have a paginated way of listing the children. > > > > > This breaks "monotonically increasing" and gains no"uniqueness". The > > cversion will wrap around again given your cases. > > It breaks "monotonically increasing" only at the point of overflow, it is > > "monotonically increasing" before overflow and after overflow. > > It gains more ""uniqueness" as the range of "uniqueness" is increased > from > > [0, 2147483647] to [-2147483648, 2147483647]. > > This is not ideal and I agree using a 64 counter would be better if we > can. > > I may be fine with a proposal that allows you to extend the range of > values, but I am not sure that allowing it > would make much difference or introduce more problems: we are seeing > that the sequence will no longer be monotonically increasing, so we. > lose a feature > > I am supporting the initiative to move the counter to be 64 bits, as > far as we are able to > ensure 100% disk and wire protocol compatibility with clusters and > applications that do not overflow. > > I am still a little bit worried about your use case. > I may be wrong that the ids generated by ZooKeeper are not meant to be > globally unique outside of the scope of "coordination" of the clients > connected to the cluster. > For instance it is not expected that you use ZooKeeper to generate a > "customer id" or a "session id" (where the session is a session with a > web browser). > > It would be better to store a "counter" and use "compare and set", > but, again, zookeeper is not meant to be used on "hot paths", there > are many systems built over zookeeper > that provide higher level features with high performance (like > BookKeeper, or let me cite HerdDB, a small but powerful project I > worked on some time ago). > > So in the end my suggestions are: > - work together on the 64 bits counter Great, opened a JIRA for it as a start. https://issues.apache.org/jira/browse/ZOOKEEPER-4706 > - rework your application to not use ZK to generate globally unique ids that are not used for coordination > Yes, we are looking into it. I will look more into both once I am back from vacation. Thanks, Li > > Enrico > > > > > Thanks, > > > > Li > > > > On Thu, Jun 15, 2023 at 9:31 PM Kezhu Wang <[email protected]> wrote: > > > > > Hi, > > > > > > > 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"). > > > > > > I wrote a test[1] and found that overflow of cversion results in > > > NODEEXISTS[2]. I think this is better than what the doc[3] says. There > > > is no wrap-around on the client side. > > > > > > > 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. > > > > > > Given that you have overflowed the cversion, I think you could take a > > > look at ZOOKEEPER-4332[4]. ZooKeeper does not work well with massive > > > children when listing. BookKeeper's HierarchicalLedgerManager[5] is a > > > real world example for this. > > > > > > > > > > New > > > > === > > > > if (parentCVersion > currentParentCVersion > > > > *|| parentCVersion == Integer.MIN_VALUE && > > > > currentParentCVersion == Integer.MAX_VALUE) *{ > > > > parent.stat.setCversion(parent > > > > CVersion); > > > > parent.stat.setPzxid(zxid); > > > > } > > > > > > This breaks "monotonically increasing" and gains no "uniqueness". The > > > cversion will wrap around again given your cases. > > > > > > > It would be better to use a longer integer. > > > > 1. Increasing to a 64-bit counter certainly solves the problem, > but > > > this might require conversion of zk data when the current counter is > stored > > > as 32-bit > > > > > > I support this, but it demands massive work and probably relates to a > > > long term goal[6]. The "int" fact of "cversion" is exposed both in > > > API(Stat) and storage(StatPersisted). > > > > > > > > > [1]: > > > > https://github.com/kezhuw/zookeeper/commit/755b1168156c28e4fc2813be593ac67514e8bdc7#diff-1af986ce48b5d4bb4b8e51374a70cc6e109a04c70d9f450be3df8f302010341cR59 > > > [2]: > > > > https://github.com/apache/zookeeper/blob/master/zookeeper-server/src/main/java/org/apache/zookeeper/server/PrepRequestProcessor.java#L675 > > > [3]: > > > > https://zookeeper.apache.org/doc/r3.8.1/zookeeperProgrammers.html#Sequence+Nodes+--+Unique+Naming > > > [4]: https://issues.apache.org/jira/browse/ZOOKEEPER-4332 > > > [5]: > > > > https://bookkeeper.apache.org/docs/getting-started/concepts/#hierarchical-ledger-manager > > > [6]: https://issues.apache.org/jira/browse/ZOOKEEPER-102 > > > > > > Kezhu Wang > > > > > > > > > On Fri, Jun 16, 2023 at 11:33 AM Josef Roehrl > > > <[email protected]> wrote: > > > > > > > > I wanted to add 2 things. > > > > > > > > > > > > 1. Increasing to a 64-bit counter certainly solves the problem, > but > > > this might require conversion of zk data when the current counter is > stored > > > as 32-bit > > > > 2. A client that relies on a unique version that it uses as a > > > reference outside of zk should verify that a version that it receives > does > > > not already exist outside zk. This applies even if 1. is considered, > should > > > a zk quorum be reset or lose its data. > > > > > > > > Josef Roehrl > > > > FuseForward | Senior Architect - Professional Services > > > > [ > > > > https://fuseforward.atlassian.net/wiki/download/attachments/512327681/image001.png?version=1&modificationDate=1537397840684&cacheVersion=1&api=v2 > > > ] > > > > Website< > > > > https://fuseforward.com/?utm_source=Email%20Signature&utm_medium=email%20signature&utm_campaign=email%20signature > > > > > | Newsletter< > > > > https://fuseforward.com/subscribe-to-our-newsletter/?utm_source=Email%20Signature&utm_medium=Email%20Signature&utm_campaign=Email%20Signature > > > > > | Twitter<https://twitter.com/fuseforward> | LinkedIn< > > > https://www.linkedin.com/company/fuseforward/?originalSubdomain=ca> > > > > > > > > ________________________________ > > > > From: Ted Dunning <[email protected]> > > > > Sent: Thursday, June 15, 2023 6:53 PM > > > > To: [email protected] <[email protected]> > > > > Subject: Re: Name of sequence node is not unique > > > > > > > > The invariant is that the value should be increasing except in > failure > > > > modes. You found a somewhat surprising failure mode. > > > > > > > > Please compute how long it would take for a 64-bit counter to > overflow if > > > > incremented a million times per second. (hint, half a million years). > > > > Remember that zk only does things at less than 100,000 per second > > > > > > > > On Thu, Jun 15, 2023, 17:03 Li Wang <[email protected]> wrote: > > > > > > > > > Thanks a lot for your inputs, Ted. > > > > > > > > > > On Thu, Jun 15, 2023 at 2:52 PM Ted Dunning <[email protected] > > > > > 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 <[email protected]> wrote: > > > > > > > > > > > > > Thanks for the response, Enrico. > > > > > > > > > > > > > > Please see comments below. > > > > > > > > > > > > > > On Thu, Jun 15, 2023 at 5:47 AM Enrico Olivelli < > > > [email protected]> > > > > > > > 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 < > > > [email protected]> > > > > > 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 > > > > > > > > > > > > > > > > > > > > > > > > > > > > > >
