Another question… So, my two current questions are:
* noting that a ZNode is a container, would you suggest the hack of a special ephemeralOwner value or would you add a new field to Stat? * is there a current mechanism in the ZK server code (for the leader in particular) to handle periodic housecleaning tasks? If so, where is that code? -Jordan On April 13, 2015 at 2:48:27 PM, Jordan Zimmerman ([email protected]) wrote: As for noting that a ZNode is a container, would you suggest the hack of a special ephemeralOwner value or would you add a new field to Stat? -Jordan On April 10, 2015 at 6:40:23 PM, Patrick Hunt ([email protected]) wrote: Adding is typically good from a b/w compact perspective. If you use the new feature (at runtime) it generally precludes rollback though. See CreateTxn and CreateTxnV0 A bit of background on convenience vs availability: Originally in ZK's life we explicitly stayed away from such operations at the API level (another example being "rm -r"). We wanted to have high availability, in the sense that a single operation performed a single discreet operation on the service. We didn't want to allow "unbounded" sets of changes that might affect availability - say a single operation that triggered a thousand discreet operations on the service, blocking out clients from doing other work. This seems pretty bounded to me though - at worst deleting the entire parent chain, which in general should be relatively small. Patrick On Thu, Apr 9, 2015 at 12:41 PM, Jordan Zimmerman < [email protected]> wrote: > You don’t even need to look at cversion. If the parent node is a container > and has no children (i.e. the node being deleted is the last child), it > gets deleted. > > The trouble I’m currently having, though, is that I don’t want to modify > the CreateTxn record. I can’t find a place to mark that the node should be > a container. I guess I’ll have to add a new record type. What are the > ramifications of that? > > -JZ > > On April 9, 2015 at 2:24:16 PM, Michi Mutsuzaki ([email protected]) > wrote: > > I see, so the container znode is a znode that gets deleted if it's > empty and it ever had a child (cversion is greater than zero). It > sounds good to me. Let's see what other people say. > > Thanks Jordan! > > On Thu, Apr 9, 2015 at 10:20 AM, Jordan Zimmerman > <[email protected]> wrote: > > This sounds great to me, but it sounds a lot like ZOOKEEPER-723. > > > > The problem with both ZOOKEEPER-723 and ZOOKEEPER-834 is that it > overloads > > the concept of EPHEMERAL. EPHEMERALs are tied to sessions. In the use > cases > > that I see, the parent node is always PERSISTENT - i.e. not tied to a > > session. > > > > I haven't looked at the patch yet, but how do you handle the "first > > child" problem? > > > > My solution applies only when a node is deleted. So, there is no need > for a > > first child check. When a node is deleted, iff it's parent has zero > children > > and is of type CONTAINER, then the parent is deleted and recursively up > the > > tree. > > > > -Jordan > > > > On April 9, 2015 at 12:15:33 PM, Michi Mutsuzaki ([email protected]) > > wrote: > > > > Hi Jordan. > > > > This sounds great to me, but it sounds a lot like ZOOKEEPER-723. > > Different people had different ideas there, but the original > > description was: > > > > "rather than changing the semantics of ephemeral nodes, i propose > > ephemeral parents: znodes that disappear when they have no more > > children. this cleanup would happen automatically when the last child > > is removed. an ephemeral parent is not tied to any particular session, > > so even if the creator goes away, the ephemeral parent will remain as > > long as there are children." > > > > I haven't looked at the patch yet, but how do you handle the "first > > child" problem? Is the container znode created with a first child to > > prevent getting deleted, or does the client rely on multi to create a > > container and its children, or something else? > > > > > > On Thu, Apr 9, 2015 at 8:00 AM, Jordan Zimmerman > > <[email protected]> wrote: > >> BACKGROUND > >> ============ > >> A recurring problem for ZooKeeper users is garbage collection of parent > >> nodes. Many recipes (e.g. locks, leaders, etc.) call for the creation > of a > >> parent node under which participants create sequential nodes. When the > >> participant is done, it deletes its node. In practice, the ZooKeeper > tree > >> begins to fill up with orphaned parent nodes that are no longer needed. > The > >> ZooKeeper APIs don't provide a way to clean these. Over time, ZooKeeper > can > >> become unstable due to the number of these nodes. > >> > >> CURRENT SOLUTIONS > >> =================== > >> Apache Curator has a workaround solution for this by providing the > Reaper > >> class which runs in the background looking for orphaned parent nodes and > >> deleting them. This isn't ideal and it would be better if ZooKeeper > >> supported this directly. > >> > >> PROPOSAL > >> ========= > >> ZOOKEEPER-723 and ZOOKEEPER-834 have been proposed to allow EPHEMERAL > >> nodes to contain child nodes. This is not optimum as EPHEMERALs are > tied to > >> a session and the general use case of parent nodes is for PERSISTENT > nodes. > >> This proposal adds a new node type, CONTAINER. A CONTAINER node is the > same > >> as a PERSISTENT node with the additional property that when its last > child > >> is deleted, it is deleted (and CONTAINER nodes recursively up the tree > are > >> deleted if empty). > >> > >> I have a first pass (untested) straw man proposal open for comment here: > >> > >> https://github.com/apache/zookeeper/pull/28 > >> > >> In order to have minimum impact on existing implementations, a container > >> node is denoted by having an ephemeralOwner id of Long.MIN_VALUE. This > is > >> pretty hackish, but I think it's the most supportable without causing > >> disruption. Also, a container behaves a "little bit" like an EPHEMERAL > node > >> so it isn't totally illogical. Alternatively, a new field could be > added to > >> STAT. > >> > >> I look forward to feedback on this. If people think it's worthwhile I'll > >> open a Jira and work on a Production quality solution. If it's > rejected, I'd > >> appreciate discussion of an alternate as this is a real need in the ZK > user > >> community. > >> > >> -Jordan > >> > >> >
