the simple leader election should work for you: both try to create an ephemeral node: /leader. the one that succeeds is the leader. sets a watch on /leader and will try to create it if it goes away. if the standby succeeds, it will become the leader and will remain so.
(i realize i'm repeating what flavio said :) i though perhaps different words would help :) is there some functionality that this solution is missing? it is one of the most popular ways to do leader election with zookeeper. ben On Sat, Jan 30, 2021 at 1:35 AM Dipti Mulay <[email protected]> wrote: > > Hi Flavio, > > I am using the Zookeepers to do a leader election. The leader election > needs to be done between entities which are actually a combination of > active/stanb-by machines. The standby is a hot standby ready to take over > when the active fails. Now when such a failure occurs I need to ensure that > if the active system was the leader then the standby system remains as > leader when it takes over and becomes active. Now I can have both active > and standby systems create their own ephemeral node. But that will not > guarantee that every time a failure happens on the active system the > standby which is going to become active will assume the role of leader; > that is because other ephemeral nodes might exist already. > > I am hoping that I explained it properly. > > Thanks for bearing with me. > -Parag > > On Fri, 29 Jan 2021 at 20:25, Flavio Junqueira <[email protected]> wrote: > > > If the session expires, then you cannot reconnect to that session and the > > associated ephemerals should be deleted. > > > > I'm not entirely clear why you want to preserve the ephemeral znode. The > > session expiring and the ephemeral being deleted is an indication that the > > active leadership has been released, which brings to me to the question of > > how the standby is checking whether the active is still up. If it is not by > > watching the ephemeral znode, then it must be by other means. > > > > If you need multiple clients to know which client is active, then you > > might consider writing an id to a persistent znode. That alone won't be > > sufficient, though. > > > > -Flavio > > > > > On 29 Jan 2021, at 15:03, Dipti Mulay <[email protected]> wrote: > > > > > > In his mail Ben said "when the session expires, the ephemeral nodes will > > > get deleted. if your standby system reconnects to zookeeper with the > > > clientid, the session will not expire and the ephemeral nodes will stay > > > active." > > > Are you suggesting that I can re-connect on a session that has already > > > expired..? Won't the eph node get deleted when the session expires? > > > > > > On Fri, 29 Jan 2021 at 18:33, Dipti Mulay <[email protected]> wrote: > > > > > >> Hi Ben, Flavio, > > >> > > >> Thanks for your replies. > > >> > > >> I have tried coding it in a way that when standby takes over from the > > >> active system only then it will establish a session with ZK server using > > >> the same client id. But there is a small window of time when both > > systems > > >> have an active session. > > >> What I want to happen is that when standby takes over and active goes > > down > > >> the ephemeral node should remain intact. The reason being that if the > > >> active server was the "leader" then after the standby takes over I would > > >> like the standby to remain as leader. > > >> I have tried it many ways but as soon as the standby takes over and > > socket > > >> from the old active closes the ephemeral node is deleted and triggers > > >> leader election again. > > >> > > >> Hope this helps. > > >> > > >> Thanks > > >> -Parag > > >> > > >> > > >> On Fri, 29 Jan 2021 at 13:26, Flavio Junqueira <[email protected]> wrote: > > >> > > >>> Yeah, what Ben said... What you might want to do is something along the > > >>> lines of: > > >>> > > >>> 1- Have a client create the ephemeral znode, let's call it say > > "/active" > > >>> 2- The other client acting as a standby sets a watch on "/active" > > >>> 3- If the active node crashes, then "/active" is removed and the > > standby > > >>> is notified and can become active by creating the "/active" znode > > >>> > > >>> Would that solve your problem? > > >>> > > >>> -Flavio > > >>> > > >>>> On 29 Jan 2021, at 07:44, Benjamin Reed <[email protected]> wrote: > > >>>> > > >>>> can you explain a bit more what you mean by "The zookeeper-client > > >>>> process on standby which uses the same clientid to initialize a > > >>>> session with zookeeper." ? if i understand correctly, you are going to > > >>>> have two clients using the same session which cannot happen. do you > > >>>> only reuse the clientid when the takeover occurs (the other node stops > > >>>> using the session)? > > >>>> > > >>>> when the session expires, the ephemeral nodes will get deleted. if > > >>>> your standby system reconnects to zookeeper with the clientid, the > > >>>> session will not expire and the ephemeral nodes will stay active. > > >>>> > > >>>> ben > > >>>> > > >>>> On Wed, Jan 27, 2021 at 3:20 AM Dipti Mulay <[email protected]> > > wrote: > > >>>>> > > >>>>> Hi Flavio, > > >>>>> > > >>>>> Thanks for your inputs let me say what I am trying to do: > > >>>>> > > >>>>> I have an active- standby system, each of these system is running on > > >>> its own VM. > > >>>>> > > >>>>> The zookpeeper client process which communicates with ZK is running > > >>> even on the stanby system. The active system creates a session with > > ZK and > > >>> also creates a ephemeral node, I use the zoo_client_id() call to get > > the > > >>> clientId_t information. > > >>>>> The active system then passes on this ClientID_t to standby system( > > >>> this is done by a memory shadow mechanism we have). The > > zookeeper-client > > >>> process on standby which uses the same clientid to initialize a session > > >>> with zookeeper. > > >>>>> > > >>>>> Now after some time I turn of the active VM. On the stanby I see logs > > >>> which tell me a socket error happened and I get a session expired event > > >>> upon which I close the zookeeper session. Does the session expire even > > >>> also mean that my ephemeral is now gone? The standard error handling > > in my > > >>> code is that when a session expired event is received I call > > >>> zookeeper_close() which will surely remove the ephemeral node. > > >>>>> Is there a way to preserve the ephemeral node and re-connect the > > >>> session ? > > >>>>> > > >>>>> Thanks > > >>>>> -Parag > > >>>>> P.S: I hope I have clearly explained the problem > > >>>>> > > >>>>> On 1/26/21, 8:17 PM, "Flavio Junqueira" <[email protected]> wrote: > > >>>>> > > >>>>> Hi Parag, > > >>>>> > > >>>>> Typically, the application knows the path of the znode that it > > >>> wants to check. Knowing the path, you can, for example, check the znode > > >>> metadata, which includes the session id indicating the owner in the > > case of > > >>> an ephemeral znode. > > >>>>> > > >>>>> -Flavio > > >>>>> > > >>>>>> On 26 Jan 2021, at 03:07, Dipti Mulay <[email protected]> wrote: > > >>>>>> > > >>>>>> Hi All, > > >>>>>> > > >>>>>> I had a question regarding ephemeral nodes. Given a particular > > >>> client_id can zookeeper return me the ephemeral node corresponding to > > the > > >>> provided client_id? > > >>>>>> I am using the C Client Library and I could not find a way ? > > >>>>>> > > >>>>>> Any suggestions ? > > >>>>>> > > >>>>>> Thanks > > >>>>>> -Parag > > >>>>> > > >>> > > >>> > > > >
