[tarantool-patches] Re: [PATCH rfc] schema: add possibility to find and throw away dead replicas

[Vladimir Davydov]

On Thu, Sep 27, 2018 at 10:37:57AM +0300, Olga Arkhangelskaia wrote:
> 
> 
> 26/09/2018 17:46, Vladimir Davydov пишет:
> > On Fri, Sep 21, 2018 at 09:25:03PM +0300, Olga Arkhangelskaia wrote:
> > > Adds possibility to get list of alive replicas in a replicaset,
> > > prune from box.space_cluster those who is not considered as alive,
> > > and if one has doubts see state of replicaset.
> > > 
> > > Replica is considered alive if it is just added, its status after
> > > timeout period is not stopped or disconnected. However it it has both
> > > roles (master and replica) we consider such instance dead only if its
> > > upstream and downstream status is stopped or disconnected.
> > > 
> > > If replica is considered dead we can prune its uuid from _cluster space.
> > > If one not sure if the replica is dead or is there is any activity on it
> > > it is possible to list replicas with its role, status and lsn
> > > statistics.
> > > 
> > > If you have some ideas how else we can/should decide whether replica is dead
> > > please share.
> > > 
> > > Closes #3110
> > > ---
> > > 
> > > https://github.com/tarantool/tarantool/issues/3110
> > > https://github.com/tarantool/tarantool/tree/OKriw/gh-3110-prune-dead-replica-from-replicaset-1.10
> > A documentation request with the new API description is missing.
> > Tests don't pass on Travis CI.
> > 
> > Regarding the code:
> > 
> >   1. Why do you add a function that lists *alive* replicas? The issue
> >      author didn't ask for that. He asked for a script that would delete
> >      dead replicas from the _cluster system space. We might want to add a
> >      function that would list *dead* replicas so that he/she could check
> >      what replicas would be deleted (aka "dry run"), but it doesn't make
> >      sense to list alive replicas.
> It is easy to change, but as I understood we need to throw away replica.

How does it contradict what I said?

> > 
> >   2. Dead replica detection is utterly ridiculuous: the functions sleeps
> >      for the given amount of time and then deletes inactive replicas.
> >      As a user, I'd want to have an ability to delete replicas that have
> >      been inactive for, say, a day. Does this mean that I have to wait
> >      for a whole day before this function completes? Obviously, no.
> >      I guess tarantool core should keep track of the time each replica
> >      was active
> 
> So we need changes in core code? About lastt time of activity, what do you
> mean? Lasn change, vclock, status?

Real time when the replica was last active.

> If replica is dead for long perion of time we can see its status. And as I
> undestand we have heartbeat to monitor the connecion, so if there is
> problems with it - we see status.

Disconnected status only means that the replica is not available
right now. We want to delete replicas that haven't been active for
the specified amount of time, say a day or even a week.

BTW, forgot to mention: this function should probably be defined in
box.ctl.

> > last time so that the function would work instantly.
> >      The time should probably be persisted so that restarts wouldn't
> >      affect the way the function works.
>