+1 for pause being explicit RPC pauses, but does it really add complexity to just add a new state (WAITING?) when no heartbeat is sent? Not being able to see that an update was blocked because of a lack of heartbeat seems like a missing feature.
On Wed, Oct 15, 2014 at 5:12 PM, Maxim Khutornenko <ma...@apache.org> wrote: > +1. Updated the doc: > > https://github.com/maxim111333/incubator-aurora/blob/hb_doc/docs/update-heartbeat.md > > On Wed, Oct 15, 2014 at 5:09 PM, Bill Farner <wfar...@apache.org> wrote: > > +1 to the scheduler not proceeding on an update when heartbeats are > absent, > > and requiring the heartbeat service to explicitly call pauseJobUpdate > when > > it detects problems. > > > > -=Bill > > > > On Wed, Oct 15, 2014 at 4:59 PM, Kevin Sweeney > <kswee...@twitter.com.invalid > >> wrote: > > > >> Chatted with Maxim and Bill, I think we figured it out > >> > >> I think the confusion stems from the fact that there are two types of > >> pauses in this system, explicit, persisted pauses generated by the > >> pauseJobUpdate RPC and implicit, volatile pauses caused due to the > absence > >> of a sufficiently fresh heartbeat (such as in the case of a network > >> partition). > >> > >> In case a monitoring service detects a problem it should call the > explicit > >> pauseJobUpdate RPC, which will cause a state change that requires an > >> explicit resumeJobUpdate RPC to resume. That feature already exists. > >> > >> But, we need one more thing to make this reliable - heartbeats to > protect > >> against network partitions between the scheduler and the monitoring > >> service. These can be volatile and lightweight - the scheduler just > checks > >> for a sufficiently fresh heartbeat before it performs an update action, > and > >> if none is present it simply refuses to perform the action. If the > >> partition heals a new heartbeat will arrive (if the update being > monitored > >> should still be allowed to proceed) and the scheduler will allow the > update > >> to proceed. > >> > >> > >> On Wed, Oct 15, 2014 at 11:56 AM, Bill Farner <wfar...@apache.org> > wrote: > >> > >> > I think we should assess that after building the rest of the feature. > >> IIUC > >> > the rest of the code doesn't care if the update is initially paused. > >> > > >> > -=Bill > >> > > >> > On Wed, Oct 15, 2014 at 11:50 AM, Maxim Khutornenko <ma...@apache.org > > > >> > wrote: > >> > > >> > > Can we get a consensus here? Looks like the only sticky point left > is > >> > > around starting an update in paused vs. non-paused state. I can > argue > >> > > either way as it's easy to add later if needed. > >> > > > >> > > On Tue, Oct 14, 2014 at 1:03 PM, Bill Farner <wfar...@apache.org> > >> wrote: > >> > > > I'm not arguing against the merits of the approach. Just feeling > out > >> > > > whether that should be done _after_ the rest of the heartbeat > >> support. > >> > > > Seems like it can be cleanly added at the end to get something > usable > >> > > > earlier. > >> > > > > >> > > > -=Bill > >> > > > > >> > > > On Tue, Oct 14, 2014 at 12:38 PM, Kevin Sweeney < > kevi...@apache.org> > >> > > wrote: > >> > > > > >> > > >> I'm +1 for using lack of heartbeats as a uniform > >> unknown-or-unhealthy > >> > > >> signal, and punting on a more complex NACK signal (which we'd > have > >> to > >> > > >> reliably persist). > >> > > >> > >> > > >> I think the only disagreement in this thread is whether the > default > >> > > state > >> > > >> for a new update should be running or waiting-for-heartbeat. I > think > >> > > >> waiting for a heartbeat is not only a more correct implementation > >> (no > >> > > risk > >> > > >> of acting after a failover but before the heartbeat timeout) but > >> > > simpler to > >> > > >> implement (initialize the PulseMonitor data structure as empty > >> rather > >> > > than > >> > > >> with a synthetic heartbeat). > >> > > >> > >> > > >> From an API consumer perspective the sequence is: > >> > > >> > >> > > >> 1. API client sends a startUpdate RPC to the scheduler > >> > > >> 2. API client receives an OK response, then arranges for > something > >> to > >> > > call > >> > > >> heartbeat with that updateId on some interval > >> > > >> 3. Whatever is supposed to send heartbeats sends one immediately, > >> then > >> > > >> starts sending them on some smaller interval > >> > > >> > >> > > >> Waiting for the first heartbeat ensures that this sequence has > been > >> > > >> completed successfully, while not waiting for it only ensure that > >> > step 1 > >> > > >> has happened. > >> > > >> > >> > > >> > >> > > >> On Tue, Oct 14, 2014 at 12:18 PM, Bill Farner < > wfar...@apache.org> > >> > > wrote: > >> > > >> > >> > > >> > Wait - simpler solution than what? We're talking about not > doing > >> > > either. > >> > > >> > > >> > > >> > -=Bill > >> > > >> > > >> > > >> > On Tue, Oct 14, 2014 at 12:16 PM, Kevin Sweeney < > >> kevi...@apache.org > >> > > > >> > > >> > wrote: > >> > > >> > > >> > > >> > > I think waiting for the first heartbeat before taking any > action > >> > is > >> > > the > >> > > >> > > simpler solution here as it allows the implementation to be > >> > entirely > >> > > >> > > soft-state and still catches the bugs I described. > >> > > >> > > > >> > > >> > > The implementation is just PulseMonitorImpl<UpdateId> - > >> heartbeat > >> > > calls > >> > > >> > > pulse and mutation operations check isAlive. I think the code > >> > might > >> > > >> > > actually work as-is. > >> > > >> > > > >> > > >> > > On Tue, Oct 14, 2014 at 12:11 PM, Maxim Khutornenko < > >> > > ma...@apache.org> > >> > > >> > > wrote: > >> > > >> > > > >> > > >> > > > Pausing update on creation seems like a logical approach > when > >> > > dealing > >> > > >> > > > with inverted dependency model. I.e. updater is happy to > act > >> as > >> > > long > >> > > >> > > > as it's greenlighted by the external signal. It's also > aligned > >> > > with a > >> > > >> > > > failover experience where coordinated updates are > rehydrated > >> in > >> > > >> paused > >> > > >> > > > state waiting for HB awakening. That said, I am OK punting > it > >> > for > >> > > the > >> > > >> > > > sake of simplicity for now. > >> > > >> > > > > >> > > >> > > > Kevin? > >> > > >> > > > > >> > > >> > > > On Tue, Oct 14, 2014 at 12:05 PM, Bill Farner < > >> > wfar...@apache.org > >> > > > > >> > > >> > > wrote: > >> > > >> > > > > If the goal is to reduce complexity now and add features > >> > later, > >> > > why > >> > > >> > not > >> > > >> > > > > nuke both for now - kick off the update right away, and > let > >> > > lack of > >> > > >> > > > > heartbeats serve as a uniform "unknown or unhealthy" > signal? > >> > > >> > > > > > >> > > >> > > > > -=Bill > >> > > >> > > > > > >> > > >> > > > > On Mon, Oct 13, 2014 at 5:25 PM, Maxim Khutornenko < > >> > > >> ma...@apache.org > >> > > >> > > > >> > > >> > > > wrote: > >> > > >> > > > > > >> > > >> > > > >> I am still +1 on the idea to have default paused state > on > >> > > >> creation. > >> > > >> > I > >> > > >> > > > >> think we could still differentiate between initially > paused > >> > and > >> > > >> > timed > >> > > >> > > > >> out states internally by looking at pause reason. It's > >> quite > >> > > >> > different > >> > > >> > > > >> if we want to store explicit NACK reasons from the > external > >> > > >> service > >> > > >> > > > >> though. That would require persistence and a bit more > >> > > complicated > >> > > >> > > > >> logic. > >> > > >> > > > >> > >> > > >> > > > >> On Mon, Oct 13, 2014 at 5:15 PM, Kevin Sweeney < > >> > > >> kevi...@apache.org> > >> > > >> > > > wrote: > >> > > >> > > > >> > I like the idea of implementing this scheduler-side > >> purely > >> > > >> through > >> > > >> > > > >> volatile > >> > > >> > > > >> > state, but the lack of feedback (generic vs specific > >> error > >> > > >> > messages > >> > > >> > > > when > >> > > >> > > > >> an > >> > > >> > > > >> > update is paused) leaves something to be desired. > Maybe > >> we > >> > > can > >> > > >> > > address > >> > > >> > > > >> that > >> > > >> > > > >> > with a metadata field in the initial call to > startUpdate > >> > > (with > >> > > >> an > >> > > >> > > > >> optional > >> > > >> > > > >> > link to a page where one can get more rich information > >> > about > >> > > the > >> > > >> > > > state of > >> > > >> > > > >> > the monitor sending/not sending heartbeats). > >> > > >> > > > >> > > >> > > >> > > > >> > The main drawback is that we may have to wait a > maximum > >> of > >> > > one > >> > > >> > > > heartbeat > >> > > >> > > > >> > interval to find out that an update should be paused. > >> > > >> > > > >> > > >> > > >> > > > >> > On Mon, Oct 13, 2014 at 4:55 PM, Maxim Khutornenko < > >> > > >> > > ma...@apache.org> > >> > > >> > > > >> wrote: > >> > > >> > > > >> > > >> > > >> > > > >> >> The main reason I preferred the lack-of-ACK approach > >> over > >> > an > >> > > >> > > explicit > >> > > >> > > > >> >> NACK one is simplicity. As Joshua pointed out there > is > >> > more > >> > > >> state > >> > > >> > > to > >> > > >> > > > >> >> handle in that case. The lack-of-ACK model can be > >> > completely > >> > > >> > > > >> >> implemented in volatile memory sidestepping the > >> persistent > >> > > >> > storage > >> > > >> > > > >> >> entirely. With the NACK we would need to reliably > >> persist > >> > > >> > external > >> > > >> > > > >> >> service call reasons to survive scheduler failovers. > >> Not a > >> > > huge > >> > > >> > > > >> >> challenge but something to keep in mind. > >> > > >> > > > >> >> > >> > > >> > > > >> >> I still think the simplicity/reliability tradeoff is > >> > > acceptable > >> > > >> > > here > >> > > >> > > > >> >> if we rely on external service to abort heartbeats in > >> case > >> > > of a > >> > > >> > > > health > >> > > >> > > > >> >> alert fired. This can be explicitly documented as an > >> > > external > >> > > >> > > > >> >> integration requirement. However, If the consensus > is to > >> > go > >> > > a > >> > > >> > more > >> > > >> > > > >> >> reliable (though more complicated) NACK route I am > happy > >> > to > >> > > >> > > > reconsider > >> > > >> > > > >> >> the current proposal. > >> > > >> > > > >> >> > >> > > >> > > > >> >> On Mon, Oct 13, 2014 at 3:50 PM, Joshua Cohen < > >> > > >> > > > jco...@twopensource.com> > >> > > >> > > > >> >> wrote: > >> > > >> > > > >> >> > "The heratbeatJobUpdate RPC serves as an ACK, but > we > >> > don't > >> > > >> > have a > >> > > >> > > > >> NACK. > >> > > >> > > > >> >> If > >> > > >> > > > >> >> > we are going to let lack-of-ACK serve as the NACK, > i > >> > don't > >> > > >> > think > >> > > >> > > > it's > >> > > >> > > > >> >> safe > >> > > >> > > > >> >> > to resume when we receive another ACK. In other > >> words, > >> > a > >> > > >> > service > >> > > >> > > > >> >> toggling > >> > > >> > > > >> >> > unhealthy might not be deemed safe to proceed." > >> > > >> > > > >> >> > > >> > > >> > > > >> >> > Lack-of-ACK is the scenario where connectivity > between > >> > the > >> > > >> > > monitor > >> > > >> > > > and > >> > > >> > > > >> >> the > >> > > >> > > > >> >> > scheduler is unavailable. Shouldn't the NACK > scenario > >> > > >> > (everything > >> > > >> > > > is > >> > > >> > > > >> not > >> > > >> > > > >> >> > ok!) be handled by the monitoring service > triggering > >> an > >> > > >> > explicit > >> > > >> > > > >> pause? > >> > > >> > > > >> >> > I.e. section 2 should be updated to say "External > >> > service > >> > > >> > detects > >> > > >> > > > >> service > >> > > >> > > > >> >> > health problems and pauses the update" and section > 4 > >> > > becomes > >> > > >> > the > >> > > >> > > > >> current > >> > > >> > > > >> >> > section 2 (i.e. "Should a heartbeat not be received > >> the > >> > > >> > scheduler > >> > > >> > > > >> pauses > >> > > >> > > > >> >> > the update."). > >> > > >> > > > >> >> > > >> > > >> > > > >> >> > I agree that it's unsafe to to resume updates after > >> > > >> receiving a > >> > > >> > > > >> heartbeat > >> > > >> > > > >> >> > after previously pausing due to a missed > heartbeat. In > >> > > that > >> > > >> > > > scenario > >> > > >> > > > >> I'd > >> > > >> > > > >> >> > think we'd want an explicit resumeJobUpdate. If the > >> > > scenario > >> > > >> > > we're > >> > > >> > > > >> trying > >> > > >> > > > >> >> > to handle is *never* received a heartbeat, that's a > >> > > separate > >> > > >> > > > matter, > >> > > >> > > > >> in > >> > > >> > > > >> >> > that case unpausing upon receiving the first > heartbeat > >> > > would > >> > > >> > make > >> > > >> > > > >> sense, > >> > > >> > > > >> >> > but it feels like that complicates things quite a > bit > >> > > (now we > >> > > >> > > need > >> > > >> > > > to > >> > > >> > > > >> >> > differentiate between heartbeat #1 and hearbeat > #N). > >> > > >> > > > >> >> > > >> > > >> > > > >> >> > On Mon, Oct 13, 2014 at 2:50 PM, Bill Farner < > >> > > >> > wfar...@apache.org > >> > > >> > > > > >> > > >> > > > >> wrote: > >> > > >> > > > >> >> > > >> > > >> > > > >> >> >> What is the guidance for deploying while the > >> heartbeat > >> > > >> service > >> > > >> > > is > >> > > >> > > > >> >> broken? > >> > > >> > > > >> >> >> I think i know the answer, but it's important to > >> spell > >> > > out. > >> > > >> > > > >> >> >> > >> > > >> > > > >> >> >> > >> > > >> > > > >> >> >> > >> > > >> > > > >> >> >> > Create a new coordinated job update in a paused > >> > > >> > > > >> (ROLL_FORWARD_PAUSED) > >> > > >> > > > >> >> >> > state to avoid any progress until the first > >> heartbeat > >> > > call > >> > > >> > > > arrives. > >> > > >> > > > >> >> >> > >> > > >> > > > >> >> >> > >> > > >> > > > >> >> >> I'm not sold on this being ultimately > beneficial. In > >> > the > >> > > >> > worst > >> > > >> > > > case, > >> > > >> > > > >> >> >> impact is still limited by the health check > >> threshold. > >> > > >> Seems > >> > > >> > > like > >> > > >> > > > >> >> >> premature optimization at best, and an odd one if > we > >> > > proceed > >> > > >> > > > without > >> > > >> > > > >> a > >> > > >> > > > >> >> >> 'NACK' signal via the heartbeatJobUpdate RPC. > >> > > >> > > > >> >> >> > >> > > >> > > > >> >> >> Allow resuming of the paused-due-to-no-heartbeat > >> update > >> > > via > >> > > >> a > >> > > >> > > > >> >> >> > resumeJobUpdate call. > >> > > >> > > > >> >> >> > >> > > >> > > > >> >> >> > >> > > >> > > > >> >> >> Are heartbeats required while rolling back? If > so, > >> > that > >> > > >> might > >> > > >> > > > impact > >> > > >> > > > >> >> the > >> > > >> > > > >> >> >> design here and in other places. > >> > > >> > > > >> >> >> > >> > > >> > > > >> >> >> Allow resuming of the paused-due-to-no-heartbeat > >> update > >> > > via > >> > > >> a > >> > > >> > > > fresh > >> > > >> > > > >> >> >> > heartbeatJobUpdate call. > >> > > >> > > > >> >> >> > >> > > >> > > > >> >> >> > >> > > >> > > > >> >> >> The heratbeatJobUpdate RPC serves as an ACK, but > we > >> > don't > >> > > >> > have a > >> > > >> > > > >> NACK. > >> > > >> > > > >> >> If > >> > > >> > > > >> >> >> we are going to let lack-of-ACK serve as the > NACK, i > >> > > don't > >> > > >> > think > >> > > >> > > > it's > >> > > >> > > > >> >> safe > >> > > >> > > > >> >> >> to resume when we receive another ACK. In other > >> > words, a > >> > > >> > > service > >> > > >> > > > >> >> toggling > >> > > >> > > > >> >> >> unhealthy might not be deemed safe to proceed. > >> > > >> > > > >> >> >> > >> > > >> > > > >> >> >> Perhaps just sending OK (or a NOOP equivalent) in > >> case > >> > > of a > >> > > >> > > > >> user-paused > >> > > >> > > > >> >> job > >> > > >> > > > >> >> >> > update would make more sense as there is nothing > >> > > >> monitoring > >> > > >> > > > service > >> > > >> > > > >> >> could > >> > > >> > > > >> >> >> > do in that case. This should work fine with > >> > > pause/resume > >> > > >> > > > >> >> -aware/-agnostic > >> > > >> > > > >> >> >> > monitoring service implementation. > >> > > >> > > > >> >> >> > >> > > >> > > > >> >> >> > >> > > >> > > > >> >> >> This seems reasonable to me - heartbeats for a > paused > >> > > update > >> > > >> > > > should > >> > > >> > > > >> not > >> > > >> > > > >> >> >> pose a risk, but can be safely ignored. > >> > > >> > > > >> >> >> > >> > > >> > > > >> >> >> > >> > > >> > > > >> >> >> > >> > > >> > > > >> >> >> -=Bill > >> > > >> > > > >> >> >> > >> > > >> > > > >> >> >> On Mon, Oct 13, 2014 at 12:48 PM, Maxim > Khutornenko < > >> > > >> > > > >> ma...@apache.org> > >> > > >> > > > >> >> >> wrote: > >> > > >> > > > >> >> >> > >> > > >> > > > >> >> >> > Agreed. That would be a logical generalization > of > >> the > >> > > post > >> > > >> > > > failover > >> > > >> > > > >> >> >> > behavior. > >> > > >> > > > >> >> >> > > >> > > >> > > > >> >> >> > I have updated the above document with the > >> following > >> > > >> > changes: > >> > > >> > > > >> >> >> > - Reply with PAUSED any time a job was paused by > >> > user; > >> > > >> > > > >> >> >> > - Start in paused state by default. > >> > > >> > > > >> >> >> > > >> > > >> > > > >> >> >> > On Mon, Oct 13, 2014 at 11:32 AM, Kevin Sweeney > < > >> > > >> > > > >> kevi...@apache.org> > >> > > >> > > > >> >> >> > wrote: > >> > > >> > > > >> >> >> > > The doc mentioned that the scheduler will > start > >> an > >> > > >> update > >> > > >> > > > >> subject to > >> > > >> > > > >> >> >> the > >> > > >> > > > >> >> >> > > heartbeat countdown, and if it doesn't > receive a > >> > > >> heartbeat > >> > > >> > > it > >> > > >> > > > >> will > >> > > >> > > > >> >> >> pause > >> > > >> > > > >> >> >> > > the update. Why not start with the update > >> > > >> > > > >> >> paused-due-to-no-heartbeat to > >> > > >> > > > >> >> >> > > fail-fast any connectivity issues between the > >> > service > >> > > >> > > > providing > >> > > >> > > > >> the > >> > > >> > > > >> >> >> > > heartbeats and the scheduler? > >> > > >> > > > >> >> >> > > > >> > > >> > > > >> >> >> > > On Fri, Oct 10, 2014 at 12:47 PM, Maxim > >> > Khutornenko < > >> > > >> > > > >> >> ma...@apache.org> > >> > > >> > > > >> >> >> > > wrote: > >> > > >> > > > >> >> >> > > > >> > > >> > > > >> >> >> > >> Hi all, > >> > > >> > > > >> >> >> > >> > >> > > >> > > > >> >> >> > >> We are proposing a new feature for the > scheduler > >> > > >> updater, > >> > > >> > > > which > >> > > >> > > > >> you > >> > > >> > > > >> >> >> > >> may find helpful. > >> > > >> > > > >> >> >> > >> > >> > > >> > > > >> >> >> > >> I have posed a brief feature summary here: > >> > > >> > > > >> >> >> > >> > >> > > >> > > > >> >> >> > >> > >> > > >> > > > >> >> >> > > >> > > >> > > > >> >> >> > >> > > >> > > > >> >> > >> > > >> > > > >> > >> > > >> > > > > >> > > >> > > > >> > > >> > > >> > > >> > >> > > > >> > > >> > https://github.com/maxim111333/incubator-aurora/blob/hb_doc/docs/update-heartbeat.md > >> > > >> > > > >> >> >> > >> > >> > > >> > > > >> >> >> > >> Please, reply with your > >> > feedback/concerns/comments. > >> > > >> > > > >> >> >> > >> > >> > > >> > > > >> >> >> > >> Thanks, > >> > > >> > > > >> >> >> > >> Maxim > >> > > >> > > > >> >> >> > >> > >> > > >> > > > >> >> >> > > >> > > >> > > > >> >> >> > >> > > >> > > > >> >> > >> > > >> > > > >> > >> > > >> > > > > >> > > >> > > > >> > > >> > > >> > > >> > >> > > > >> > > >> > >> > >> > >> -- > >> Kevin Sweeney > >> @kts > >> >
