+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 >
