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 >> > > > >> >> >> > >> >> > > > >> >> >> > >> > > > >> >> >> >> > > > >> >> >> > > > >> >> > > > >> > > >> > >>
