[openstack-dev] [Heat] Convergence proof-of-concept showdown

Jastrzebski, Michal michal.jastrzebski at intel.com
Thu Nov 27 08:32:16 UTC 2014

	W dniu 11/27/2014 o 5:15 AM, Angus Salkeld pisze:> On Thu, Nov 27, 2014 at 12:20 PM, Zane Bitter <zbitter at redhat.com 
> <mailto:zbitter at redhat.com>> wrote:
>     A bunch of us have spent the last few weeks working independently on
>     proof of concept designs for the convergence architecture. I think
>     those efforts have now reached a sufficient level of maturity that
>     we should start working together on synthesising them into a plan
>     that everyone can forge ahead with. As a starting point I'm going to
>     summarise my take on the three efforts; hopefully the authors of the
>     other two will weigh in to give us their perspective.
>     Zane's Proposal
>     ===============
>     https://github.com/zaneb/heat-__convergence-prototype/tree/__distributed-graph
>     <https://github.com/zaneb/heat-convergence-prototype/tree/distributed-graph>
>     I implemented this as a simulator of the algorithm rather than using
>     the Heat codebase itself in order to be able to iterate rapidly on
>     the design, and indeed I have changed my mind many, many times in
>     the process of implementing it. Its notable departure from a
>     realistic simulation is that it runs only one operation at a time -
>     essentially giving up the ability to detect race conditions in
>     exchange for a completely deterministic test framework. You just
>     have to imagine where the locks need to be. Incidentally, the test
>     framework is designed so that it can easily be ported to the actual
>     Heat code base as functional tests so that the same scenarios could
>     be used without modification, allowing us to have confidence that
>     the eventual implementation is a faithful replication of the
>     simulation (which can be rapidly experimented on, adjusted and
>     tested when we inevitably run into implementation issues).
>     This is a complete implementation of Phase 1 (i.e. using existing
>     resource plugins), including update-during-update, resource
>     clean-up, replace on update and rollback; with tests.
>     Some of the design goals which were successfully incorporated:
>     - Minimise changes to Heat (it's essentially a distributed version
>     of the existing algorithm), and in particular to the database
>     - Work with the existing plugin API
>     - Limit total DB access for Resource/Stack to O(n) in the number of
>     resources
>     - Limit overall DB access to O(m) in the number of edges
>     - Limit lock contention to only those operations actually contending
>     (i.e. no global locks)
>     - Each worker task deals with only one resource
>     - Only read resource attributes once
>     Open questions:
>     - What do we do when we encounter a resource that is in progress
>     from a previous update while doing a subsequent update? Obviously we
>     don't want to interrupt it, as it will likely be left in an unknown
>     state. Making a replacement is one obvious answer, but in many cases
>     there could be serious down-sides to that. How long should we wait
>     before trying it? What if it's still in progress because the engine
>     processing the resource already died?
> Also, how do we implement resource level timeouts in general?
>     Michał's Proposal
>     =================
>     https://github.com/inc0/heat-__convergence-prototype/tree/__iterative <https://github.com/inc0/heat-convergence-prototype/tree/iterative>
>     Note that a version modified by me to use the same test scenario
>     format (but not the same scenarios) is here:
>     https://github.com/zaneb/heat-__convergence-prototype/tree/__iterative-adapted
>     <https://github.com/zaneb/heat-convergence-prototype/tree/iterative-adapted>
>     This is based on my simulation framework after a fashion, but with
>     everything implemented synchronously and a lot of handwaving about
>     how the actual implementation could be distributed. The central
>     premise is that at each step of the algorithm, the entire graph is
>     examined for tasks that can be performed next, and those are then
>     started. Once all are complete (it's synchronous, remember), the
>     next step is run. Keen observers will be asking how we know when it
>     is time to run the next step in a distributed version of this
>     algorithm, where it will be run and what to do about resources that
>     are in an intermediate state at that time. All of these questions
>     remain unanswered.

Q1. When is time to run next step?
There is no really next step or previous step, there is step which can be run *right now*, which effectively means when current step finishes, because then and only then all requirements are met and we can proceed.

Q2. I can see 3 main processes there:

* Converger (I'd assume thats current engine):
Process which parses graph and schedules next steps, it will be run after change in reality is detected.

* Observer:
Process which keeps reality_db and actual resources aligned. Its mostly for phase2. This one will call GET methods on different APIs

* Worker:
Takes one scheduled task from queue and performs it (handle_create would be such task)

Q3. Resources in IN_PROGRESS doesn't concern us, we just do nothing about them (unless we set up a timeout there, then after a while we might give them ERROR state)
> Yes, I was struggling to figure out how it could manage an IN_PROGRESS 
> state as it's stateless. So you end up treading on the other action's toes.
> Assuming we use the resource's state (IN_PROGRESS) you could get around 
> that. Then you kick off a converge when ever an action completes (if 
> there is nothing new to be
> done then do nothing).

Exactly. We'll need notion of IN_PROGRESS there. So when we detect diff - IN_PROGRESS differs from COMPLETE, action would be "do nothing". And when we detect state change, we simply run convergence again. Since dependencies requires resource id depends on to be COMPLETE, these won't be performed either.

>     A non-exhaustive list of concerns I have:
>     - Replace on update is not implemented yet

Really its just matter of implementing one more if to check_delete_ready - resource is not delete_ready until its replacement is COMPLETE

>     - AFAIK rollback is not implemented yet

Rollback is just an update to one version back.

>     - The simulation doesn't actually implement the proposed architecture
>     - This approach is punishingly heavy on the database - O(n^2) or worse

I admit, during PoC I haven't really care about limiting db queries there. I just didn't really have time to do that. My estimation is that every convergence run will be n*logn or less.

> Yes, re-reading the state of all resources when ever run a new converge 
> is worrying, but I think Michal had some ideas to minimize this.

Yes, I was thinking of having an observer process listening on notifications about resource state update, and saving new state in local database, then kick off convergence. Convergence itself will use local db for its magic.

>     - A lot of phase 2 is mixed in with phase 1 here, making it
>     difficult to evaluate which changes need to be made first and
>     whether this approach works with existing plugins

I can't see why it wouldn't work with existing plugins. You can put synchronous code to async mechanism, you'll just don't have benefits from async.

>     - The code is not really based on how Heat works at the moment, so
>     there would be either a major redesign required or lots of radical
>     changes in Heat or both
>     I think there's a fair chance that given another 3-4 weeks to work
>     on this, all of these issues and others could probably be resolved.
>     The question for me at this point is not so much "if" but "why".

It will allow implementing phase2, as you pointed out. It will make heat mechanisms effectively stateless at nearly every level, which is great for scallability. Policy when we call stack "failed", retry parts of it or rollback whole thing will be placed in "update goal" mechanism, so it will be very easy to tinker with. Core of heat will remain stable as all its really have to do is update (creation is an update from empty to full stack). Also, because it relies on check_*_ready where * is action (like create), we can implement that on resource level (with defaults in base resource class), and then later on implement different logic (validation) for different resource.

>     Michał believes that this approach will make Phase 2 easier to
>     implement, which is a valid reason to consider it. However, I'm not
>     aware of any particular issues that my approach would cause in
>     implementing phase 2 (note that I have barely looked into it at all
>     though). In fact, I very much want Phase 2 to be entirely
>     encapsulated by the Resource class, so that the plugin type (legacy
>     vs. convergence-enabled) is transparent to the rest of the system.
>     Only in this way can we be sure that we'll be able to maintain
>     support for legacy plugins. So a phase 1 that mixes in aspects of
>     phase 2 is actually a bad thing in my view.

It does mix a bit of phase 2 to phase 1, I give you that. But I don't see why its a bad thing to do, as long as we'll be careful to not break anything from phase-1 compatible plugins (which should be pretty easy really...).

>     I really appreciate the effort that has gone into this already, but
>     in the absence of specific problems with building phase 2 on top of
>     another approach that are solved by this one, I'm ready to call this
>     a distraction.
> In it's defence, I like the simplicity of it. The concepts and code are 
> easy to understand - tho' part of this is doesn't implement all the 
> stuff on your list yet.

Sorry :( as Zane have said, give me 3 or 4 weeks and it all will be resolved in a way I described (or better). Not that I want that, I'd really get to actual implementation, we can do PoCs forever.

Also, my code doesn't have issues with concurrent updates;) You just set a new goal, which is atomic function and convergence tries to match to it...no matter what happened before.

>     Anant & Friends' Proposal
>     =========================
>     First off, I have found this very difficult to review properly since
>     the code is not separate from the huge mass of Heat code and nor is
>     the commit history in the form that patch submissions would take
>     (but rather includes backtracking and iteration on the design). As a
>     result, most of the information here has been gleaned from
>     discussions about the code rather than direct review. I have
>     repeatedly suggested that this proof of concept work should be done
>     using the simulator framework instead, unfortunately so far to no avail.
>     The last we heard on the mailing list about this, resource clean-up
>     had not yet been implemented. That was a major concern because that
>     is the more difficult half of the algorithm. Since then there have
>     been a lot more commits, but it's not yet clear whether resource
>     clean-up, update-during-update, replace-on-update and rollback have
>     been implemented, though it is clear that at least some progress has
>     been made on most or all of them. Perhaps someone can give us an update.
> https://github.com/anantpatil/heat-convergence-poc
>     AIUI this code also mixes phase 2 with phase 1, which is a concern.
>     For me the highest priority for phase 1 is to be sure that it works
>     with existing plugins. Not only because we need to continue to
>     support them, but because converting all of our existing
>     'integration-y' unit tests to functional tests that operate in a
>     distributed system is virtually impossible in the time frame we have
>     available. So the existing test code needs to stick around, and the
>     existing stack create/update/delete mechanisms need to remain in
>     place until such time as we have equivalent functional test coverage
>     to begin eliminating existing unit tests. (We'll also, of course,
>     need to have unit tests for the individual elements of the new
>     distributed workflow, functional tests to confirm that the
>     distributed workflow works in principle as a whole - the scenarios
>     from the simulator can help with _part_ of this - and, not least, an
>     algorithm that is as similar as possible to the current one so that
>     our existing tests remain at least somewhat representative and don't
>     require too many major changes themselves.)
>     Speaking of tests, I gathered that this branch included tests, but I
>     don't know to what extent there are automated end-to-end functional
>     tests of the algorithm?
>      From what I can gather, the approach seems broadly similar to the
>     one I eventually settled on also. The major difference appears to be
>     in how we merge two or more streams of execution (i.e. when one
>     resource depends on two or more others). In my approach, the
>     dependencies are stored in the resources and each joining of streams
>     creates a database row to track it, which is easily locked with
>     contention on the lock extending only to those resources which are
>     direct dependencies of the one waiting. In this approach, both the
>     dependencies and the progress through the graph are stored in a
>     database table, necessitating (a) reading of the entire table (as it
>     relates to the current stack) on every resource operation, and (b)
>     locking of the entire table (which is hard) when marking a resource
>     operation complete.
>     I chatted to Anant about this today and he mentioned that they had
>     solved the locking problem by dispatching updates to a queue that is
>     read by a single engine per stack.
>     My approach also has the neat side-effects of pushing the data
>     required to resolve get_resource and get_att (without having to
>     reload the resources again and query them) as well as to update
>     dependencies (e.g. because of a replacement or deletion) along with
>     the flow of triggers. I don't know if anything similar is at work here.
>     It's entirely possible that the best design might combine elements
>     of both approaches.
>     The same open questions I detailed under my proposal also apply to
>     this one, if I understand correctly.
>     I'm certain that I won't have represented everyone's work fairly
>     here, so I encourage folks to dive in and correct any errors about
>     theirs and ask any questions you might have about mine. (In case you
>     have been living under a rock, note that I'll be out of the office
>     for the rest of the week due to Thanksgiving so don't expect
>     immediate replies.)
>     I also think this would be a great time for the wider Heat community
>     to dive in and start asking questions and suggesting ideas. We need
>     to, ahem, converge on a shared understanding of the design so we can
>     all get to work delivering it for Kilo.
> Agree, we need to get moving on this.
> -Angus
>     cheers,
>     Zane.
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