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

Anant Patil anant.patil at hp.com
Mon Dec 15 15:15:30 UTC 2014

On 13-Dec-14 05:42, Zane Bitter wrote:
> On 12/12/14 05:29, Murugan, Visnusaran wrote:
>>> -----Original Message-----
>>> From: Zane Bitter [mailto:zbitter at redhat.com]
>>> Sent: Friday, December 12, 2014 6:37 AM
>>> To: openstack-dev at lists.openstack.org
>>> Subject: Re: [openstack-dev] [Heat] Convergence proof-of-concept
>>> showdown
>>> On 11/12/14 08:26, Murugan, Visnusaran wrote:
>>>>>> [Murugan, Visnusaran]
>>>>>> In case of rollback where we have to cleanup earlier version of
>>>>>> resources,
>>>>> we could get the order from old template. We'd prefer not to have a
>>>>> graph table.
>>>>> In theory you could get it by keeping old templates around. But that
>>>>> means keeping a lot of templates, and it will be hard to keep track
>>>>> of when you want to delete them. It also means that when starting an
>>>>> update you'll need to load every existing previous version of the
>>>>> template in order to calculate the dependencies. It also leaves the
>>>>> dependencies in an ambiguous state when a resource fails, and
>>>>> although that can be worked around it will be a giant pain to implement.
>>>> Agree that looking to all templates for a delete is not good. But
>>>> baring Complexity, we feel we could achieve it by way of having an
>>>> update and a delete stream for a stack update operation. I will
>>>> elaborate in detail in the etherpad sometime tomorrow :)
>>>>> I agree that I'd prefer not to have a graph table. After trying a
>>>>> couple of different things I decided to store the dependencies in the
>>>>> Resource table, where we can read or write them virtually for free
>>>>> because it turns out that we are always reading or updating the
>>>>> Resource itself at exactly the same time anyway.
>>>> Not sure how this will work in an update scenario when a resource does
>>>> not change and its dependencies do.
>>> We'll always update the requirements, even when the properties don't
>>> change.
>> Can you elaborate a bit on rollback.
> I didn't do anything special to handle rollback. It's possible that we 
> need to - obviously the difference in the UpdateReplace + rollback case 
> is that the replaced resource is now the one we want to keep, and yet 
> the replaced_by/replaces dependency will force the newer (replacement) 
> resource to be checked for deletion first, which is an inversion of the 
> usual order.

This is where the version is so handy! For UpdateReplaced ones, there is
an older version to go back to. This version could just be template ID,
as I mentioned in another e-mail. All resources are at the current
template ID if they are found in the current template, even if they is
no need to update them. Otherwise, they need to be cleaned-up in the
order given in the previous templates.

I think the template ID is used as version as far as I can see in Zane's
PoC. If the resource template key doesn't match the current template
key, the resource is deleted. The version is misnomer here, but that
field (template id) is used as though we had versions of resources.

> However, I tried to think of a scenario where that would cause problems 
> and I couldn't come up with one. Provided we know the actual, real-world 
> dependencies of each resource I don't think the ordering of those two 
> checks matters.
> In fact, I currently can't think of a case where the dependency order 
> between replacement and replaced resources matters at all. It matters in 
> the current Heat implementation because resources are artificially 
> segmented into the current and backup stacks, but with a holistic view 
> of dependencies that may well not be required. I tried taking that line 
> out of the simulator code and all the tests still passed. If anybody can 
> think of a scenario in which it would make a difference, I would be very 
> interested to hear it.
> In any event though, it should be no problem to reverse the direction of 
> that one edge in these particular circumstances if it does turn out to 
> be a problem.
>> We had an approach with depends_on
>> and needed_by columns in ResourceTable. But dropped it when we figured out
>> we had too many DB operations for Update.
> Yeah, I initially ran into this problem too - you have a bunch of nodes 
> that are waiting on the current node, and now you have to go look them 
> all up in the database to see what else they're waiting on in order to 
> tell if they're ready to be triggered.
> It turns out the answer is to distribute the writes but centralise the 
> reads. So at the start of the update, we read all of the Resources, 
> obtain their dependencies and build one central graph[1]. We than make 
> that graph available to each resource (either by passing it as a 
> notification parameter, or storing it somewhere central in the DB that 
> they will all have to read anyway, i.e. the Stack). But when we update a 
> dependency we don't update the central graph, we update the individual 
> Resource so there's no global lock required.
> [1] 
> https://github.com/zaneb/heat-convergence-prototype/blob/distributed-graph/converge/stack.py#L166-L168

A centralized graph and decision making will make the implementation far
more simpler than distributed. This looks academic, but the simplicity
beats everything! When each worker has to decide, there needs to be
lock, only DB transactions are not enough. In contrast, when the
decision making is centralized, that particular critical section can be
attempted with transaction and re-attempted if needed.

With the distributed approach, I see following drawbacks:
1. Every time a resource is done, the peer resources (siblings) are
checked to see if they are done and the parent is propagated. This
happens for each resource.
2. The worker has to run through all the resources to see if the stack
is done, to mark it as completed.
3. The decision to converge is made by each worker resulting in lot of
contention. The centralized graph restricts the contention point to one
place where we can use DB transactions. It is easier to maintain code
where particular decisions are made at a place rather than at many
4. The complex part we are trying to solve is to decide on what to do
next when a resource is done. With centralized graph, this is abstracted
out to the DB API. The API will return the next set of nodes. A smart
SQL query can reduce a lot of logic currently being coded in
5. What would be the starting point for resource clean-up? The clean-up
has to start when all the resources are updated. With no centralized
graph, the DB has to be searched for all the resources with no
dependencies and with older versions (or having older template keys) and
start removing them. With centralized graph, this would be a simpler
with a SQL queries returning what needs to be done. The search space for
where to start with clean-up will be huge.
6. When engine restarts, the search space on where to start will be
huge. With a centralized graph, the abstracted API to get next set of
nodes makes the implementation of decision simpler.

I am convinced enough that it is simpler to assign the responsibility to
engine on what needs to be done next. No locks will be required, not
even resource locks! It is simpler from implementation, understanding
and maintenance perspective.

>>>> Also taking care of deleting resources in order will be an issue.
>>> It works fine.
>>>> This implies that there will be different versions of a resource which
>>>> will even complicate further.
>>> No it doesn't, other than the different versions we already have due to
>>> UpdateReplace.
>>>>>>> This approach reduces DB queries by waiting for completion
>>>>>>> notification
>>>>> on a topic. The drawback I see is that delete stack stream will be
>>>>> huge as it will have the entire graph. We can always dump such data
>>>>> in ResourceLock.data Json and pass a simple flag
>>>>> "load_stream_from_db" to converge RPC call as a workaround for delete
>>> operation.
>>>>>> This seems to be essentially equivalent to my 'SyncPoint'
>>>>>> proposal[1], with
>>>>> the key difference that the data is stored in-memory in a Heat engine
>>>>> rather than the database.
>>>>>> I suspect it's probably a mistake to move it in-memory for similar
>>>>>> reasons to the argument Clint made against synchronising the marking
>>>>>> off
>>>>> of dependencies in-memory. The database can handle that and the
>>>>> problem of making the DB robust against failures of a single machine
>>>>> has already been solved by someone else. If we do it in-memory we are
>>>>> just creating a single point of failure for not much gain. (I guess
>>>>> you could argue it doesn't matter, since if any Heat engine dies
>>>>> during the traversal then we'll have to kick off another one anyway,
>>>>> but it does limit our options if that changes in the
>>>>> future.) [Murugan, Visnusaran] Resource completes, removes itself
>>>>> from resource_lock and notifies engine. Engine will acquire parent
>>>>> lock and initiate parent only if all its children are satisfied (no child entry in
>>> resource_lock).
>>>>> This will come in place of Aggregator.
>>>>> Yep, if you s/resource_lock/SyncPoint/ that's more or less exactly what I
>>> did.
>>>>> The three differences I can see are:
>>>>> 1) I think you are proposing to create all of the sync points at the
>>>>> start of the traversal, rather than on an as-needed basis. This is
>>>>> probably a good idea. I didn't consider it because of the way my
>>>>> prototype evolved, but there's now no reason I can see not to do this.
>>>>> If we could move the data to the Resource table itself then we could
>>>>> even get it for free from an efficiency point of view.
>>>> +1. But we will need engine_id to be stored somewhere for recovery
>>> purpose (easy to be queried format).
>>> Yeah, so I'm starting to think you're right, maybe the/a Lock table is the right
>>> thing to use there. We could probably do it within the resource table using
>>> the same select-for-update to set the engine_id, but I agree that we might
>>> be starting to jam too much into that one table.
>> yeah. Unrelated values in resource table. Upon resource completion we have to
>> unset engine_id as well as compared to dropping a row from resource lock.
>> Both are good. Having engine_id in resource_table will reduce db operaions
>> in half. We should go with just resource table along with engine_id.
> OK
>>>> Sync points are created as-needed. Single resource is enough to restart
>>> that entire stream.
>>>> I think there is a disconnect in our understanding. I will detail it as well in
>>> the etherpad.
>>> OK, that would be good.
>>>>> 2) You're using a single list from which items are removed, rather
>>>>> than two lists (one static, and one to which items are added) that get
>>> compared.
>>>>> Assuming (1) then this is probably a good idea too.
>>>> Yeah. We have a single list per active stream which work by removing
>>>> Complete/satisfied resources from it.
>>> I went to change this and then remembered why I did it this way: the sync
>>> point is also storing data about the resources that are triggering it. Part of this
>>> is the RefID and attributes, and we could replace that by storing that data in
>>> the Resource itself and querying it rather than having it passed in via the
>>> notification. But the other part is the ID/key of those resources, which we
>>> _need_ to know in order to update the requirements in case one of them
>>> has been replaced and thus the graph doesn't reflect it yet. (Or, for that
>>> matter, we need it to know where to go looking for the RefId and/or
>>> attributes if they're in the
>>> DB.) So we have to store some data, we can't just remove items from the
>>> required list (although we could do that as well).
>>>>> 3) You're suggesting to notify the engine unconditionally and let the
>>>>> engine decide if the list is empty. That's probably not a good idea -
>>>>> not only does it require extra reads, it introduces a race condition
>>>>> that you then have to solve (it can be solved, it's just more work).
>>>>> Since the update to remove a child from the list is atomic, it's best
>>>>> to just trigger the engine only if the list is now empty.
>>>> No. Notify only if stream has something to be processed. The newer
>>>> Approach based on db lock will be that the last resource will initiate its
>>> parent.
>>>> This is opposite to what our Aggregator model had suggested.
>>> OK, I think we're on the same page on this one then.
>> Yeah.
>>>>>> It's not clear to me how the 'streams' differ in practical terms
>>>>>> from just passing a serialisation of the Dependencies object, other
>>>>>> than being incomprehensible to me ;). The current Dependencies
>>>>>> implementation
>>>>>> (1) is a very generic implementation of a DAG, (2) works and has
>>>>>> plenty of
>>>>> unit tests, (3) has, with I think one exception, a pretty
>>>>> straightforward API,
>>>>> (4) has a very simple serialisation, returned by the edges() method,
>>>>> which can be passed back into the constructor to recreate it, and (5)
>>>>> has an API that is to some extent relied upon by resources, and so
>>>>> won't likely be removed outright in any event.
>>>>>> Whatever code we need to handle dependencies ought to just build on
>>>>> this existing implementation.
>>>>>> [Murugan, Visnusaran] Our thought was to reduce payload size
>>>>> (template/graph). Just planning for worst case scenario (million
>>>>> resource
>>>>> stack) We could always dump them in ResourceLock.data to be loaded by
>>>>> Worker.
> With the latest updates to the Etherpad, I'm even more confused by 
> streams than I was before.
> One thing I never understood is why do you need to store the whole path 
> to reach each node in the graph? Surely you only need to know the nodes 
> this one is waiting on, the nodes waiting on this one and the ones those 
> are waiting on, not the entire history up to this point. The size of 
> each stream is theoretically up to O(n^2) and you're storing n of them - 
> that's going to get painful in this million-resource stack.
>>>>> If there's a smaller representation of a graph than a list of edges
>>>>> then I don't know what it is. The proposed stream structure certainly
>>>>> isn't it, unless you mean as an alternative to storing the entire
>>>>> graph once for each resource. A better alternative is to store it
>>>>> once centrally - in my current implementation it is passed down
>>>>> through the trigger messages, but since only one traversal can be in
>>>>> progress at a time it could just as easily be stored in the Stack table of the
>>> database at the slight cost of an extra write.
>>>> Agree that edge is the smallest representation of a graph. But it does
>>>> not give us a complete picture without doing a DB lookup. Our
>>>> assumption was to store streams in IN_PROGRESS resource_lock.data
>>>> column. This could be in resource table instead.
>>> That's true, but I think in practice at any point where we need to look at this
>>> we will always have already loaded the Stack from the DB for some other
>>> reason, so we actually can get it for free. (See detailed discussion in my reply
>>> to Anant.)
>> Aren't we planning to stop loading stack with all resource objects in future to
>> Address scalability concerns we currently have?
> We plan on not loading all of the Resource objects each time we load the 
> Stack object, but I think we will always need to have loaded the Stack 
> object (for example, we'll need to check the current traversal ID, 
> amongst other reasons). So if the serialised dependency graph is stored 
> in the Stack it will be no big deal.
>>>>> I'm not opposed to doing that, BTW. In fact, I'm really interested in
>>>>> your input on how that might help make recovery from failure more
>>>>> robust. I know Anant mentioned that not storing enough data to
>>>>> recover when a node dies was his big concern with my current approach.
>>>> With streams, We feel recovery will be easier. All we need is a
>>>> trigger :)
>>>>> I can see that by both creating all the sync points at the start of
>>>>> the traversal and storing the dependency graph in the database
>>>>> instead of letting it flow through the RPC messages, we would be able
>>>>> to resume a traversal where it left off, though I'm not sure what that buys
>>> us.
>>>>> And I guess what you're suggesting is that by having an explicit lock
>>>>> with the engine ID specified, we can detect when a resource is stuck
>>>>> in IN_PROGRESS due to an engine going down? That's actually pretty
>>> interesting.
>>>> Yeah :)
>>>>>> Based on our call on Thursday, I think you're taking the idea of the
>>>>>> Lock
>>>>> table too literally. The point of referring to locks is that we can
>>>>> use the same concepts as the Lock table relies on to do atomic
>>>>> updates on a particular row of the database, and we can use those
>>>>> atomic updates to prevent race conditions when implementing
>>>>> SyncPoints/Aggregators/whatever you want to call them. It's not that
>>>>> we'd actually use the Lock table itself, which implements a mutex and
>>>>> therefore offers only a much slower and more stateful way of doing
>>>>> what we want (lock mutex, change data, unlock mutex).
>>>>>> [Murugan, Visnusaran] Are you suggesting something like a
>>>>>> select-for-
>>>>> update in resource table itself without having  a lock table?
>>>>> Yes, that's exactly what I was suggesting.
>>>> DB is always good for sync. But we need to be careful not to overdo it.
>>> Yeah, I see what you mean now, it's starting to _feel_ like there'd be too
>>> many things mixed together in the Resource table. Are you aware of some
>>> concrete harm that might cause though? What happens if we overdo it? Is
>>> select-for-update on a huge row more expensive than the whole overhead
>>> of manipulating the Lock?
>>> Just trying to figure out if intuition is leading me astray here.
>> You are right. There should be no difference apart from little bump
>> In memory usage. But I think it should be fine.
>>>> Will update etherpad by tomorrow.
>>> OK, thanks.
>>> cheers,
>>> Zane.
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