[openstack-dev] [TripleO] Is Swift a good choice of database for the TripleO API?

Jiri Tomasek jtomasek at redhat.com
Tue Jan 5 17:09:29 UTC 2016

On 12/23/2015 07:40 PM, Steven Hardy wrote:
> On Wed, Dec 23, 2015 at 11:05:05AM -0600, Ben Nemec wrote:
>> On 12/23/2015 10:26 AM, Steven Hardy wrote:
>>> On Wed, Dec 23, 2015 at 09:28:59AM -0600, Ben Nemec wrote:
>>>> On 12/23/2015 03:19 AM, Dougal Matthews wrote:
>>>>> On 22 December 2015 at 17:59, Ben Nemec <openstack at nemebean.com
>>>>> <mailto:openstack at nemebean.com>> wrote:
>>>>>      Can we just do git like I've been suggesting all along? ;-)
>>>>>      More serious discussion inline. :-)
>>>>>      On 12/22/2015 09:36 AM, Dougal Matthews wrote:
>>>>>      > Hi all,
>>>>>      >
>>>>>      > This topic came up in the 2015-12-15 meeting[1], and again briefly
>>>>>      today.
>>>>>      > After working with the code that came out of the deployment library
>>>>>      > spec[2] I
>>>>>      > had some concerns with how we are storing the templates.
>>>>>      >
>>>>>      > Simply put, when we are dealing with 100+ files from
>>>>>      tripleo-heat-templates
>>>>>      > how can we ensure consistency in Swift without any atomicity or
>>>>>      > transactions.
>>>>>      > I think this is best explained with a couple of examples.
>>>>>      >
>>>>>      >  - When we create a new deployment plan (upload all the templates
>>>>>      to swift)
>>>>>      >    how do we handle the case where there is an error? For example,
>>>>>      if we are
>>>>>      >    uploading 10 files - what do we do if the 5th one fails for
>>>>>      some reason?
>>>>>      >    There is a patch to do a manual rollback[3], but I have
>>>>>      concerns about
>>>>>      >    doing this in Python. If Swift is completely inaccessible for a
>>>>>      short
>>>>>      >    period the rollback wont work either.
>>>>>      >
>>>>>      >  - When deploying to Heat, we need to download all the YAML files from
>>>>>      > Swift.
>>>>>      >    This can take a couple of seconds. What happens if somebody
>>>>>      starts to
>>>>>      >    upload a new version of the plan in the middle? We could end up
>>>>>      trying to
>>>>>      >    deploy half old and half new files. We wouldn't have a
>>>>>      consistent view of
>>>>>      >    the database.
>>>>>      >
>>>>>      > We had a few suggestions in the meeting:
>>>>>      >
>>>>>      >  - Add a locking mechanism. I would be concerned about deadlocks or
>>>>>      > having to
>>>>>      >    lock for the full duration of a deploy.
>>>>>      There should be no need to lock the plan for the entire deploy.  It's
>>>>>      not like we're re-reading the templates at the end of the deploy today.
>>>>>       It's a one-shot read and then the plan could be unlocked, at least as
>>>>>      far as I know.
>>>>> Good point. That would be holding the lock for longer than we need.
>>>>>      The only option where we wouldn't need locking at all is the
>>>>>      read-copy-update model Clint mentions, which might be a valid option as
>>>>>      well.  Whatever we do, there are going to be concurrency issues though.
>>>>>       For example, what happens if two users try to make updates to the plan
>>>>>      at the same time?  If you don't either merge the changes or disallow one
>>>>>      of them completely then one user's changes might be lost.
>>>>>      TBH, this is further convincing me that we should just make this git
>>>>>      backed and let git handle the merging and conflict resolution (never
>>>>>      mind the fact that it gets us a well-understood version control system
>>>>>      for "free").  For updates that don't conflict with other changes, git
>>>>>      can merge them automatically, but for merge conflicts you just return a
>>>>>      rebase error to the user and make them resolve it.  I have a feeling
>>>>>      this is the behavior we'll converge on eventually anyway, and rather
>>>>>      than reimplement git, let's just use the real thing.
>>>>> I'd be curious to hear more how you would go about doing this with git. I've
>>>>> never automated git to this level, so I am concerned about what issues we
>>>>> might hit.
>>>> TBH I haven't thought it through to that extent yet.  I'm mostly
>>>> suggesting it because it seems like a fit for the template storage
>>>> requirements - we know we want version control, we want to be able to
>>>> merge changes from multiple sources, and we want some way to handle
>>>> merge conflicts.  Git does all of this already.
>>>> That said, I'm not sure about everything here.  For example, how would
>>>> you expose merge conflicts to the user?  I don't know that I would want
>>>> to force a user to learn git in order to use TripleO (although that
>>>> would be the devops-y thing to do), but maybe just passing them back the
>>>> files with the merge conflict markers and having them resolve those
>>>> locally and retry the update would work.  I'm not sure how that would
>>>> map to the current version of the API though.  Do we provide any way to
>>>> pass templates back to the user?  I feel like that was kind of a one-way
>>>> street.
>>> What part of the deployment API workflow could result in merge conflicts?
>>> My understanding was that it's something like:
>>> 1. Take copy of reference templates tree
>>> 2. Introspect tempalates, expose required parameters so user can be
>>> prompted for them
>>> 3. Create environment files(s) derived from the user input
>>> 4. Validate the combination of (1) and (3)
>>> 5. Deploy the templates+environments
>>> On update, (1) would be "overwrite existing version of templates"
>> This update policy means you may have just blown away someone else's
>> work, unless you rebase on the plan's templates immediately before
>> updating (and even then there's a race if two people submit updates at
>> the same time).
> What has been proposed to date is somewhat more limited in scope than what
> you're hinting at (which I think is more of a colloborate-on-templates
> requirement?)
> https://github.com/openstack/tripleo-specs/blob/master/specs/mitaka/tripleo-overcloud-deployment-library.rst
> Here, you would expect any template collaboration to happen outside of the
> scope of the actual deployment workflow, so e.g step 1 above consumes
> either a packaged version of tripleo-heat-templates (which we don't expect
> to be routinely modified), or another location on the local filesystem
> (such as a repository managed by e.g git, outside of the deployment
> workflow).
> The "plan" then takes a copy of the golden tree, prompts for additional
> inputs, validates and deploys it.
> You are right though, if we allow concurrent update of the plan, it's
> possible that environments added to two versions of the plan would have to
> be merged, which could mean either conflicts or validation errors (if two
> operators select mutually exclusive configurations for example).
>> Possible example: Two operators are working on enabling separate
>> features in their cloud, and need to make configuration changes to the
>> plan to do so.  Let's say one decides they need to enable the Storage
>> network, while the other decides to enable the Tenant network.  The
>> first operator makes their changes, sends the update and thinks their
>> work is done.  The second operator, working from the same base set of
>> templates as the first, makes their changes and sends the update.  Using
>> the "overwrite" method of conflict resolution the first operator's
>> changes have just been silently destroyed with no indication to either
>> user that anything bad happened.
> Ok, so separating the two requirements alluded to here may help improve
> clarity:
> 1. Multiple users collaborating on the t-h-t tree as a whole.
> 2. Enabling multiple features via updates and avoiding mid-air-collisions
> I think (2) may simpler problem to consider, particularly if a lock
> of some sort is considered acceptable, e.g we explcitly do not allow multiple
> operators actively modifying the cloud concurrently.
> That would also be consistent with the current heat behavior, e.g even if
> you did allow multiple operators to concurrently change a plan, they cannot
> concurrently update the overcloud via heat anyway (this will change
> eventually with convergence).
> (1) is a much harder problem, and I can't help thinking it'd be better
> solved with existing tools (e.g document how to use git, gerrit, jenkins &
> CI test your own t-h-t tree, potentially allowing for semi-automated
> promotion of things between environments, a staging workflow).
>> I guess you could tell users "don't do that", but unless you have
>> exactly one person making updates to the templates there's going to be
>> the possibility of conflicts, and in the Swift case all it takes is two
>> people editing the same file, even in completely different areas, for
>> someone's changes to be lost.
> Ok, good point, I think I'd been assuming more of a serialized workflow as
> a given, so it's definitely something to consider, thanks for clarifying.
> Steve
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To add the information here and maybe (hopefully) clear things a bit,  
the current workflow does not manipulate the templates and environments 
We only set the metadata about certain templates/environments and create 
single temporary environment file:

1. Upload files (using git, it means provide git url) and identify 
capabilities-map file (capabilities_map.yaml) and set it's 'type' 
metadata to 'capabilities-map'
2. based on the capabilities-map information, identify 'root-template' 
(overcloud.yaml), 'root-environment' 
(overcloud-resource-registry-puppet.yaml), 'environment' 
(environments/*.yaml) and store this information in those files 'type' 
3. Let user select from optional environments ('type' is 'environment') 
based on the constraints defined in capabilities-map. Store the 
information about selected environments in 'enabled' meta.
4. Generate a list of parameters by sending templates, root-environment 
and _enabled_ optional environments to heat-validate (nested). Let user 
set values for those parameters and store the parameter values in newly 
created temporary environment's parameter_defaults block. Upload this 
template to Swift and set it's 'type' meta to 'temp-environment'.
5. Deploy - take everything from Swift, process templates (to resolve 
the urls in get_file etc.) and merge environments in order: root 
environment < enabled optional environments < temporary environment. And 
send this to Heat API's Stack Create.

So you can see, that we don't really manipulate the template files, we 
just add a metadata and create single temporary environment that holds 
the parameter values, although this is not really necessary and can be 
replaced by storing the parameter values in DB and then send this as 
'parameters' param to Heat.  I think that storing files in Git is good 
idea as it is what we already have (t-h-t) but we probably need to use 
DB to store the metadata because the metadata are plan-specific, whereas 
the Git repository is not (or is it meant to be? That would mean 
creating separate git repo for every deploymeny attempt.)

To make sure, that Plan is in sync with Git repo (t-h-t) we can create 
the Plan is tied to not just specific repository, but also to a specific 
tag or commit. This way if the user updates the templates repository 
with changes he wants to use, he needs to create a new Plan and start 
over the deployment process.

Correct me if I am wrong, but I think this approach resolves the 
problems with merge conflicts. The Files and Plan (Deployment) are 
separate thing - Files are stored in Git and Plan is stored in DB, holds 
the files metadata and is tied to a Git repo commit/tag.

Any changes that involve the changes in templates themself should be 
done in Git repo and I am not convinced that we want to introduce 
anything like that in GUI/CLI deployment workflow, as as it was agreed 
before, Git is best tool for doing/tracking such changes.


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