[openstack-dev] [trove][all][tc] A proposal to rearchitect Trove

Zane Bitter zbitter at redhat.com
Tue Jun 20 14:21:24 UTC 2017

On 18/06/17 07:35, Amrith Kumar wrote:
> Trove has evolved rapidly over the past several years, since integration 
> in IceHouse when it only supported single instances of a few databases. 
> Today it supports a dozen databases including clusters and replication.
> The user survey [1] indicates that while there is strong interest in the 
> project, there are few large production deployments that are known of 
> (by the development team).
> Recent changes in the OpenStack community at large (company 
> realignments, acquisitions, layoffs) and the Trove community in 
> particular, coupled with a mounting burden of technical debt have 
> prompted me to make this proposal to re-architect Trove.
> This email summarizes several of the issues that face the project, both 
> structurally and architecturally. This email does not claim to include a 
> detailed specification for what the new Trove would look like, merely 
> the recommendation that the community should come together and develop 
> one so that the project can be sustainable and useful to those who wish 
> to use it in the future.
> Trove, with support for a dozen or so databases today, finds itself in a 
> bind because there are few developers, and a code-base with a 
> significant amount of technical debt.
> Some architectural choices which the team made over the years have 
> consequences which make the project less than ideal for deployers.
> Given that there are no major production deployments of Trove at 
> present, this provides us an opportunity to reset the project, learn 
> from our v1 and come up with a strong v2.
> An important aspect of making this proposal work is that we seek to 
> eliminate the effort (planning, and coding) involved in migrating 
> existing Trove v1 deployments to the proposed Trove v2. Effectively, 
> with work beginning on Trove v2 as proposed here, Trove v1 as released 
> with Pike will be marked as deprecated and users will have to migrate to 
> Trove v2 when it becomes available.

I'm personally fine with not having a migration path (because I'm not 
personally running Trove v1 ;) although Thierry's point about choosing a 
different name is valid and surely something the TC will want to weigh 
in on.

However, I am always concerned about throwing out working code and 
rewriting from scratch. I'd be more comfortable if I saw some value 
being salvaged from the existing Trove project, other than as just an 
extended PoC/learning exercise. Would the API be similar to the current 
Trove one? Can at least some tests be salvaged to rapidly increase 
confidence that the new code works as expected?

> While I would very much like to continue to support the users on Trove 
> v1 through this transition, the simple fact is that absent community 
> participation this will be impossible. Furthermore, given that there are 
> no production deployments of Trove at this time, it seems pointless to 
> build that upgrade path from Trove v1 to Trove v2; it would be the 
> proverbial bridge from nowhere.
> This (previous) statement is, I realize, contentious. There are those 
> who have told me that an upgrade path must be provided, and there are 
> those who have told me of unnamed deployments of Trove that would 
> suffer. To this, all I can say is that if an upgrade path is of value to 
> you, then please commit the development resources to participate in the 
> community to make that possible. But equally, preventing a v2 of Trove 
> or delaying it will only make the v1 that we have today less valuable.
> We have learned a lot from v1, and the hope is that we can address that 
> in v2. Some of the more significant things that I have learned are:
> - We should adopt a versioned front-end API from the very beginning; 
> making the REST API versioned is not a ‘v2 feature’
> - A guest agent running on a tenant instance, with connectivity to a 
> shared management message bus is a security loophole; encrypting 
> traffic, per-tenant-passwords, and any other scheme is merely lipstick 
> on a security hole

Totally agree here, any component of the architecture that is accessed 
directly by multiple tenants needs to be natively multi-tenant. I 
believe this has been one of the barriers to adoption.

> - Reliance on Nova for compute resources is fine, but dependence on Nova 
> VM specific capabilities (like instance rebuild) is not; it makes things 
> like containers or bare-metal second class citizens
> - A fair portion of what Trove does is resource orchestration; don’t 
> reinvent the wheel, there’s Heat for that. Admittedly, Heat wasn’t as 
> far along when Trove got started but that’s not the case today and we 
> have an opportunity to fix that now

+1, obviously ;)

Although I also think Kevin's suggestion is worthy of serious consideration.

> - A similarly significant portion of what Trove does is to implement a 
> state-machine that will perform specific workflows involved in 
> implementing database specific operations. This makes the Trove 
> taskmanager a stateful entity. Some of the operations could take a fair 
> amount of time. This is a serious architectural flaw
> - Tenants should not ever be able to directly interact with the 
> underlying storage and compute used by database instances; that should 
> be the default configuration, not an untested deployment alternative
> - The CI should test all databases that are considered to be ‘supported’ 
> without excessive use of resources in the gate; better code 
> modularization will help determine the tests which can safely be skipped 
> in testing changes
> - Clusters should be first class citizens not an afterthought, single 
> instance databases may be the ‘special case’, not the other way around
> - The project must provide guest images (or at least complete tooling 
> for deployers to build these); while the project can’t distribute 
> operating systems and database software, the current deployment model 
> merely impedes adoption
> - Clusters spanning OpenStack deployments are a real thing that must be 
> supported
> This might sound harsh, that isn’t the intent. Each of these is the 
> consequence of one or more perfectly rational decisions. Some of those 
> decisions have had unintended consequences, and others were made knowing 
> that we would be incurring some technical debt; debt we have not had the 
> time or resources to address. Fixing all these is not impossible, it 
> just takes the dedication of resources by the community.
> I do not have a complete design for what the new Trove would look like. 
> For example, I don’t know how we will interact with other projects (like 
> Heat). Many questions remain to be explored and answered.
> Would it suffice to just use the existing Heat resources and build 
> templates around those, or will it be better to implement custom Trove 
> resources and then orchestrate things based on those resources?

(Context: Amrith and I discussed this already)

The idea here is that there are some things that the Heat 'workflow' 
doesn't handle by itself - for example, quiescing a server prior to 
rebuilding (as opposed to replacing) it. The most obvious way to do that 
(discussed in Amrith's next paragraph) is to drive it from some workflow 
outside of Heat, with a Heat stack update to rebuild the server as one 
of the steps. However, an alternative might be to implement custom Heat 
resources that codify the required workflow.

IMHO this doesn't really improve the problem described above ("This 
makes the Trove taskmanager a stateful entity. Some of the operations 
could take a fair amount of time. This is a serious architectural flaw") 
so much as move it around - Heat persists state at the resource level, 
but isn't really well set up to handle a lot of state within a resource.

> Would Trove implement the workflows required for multi-stage database 
> operations by itself,

One option to look at here is the taskflow library that Josh and others 
wrote. It works well for the case where the workflow can be hard-coded 
in code (which I think may fit this use case). It's already used by 
Cinder, and perhaps other projects.

> or would it rely on some other project (say 
> Mistral) for this? Is Mistral really a workflow service, or just cron on 
> steroids? I don’t know the answer but I would like to find out.

Mistral really is a workflow service. It uses YAML rather than Python to 
define workflows, so it's better than taskflow for the case where the 
workflow needs to be generated at runtime. Obviously it also has the 
advantage of a multi-tenant REST API, so it can provide a plugability 
point for users to customise. It's possible that neither of those 
advantages are relevant in this situation.

One potential advantage of Mistral is that the workflows can be set up 
as part of a Heat template. If all of the workflows were set up like 
that, it would be easy for users to use the generated templates as a 
private database management layer on a cloud that didn't offer it 

The disadvantage, obviously, is that it requires the cloud to offer 
Mistral as-a-Service, which currently doesn't include nearly as many 
clouds as I'd like.

> While we don’t have the answers to these questions, I think this is a 
> conversation that we must have, one that we must decide on, and then as 
> a community commit the resources required to make a Trove v2 which 
> delivers on the mission of the project; “To provide scalable and 
> reliable Cloud Database as a Service provisioning functionality for both 
> relational and non-relational database engines, and to continue to 
> improve its fully-featured and extensible open source framework.”[2]



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