[openstack-dev] [trove][all][tc] A proposal to rearchitect Trove
Fox, Kevin M
Kevin.Fox at pnnl.gov
Mon Jun 19 19:34:02 UTC 2017
Thanks for starting this difficult discussion.
I think I agree with all the lessons learned except the nova one. while you can treat containers and vm's the same, after years of using both, I really don't think its a good idea to treat them equally. Containers can't work properly if used as a vm. (really, really.)
I agree whole heartedly with your statement that its mostly an orchestration problem and should reuse stuff now that there are options.
I would propose the following that I think meets your goals and could widen your contributor base substantially:
Look at the Kubernetes (k8s) concept of Operator -> https://coreos.com/blog/introducing-operators.html
They allow application specific logic to be added to Kubernetes while reusing the rest of k8s to do what its good at. Container Orchestration. etcd is just a clustered database and if the operator concept works for it, it should also work for other databases such as Gallera.
Where I think the containers/vm thing is incompatible is the thing I think will make Trove's life easier. You can think of a member of the database as few different components, such as:
* main database process
* metrics gatherer (such as https://github.com/prometheus/mysqld_exporter)
With the current approach, all are mixed into the same vm image, making it very difficult to update the trove_guest_agent without touching the main database process. (needed when you upgrade the trove controllers). With the k8s sidecar concept, each would be a separate container loaded into the same pod.
So rather then needing to maintain a trove image for every possible combination of db version, trove version, etc, you can reuse upstream database containers along with trove provided guest agents.
There's a secure channel between kube-apiserver and kubelet so you can reuse it for secure communications. No need to add anything for secure communication. trove engine -> kubectl exec xxxxx-db -c guest_agent some command.
There is k8s federation, so if the operator was started at the federation level, it can cross multiple OpenStack regions.
Another big feature I that hasn't been mentioned yet that I think is critical. In our performance tests, databases in VM's have never performed particularly well. Using k8s as a base, bare metal nodes could be pulled in easily, with dedicated disk or ssd's that the pods land on that are very very close to the database. This should give native performance.
So, my suggestion would be to strongly consider basing Trove v2 on Kubernetes. It can provide a huge bang for the buck, simplifying the Trove architecture substantially while gaining the new features your list as being important. The Trove v2 OpenStack api can be exposed as a very thin wrapper over k8s Third Party Resources (TPR) and would make Trove entirely stateless. k8s maintains all state for everything in etcd.
Please consider this architecture.
From: Amrith Kumar [amrith.kumar at gmail.com]
Sent: Sunday, June 18, 2017 4:35 AM
To: OpenStack Development Mailing List (not for usage questions)
Subject: [openstack-dev] [trove][all][tc] A proposal to rearchitect Trove
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  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.
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
- 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
- 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?
Would Trove implement the workflows required for multi-stage database operations by itself, 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.
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.”
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