Last week was the OpenStack Summit, which was held in Sydney, NSW, Australia. This was my first summit since the split with the PTG, and it felt very different than previous summits. In the past there was a split between the business community part of the summit and the Design Summit, which was where the dev teams met to plan the work for the upcoming cycle. With the shift to the PTG, there is no move developer-centric work at the summit, so I was free to attend sessions instead of being buried in the Nova room the whole time. That also meant that I was free to explore the hallway track more than in the past, and as a result I had many interesting conversations with fellow OpenStackers.
There was also only one keynote session on Monday morning. I found this a welcome change, because despite getting some really great information, there are the inevitable vendor keynotes that bore you to tears. Some vendors get it right: they showed the cool scientific research that their OpenStack cloud was enabling, and knowing that I’m helping to make that happen is always a positive feeling. But other vendors just drone about things like the number of cores they are running, and the tools that they use to get things running and keep them running. Now don’t get me wrong: that’s very useful information, but it’s not keynote material. I’d rather see it written up on their website as a reference document.
On Monday after the keynote we had a lively session for the API-SIG, with a lot of SDK developers participating. One issue was that of keeping up with API changes and deprecating older API versions. In many cases, though, the reason people use an SDK is to be insulated from that sort of minutiae; they just want it to work. Sometimes that comes at a price of not having access to the latest features offered by the API. This is where the SDK developer has to determine what would work best for their target users.
Another discussion was how to best use microversions within an SDK. The consensus was to pin each request to the particular microversion that provides the desired functionality, rather than make all requests at the same version. There was a suggestion to have aliases for the latest microversion for each release; e.g., “OpenStack-API-Version: compute pike” would return the latest behaviors that were available for the Nova Pike release. This idea was rejected, as it dilutes the meaning and utility of what a microversion is.
On the Tuesday I helped with the Nova onboarding session, along with Dan Smith and Melanie Witt. We covered things like the layout of code in the Nova repository, and also some of the “magic” that handles the RPC communication among services within Nova. While the people attending seemed to be interested in this, it was hard to gauge the effectiveness for them, as we got precious few questions, and those we did get really didn’t have much to do with what we covered.
That evening the folks from Aptira hired a fairly large party boat, and invited several people to attend. I was fortunate enough to be invited along with my wife, and we had a wonderful evening cruising around Sydney Harbour, with some delicious food and drink provided. I also got to meet and converse with several other IBMers.
There were other sessions I attended, but mostly out of curiosity about the subject. The only other session with anything worth reporting was with the Ironic team and their concerns about the change to scheduling by resource classes and traits. There was still a significant lack of understanding about how this will work for many in the room, which I interpret to mean that we who are creating the Placement service are not communicating this well enough. I was glad that I was able to clarify several things for those who had concerns, and I think that everyone had a better understanding of both how things are supposed to work, as well as what will be required to move their deployments forward.
One development I was especially interested in was the announcement of OpenLab, which will be especially useful for testing SDKs across multiple clouds. Many people attending the API-SIG session thought that they would want to take advantage of that for their SDK work.
My overall impression of the new Summit format is that, as a developer, it leaves a lot to be desired. Perhaps it was because the PTGs have become the place where all the real development planning happens, and so many of the people who I normally would have a chance to interact with simply didn’t come. The big benefit of in-person conferences is getting to know the new people who have joined the project, and re-establishing ties with those with whom you have worked for a while. If you are an OpenStack developer, the PTGs are essential; the Summits, no so much. It will be interesting to see how this new format evolves in the future.
If you’re interested in more in-depth coverage of what went on at the Summit, be sure to read the summary from Superuser.
The location was far away for me, but Sydney was wonderful! We took a few days afterwards to holiday down in Hobart, Tasmania, which made the long journey that much more worth the effort.
Last week was the second-ever OpenStack Project Teams Gathering, or PTG. It’s still an awkward name for a very productive conference.
This time the PTG was held in Denver, Colorado, at a hotel several miles outside of downtown Denver.
It was clear that the organizers from the OpenStack Foundation took the comments from the attendees of the first PTG in Atlanta to heart, as it seemed that none of the annoyances from Atlanta were an issue: there was no loud air conditioning, and the rooms were much less echo-y. The food was also a lot better!
As in Atlanta, Monday and Tuesday were set aside for cross-project sessions, with team sessions on Wednesday–Friday. Most of the first two days was taken up by the API-SIG discussions. There was a lot to talk about, and we managed to cover most of it. One main focus was how to expand our outreach to various groups, now that we have transitioned from a Working Group (WG) to a Special Interest Group (SIG). That may sound like a simple name change, but it represents the shift in direction from being only API developer-focused to reaching out to SDK developers and users.
We discussed several issues that had been identified ahead of time. The first was the format for single resources. The format for multiple resources has not been contentious; it looks like:
In English, a list of the returned resources in a dictionary with the resource type/name as the key. But for a single resource, there are several possibilities:
# Singular resource
# One-element list
# Dictionary keyed by resource name, single value
# Dictionary keyed by resource name, list of one value
None of these stood out as a clear winner, as we could come up with pros and cons for each. When that happens, we make consistency with the rest of OpenStack a priority, so elmiko agreed to survey the code base to get some numbers. If there is a clear preference within OpenStack, we can make that the recommended form.
Next was a very quick discussion of the microversion-parse library, and whether we should recommend it as an “official” tool for projects to use (we did). This would mean that the API-SIG would be undertaking ownership of the library, but as it’s very simple, this was not felt to be a significant burden.
We moved on to the topic of API testing tools. This idea had come up in the past: create a tool that would check how well an API conformed to the guidelines. We agreed once again that that would be a huge effort with very little practical benefit, and that we would not entertain that idea again.
Next up were some people from the Ironic team who had questions about what we would recommend for an API call that was expected to take a long time to complete. Blocking while the call completes could take several minutes, so that was not a good option. The two main options were to use a GET with an “action” as the resource, or POST with the action in the body. Using GET for this doesn’t fit well with RESTful principles, so POST was really the only option, as it is semantically fluid. The response should be a 202 Accepted, and contain the URI that can be called with GET to determine the status of the request. The Ironic team agreed to write up a more detailed description of their use case, which the API-SIG could then use as the base for an example of a guided review discussion.
Another topic that got a lot of discussion was Capabilities. This term is used in many contexts, so we were sure to distinguish among them.
What is this cloud capable of doing?
What actions are possible for this particular resource?
What actions are possible for this particular authenticated user?
We focused on the first type of capability, as it is important for cloud interoperability. There are ways to determine these things, but they might require a dozen API calls to get the information needed. There already is a proposal for creating a static file for clouds, so perhaps this can be expanded to cover all the capabilities that may be of interest to consumers of multiple clouds. This sort of root document would be very static and thus highly cacheable.
For the latter two types of capabilities, it was felt that there was no alternative to making the calls as needed. For example, a user might be able to create an instance of a certain size one minute, but a little later they would not because they’ve exceeded their quota. So for user interfaces such as Horizon, where, say, a button in the UI might be disabled if the user cannot perform that action, there does not seem to be a good way to simplify things.
We spent a good deal of time with a few SDK authors about some of the issues they are having, and how the API-SIG can help. As someone who works on the API creation side of things but who has also created an SDK, these discussions were of particular interest. Since this topic is fairly recent, most of the time was spent getting a feel for the issues that may be of interest. There was some talk of creating SDK guidelines, similar to the API guidelines, but that doesn’t seem like the best way to go. APIs have to be consumed by all sorts of different applications, so consistency is important. SDKs, on the other hand, are consumed by developers for that particular language. The best advice is to make your SDK as idiomatic as possible for the language so that the developers using your SDK will find it as usable as the rest of the language.
After the sessions on Tuesday, there was a pleasant happy hour, with the refreshments sponsored by IBM. It gave everyone a chance to talk to each other, and I had several interesting conversations with people working on different parts of OpenStack.
Starting Wednesday I was in the Nova room for most of the time. The day started off with the Pike retrospective, where we ideally take a look at how things went during the last cycle, and identify the things that we could do better. This should then be used to help make the next cycle go more smoothly. The Nova team can certainly be pretty dysfunctional at times, and in past retrospectives people have tried to address that. But rather than help people understand the effects of their actions better, such comments were typically met by sheer defensiveness, and as a result none of the negative behaviors changed. So this time no one brought up the problems with personal interactions, and we settled on a vague “do shit earlier” motto. What this means is that some people felt that the spec process dragged on for much too long, and that we would be better off if we kept that short and started coding sooner. No process for cutting short the time spent on specs was discussed, though, so it isn’t clear how this will be carried out. The main advantage of coding sooner is that many of these changes will break existing behaviors, and it is better to find that out early in the cycle rather than just before freeze. The downside is that we may start down a particular path early, and due to shortening the spec process, not realize that it isn’t the right (or best) path until we have already written a bunch of code. This will most likely result in a sunk cost fallacy argument in favor of patching the code and taking on more technical debt. Let’s hope that I’m wrong about this.
We moved on to Cells V2. On of the top priorities is listing instances in a multi-cell deployment. One proposed solution was to have Searchlight monitor instance notifications from the cells, and aggregate that information so that the API layer could have access to all cell instance info. That approach was discarded in favor of doing cross-cell DB queries. Another priority was the addition of alternate build candidates being sent to the cell, so that after a request to build an instance is scheduled to a cell, the local cell conductor can retry a failed build without having to go back through the entire scheduling process. I’ve already got some code for doing this, and will be working on it in the coming weeks.
In the afternoon we discussed Placement. One of the problems we uncovered late in the Pike cycle was that the Placement model we created didn’t properly handle migrations, as migrations involve resources from two separate hosts being “in use” at the same time for a single instance. While we got some quick fixes in Pike, we want to implement a better solution early in Queens. The plan is to add a migration UUID, and make that the consumer of the resources on the target provider. This will greatly simplify the accounting necessary to handle resources during migrations.
We moved on to discuss the status of Traits. Traits are the qualitative part of resources, and we have continued to make progress in being able to select resource providers who have particular traits. There is also work being done to have the virt drivers report traits on things such as CPUs.
We moved on to the biggest subject in Placement: nested resource providers. Implementing this will enable us to model resources such as PCI devices that have a number of Physical Functions (PFs), each of which can supply a number of Virtual Functions (VFs). That much is easy enough to understand, but when you start linking particular VCPUs to particular NUMA nodes, it gets messy very quickly. So while we outlined several of these complex relationships during the session, we all agreed that completing all that was not realistic for Queens. We do want to keep those complex cases in mind, though, so that anything we do in Queens won’t have to be un-done in Rocky.
We briefly touched on the question of when we would separate Placement out into its own service. This has been the plan from the beginning, and once again we decided to punt this to a future cycle. That’s too bad, as keeping it as part of Nova is beginning to blur the boundaries of things a bit. But it’s not super-critical, so…
We then moved on to discuss Ironic, and the discussion centered mainly on the changes in how an Ironic node is represented in Placement. To recap, we used to use a hack that pretended that an Ironic node, which must be consumed as a single unit, was a type of VM, so that the existing paradigm of selection based on CPU/RAM/disk would work. So in Ocata we started allowing operators to configure a node’s resource_class attribute; all nodes having the same physical hardware would be the same class, and there would always be an inventory of 1 for each node. Flavors were modified in Pike to accept an Ironic custom resource class or the old VM-ish method of selection, but in Queens, Ironic nodes will only be selected based on this class. This has been a request from operators of large Ironic deployments for some time, and we’re close to realizing this goal. But, of course, not everyone is happy about this. There are some operators who want to be able to select nodes based on “fuzzy” criteria, like they were able to in the “old days”. Their use cases were put forth, but they weren’t considered compelling enough. You can’t just consume 2 GPUs on a 4-GPU node: you must consume them all. There may be ways to accomplish what these operators want using traits, but in order to determine that, they will have to detail their use cases much more completely.
Thursday began with a Nova-Cinder discussion, which I confess I did not pay a lot of attention to, except for the parts about evolving and maintaining the API between the two. The afternoon was focused on Nova-Neutron, with a lot of discussion about improving the interaction between the two services during instance migration. There was some discussion about bandwidth-based scheduling, but as this depends on Placement getting nested resource providers done, it was agreed that we would hold off on that for now.
We wrapped up Thursday with another deep-dive into Placement; this time focusing on Generic Device Management, which has as its goal to be able to model all devices, not just PCI devices, as being attached to instances. This would involve the virt driver being able to report all such devices to the placement service in such as way as to correctly model any sort of nested relationships, and determine the inventory for each such item. Things began to get pretty specific, from the “I need a GPU” to “I need a particular GPU on a particular host”, which, in my opinion, is a cloud anti-pattern. One thing that stuck out for me was the request to be able to ask for multiple things of the same class, but each having a different trait. While this is certainly possible, it wasn’t one of the use cases considered when creating the queries that make placement work, and will require some more thought. There was much more discussed, and I think I wasn’t the only one whose brain was hurting afterwards. If you’re interested, you can read the notes from the session.
Friday was reserved for all the things that didn’t fit into one of the big topics covered on Wednesday or Thursday. You can see the variety of things covered on this etherpad, starting around line 189. We actually managed to get through the majority of those, as most people were able to stay for the last day of PTG. I’m not going to summarize them here, as that would make this post interminably long, but it was satisfying to accomplish as much as we did.
After the conference, my wife joined me, and we spent the weekend out in the nearby Rockies. We visited Rocky Mountain National Park, and to describe the views as breathtaking would be an understatement.
I would certainly say that the week was a success! It took me a few days upon returning to decompress after a week of intense meetings, but I think we laid the groundwork for a productive Queens release!
“Fanatical Support®” – that’s the slogan for my former employer, Rackspace. It meant that they would do whatever it took to make their customers successful. From their own website:
Fanatical Support® Happens Anytime, Anywhere, and Any Way Imaginable at Rackspace
It’s the no excuses, no exceptions, can-do way of thinking that Rackers (our employees) bring to work every day. Your complete satisfaction is our sole ambition. Anything less is unacceptable.
Sounds great, right? This sort of approach to customer service is something I have always believed in. And it was my philosophy when I ran my own companies, too. Conversely, nothing annoys me more than a company that won’t give good service to their customers. So when I joined Rackspace, I felt right at home.
Back in 2012 I was asked to create an SDK in Python for the Rackspace Cloud, which was based on OpenStack. This would allow our customers to more easily develop applications that used the cloud, as the SDK would handle the minutiae of dealing with the API, and allow developers to focus on the tasks they needed to carry out. This SDK, called pyrax, was very popular, and when I eventually left Rackspace in 2014, it was quite stable, with maybe a few outstanding small bugs.
Our team at Rackspace promoted pyrax, as well as our SDKs for other languages, as “officially supported” products. Prior to the development of official SDKs, some people within the company had developed some quick and dirty toolkits in their spare time that customers began using, only to find out some time later when they had an issue that the original developer had moved on, and no one knew how to correct problems. So we told developers to use these official SDKs, and they would always be supported.
However, a few years later there was a movement within the OpenStack community to build a brand-new SDK for Python, so being good community citizens, we planned on supporting that tool, and helping our customers transition from pyrax to the OpenStackSDK for Python. That was in January of 2014. Three and a half years later, this has still not been done. The OpenStackSDK has still not reached a 1.0 release, which in itself is not that big a deal to me. What is a big deal is that the promise for transitioning customers from pyrax to this new tool was never kept. A few years ago the maintainers began replying to issues and pull requests stating that pyrax was deprecated in favor of the OpenStackSDK, but no tools or documentation to help move to the new tool have been released.
What’s worse, is that Rackspace now actively refuses to make even the smallest of fixes to pyrax, even though they would require no significant developer time to verify. At this point, I take this personally. For years I went to conference after conference promoting this tool, and personally promising people that we would always support it. I fought internally at Rackspace to have upper management commit to supporting these tools with guaranteed headcount backing them before we would publish them as officially supported tools. And now I’m extremely sad to see Rackspace abandon these people who trusted my words.
So here’s what I will do: I have a fork of pyax on my GitHub account. While my current job doesn’t afford me the time to actively contribute much to pyrax, I will review and accept pull requests, and try to answer support questions.
Rackspace may have broken its promises and abandoned its customers, but I cannot do that. These may not be my customers, but they are my community.
One of the shortcomings of the current scheduler in OpenStack Nova is that there is a long interval from when the scheduler selects a suitable host for a new instance until the resources on that host are claimed so that they are no longer available. Now that resources are tracked in the Placement service, we want to move the claim closer to the time of host selection, in order to avoid (or eliminate) the race condition. I’m not going to explain the race condition here; if you’re reading this, I’m assuming this is well understood, so let me just summarize my concern: the current proposed design, as seen in the series starting with https://review.openstack.org/#/c/465175/, could be made much better with some design changes.
At the recent Boston Summit, which I was unable to attend due to lack of funding by my employer, the design for this change was discussed, and the consensus was to have the scheduler return a list of hosts for each instance to the super conductor, and then have the super conductor attempt to claim the resources for the first host returned. If the allocation fails, the super conductor discards that host and tries to claim the resources on the second host. When it finally succeeds in a claim, it sends a message to that host to start building the instance, and that message will include the list of alternative hosts. If something happens that causes the build to fail, the compute node sends it back to its local conductor, which will unclaim the resources, and then try each of the alternates in order by first claiming the resources on that host, and if successful, sending the build request to that host. Only if all of the alternates fail will the request fail.
I believe that while this is an improvement, it could be better. I’d like to do two things differently:
Have the scheduler claim the resources on the first selected host. If it fails, discard it and try the next. When it succeeds, find other hosts in the list of weighed hosts that are in the same cell as the selected host in order to provide the number of alternates, and return that list.
Have the process asking the scheduler to select a host also provide the number of alternates, instead of having the scheduler use the current max_attempts config option value.
On the first point: the scheduler already has a representation of the resources that need to be claimed. If the super conductor does the claiming, it will have to re-generate that representation. Sure, that’s not all that demanding, but it sure makes for cleaner design to not repeat things. It also ensures that the super conductor gets a good host from the start. Let me give an example. If the scheduler returns a chosen host (without claiming) and two alternates (which is the standard behavior using the config option default), the conductor has no guarantee of getting a good host. In the event of a race, the first host may fail to allocate resources, and now there are only the two alternates to try. If the claim was done in the scheduler, though, when that first host failed it would have been discarded, and the the next host tried, until the allocation succeeded. Only then would the alternates be determined, and the super conductor could confidently pass on that build request to the chosen host. Simply put: by having the scheduler do the initial claim, the super conductor is guaranteed to get a good host.
Another problem, although much less critical, is that the scheduler still has the host do consume_from_request(). With the claim done in the conductor, there is no way to keep this working if the initial host fails. We will have consumed on that host, even though we aren’t building on it, and have not consumed on the host we actually select.
On the second point: we have spent a lot of time over the past few years trying to clean up the interface between Nova and the scheduler, and have made a great deal of progress on that front. Now I know that the dream of an independent scheduler is still just that: a dream. But I also know that the scheduler code has been greatly improved by defining a cleaner interface between it an Nova. One of the items that has been discussed is that the config option max_attempts doesn’t belong in the scheduler; instead, it really belongs in the conductor, and now that the conductor will be getting a list of hosts from the scheduler, the scheduler is out of the picture when it comes to retrying a failed build. The current proposal to not only leave that config option in the scheduler, but to make it dependent on it for its functioning, is something that once again makes the scheduler Nova-centric (and Nova-exclusive). It would be a much cleaner design to simply have the conductor ask for the number of hosts (chosen + alternates), and have the scheduler’s behavior use that number. Yes, it requires a change to the RPC interface, but that is to be expected if you are changing a fundamental behavior of the scheduler. And if the scheduler is ever moved into a module, all it is is another parameter. Really, that’s not a good reason to follow a poor design.
Since some of the principal people involved in this discussion are not available now, and I’m going to be away at PyCon for the next few days, Dan Smith suggested that I post a summary of my concerns so that all can read it and have an idea what the issues are. Then next week sometime when we are all around and have the time to discuss this, we can hash it out on #openstack-nova, or maybe in a hangout. I also have pushed a series that has all of the steps needed to make this happen, since it’s one thing to talk about a design, and it’s another to see the actual code. The series starts here: https://review.openstack.org/#/c/464086/. For some of the later patches I haven’t finished updating the tests to match the change in method signatures and returned value structures, but you should be able to get a good idea of the code changes I’m proposing.
Recently in the OpenStack API Working Group we have been spending a lot of time and energy on establishing the API Stability guidelines that will serve as the basis for the supports-api-stability tag proposed by the OpenStack Technical Committee. Tags are a way for consumers of OpenStack to get a better idea as to the state of the various projects, and this particular tag is intended to reassure consumers that the API for a project with this tag would not change in a breaking way. The problem with that is defining what exactly constitutes a “breaking change”.
While there are about as many opinions as there are participants in the discussion, they all roughly fall into one of two camps:
A change that simply adds to the existing API, such as returning additional values in addition to the current ones, isn’t breaking stability, as existing clients will still receive all the information they expect, and will ignore the additional stuff.
Any cloud that says it is running a particular version of an API should return the exact same information. In other words, a client written for Cloud A will work without modification with Cloud B. If something changes that would make these responses different, that change must be reflected in a new version, and the old version should remain available for a “long time” (precisely how long a “long time” is is a completely separate discussion in itself!).
I wrote about the second point above in an earlier post, which attempted to summarize that position after some discussion with many in the community who were pushing cloud interoperability (or “interop”). And at the recent Atlanta PTG (which I recapped here), we discussed this issue at length. The problem was that those who fell into Camp #1 above were at the morning session, while Camp #2 was there in the afternoon. So while the discussions were fruitful, they were not decisive. The discussions and comments on the Gerrit review for the proposed change to the API Stability Guidelines since the PTG reflect this division of opinion and lack of resolution.
But today during discussions in the API-WG meeting on IRC, it dawned on me that there is a fundamental reason we can’t reconcile these two points of view: we’re talking about 2 different goals. Camp #1 is concerned with not breaking clients whose applications rely on an OpenStack service’s API, while Camp #2 is concerned with not having different cloud deployments vary from each other.
The latter goal, while admirable, is very difficult to achieve in practice for anything but the most basic stuff. For one thing, any service that uses extensions will almost certainly fail, because there is no way to guarantee that deployments will always install and run the same extensions – that’s sort of the point of extensibility, after all. And during the discussions at the PTG, we tried to identify versioning systems that could meet the interop requirements, and the only one anyone could describe was microversions. So that means to satisfy Camp #2, a service would have to use microversions, period.
So I propose a slightly different route forward: let’s define 2 tags to reflect these two different types of “stability”. Let’s use the original tag “assert:supports-api-compatibility” to mean the Camp #2 standard, as its emphasis is interoperability. Then add a separate “assert:supports-api-stability”, which reflects the Camp #1 understanding of never breaking clients.
It is important to note that this second tag is not meant to indicate a “light” version of the first, just because the requirements wouldn’t be as difficult to attain. It reflects support for a different, but still important, continuity for their users. Each project can decide which of these goals are relevant to it, and will make their APIs better by achieving either (or both!) goals.