Addressing Abstraction Hell in oVirt
When I was a kid we used to play this game in which someone thought about a message and told it to his friend. The latter told the message he heard to another kid, and so on. Eventually, we were amused to see the difference between the original message and the one that the last kid has heard. This post describes our approach for addressing a similar problem that was caused by having too many abstraction layers in oVirt. Each layer converted its input in order to report it to the next layer “in its own words”, resulting in cumbersome and error-prone business flows in our platform.
oVirt was originated from a management platform developed by a startup company that created the KVM hypervisor. Back at its early days, not only that the technology it was implemented in was different than the one we use today, but it was also designed differently.
Initially, the agent that resides on the distributed hosts, VDSM, was expected to interact directly with the hypervisor. Another protocol was defined for the communication between the central management unit (called ovirt-engine nowadays) and VDSM.
Consequently, flows that included interaction with the hypervisor required two data conversions. Figure 1 depicts such a flow. The management unit had its own representation of business entities that front-end clients used in order to communicate with the back-end. The management unit needed to convert those entities to the language VDSM speaks which is dictionary-based. VDSM, in turn, needed to convert the dictionary it received to the set of parameters that conform with the language of the hypervisor.
Figure 1: Architecture Before Using Libvirt
Later, Libvirt got into the picture. Libvirt provides an API for managing virtualization hosts. Its API is the de-facto standard in the industry, supporting a variety of hypervisors such as hyperv, xen, esx and qemu. Although focusing on qemu-kvm, it was a natural decision to leverage that simpler, more general and widely supported API in oVirt despite the downsides in having yet-another-abstraction-layer.
And so, an additional layer was added. Figure 2 depicts the previously mentioned flow with that new design. Now, VDSM converts its input into the language that Libvirt speaks (that is mostly XML-based) and Libvirt converts that to the language of the hypervisor.
Figure 2: Architecture with Libvirt
Since Libvirt is treated as a third-party tool in oVirt, we remained with two conversions in the scope of our platform. First, ovirt-engine converts its business entities into a dictionary. Second, VDSM converts the dictionary into Libvirt’s XMLs.
The need to convert data twice, in two different components, introduced several challenges. First, it required one to code in two different programming languages, as ovirt-engine is written in Java while VDSM is written in Python. In practice, many times several developers were involved in every feature, each was responsible for a part of the implementation within a certain component. These developers had to always be in-sync. Second, flows were more buggy and harder to debug as more code that is spread over different repositories and deployed in different places was required. Third, and maybe most importantly, the development process required reviews by different people that maintain the different components. It generally reduced the pace of the development process, mainly due to the review process in VDSM that has traditionally been slower largely due to its maintainership model.
We observed that many of our features required changes on the client side, UI or REST-API based clients, as well as on the back-end side but only minimal changes were needed on the agent side. The latter part mostly included conversion of data into Libvirt’s XMLs.
Thus, by letting ovirt-engine speak the language of Libvirt rather than that of VDSM, i.e., converting its business entities directly to Libvirt’s XMLs, not only that we use a more standard API in ovirt-engine but we often avoid any change in VDSM. Furthermore, this reduces the chances of people making hacks on the host side and increases the chances of the data representation in ovirt-engine being better aligned with the one in Libvirt.
We modified ovirt-engine to both generate Libvirt’s Domain XML on run VM and to consume Libvirt’s Domain XML when monitoring the devices of the VM.
In run VM flow (figure 3), ovirt-engine now generates a full Libvirt’s Domain XML. Since the dictionary used to contain data that is required for VDSM but is not included in the specification of the Domain XML, the XML is extended with a metadata section that contains this kind of data. VDSM in turn inspects the metadata section to gather the information it needs for preparing the host for running the VM (e.g., creates payload devices, activates LVs) and then passes the XML to Libvirt.
Figure 3: Run VM Flow
In the monitoring process (figure 4), ovirt-engine now queries the Domain XML of the running VMs whose devices hash has changed. Then, ovirt-engine matches the reported devices with the ones in the database, the kind of matching that VDSM used to do, and passes the reported devices along with their correlation with the devices that exist in the database as a dictionary to the legacy devices monitoring code. This is done in a conversion layer (class) that was added to ovirt-engine.
Figure 4: VM Devices Monitoring Flow
In oVirt 4.2 we made a major change behind the scenes in the way ovirt-engine and VDSM interact. Now, ovirt-engine uses the commonly used API of Libvirt and VDSM mostly routes data from/to Libvirt in virt (i.e., VM-lifecycle) flows. This way, we eliminate the costly conversion of the data (in terms of development effort) by VDSM in these flows.
This change is supposed to be invisible for most of our users. However, while debugging issues that involve running a VM, one should be aware that the Domain XML is now generated by ovirt-engine and is printed to engine.log. In addition, while debugging issues with devices monitoring, one should now be aware that ovirt-engine matches the reported devices with the ones from the database rather than VDSM.
We have already been able to add new functionality to oVirt 4.2 more easily with this change and we expect it to simplify future changes as well.