What is ZeroVM?

ZeroVM is a lightweight virtualization technology based on Google Native Client (NaCl). While it shares some similarities with traditional hypervisors and container technologies, it is unique in a number of respects. Unlike KVM and LXC, which provide an entire virtualized operating system environment, it isolates single processes and provides no operating system or kernel. This allows instances to start up in a very short time: about five milliseconds. Combined with a high level of security and zero execution overhead, ZeroVM is well-suited to ephemeral processes running untrusted code in multi-tenant environments.

There are of course some limitations inherent in the design. ZeroVM cannot be used as a drop-in replacement for something like KVM or LXC. These limitations, however, were the deliberate design decisions necessary in order to create a virtualization platform specifically for building cloud applications.

How ZeroVM is different to other virtualization tools

Blake Yeager and Camuel Gilyadov gave a talk at the 2014 OpenStack Summit in Atlanta which summed up nicely the main differences between hypervisor-based virtual machines (KVM, Xen, and so on), containers (LXC, Docker, and so on), and ZeroVM. Here are the key differences they outlined:

Traditional VMs and containers provide a way to partition and schedule shared server resources for multiple tenants. ZeroVM accomplishes the same goal using a different approach and with finer granularity. Instead of running one or more application processes in a traditional virtual machine, applications written for ZeroVM must be decomposed in microprocesses, and each one gets its own instance. The advantage of in this case is that you can avoid long running VMs/processes which accumulate state (leading to memory leaks and cache problems). The disadvantage, however, is that it can be difficult to port existing applications. Each process running on ZeroVM is a single stateless unit of computation (much like a function in the “purely functional” sense; more on that to follow), and applications need to be structured specifically to fit this model. Some applications, such as long-running server applications, would arguably be impossible to re-implement entirely on ZeroVM, although some parts could be abstracted away to run inside ZeroVM instances. Applications that are predominantly parallel and involve many small units of computation are better suited to run on ZeroVM.

Determinism

ZeroVM provides a guarantee of functional determinism. What this means in practice is that with a given set of inputs (parameters, data, and so on), outputs are guaranteed to always be the same. This works because there are no sources of entropy. For example, the ZeroVM toolchain includes a port of glibc, which has a custom implementation of time functions such that time advances in a deterministic way for CPU and I/O operations.

No state is accumulated during execution and no instances can be reused. The ZeroVM Run-Time environment (ZRT) does provide an in-memory virtual file system which can be used to read/write files during execution, but all writes are discarded when the instance terminates unless an output “channel” is used to pipe data to the host OS or elsewhere.

Channels and I/O

“Channels” are the basic I/O abstraction for ZeroVM instances. All I/O between the host OS and ZeroVM must occur over channels, and channels must be declared explicitly in advance. On the host, a channel can map to a file, character device, pipe, or socket. Inside an instance, all channels are presented as files that can be written to/read from, including devices like stdin, stdout, and stderr.

Channels can also be used to connect multiple instances together to create arbitrary multi-stage job pipelines. For example, a MapReduce-style search application with multiple filters could be implemented on ZeroVM by writing each filter as a separate application/script and piping data from one to the next.

Security

ZeroVM has two key security components: static binary validation and a limited system call API.

Static validation occurs before “untrusted” user code is executed to ensure that there are no accidental or malicious instructions that could break out of the sandbox and compromise the host system. Binary validation in this instance is largely based on the NaCl validator. (For more information about NaCl and its validation, you can read the following whitepaper http://static.googleusercontent.com/media/research.google.com/en//pubs/archive/34913.pdf.)

To further lock down the execution environment, ZeroVM only supports six system calls via a "trap" interface: pread, pwrite, jail, unjail, fork, and exit. By comparison, containers (LXC) expose the entire Linux system call API which presents a larger attack surface and more potential for exploitation.

ZeroVM is lightweight

ZeroVM is very lightweight. It can start in about five milliseconds. After the initial validation, program code is executed directly on the hardware without interpretation overhead or hardware virtualization.

It's easy to embed in existing systems

The security and lightweight nature of ZeroVM makes it ideal to embed in existing systems. For example, it can be used for arbitrary data-local computation in any kind of data store, akin to stored procedures. In this scenario, untrusted code provided by any user with access to the system can be executed safely. Because inputs and outputs must be declared explicitly upfront, the only concerns remaining are data access rules and quotas for storage and computation.

Contrasted with a traditional model, where storage and compute nodes are separate, data-local computing can be a more efficient model when the cost of transferring data over the network to/from compute nodes outweighs the actual computation time itself.

The tool has already been integrated with OpenStack Swift using ZeroCloud (middleware for Swift). This turns Swift into a “smart” data store, which can be used to scale parallel computations (such as multi-stage MapReduce jobs) across large collections of objects.

Language support

C and C++ applications can run on ZeroVM, provided that they are cross-compiled to NaCl using the provided toolchain. At present there is also support for Python 2.7 and Lua.

Licensing

All projects under the ZeroVM umbrella are licensed under Apache 2.0, which makes ZeroVM suitable for both commercial and non-commercial applications (the same as OpenStack).