agent

Kata Agent

Overview

The Kata agent is a long running process that runs inside the Virtual Machine (VM) (also known as the "pod" or "sandbox").

The agent is packaged inside the Kata Containers guest image which is used to boot the VM. Once the runtime has launched the configured hypervisor to create a new VM, the agent is started. From this point on, the agent is responsible for creating and managing the life cycle of the containers inside the VM.

For further details, see the architecture document.

Audience

If you simply wish to use Kata Containers, it is not necessary to understand the details of how the agent operates. Please see the installation documentation for details of how deploy Kata Containers (which will include the Kata agent).

The remainder of this document is only useful for developers and testers.

Build from Source

Since the agent is written in the Rust language this section assumes the tool chain has been installed using standard Rust rustup tool.

Build with musl

If you wish to build the agent with the musl C library, you need to run the following commands:

$ arch=$(uname -m)
$ rustup target add "${arch}-unknown-linux-musl"
$ sudo ln -s /usr/bin/g++ /bin/musl-g++

Note:

It is not currently possible to build using musl on ppc64le and s390x since both platforms lack the musl target.

Build the agent binary

The following steps download the Kata Containers source files and build the agent:

$ GOPATH="${GOPATH:-$HOME/go}"
$ dir="$GOPATH/src/github.com/kata-containers"
$ git -C ${dir} clone --depth 1 https://github.com/kata-containers/kata-containers
$ make -C ${dir}/kata-containers/src/agent

Change the agent API

The Kata runtime communicates with the Kata agent using a ttRPC based API protocol.

This ttRPC API is defined by a set of protocol buffers files. The protocol files are used to generate the bindings for the following components:

Component	Language	Generation method [*]	Tooling required
runtime	Golang	Run, make generate-protocols	protoc
agent	Rust	Run, make

Key:

[*] - All commands must be run in the agent repository.

If you wish to change the API, these files must be regenerated. Although the rust code will be automatically generated by the build script, the Golang code generation requires the external protoc command to be available in $PATH.

To install the protoc command on a Fedora/CentOS/RHEL system:

$ sudo dnf install -y protobuf-compiler

Custom guest image and kernel assets

If you wish to develop or test changes to the agent, you will need to create a custom guest image using the osbuilder tool. You may also wish to create a custom guest kernel.

Once created, configure Kata Containers to use these custom assets to allow you to test your changes.

Note:

To simplify development and testing, you may wish to run the agent stand alone initially.

Tracing

For details of tracing the operation of the agent, see the tracing documentation.

Run the agent stand alone

Although the agent is designed to run in a VM environment, for development and testing purposes it is possible to run it as a normal application.

When run in this way, the agent can be controlled using the low-level Kata agent control tool, rather than the Kata runtime.

For further details, see the agent control tool documentation.