containerd is an industry-standard container runtime with an emphasis on simplicity, robustness and portability. It is a available as a daemon for Linux and Windows, which can manage the complete container lifecycle of its host system: image transfer and storage, container execution and supervision, low-level storage and network attachments, etc..
Containerd is designed to be embedded into a larger system, rather than being used directly by developers or end-users.
Components should not have tight dependencies on each other so that they are unable to be used independently.
The APIs for images and containers should be designed in a way that when used together the components have a natural flow but still be useful independently.
The execution layer and overlay filesystems can be used independently but if you were to use both, they share a common `Mount` struct that the filesystems produce and the execution layer consumes.
Breaking up the filesystem APIs to allow snapshots, copy functionality, and mounts allow people implementing build at the higher levels more flexibility.
For the various components in containerd there should be defined extension points where implementations can be swapped for alternatives.
The best example of this is that containerd will use `runc` from OCI as the default runtime in the execution layer but other runtimes conforming to the OCI Runtime specification they can be easily added to containerd.
Additional implementations will not be accepted into the core repository and should be developed in a separate repository not maintained by the containerd maintainers.
Containerd will be released with a 1.0 when feature complete and this version will be supported for 1 year with security and bug fixes applied and released.
The upgrade path for containerd is that the 0.0.x patch relases are always backward compatible with its major and minor version.
Minor (0.x.0) version will always be compatible with the previous minor release. i.e. 1.2.0 is backwards compatible with 1.1.0 and 1.1.0 is compatible with 1.0.0.
There is no compatiability guarentes with upgrades from two minor relases. i.e. 1.0.0 to 1.2.0.
| execution | Provide an extensible execution layer for executing a container | in | Create,start, stop pause, resume exec, signal, delete |
| cow filesystem | Built in functionality for overlay, aufs, and other copy on write filesystems for containers | in | |
| distribution | Having the ability to push and pull images as well as operations on images as a first class api object | in | Containerd will fully support the management and retrieval of images |
| low-level networking drivers | Providing network functionality to containers along with configuring their network namespaces | in | Network support will be added via interface and network namespace operations, not service discovery and service abstractions. |
| build | Building images as a first class API | out | Build is a higher level tooling feature and can be implemented in many different ways on top of containerkit |
| volumes | Volume management for external data | out | The api supports mounts, binds, etc where all volumes type systems can be built on top of. |
| logging | Persisting container logs | out | Logging can be build on top of containerd because the container’s STDIO will be provided to the clients and they can persist any way they see fit.,There is no io copying of container STDIO in containerd. |
containerd should provide the primitives to create, add, remove, or manage network interfaces and network namespaces for a container but ip allocation, discovery, and DNS should be handled at higher layers.
