Imperative app-level API for pod manipulation

To provide an API for imperative application operations (e.g. start|stop|add) inside a pod for finer-grained control over rkt containerization concepts and debugging needs, this proposal introduces new stage1 entrypoints and a subcommand CLI API that will be used for manipulating applications inside pods.

The primary motivation behind this change is to facilitate the new direction orchestration systems are taking in how they integrate with container runtimes. For more details, see kubernetes#25899.

API

The envisioned workflow for the app-level API is that after a pod has been started, users will invoke the rkt CLI to manipulate the pod. The implementation behaviour consists of application logic on top of the aforementioned stage1 entrypoints.

The proposed app-level commands are described below.

rkt app sandbox

Initializes an empty pod having no applications. This returns a single line containing the pod-uuid which can be used to perform application operations specified below. This also implies the started pod will be injectable.

rkt app sandbox

rkt app add

Injects an application image into a running pod. After this has been called, the app is prepared and ready to be run via rkt app start.

It first prepares an application rootfs for the application image, creates a runtime manifest for the app, and then injects the prepared app via an entrypoint.

rkt app add <pod-uuid> --app=<app-name> <image-name/hash/address/registry-URL> <arguments>

Note: Not every pod will be injectable; it will be configured through an option when the pod is created..

rkt app start

Starts an application that was previously added (injected) to a pod. This operation is idempotent; if the specified application is already started, it will have no effect.

rkt app start <pod-uuid> --app=<app-name> <arguments>

rkt app stop

Stops a running application gracefully. Grace is defined in the app/stop entrypoint section. This does not remove associated resources (see app rm).

rkt app stop <pod-uuid> --app=<app-name>

rkt app rm

Removes a stopped application from a running pod, including all associated resources.

rkt app rm <pod-uuid> --app=<app-name> <arguments>

Note: currently, when a pod becomes empty (no apps are running), it will terminate. This proposal will introduce a --mutable or --allow-empty or --dumb flag to be used when starting pods, so that the lifecycle management of the pod is configurable by the user (i.e. it will be possible to create a pod that won't be terminated when it is empty).

Resources left over by a stopped application (in default stage1 flavor)

  • Rootfs (e.g. /opt/stage2/<app-name>)
  • Mounts from volumes (e.g. /opt/stage2/<app-name>/<volume-name>)
  • Mounts related to rkt operations (e.g. /opt/stage2/<app-name>/dev/null)
  • systemd service files (e.g. <app-name>.service and reaper-<app-name>.service)
  • Miscellaneous files (e.g. /rkt/<app-name>.env, /rkt/status...)

rkt app list

Lists the applications that are inside a pod, running or stopped.

rkt app list <pod-uuid> <arguments>

Note: The information returned by list should consist of an app specifier and status at the very least, the rest is up for discussion.

rkt app status

Returns the execution status of an application inside a pod.

rkt app status <pod-uuid> --app=<app-name> <arguments>

The returned status information for an application would contain the following details (output format is up for discussion):

type AppStatus struct {
	Name       string
	State      AppState
	CreatedAt  time.Time
	StartedAt  time.Time
	FinishedAt time.Time
	ExitCode   int64 
}

Note: status will be obtained from an annotated JSON file residing in stage1 that contains the required information. OPEN QUESTION: what is responsible for updating this file? How is concurrent access handled?

rkt app exec

Executes a command inside an application.

rkt app exec <pod-uuid> --app=<app-name> <arguments> -- <command> <command-arguments>

Entrypoints

In order to facilitate the app-level operations API, four new stage1 entrypoints are introduced. Entrypoints are resolved via annotations found within a pod's stage1 manifest (e.g. /var/lib/rkt/pods/run/$uuid/stage1/manifest).

coreos.com/rkt/stage1/app/add

The responsibility of this entrypoint is to receive a prepared app and inject it into the pod, where it will be started using the app/start entrypoint.

The entrypoint should receive a reference to a runtime manifest of the prepared app, and perform any necessary setup based on that runtime manifest.

coreos.com/rkt/stage1/app/rm

The responsibility of this entrypoint is to remove an app from a pod. After rm, starting the application again is not possible - the app must be re-injected to be re-used.

  1. receive a reference to an application that resides inside the pod (running or stopped)
  2. stop the application if its running.
  3. remove the contents of the application (rootfs) from the pod (keep the logs?) and delete references to it (e.g. service files).

coreos.com/rkt/stage1/app/start

The responsibility of this entrypoint is to start an application that is in the Prepared state, which is an app that was recently injected.

coreos.com/rkt/stage1/app/stop

The responsibility of this entrypoint is to stop an application that is in the Running state, by instructing the stage1.

rkt will attempt a graceful shutdown: sending a termination signal (i.e. SIGTERM) to application and waiting for a grace period for the application to exit. If the application does not terminate by the end of the grace period, rkt will forcefully shut it down (i.e. SIGKILL).

App States

Expected set of app states are listed below:

type AppState string

const (
	UnknownAppState AppState = "unknown"

	PreparingAppState AppState = "preparing"

	// Apps that are ready to be used by `app start`.
	PreparedAppState AppState = "prepared"

	RunningAppState AppState = "running"

	// Apps stopped by `app stop`.
	StoppingAppState AppState = "stopping"

	// Apps that finish their execution naturally.
	ExitedAppState AppState = "exited"

	// Once an app is marked for removal, while the removal is being
	// performed, no further operations can be done on that app.
	DeletingAppState AppState = "deleting"
)

Note: State transitions are linear; an app that is in state Exited cannot transition into Running state. OPEN QUESTION can a stopped app not be restarted?

Use Cases

Low-level Pod Control

Grant granular access to pods for orchestration systems and allow orchestration systems to develop their own pod concept on top of the exposed app-level operations.

Example Workflow

  1. Create an empty pod.
  2. Inject applications into the pod.
  3. Orchestrate the workflow of applications (e.g. app1 has to terminate successfully before app2).

Updates

Enable in-place updates of a pod without disrupting the operations of the pod.

Example Workflow

  1. Remove old applications without disturbing/restarting the whole pod.
  2. Inject updated applications.
  3. Start the updated applications.

Debugging Pods

Allow users to inject debug applications into a pod in production.

Example Workflow

  1. Deploy an application containing only a Go web service binary.
  2. Encounter an error not decipherable via the available information (e.g. status info, logs, etc.).
  3. Add a debug app image containing binaries (e.g. lsof) for debugging the service.
  4. Enter the pod namespace and use the debug binaries.