Engines

Edgelet routes all container operations through the ContainerEngine interface (pkg/engine). The internal/engines package is the factory that constructs the correct implementation from containerEngine config. Platform build tags determine which engines are linked.

Code: internal/engines/, implementations in pkg/engine/{edgelet,docker,podman}/

Operator guide: Container engines

Purpose

• Select engine at runtime: edgelet, docker, or podman
• Apply consistent EngineConfig (socket URL, API version, log dir)
• Wrap external engines with structured debug logging
• Warn when Docker/Podman sockets exist alongside containerEngine=edgelet

Dependencies

Depends on	Reason
config	containerEngine, containerEngineUrl
pkg/engine/*	Concrete engine implementations

Used by	Reason
supervisor	Creates engine before Process Manager start
processmanager	All container CRUD
pruning	Image prune delegation
healthcheck	Exec-based checks (edgelet engine)
runtimeapi	Image pull/list/load

Factory behavior

Linux (factory_linux.go)

containerEngine	Implementation
edgelet	pkg/engine/edgelet: embedded containerd + CNI + DNS
docker	pkg/engine/docker
podman	pkg/engine/podman

NewContainerEngine(type, cfg) returns (ContainerEngine, error).

WrapWithLoggingIfExternal() adds API call logging for docker/podman only.

Desktop (factory_desktop.go)

Only docker and podman. No embedded edgelet engine.

Edgelet engine specifics

When containerEngine=edgelet:

• Uses isolated paths: /var/lib/edgelet-containerd/, /run/edgelet/containerd.sock
• Embedded containerd started in bootstrap before Supervisor (not by Supervisor)
• Integrates dnsresolver, RuntimeClass shims, pause image, CNI
• Supports HealthcheckEngine exec path

warnIfExternalRuntimePresent() logs if host Docker/Podman sockets are visible (informational; no interference).

External engine degraded mode

Supervisor retries docker/podman init; if socket unavailable:

• Daemon continues in WARNING state
• Background recovery goroutine retries until engine connects
• Process Manager reconcile may quiesce during engine switch

Configuration

Key	Effect
containerEngine	Factory selector
containerEngineUrl	Socket/endpoint for external engines
dockerApiVersion	Docker API version string
logDirectory	Container log root passed to edgelet engine

Interface surface

Key ContainerEngine operations (see pkg/engine/engine.go):

• Init, PullImage, CreateContainer, Start, Stop, Remove
• ListContainers, Inspect, Logs, Exec
• PruneImages (used by pruning manager)

Observability

• Factory log module: "ContainerEngine"
• Engine name recorded in runtime.GetState() at startup
• External engine API logs via LoggingEngine wrapper

Failure modes

Symptom	Typical cause
Start failure on linux edgelet	Containerd not prestarted
WARNING daemon	Docker/Podman socket down
Wrong network/DNS	Engine mismatch vs config reload
Crash looks empty on docker/podman	Inspect must include exitCode= / oomKilled= / error= when not healthily running; embedded engine keeps CRI reason=...

Code map

File	Role
factory_linux.go	Linux engine construction
factory_desktop.go	Desktop engine construction
factory_common.go	Shared warnings

Implementations: pkg/engine/edgelet/, pkg/engine/docker/, pkg/engine/podman/.

Related: Supervisor, Process manager, DNS resolver, Health check, Engine lifecycle.

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