Where does docker ps Command look for current containers?

Question

I need to know how it works and how it finds the current container and where it looks for containers by default.

Don\'t get the wrong idea of the question. I am very

Answer 1

TLDR; the docker daemon keeps the list of containers in memory.
On a docker ps, there is no filesystem access needed.
On a docker create/run/start, the list of container can be derived from the layer found in /var/lib/docker/aufs (if you are using the aufs storage driver)

The containers themselves are under /var/lib/docker/containers/{id}
As illsutrated by issue 15047, that path is loaded when the docker daemon is started. After, this is managed in memory.

---

docker ps asks for containers in api/client/ps.go#L59:

containers, err := cli.client.ContainerList(options)

It asks api/client/lib/container_list.go#L45

resp, err := cli.get("/containers/json", query, nil)

That is managed in api/server/router/container/container.go#L34

local.NewGetRoute("/containers/json", r.getContainersJSON),

It then asks to the backend in api/server/router/container/container_routes.go#L48

containers, err := s.backend.Containers(config)

Which is an interface in api/server/router/container/backend.go#L58

Containers(config *daemon.ContainersConfig) ([]*types.Container, error)

implemented by the docker daemon in daemon/list.go#L83-L86:

// Containers returns the list of containers to show given the user's filtering.
func (daemon *Daemon) Containers(config *ContainersConfig) ([]*types.Container, error) {
    return daemon.reduceContainers(config, daemon.transformContainer)
}

The daemon has the list of containers: daemon/list.go#L37-L40

// List returns an array of all containers registered in the daemon.
func (daemon *Daemon) List() []*container.Container {
    return daemon.containers.List()
}

From a contStore: daemon/daemon.go#L123-L132

func (c *contStore) List() []*container.Container {
    containers := new(History)
    c.Lock()
    for _, cont := range c.s {
        containers.Add(cont)
    }
    c.Unlock()
    containers.sort()
    return *containers
}


type contStore struct {
    s map[string]*container.Container
    sync.Mutex
}