Setup plugin ids and dependencies

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

client.go

@@ -2,6 +2,7 @@ package containerd
 
 import (
 	"context"
+	"fmt"
 	"io/ioutil"
 	"log"
 	"net/http"
@@ -19,6 +20,7 @@ import (
 	versionservice "github.com/containerd/containerd/api/services/version"
 	"github.com/containerd/containerd/content"
 	"github.com/containerd/containerd/images"
+	"github.com/containerd/containerd/plugin"
 	"github.com/containerd/containerd/remotes"
 	"github.com/containerd/containerd/remotes/docker"
 	contentservice "github.com/containerd/containerd/services/content"
@@ -83,7 +85,7 @@ func New(address string, opts ...ClientOpt) (*Client, error) {
 	}
 	return &Client{
 		conn:    conn,
-		runtime: runtime.GOOS,
+		runtime: fmt.Sprintf("%s.%s", plugin.RuntimePlugin, runtime.GOOS),
 	}, nil
 }
 

cmd/containerd/config.go

@@ -37,8 +37,6 @@ func loadConfig(path string) error {
 // config specifies the containerd configuration file in the TOML format.
 // It contains fields to configure various subsystems and containerd as a whole.
 type config struct {
-	// State is the path to a directory where containerd will store runtime state
-	State string `toml:"state"`
 	// Root is the path to a directory where containerd will store persistent data
 	Root string `toml:"root"`
 	// GRPC configuration settings

cmd/containerd/config_linux.go

@@ -2,8 +2,7 @@ package main
 
 func defaultConfig() *config {
 	return &config{
-		Root:  "/var/lib/containerd",
-		State: "/run/containerd",
+		Root: "/var/lib/containerd",
 		GRPC: grpcConfig{
 			Address: "/run/containerd/containerd.sock",
 		},
@@ -11,7 +10,7 @@ func defaultConfig() *config {
 			Level:   "info",
 			Address: "/run/containerd/debug.sock",
 		},
-		Snapshotter: "overlay",
-		Differ:      "base",
+		Snapshotter: "io.containerd.snapshotter.v1.overlayfs",
+		Differ:      "io.containerd.differ.v1.base-diff",
 	}
 }

cmd/containerd/config_unix.go

@@ -4,8 +4,7 @@ package main
 
 func defaultConfig() *config {
 	return &config{
-		Root:  "/var/lib/containerd",
-		State: "/run/containerd",
+		Root: "/var/lib/containerd",
 		GRPC: grpcConfig{
 			Address: "/run/containerd/containerd.sock",
 		},
@@ -13,7 +12,7 @@ func defaultConfig() *config {
 			Level:   "info",
 			Address: "/run/containerd/debug.sock",
 		},
-		Snapshotter: "naive",
-		Differ:      "base",
+		Snapshotter: "io.containerd.snapshotter.v1.naive",
+		Differ:      "io.containerd.differ.v1.base-diff",
 	}
 }

cmd/containerd/config_windows.go

@@ -7,8 +7,7 @@ import (
 
 func defaultConfig() *config {
 	return &config{
-		Root:  filepath.Join(os.Getenv("programfiles"), "containerd", "root"),
-		State: filepath.Join(os.Getenv("programfiles"), "containerd", "state"),
+		Root: filepath.Join(os.Getenv("programfiles"), "containerd", "root"),
 		GRPC: grpcConfig{
 			Address: `\\.\pipe\containerd-containerd`,
 		},
@@ -16,7 +15,7 @@ func defaultConfig() *config {
 			Level:   "info",
 			Address: `\\.\pipe\containerd-debug`,
 		},
-		Snapshotter: "windows",
-		Differ:      "base",
+		Snapshotter: "io.containerd.snapshotter.v1.windows",
+		Differ:      "io.containerd.differ.v1.base-diff",
 	}
 }

cmd/containerd/main.go

@@ -29,7 +29,6 @@ import (
 	"github.com/containerd/containerd/content"
 	"github.com/containerd/containerd/log"
 	"github.com/containerd/containerd/plugin"
-	"github.com/containerd/containerd/snapshot"
 	"github.com/containerd/containerd/sys"
 	"github.com/containerd/containerd/version"
 	metrics "github.com/docker/go-metrics"
@@ -105,41 +104,42 @@ func main() {
 		if err := plugin.Load(filepath.Join(conf.Root, "plugins")); err != nil {
 			return err
 		}
+		registerContentStore()
+		registerMetaDB()
 		// start debug and metrics APIs
 		if err := serveDebugAPI(); err != nil {
 			return err
 		}
-		monitor, err := loadMonitor()
-		if err != nil {
-			return err
-		}
-		runtimes, err := loadRuntimes(monitor)
-		if err != nil {
-			return err
-		}
-		store, err := resolveContentStore()
-		if err != nil {
-			return err
-		}
-		meta, err := resolveMetaDB(context)
-		if err != nil {
-			return err
-		}
-		defer meta.Close()
-		snapshotter, err := loadSnapshotter(store)
-		if err != nil {
-			return err
+
+		var (
+			services []plugin.Service
+			plugins  = make(map[plugin.PluginType][]interface{})
+		)
+		for _, init := range plugin.Graph() {
+			id := init.URI()
+			log.G(global).WithField("type", init.Type).Infof("loading plugin %q...", id)
+			if !shouldLoad(init) {
+				continue
+			}
+			ic := plugin.NewContext(plugins)
+			ic.Root = filepath.Join(conf.Root, id)
+			ic.Context = log.WithModule(global, id)
+			if init.Config != nil {
+				if err := loadPluginConfig(init.ID, init.Config, ic); err != nil {
+					return err
+				}
+			}
+
+			p, err := init.Init(ic)
+			if err != nil {
+				return err
+			}
+			plugins[init.Type] = append(plugins[init.Type], p)
+			if s, ok := p.(plugin.Service); ok {
+				services = append(services, s)
+			}
 		}
 
-		differ, err := loadDiffer(snapshotter, store)
-		if err != nil {
-			return err
-		}
-
-		services, err := loadServices(runtimes, store, snapshotter, meta, differ)
-		if err != nil {
-			return err
-		}
 		// start the GRPC api with the execution service registered
 		server := newGRPCServer()
 		for _, service := range services {
@@ -159,7 +159,6 @@ func main() {
 		log.G(global).Infof("containerd successfully booted in %fs", time.Since(start).Seconds())
 		return handleSignals(signals, server)
 	}
-
 	if err := app.Run(os.Args); err != nil {
 		fmt.Fprintf(os.Stderr, "containerd: %s\n", err)
 		os.Exit(1)
@@ -184,10 +183,6 @@ func before(context *cli.Context) error {
 			name: "root",
 			d:    &conf.Root,
 		},
-		{
-			name: "state",
-			d:    &conf.State,
-		},
 		{
 			name: "address",
 			d:    &conf.GRPC.Address,
@@ -254,139 +249,27 @@ func serveDebugAPI() error {
 	return nil
 }
 
-func resolveContentStore() (content.Store, error) {
-	cp := filepath.Join(conf.Root, "content")
-	return content.NewStore(cp)
+func registerContentStore() {
+	plugin.Register(&plugin.Registration{
+		Type: plugin.ContentPlugin,
+		ID:   "content",
+		Init: func(ic *plugin.InitContext) (interface{}, error) {
+			return content.NewStore(ic.Root)
+		},
+	})
 }
 
-func resolveMetaDB(ctx *cli.Context) (*bolt.DB, error) {
-	path := filepath.Join(conf.Root, "meta.db")
-
-	db, err := bolt.Open(path, 0644, nil)
-	if err != nil {
-		return nil, err
-	}
-
-	return db, nil
-}
-
-func loadRuntimes(monitor plugin.TaskMonitor) (map[string]plugin.Runtime, error) {
-	o := make(map[string]plugin.Runtime)
-	for name, rr := range plugin.Registrations() {
-		if rr.Type != plugin.RuntimePlugin {
-			continue
-		}
-		log.G(global).Infof("loading runtime plugin %q...", name)
-		ic := &plugin.InitContext{
-			Root:    conf.Root,
-			State:   conf.State,
-			Context: log.WithModule(global, fmt.Sprintf("runtime-%s", name)),
-			Monitor: monitor,
-		}
-		if rr.Config != nil {
-			if err := conf.decodePlugin(name, rr.Config); err != nil {
+func registerMetaDB() {
+	plugin.Register(&plugin.Registration{
+		Type: plugin.MetadataPlugin,
+		ID:   "bolt",
+		Init: func(ic *plugin.InitContext) (interface{}, error) {
+			if err := os.MkdirAll(ic.Root, 0700); err != nil {
 				return nil, err
 			}
-			ic.Config = rr.Config
-		}
-		vr, err := rr.Init(ic)
-		if err != nil {
-			return nil, err
-		}
-		o[name] = vr.(plugin.Runtime)
-	}
-	return o, nil
-}
-
-func loadMonitor() (plugin.TaskMonitor, error) {
-	var monitors []plugin.TaskMonitor
-	for name, m := range plugin.Registrations() {
-		if m.Type != plugin.TaskMonitorPlugin {
-			continue
-		}
-		log.G(global).Infof("loading monitor plugin %q...", name)
-		ic := &plugin.InitContext{
-			Root:    conf.Root,
-			State:   conf.State,
-			Context: log.WithModule(global, fmt.Sprintf("monitor-%s", name)),
-		}
-		mm, err := m.Init(ic)
-		if err != nil {
-			return nil, err
-		}
-		monitors = append(monitors, mm.(plugin.TaskMonitor))
-	}
-	if len(monitors) == 0 {
-		return plugin.NewNoopMonitor(), nil
-	}
-	return plugin.NewMultiTaskMonitor(monitors...), nil
-}
-
-func loadSnapshotter(store content.Store) (snapshot.Snapshotter, error) {
-	for name, sr := range plugin.Registrations() {
-		if sr.Type != plugin.SnapshotPlugin {
-			continue
-		}
-		moduleName := fmt.Sprintf("snapshot-%s", conf.Snapshotter)
-		if name != moduleName {
-			continue
-		}
-
-		log.G(global).Infof("loading snapshot plugin %q...", name)
-		ic := &plugin.InitContext{
-			Root:    conf.Root,
-			State:   conf.State,
-			Content: store,
-			Context: log.WithModule(global, moduleName),
-		}
-		if sr.Config != nil {
-			if err := conf.decodePlugin(name, sr.Config); err != nil {
-				return nil, err
-			}
-			ic.Config = sr.Config
-		}
-		sn, err := sr.Init(ic)
-		if err != nil {
-			return nil, err
-		}
-
-		return sn.(snapshot.Snapshotter), nil
-	}
-	return nil, fmt.Errorf("snapshotter not loaded: %v", conf.Snapshotter)
-}
-
-func loadDiffer(snapshotter snapshot.Snapshotter, store content.Store) (plugin.Differ, error) {
-	for name, sr := range plugin.Registrations() {
-		if sr.Type != plugin.DiffPlugin {
-			continue
-		}
-		moduleName := fmt.Sprintf("diff-%s", conf.Differ)
-		if name != moduleName {
-			continue
-		}
-
-		log.G(global).Infof("loading differ plugin %q...", name)
-		ic := &plugin.InitContext{
-			Root:        conf.Root,
-			State:       conf.State,
-			Content:     store,
-			Snapshotter: snapshotter,
-			Context:     log.WithModule(global, moduleName),
-		}
-		if sr.Config != nil {
-			if err := conf.decodePlugin(name, sr.Config); err != nil {
-				return nil, err
-			}
-			ic.Config = sr.Config
-		}
-		sn, err := sr.Init(ic)
-		if err != nil {
-			return nil, err
-		}
-
-		return sn.(plugin.Differ), nil
-	}
-	return nil, fmt.Errorf("differ not loaded: %v", conf.Differ)
+			return bolt.Open(filepath.Join(ic.Root, "meta.db"), 0644, nil)
+		},
+	})
 }
 
 func newGRPCServer() *grpc.Server {
@@ -397,40 +280,6 @@ func newGRPCServer() *grpc.Server {
 	return s
 }
 
-func loadServices(runtimes map[string]plugin.Runtime,
-	store content.Store, sn snapshot.Snapshotter,
-	meta *bolt.DB, differ plugin.Differ) ([]plugin.Service, error) {
-	var o []plugin.Service
-	for name, sr := range plugin.Registrations() {
-		if sr.Type != plugin.GRPCPlugin {
-			continue
-		}
-		log.G(global).Infof("loading grpc service plugin %q...", name)
-		ic := &plugin.InitContext{
-			Root:        conf.Root,
-			State:       conf.State,
-			Context:     log.WithModule(global, fmt.Sprintf("service-%s", name)),
-			Runtimes:    runtimes,
-			Content:     store,
-			Meta:        meta,
-			Snapshotter: sn,
-			Differ:      differ,
-		}
-		if sr.Config != nil {
-			if err := conf.decodePlugin(name, sr.Config); err != nil {
-				return nil, err
-			}
-			ic.Config = sr.Config
-		}
-		vs, err := sr.Init(ic)
-		if err != nil {
-			return nil, err
-		}
-		o = append(o, vs.(plugin.Service))
-	}
-	return o, nil
-}
-
 func serveGRPC(server *grpc.Server) error {
 	path := conf.GRPC.Address
 	if path == "" {
@@ -494,3 +343,23 @@ func dumpStacks() {
 	buf = buf[:stackSize]
 	logrus.Infof("=== BEGIN goroutine stack dump ===\n%s\n=== END goroutine stack dump ===", buf)
 }
+
+func loadPluginConfig(name string, c interface{}, ic *plugin.InitContext) error {
+	if err := conf.decodePlugin(name, c); err != nil {
+		return err
+	}
+	ic.Config = c
+	return nil
+}
+
+func shouldLoad(r *plugin.Registration) bool {
+	// only load certain plugins based on the config values
+	switch r.Type {
+	case plugin.SnapshotPlugin:
+		return r.URI() == conf.Snapshotter
+	case plugin.DiffPlugin:
+		return r.URI() == conf.Differ
+	default:
+		return true
+	}
+}

cmd/containerd/main_linux.go

@@ -12,13 +12,14 @@ import (
 	"google.golang.org/grpc"
 )
 
-const (
-	defaultConfigPath = "/etc/containerd/config.toml"
-)
+const defaultConfigPath = "/etc/containerd/config.toml"
 
-var (
-	handledSignals = []os.Signal{unix.SIGTERM, unix.SIGINT, unix.SIGUSR1, unix.SIGCHLD}
-)
+var handledSignals = []os.Signal{
+	unix.SIGTERM,
+	unix.SIGINT,
+	unix.SIGUSR1,
+	unix.SIGCHLD,
+}
 
 func platformInit(context *cli.Context) error {
 	if conf.Subreaper {
@@ -33,12 +34,6 @@ func platformInit(context *cli.Context) error {
 			return err
 		}
 	}
-	if err := os.MkdirAll(conf.State, 0750); err != nil {
-		return err
-	}
-	if err := os.Chown(conf.State, conf.GRPC.Uid, conf.GRPC.Gid); err != nil {
-		return err
-	}
 	return nil
 }
 

cmd/containerd/main_unix.go

@@ -22,12 +22,6 @@ var (
 )
 
 func platformInit(context *cli.Context) error {
-	if err := os.MkdirAll(conf.State, 0750); err != nil {
-		return err
-	}
-	if err := os.Chown(conf.State, conf.GRPC.Uid, conf.GRPC.Gid); err != nil {
-		return err
-	}
 	return nil
 }
 

cmd/ctr/run.go

@@ -55,8 +55,8 @@ var runCommand = cli.Command{
 		},
 		cli.StringFlag{
 			Name:  "runtime",
-			Usage: "runtime name (linux, windows, vmware-linux)",
-			Value: "linux",
+			Usage: "runtime name (io.containerd.runtime.v1.linux, io.containerd.runtime.v1.windows, io.containerd.runtime.v1.com.vmware.linux)",
+			Value: "io.containerd.runtime.v1.linux",
 		},
 		cli.BoolFlag{
 			Name:  "readonly",

differ/differ.go

@@ -18,10 +18,23 @@ import (
 )
 
 func init() {
-	plugin.Register("diff-base", &plugin.Registration{
+	plugin.Register(&plugin.Registration{
 		Type: plugin.DiffPlugin,
+		ID:   "base-diff",
+		Requires: []plugin.PluginType{
+			plugin.ContentPlugin,
+			plugin.SnapshotPlugin,
+		},
 		Init: func(ic *plugin.InitContext) (interface{}, error) {
-			return newBaseDiff(ic.Content, ic.Snapshotter)
+			c, err := ic.Get(plugin.ContentPlugin)
+			if err != nil {
+				return nil, err
+			}
+			s, err := ic.Get(plugin.SnapshotPlugin)
+			if err != nil {
+				return nil, err
+			}
+			return newBaseDiff(c.(content.Store), s.(snapshot.Snapshotter))
 		},
 	})
 }

linux/runtime.go

@@ -32,19 +32,23 @@ import (
 var (
 	ErrTaskNotExists     = errors.New("task does not exist")
 	ErrTaskAlreadyExists = errors.New("task already exists")
+	pluginID             = fmt.Sprintf("%s.%s", plugin.RuntimePlugin, "linux")
 )
 
 const (
-	runtimeName    = "linux"
 	configFilename = "config.json"
 	defaultRuntime = "runc"
 	defaultShim    = "containerd-shim"
 )
 
 func init() {
-	plugin.Register(runtimeName, &plugin.Registration{
+	plugin.Register(&plugin.Registration{
 		Type: plugin.RuntimePlugin,
+		ID:   "linux",
 		Init: New,
+		Requires: []plugin.PluginType{
+			plugin.TaskMonitorPlugin,
+		},
 		Config: &Config{
 			Shim:    defaultShim,
 			Runtime: defaultRuntime,
@@ -129,25 +133,28 @@ func (l *taskList) delete(ctx context.Context, t *Task) {
 }
 
 func New(ic *plugin.InitContext) (interface{}, error) {
-	path := filepath.Join(ic.State, runtimeName)
-	if err := os.MkdirAll(path, 0700); err != nil {
+	if err := os.MkdirAll(ic.Root, 0700); err != nil {
+		return nil, err
+	}
+	monitor, err := ic.Get(plugin.TaskMonitorPlugin)
+	if err != nil {
 		return nil, err
 	}
 	cfg := ic.Config.(*Config)
 	c, cancel := context.WithCancel(ic.Context)
 	r := &Runtime{
-		root:          path,
+		root:          ic.Root,
 		remote:        !cfg.NoShim,
 		shim:          cfg.Shim,
 		runtime:       cfg.Runtime,
 		events:        make(chan *plugin.Event, 2048),
 		eventsContext: c,
 		eventsCancel:  cancel,
-		monitor:       ic.Monitor,
+		monitor:       monitor.(plugin.TaskMonitor),
 		tasks:         newTaskList(),
 	}
 	// set the events output for a monitor if it generates events
-	ic.Monitor.Events(r.events)
+	r.monitor.Events(r.events)
 	tasks, err := r.loadAllTasks(ic.Context)
 	if err != nil {
 		return nil, err
@@ -157,7 +164,6 @@ func New(ic *plugin.InitContext) (interface{}, error) {
 			return nil, err
 		}
 	}
-	// load all tasks from disk
 	return r, nil
 }
 
@@ -174,6 +180,10 @@ type Runtime struct {
 	tasks         *taskList
 }
 
+func (r *Runtime) ID() string {
+	return pluginID
+}
+
 func (r *Runtime) Create(ctx context.Context, id string, opts plugin.CreateOpts) (t plugin.Task, err error) {
 	namespace, err := namespaces.NamespaceRequired(ctx)
 	if err != nil {
@@ -358,7 +368,7 @@ func (r *Runtime) forward(events shim.Shim_EventsClient) {
 		}
 		r.events <- &plugin.Event{
 			Timestamp:  time.Now(),
-			Runtime:    runtimeName,
+			Runtime:    r.ID(),
 			Type:       et,
 			Pid:        e.Pid,
 			ID:         e.ID,

linux/task.go

@@ -35,7 +35,7 @@ func (c *Task) Info() plugin.TaskInfo {
 	return plugin.TaskInfo{
 		ID:          c.containerID,
 		ContainerID: c.containerID,
-		Runtime:     runtimeName,
+		Runtime:     pluginID,
 		Spec:        c.spec,
 		Namespace:   c.namespace,
 	}

metrics/cgroups/cgroups.go

@@ -13,11 +13,10 @@ import (
 	"golang.org/x/net/context"
 )
 
-const name = "cgroups"
-
 func init() {
-	plugin.Register(name, &plugin.Registration{
+	plugin.Register(&plugin.Registration{
 		Type: plugin.TaskMonitorPlugin,
+		ID:   "cgroups",
 		Init: New,
 	})
 }

plugin/context.go

@@ -0,0 +1,36 @@
+package plugin
+
+import (
+	"context"
+	"fmt"
+)
+
+func NewContext(plugins map[PluginType][]interface{}) *InitContext {
+	return &InitContext{
+		plugins: plugins,
+	}
+}
+
+type InitContext struct {
+	Root    string
+	Context context.Context
+	Config  interface{}
+
+	plugins map[PluginType][]interface{}
+}
+
+func (i *InitContext) Get(t PluginType) (interface{}, error) {
+	p := i.plugins[t]
+	if len(p) == 0 {
+		return nil, fmt.Errorf("no plugins registered for %s", t)
+	}
+	return p[0], nil
+}
+
+func (i *InitContext) GetAll(t PluginType) ([]interface{}, error) {
+	p, ok := i.plugins[t]
+	if !ok {
+		return nil, fmt.Errorf("no plugins registered for %s", t)
+	}
+	return p, nil
+}

plugin/errors.go

@@ -3,7 +3,6 @@ package plugin
 import "errors"
 
 var (
-	ErrUnknownRuntime    = errors.New("unknown runtime")
 	ErrContainerExists   = errors.New("container with id already exists")
 	ErrContainerNotExist = errors.New("container does not exist")
 	ErrRuntimeNotExist   = errors.New("runtime does not exist")

plugin/plugin.go

@@ -1,45 +1,42 @@
 package plugin
 
 import (
+	"errors"
 	"fmt"
 	"sync"
 
-	"github.com/boltdb/bolt"
-	"github.com/containerd/containerd/content"
-	"github.com/containerd/containerd/snapshot"
-
-	"golang.org/x/net/context"
 	"google.golang.org/grpc"
 )
 
-type PluginType int
+var (
+	ErrNoPluginType = errors.New("plugin: no type")
+	ErrNoPluginID   = errors.New("plugin: no id")
+)
+
+type PluginType string
 
 const (
-	RuntimePlugin PluginType = iota + 1
-	GRPCPlugin
-	SnapshotPlugin
-	TaskMonitorPlugin
-	DiffPlugin
+	RuntimePlugin     PluginType = "io.containerd.runtime.v1"
+	GRPCPlugin        PluginType = "io.containerd.grpc.v1"
+	SnapshotPlugin    PluginType = "io.containerd.snapshotter.v1"
+	TaskMonitorPlugin PluginType = "io.containerd.monitor.v1"
+	DiffPlugin        PluginType = "io.containerd.differ.v1"
+	MetadataPlugin    PluginType = "io.containerd.metadata.v1"
+	ContentPlugin     PluginType = "io.containerd.content.v1"
 )
 
 type Registration struct {
-	Type   PluginType
-	Config interface{}
-	Init   func(*InitContext) (interface{}, error)
+	Type     PluginType
+	ID       string
+	Config   interface{}
+	Requires []PluginType
+	Init     func(*InitContext) (interface{}, error)
+
+	added bool
 }
 
-// TODO(@crosbymichael): how do we keep this struct from growing but support dependency injection for loaded plugins?
-type InitContext struct {
-	Root        string
-	State       string
-	Runtimes    map[string]Runtime
-	Content     content.Store
-	Meta        *bolt.DB
-	Snapshotter snapshot.Snapshotter
-	Differ      Differ
-	Config      interface{}
-	Context     context.Context
-	Monitor     TaskMonitor
+func (r *Registration) URI() string {
+	return fmt.Sprintf("%s.%s", r.Type, r.ID)
 }
 
 type Service interface {
@@ -48,10 +45,8 @@ type Service interface {
 
 var register = struct {
 	sync.Mutex
-	r map[string]*Registration
-}{
-	r: make(map[string]*Registration),
-}
+	r []*Registration
+}{}
 
 // Load loads all plugins at the provided path into containerd
 func Load(path string) (err error) {
@@ -67,16 +62,39 @@ func Load(path string) (err error) {
 	return loadPlugins(path)
 }
 
-func Register(name string, r *Registration) error {
+func Register(r *Registration) {
 	register.Lock()
 	defer register.Unlock()
-	if _, ok := register.r[name]; ok {
-		return fmt.Errorf("plugin already registered as %q", name)
+	if r.Type == "" {
+		panic(ErrNoPluginType)
 	}
-	register.r[name] = r
-	return nil
+	if r.ID == "" {
+		panic(ErrNoPluginID)
+	}
+	register.r = append(register.r, r)
 }
 
-func Registrations() map[string]*Registration {
-	return register.r
+func Graph() (ordered []*Registration) {
+	for _, r := range register.r {
+		children(r.Requires, &ordered)
+		if !r.added {
+			ordered = append(ordered, r)
+			r.added = true
+		}
+	}
+	return ordered
+}
+
+func children(types []PluginType, ordered *[]*Registration) {
+	for _, t := range types {
+		for _, r := range register.r {
+			if r.Type == t {
+				children(r.Requires, ordered)
+				if !r.added {
+					*ordered = append(*ordered, r)
+					r.added = true
+				}
+			}
+		}
+	}
 }

plugin/runtime.go

@@ -32,6 +32,8 @@ type Exit struct {
 // Runtime is responsible for the creation of containers for a certain platform,
 // arch, or custom usage.
 type Runtime interface {
+	// ID of the runtime
+	ID() string
 	// Create creates a container with the provided id and options
 	Create(ctx context.Context, id string, opts CreateOpts) (Task, error)
 	// Get returns a container

services/containers/service.go

@@ -13,10 +13,18 @@ import (
 )
 
 func init() {
-	plugin.Register("containers-grpc", &plugin.Registration{
+	plugin.Register(&plugin.Registration{
 		Type: plugin.GRPCPlugin,
+		ID:   "containers",
+		Requires: []plugin.PluginType{
+			plugin.MetadataPlugin,
+		},
 		Init: func(ic *plugin.InitContext) (interface{}, error) {
-			return NewService(ic.Meta), nil
+			m, err := ic.Get(plugin.MetadataPlugin)
+			if err != nil {
+				return nil, err
+			}
+			return NewService(m.(*bolt.DB)), nil
 		},
 	})
 }

services/content/service.go

@@ -30,15 +30,23 @@ var bufPool = sync.Pool{
 var _ api.ContentServer = &Service{}
 
 func init() {
-	plugin.Register("content-grpc", &plugin.Registration{
+	plugin.Register(&plugin.Registration{
 		Type: plugin.GRPCPlugin,
+		ID:   "content",
+		Requires: []plugin.PluginType{
+			plugin.ContentPlugin,
+		},
 		Init: NewService,
 	})
 }
 
 func NewService(ic *plugin.InitContext) (interface{}, error) {
+	c, err := ic.Get(plugin.ContentPlugin)
+	if err != nil {
+		return nil, err
+	}
 	return &Service{
-		store: ic.Content,
+		store: c.(content.Store),
 	}, nil
 }
 

services/diff/service.go

@@ -10,11 +10,19 @@ import (
 )
 
 func init() {
-	plugin.Register("diff-grpc", &plugin.Registration{
+	plugin.Register(&plugin.Registration{
 		Type: plugin.GRPCPlugin,
+		ID:   "diff",
+		Requires: []plugin.PluginType{
+			plugin.DiffPlugin,
+		},
 		Init: func(ic *plugin.InitContext) (interface{}, error) {
+			d, err := ic.Get(plugin.DiffPlugin)
+			if err != nil {
+				return nil, err
+			}
 			return &service{
-				diff: ic.Differ,
+				diff: d.(plugin.Differ),
 			}, nil
 		},
 	})

services/execution/service.go

@@ -35,22 +35,45 @@ var (
 )
 
 func init() {
-	plugin.Register("tasks-grpc", &plugin.Registration{
+	plugin.Register(&plugin.Registration{
 		Type: plugin.GRPCPlugin,
+		ID:   "tasks",
+		Requires: []plugin.PluginType{
+			plugin.RuntimePlugin,
+			plugin.MetadataPlugin,
+			plugin.ContentPlugin,
+		},
 		Init: New,
 	})
 }
 
 func New(ic *plugin.InitContext) (interface{}, error) {
-	c, err := newCollector(ic.Context, ic.Runtimes)
+	rt, err := ic.GetAll(plugin.RuntimePlugin)
+	if err != nil {
+		return nil, err
+	}
+	m, err := ic.Get(plugin.MetadataPlugin)
+	if err != nil {
+		return nil, err
+	}
+	ct, err := ic.Get(plugin.ContentPlugin)
+	if err != nil {
+		return nil, err
+	}
+	runtimes := make(map[string]plugin.Runtime)
+	for _, rr := range rt {
+		r := rr.(plugin.Runtime)
+		runtimes[r.ID()] = r
+	}
+	c, err := newCollector(ic.Context, runtimes)
 	if err != nil {
 		return nil, err
 	}
 	return &Service{
-		runtimes:  ic.Runtimes,
-		db:        ic.Meta,
+		runtimes:  runtimes,
+		db:        m.(*bolt.DB),
 		collector: c,
-		store:     ic.Content,
+		store:     ct.(content.Store),
 	}, nil
 }
 
@@ -461,7 +484,7 @@ func (s *Service) getTask(ctx context.Context, id string) (plugin.Task, error) {
 func (s *Service) getRuntime(name string) (plugin.Runtime, error) {
 	runtime, ok := s.runtimes[name]
 	if !ok {
-		return nil, plugin.ErrUnknownRuntime
+		return nil, fmt.Errorf("unknown runtime %q", name)
 	}
 	return runtime, nil
 }

services/healthcheck/service.go

@@ -13,8 +13,9 @@ type Service struct {
 }
 
 func init() {
-	plugin.Register("healthcheck-grpc", &plugin.Registration{
+	plugin.Register(&plugin.Registration{
 		Type: plugin.GRPCPlugin,
+		ID:   "healthcheck",
 		Init: NewService,
 	})
 }

services/images/service.go

@@ -12,10 +12,18 @@ import (
 )
 
 func init() {
-	plugin.Register("images-grpc", &plugin.Registration{
+	plugin.Register(&plugin.Registration{
 		Type: plugin.GRPCPlugin,
+		ID:   "images",
+		Requires: []plugin.PluginType{
+			plugin.MetadataPlugin,
+		},
 		Init: func(ic *plugin.InitContext) (interface{}, error) {
-			return NewService(ic.Meta), nil
+			m, err := ic.Get(plugin.MetadataPlugin)
+			if err != nil {
+				return nil, err
+			}
+			return NewService(m.(*bolt.DB)), nil
 		},
 	})
 }

services/namespaces/service.go

@@ -15,10 +15,18 @@ import (
 )
 
 func init() {
-	plugin.Register("namespaces-grpc", &plugin.Registration{
+	plugin.Register(&plugin.Registration{
 		Type: plugin.GRPCPlugin,
+		ID:   "namespaces",
+		Requires: []plugin.PluginType{
+			plugin.MetadataPlugin,
+		},
 		Init: func(ic *plugin.InitContext) (interface{}, error) {
-			return NewService(ic.Meta), nil
+			m, err := ic.Get(plugin.MetadataPlugin)
+			if err != nil {
+				return nil, err
+			}
+			return NewService(m.(*bolt.DB)), nil
 		},
 	})
 }

services/snapshot/service.go

@@ -16,10 +16,18 @@ import (
 )
 
 func init() {
-	plugin.Register("snapshots-grpc", &plugin.Registration{
+	plugin.Register(&plugin.Registration{
 		Type: plugin.GRPCPlugin,
+		ID:   "snapshots",
+		Requires: []plugin.PluginType{
+			plugin.SnapshotPlugin,
+		},
 		Init: func(ic *plugin.InitContext) (interface{}, error) {
-			return newService(ic.Snapshotter)
+			s, err := ic.Get(plugin.SnapshotPlugin)
+			if err != nil {
+				return nil, err
+			}
+			return newService(s.(snapshot.Snapshotter))
 		},
 	})
 }

services/version/service.go

@@ -12,8 +12,9 @@ import (
 var _ api.VersionServer = &Service{}
 
 func init() {
-	plugin.Register("version-grpc", &plugin.Registration{
+	plugin.Register(&plugin.Registration{
 		Type: plugin.GRPCPlugin,
+		ID:   "version",
 		Init: New,
 	})
 }

snapshot/btrfs/btrfs.go

@@ -20,11 +20,11 @@ import (
 )
 
 func init() {
-	plugin.Register("snapshot-btrfs", &plugin.Registration{
+	plugin.Register(&plugin.Registration{
+		ID:   "btrfs",
 		Type: plugin.SnapshotPlugin,
 		Init: func(ic *plugin.InitContext) (interface{}, error) {
-			root := filepath.Join(ic.Root, "snapshot", "btrfs")
-			return NewSnapshotter(root)
+			return NewSnapshotter(ic.Root)
 		},
 	})
 }

snapshot/naive/naive.go

@@ -16,10 +16,11 @@ import (
 )
 
 func init() {
-	plugin.Register("snapshot-naive", &plugin.Registration{
+	plugin.Register(&plugin.Registration{
 		Type: plugin.SnapshotPlugin,
+		ID:   "naive",
 		Init: func(ic *plugin.InitContext) (interface{}, error) {
-			return NewSnapshotter(filepath.Join(ic.Root, "snapshot", "naive"))
+			return NewSnapshotter(ic.Root)
 		},
 	})
 }

snapshot/overlay/overlay.go

@@ -20,10 +20,11 @@ import (
 )
 
 func init() {
-	plugin.Register("snapshot-overlay", &plugin.Registration{
+	plugin.Register(&plugin.Registration{
 		Type: plugin.SnapshotPlugin,
+		ID:   "overlayfs",
 		Init: func(ic *plugin.InitContext) (interface{}, error) {
-			return NewSnapshotter(filepath.Join(ic.Root, "snapshot", "overlay"))
+			return NewSnapshotter(ic.Root)
 		},
 	})
 }

snapshot/windows/windows.go

@@ -4,7 +4,6 @@ package windows
 
 import (
 	"context"
-	"path/filepath"
 
 	"github.com/containerd/containerd/mount"
 	"github.com/containerd/containerd/plugin"
@@ -17,10 +16,11 @@ var (
 )
 
 func init() {
-	plugin.Register("snapshot-windows", &plugin.Registration{
+	plugin.Register(&plugin.Registration{
 		Type: plugin.SnapshotPlugin,
+		ID:   "windows",
 		Init: func(ic *plugin.InitContext) (interface{}, error) {
-			return NewSnapshotter(filepath.Join(ic.Root, "snapshot", "windows"))
+			return NewSnapshotter(ic.Root)
 		},
 	})
 }

windows/runtime.go

@@ -27,14 +27,15 @@ const (
 var _ = (plugin.Runtime)(&Runtime{})
 
 func init() {
-	plugin.Register(runtimeName, &plugin.Registration{
+	plugin.Register(&plugin.Registration{
+		ID:   "windows",
 		Type: plugin.RuntimePlugin,
 		Init: New,
 	})
 }
 
 func New(ic *plugin.InitContext) (interface{}, error) {
-	rootDir := filepath.Join(ic.Root, runtimeName)
+	rootDir := filepath.Join(ic.Root)
 	if err := os.MkdirAll(rootDir, 0755); err != nil {
 		return nil, errors.Wrapf(err, "could not create state directory at %s", rootDir)
 	}
@@ -63,7 +64,7 @@ func New(ic *plugin.InitContext) (interface{}, error) {
 	}
 
 	// Try to delete the old state dir and recreate it
-	stateDir := filepath.Join(ic.State, runtimeName)
+	stateDir := filepath.Join(ic.Root, "state")
 	if err := os.RemoveAll(stateDir); err != nil {
 		log.G(c).WithError(err).Warnf("failed to cleanup old state directory at %s", stateDir)
 	}
@@ -99,6 +100,10 @@ type RuntimeSpec struct {
 	hcs.Configuration
 }
 
+func (r *Runtime) ID() string {
+	return fmt.Sprintf("%s.%s", plugin.RuntimePlugin, runtimeName)
+}
+
 func (r *Runtime) Create(ctx context.Context, id string, opts plugin.CreateOpts) (plugin.Task, error) {
 	var rtSpec RuntimeSpec
 	if err := json.Unmarshal(opts.Spec, &rtSpec); err != nil {