4f2b443a27
This rewrites the Go imports after switching to the new github org. Signed-off-by: Michael Crosby <crosbymichael@gmail.com>
209 lines
8.6 KiB
Go
209 lines
8.6 KiB
Go
package snapshot
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import (
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"context"
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"github.com/containerd/containerd"
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)
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// Kind identifies the kind of snapshot.
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type Kind int
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// definitions of snapshot kinds
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const (
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KindActive Kind = iota
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KindCommitted
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)
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// Info provides information about a particular snapshot.
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type Info struct {
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Name string // name or key of snapshot
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Parent string // name of parent snapshot
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Kind Kind // active or committed snapshot
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Readonly bool // true if readonly, only valid for active
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}
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// Snapshotter defines the methods required to implement a snapshot snapshotter for
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// allocating, snapshotting and mounting filesystem changesets. The model works
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// by building up sets of changes with parent-child relationships.
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//
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// A snapshot represents a filesystem state. Every snapshot has a parent, where
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// the empty parent is represented by the empty string. A diff can be taken
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// between a parent and its snapshot to generate a classic layer.
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//
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// An active snapshot is created by calling `Prepare`. After mounting, changes
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// can be made to the snapshot. The act of commiting creates a committed
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// snapshot. The committed snapshot will get the parent of active snapshot. The
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// committed snapshot can then be used as a parent. Active snapshots can never
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// act as a parent.
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//
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// Snapshots are best understood by their lifecycle. Active snapshots are
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// always created with Prepare or View. Committed snapshots are always created
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// with Commit. Active snapshots never become committed snapshots and vice
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// versa. All snapshots may be removed.
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//
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// For consistency, we define the following terms to be used throughout this
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// interface for snapshotter implementations:
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//
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// `key` - refers to an active snapshot
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// `name` - refers to a committed snapshot
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// `parent` - refers to the parent in relation
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//
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// Most methods take various combinations of these identifiers. Typically,
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// `name` and `parent` will be used in cases where a method *only* takes
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// committed snapshots. `key` will be used to refer to active snapshots in most
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// cases, except where noted. All variables used to access snapshots use the
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// same key space. For example, an active snapshot may not share the same key
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// with a committed snapshot.
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//
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// We cover several examples below to demonstrate the utility of a snapshot
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// snapshotter.
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//
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// Importing a Layer
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//
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// To import a layer, we simply have the Snapshotter provide a list of
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// mounts to be applied such that our dst will capture a changeset. We start
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// out by getting a path to the layer tar file and creating a temp location to
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// unpack it to:
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//
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// layerPath, tmpDir := getLayerPath(), mkTmpDir() // just a path to layer tar file.
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//
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// We start by using a Snapshotter to Prepare a new snapshot transaction, using a
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// key and descending from the empty parent "":
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//
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// mounts, err := snapshotter.Prepare(key, "")
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// if err != nil { ... }
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//
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// We get back a list of mounts from Snapshotter.Prepare, with the key identifying
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// the active snapshot. Mount this to the temporary location with the
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// following:
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//
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// if err := MountAll(mounts, tmpDir); err != nil { ... }
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//
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// Once the mounts are performed, our temporary location is ready to capture
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// a diff. In practice, this works similar to a filesystem transaction. The
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// next step is to unpack the layer. We have a special function unpackLayer
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// that applies the contents of the layer to target location and calculates the
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// DiffID of the unpacked layer (this is a requirement for docker
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// implementation):
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//
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// layer, err := os.Open(layerPath)
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// if err != nil { ... }
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// digest, err := unpackLayer(tmpLocation, layer) // unpack into layer location
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// if err != nil { ... }
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//
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// When the above completes, we should have a filesystem the represents the
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// contents of the layer. Careful implementations should verify that digest
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// matches the expected DiffID. When completed, we unmount the mounts:
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//
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// unmount(mounts) // optional, for now
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//
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// Now that we've verified and unpacked our layer, we commit the active
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// snapshot to a name. For this example, we are just going to use the layer
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// digest, but in practice, this will probably be the ChainID:
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//
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// if err := snapshotter.Commit(digest.String(), key); err != nil { ... }
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//
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// Now, we have a layer in the Snapshotter that can be accessed with the digest
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// provided during commit. Once you have committed the snapshot, the active
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// snapshot can be removed with the following:
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//
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// snapshotter.Remove(key)
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//
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// Importing the Next Layer
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//
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// Making a layer depend on the above is identical to the process described
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// above except that the parent is provided as parent when calling
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// Manager.Prepare, assuming a clean tmpLocation:
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//
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// mounts, err := snapshotter.Prepare(tmpLocation, parentDigest)
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//
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// We then mount, apply and commit, as we did above. The new snapshot will be
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// based on the content of the previous one.
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//
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// Running a Container
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//
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// To run a container, we simply provide Snapshotter.Prepare the committed image
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// snapshot as the parent. After mounting, the prepared path can
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// be used directly as the container's filesystem:
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//
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// mounts, err := snapshotter.Prepare(containerKey, imageRootFSChainID)
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//
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// The returned mounts can then be passed directly to the container runtime. If
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// one would like to create a new image from the filesystem, Manager.Commit is
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// called:
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//
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// if err := snapshotter.Commit(newImageSnapshot, containerKey); err != nil { ... }
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//
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// Alternatively, for most container runs, Snapshotter.Remove will be called to
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// signal the Snapshotter to abandon the changes.
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type Snapshotter interface {
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// Stat returns the info for an active or committed snapshot by name or
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// key.
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//
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// Should be used for parent resolution, existence checks and to discern
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// the kind of snapshot.
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Stat(ctx context.Context, key string) (Info, error)
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// Mounts returns the mounts for the active snapshot transaction identified
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// by key. Can be called on an read-write or readonly transaction. This is
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// available only for active snapshots.
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//
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// This can be used to recover mounts after calling View or Prepare.
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Mounts(ctx context.Context, key string) ([]containerd.Mount, error)
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// Prepare creates an active snapshot identified by key descending from the
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// provided parent. The returned mounts can be used to mount the snapshot
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// to capture changes.
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//
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// If a parent is provided, after performing the mounts, the destination
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// will start with the content of the parent. The parent must be a
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// committed snapshot. Changes to the mounted destination will be captured
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// in relation to the parent. The default parent, "", is an empty
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// directory.
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//
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// The changes may be saved to a committed snapshot by calling Commit. When
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// one is done with the transaction, Remove should be called on the key.
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//
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// Multiple calls to Prepare or View with the same key should fail.
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Prepare(ctx context.Context, key, parent string) ([]containerd.Mount, error)
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// View behaves identically to Prepare except the result may not be
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// committed back to the snapshot snapshotter. View returns a readonly view on
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// the parent, with the active snapshot being tracked by the given key.
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//
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// This method operates identically to Prepare, except that Mounts returned
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// may have the readonly flag set. Any modifications to the underlying
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// filesystem will be ignored. Implementations may perform this in a more
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// efficient manner that differs from what would be attempted with
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// `Prepare`.
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//
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// Commit may not be called on the provided key and will return an error.
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// To collect the resources associated with key, Remove must be called with
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// key as the argument.
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View(ctx context.Context, key, parent string) ([]containerd.Mount, error)
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// Commit captures the changes between key and its parent into a snapshot
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// identified by name. The name can then be used with the snapshotter's other
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// methods to create subsequent snapshots.
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//
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// A committed snapshot will be created under name with the parent of the
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// active snapshot.
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//
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// Commit may be called multiple times on the same key. Snapshots created
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// in this manner will all reference the parent used to start the
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// transaction.
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Commit(ctx context.Context, name, key string) error
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// Remove the committed or active snapshot by the provided key.
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//
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// All resources associated with the key will be removed.
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//
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// If the snapshot is a parent of another snapshot, its children must be
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// removed before proceeding.
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Remove(ctx context.Context, key string) error
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// Walk the committed snapshots. For each snapshot in the snapshotter, the
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// function will be called.
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Walk(ctx context.Context, fn func(context.Context, Info) error) error
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}
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