atomic

Documentation

Index

• type FileProjection
• type Writer
  • func NewAtomicWriter(targetDir string) (*Writer, error)
  • func (w *Writer) Write(payload map[string]FileProjection) error

Types

type FileProjection

type FileProjection struct {
	Data []byte
	Mode int32
}

type Writer

type Writer struct {
	// contains filtered or unexported fields
}

Writer handles atomically projecting content for a set of files into a target directory.

Note:

1. Writer reserves the set of pathnames starting with `..`.
2. Writer offers no concurrency guarantees and must be synchronized by the caller.

The visible files in this volume are symlinks to files in the writer's data directory. Actual files are stored in a hidden timestamped directory which is symlinked to the data directory. The timestamped directory and data directory symlink are created in the writer's target dir.  This scheme allows the files to be atomically updated by changing the target of the data directory symlink.

Consumers of the target directory can monitor the ..data symlink using inotify or fanotify to receive events when the content in the volume is updated.

func NewAtomicWriter

func NewAtomicWriter(targetDir string) (*Writer, error)

NewAtomicWriter creates a new Writer configured to write to the given target directory, or returns an error if the target directory does not exist.

func (*Writer) Write

func (w *Writer) Write(payload map[string]FileProjection) error

Write does an atomic projection of the given payload into the writer's target directory. Input paths must not begin with '..'.

The Write algorithm is:

1. The payload is validated; if the payload is invalid, the function returns 2.  The current timestamped directory is detected by reading the data directory symlink

3. The old version of the volume is walked to determine whether any portion of the payload was deleted and is still present on disk.

4. The data in the current timestamped directory is compared to the projected data to determine if an update is required. 5.  A new timestamped dir is created

6. The payload is written to the new timestamped directory 7.  Symlinks and directory for new user-visible files are created (if needed).

   For example, consider the files: <target-dir>/podName <target-dir>/user/labels <target-dir>/k8s/annotations

   The user visible files are symbolic links into the internal data directory: <target-dir>/podName -> ..data/podName <target-dir>/usr -> ..data/usr <target-dir>/k8s -> ..data/k8s

   The data directory itself is a link to a timestamped directory with the real data: <target-dir>/..data -> ..2016_02_01_15_04_05.12345678/ 8.  A symlink to the new timestamped directory ..data_tmp is created that will become the new data directory 9.  The new data directory symlink is renamed to the data directory; rename is atomic

10. Old paths are removed from the user-visible portion of the target directory 11.  The previous timestamped directory is removed, if it exists