atomic_writer

package module
Version: v0.0.2 Latest Latest
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Published: May 11, 2021 License: Apache-2.0 Imports: 11 Imported by: 2

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type AtomicWriter

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

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

Note:

1. AtomicWriter reserves the set of pathnames starting with `..`. 2. AtomicWriter 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 by 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, logContext string) (*AtomicWriter, error)

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

func (*AtomicWriter) Write

func (w *AtomicWriter) Write(payload map[string]FileProjection) (bool, 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

type FileProjection

type FileProjection struct {
	Data   []byte
	Mode   int32
	FsUser *int64
}

FileProjection contains file Data and access Mode

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