[jira] [Created] (FLINK-5763) Make savepoints self-contained and relocatable

["Ufuk Celebi (JIRA)" <ji...@apache.org>]

Ufuk Celebi created FLINK-5763:
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             Summary: Make savepoints self-contained and relocatable
                 Key: FLINK-5763
                 URL: https://issues.apache.org/jira/browse/FLINK-5763
             Project: Flink
          Issue Type: Improvement
          Components: State Backends, Checkpointing
            Reporter: Ufuk Celebi
            Assignee: Ufuk Celebi


After a user has triggered a savepoint, a single savepoint file will be returned as a handle to the savepoint. A savepoint to {{<target>}} creates a savepoint file like {{<target>/savepoint-<randomSuffix>}}.

This file contains the metadata of the corresponding checkpoint, but not the actual program state. While this works well for short term management (pause-and-resume a job), it makes it hard to manage savepoints over longer periods of time.

h4. Problems

h5. Scattered Checkpoint Files

For file system based checkpoints (FsStateBackend, RocksDBStateBackend) this results in the savepoint referencing files from the checkpoint directory (usually different than <target>). For users, it is virtually impossible to tell which checkpoint files belong to a savepoint and which are lingering around. This can easily lead to accidentally invalidating a savepoint by deleting checkpoint files.

h5. Savepoints Not Relocatable

Even if a user is able to figure out which checkpoint files belong to a savepoint, moving these files will invalidate the savepoint as well, because the metadata file references absolute file paths.

h5. Forced to Use CLI for Disposal

Because of the scattered files, the user is in practice forced to use Flink’s CLI to dispose a savepoint. This should be possible to handle in the scope of the user’s environment via a file system delete operation.

h4. Proposal

In order to solve the described problems, savepoints should contain all their state, both metadata and program state, inside a single directory. Furthermore the metadata must only hold relative references to the checkpoint files. This makes it obvious which files make up the state of a savepoint and it is possible to move savepoints around by moving the savepoint directory.

h5. Desired File Layout

Triggering a savepoint to {{<target>}} creates a directory as follows:

{code}
<target>/savepoint-<jobId>-<randomSuffix>
  +-- _metadata
  +-- data-<randomSuffix> [1 or more]
{code}

We include the JobID in the savepoint directory name in order to give some hints about which job a savepoint belongs to.

h5. CLI

- Trigger: When triggering a savepoint to {{<target>}} the savepoint directory will be returned as the handle to the savepoint.
- Restore: Users can restore by pointing to the directory or the _metadata file. The data files should be required to be in the same directory as the _metadata file.
- Dispose: The disposal command should be deprecated and eventually removed. While deprecated, disposal can happen by specifying the directory or the _metadata file (same as restore).




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