[FLINK-5763] Make savepoints self-contained and relocatable

• Type: Improvement
• Status: Closed
• Priority: Critical
• Resolution: Fixed
• Fix Version/s: 1.11.0
• Component/s: Runtime / State Backends
• Labels: pull-request-available usability

Release Note

After FLINK-5763, we made savepoint self-contained and relocatable so that users can migrate savepoint from one place to another without any other processing manually. Currently do not support this feature after Entropy Injection enabled.

• Assignee: Congxian Qiu
• Reporter: Ufuk Celebi
• Created: 09/Feb/17 16:23
• Updated: 08/Dec/21 13:33
• Resolved: 16/May/20 15:28

Description

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.

Problems

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.

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.

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.

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.

Desired File Layout

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

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

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

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).

Issue Links

• blocks: FLINK-5820 Extend State Backend Abstraction to support Global Cleanup Hooks
• breaks: FLINK-18744 resume from modified savepoint dirctionary: No such file or directory
• is duplicated by: FLINK-14357 Request for the ability to move checkpoints/savepoints
• is related to: FLINK-18091 Test Relocatable Savepoints
• relates to: FLINK-24149 Make checkpoint self-contained and relocatable
• links to: GitHub Pull Request #3345
• links to: GitHub Pull Request #3346
• links to: GitHub Pull Request #11722
• links to: GitHub Pull Request #12187
• links to: High level description of the implementation approach