Github user StefanRRichter commented on a diff in the pull request: https://github.com/apache/flink/pull/5239#discussion_r168753015 --- Diff: docs/ops/state/large_state_tuning.md --- @@ -234,4 +234,97 @@ Compression can be activated through the `ExecutionConfig`: **Notice:** The compression option has no impact on incremental snapshots, because they are using RocksDB's internal format which is always using snappy compression out of the box. +## Task-Local Recovery + +### Motivation + +In Flink's checkpointing, each task produces a snapshot of its state that is then written to a distributed store. Each task acknowledges +a successful write of the state to the job manager by sending a handle that describes the location of the state in the distributed store. +The job manager, in turn, collects the handles from all tasks and bundles them into a checkpoint object. + +In case of recovery, the job manager opens the latest checkpoint object and sends the handles back to the corresponding tasks, which can +then restore their state from the distributed storage. Using a distributed storage to store state has two important advantages. First, the storage +is fault tolerant and second, all state in the distributed store is accessible to all nodes and can be easily redistributed (e.g. for rescaling). + +However, using a remote distributed store has also one big disadvantage: all tasks must read their state from a remote location, over the network. +In many scenarios, recovery could reschedule failed tasks to the same task manager as in the previous run (of course there are exceptions like machine +failures), but we still have to read remote state. This can result in *long recovery times for large states*, even if there was only a small failure on +a single machine. + +### Approach + +Task-local state recovery targets exactly this problem of long recovery times and the main idea is the following: for every checkpoint, we do not +only write task states to the distributed storage, but also keep *a secondary copy of the state snapshot in a storage that is local to the task* +(e.g. on local disk or in memory). Notice that the primary store for snapshots must still be the distributed store, because local storage does not +ensure durability under node failures abd also does not provide access for other nodes to redistribute state, this functionality still requires the +primary copy. + +However, for each task that can be rescheduled to the previous location for recovery, we can restore state from the secondary, local +copy and avoid the costs of reading the state remotely. Given that *many failures are not node failures and node failures typically only affect one +or very few nodes at a time*, it is very likely that in a recovery most tasks can return to their previous location and find their local state intact. +This is what makes local recovery effective in reducing recovery time. + +Please note that this can come at some additional costs per checkpoint for creating and storing the secondary local state copy, depending on the +chosen state backend and checkpointing strategy. For example, in most cases the implementation will simply duplicate the writes to the distributed +store to a local file. + +<img src="../../fig/local_recovery.png" class="center" width="80%" alt="Illustration of checkpointing with task-local recovery."/> + +### Relationship of primary (distributed store) and secondary (task-local) state snapshots + +Task-local state is always considered a secondary copy, the ground truth of the checkpoint state is the primary copy in the distributed store. This +has implications for problems with local state during checkpointing and recovery: + +- For checkpointing, the *primary copy must be successful* and a failure to produce the *secondary, local copy will not fail* the checkpoint. A checkpoint +will fail if the primary copy could not be created, even if the secondary copy was successfully created. + +- Only the primary copy is acknowledged and managed by the job manager, secondary copies are owned by task managers and their life cycle can be +independent from their primary copy. For example, it is possible to retain a history of the 3 latest checkpoints as primary copies and only keep +the task-local state of the latest checkpoint. + +- For recovery, Flink will always *attempt to restore from task-local state first*, if a matching secondary copy is available. If any problem occurs during +the recovery from the secondary copy, Flink will *transparently retry to recovery the task from the primary copy*. Recovery only fails, if primary +and the (optional) secondary copy failed. In this case, depending on the configuration Flink could still fall back to an older checkpoint. + +- It is possible that the task-local copy contains only parts of the full task state (e.g. exception while writing one local file). In this case, +Flink will first try to recover local parts locally, non-local state is restored from the primary copy. Primary state must always be complete and is +a *superset of the task-local state*. + +- Task-local state can have a different format than the primary state, they are not required to be byte identical. For example, it could even possible that +the task-local state is an in-memory consisting of heap objects, and not stored in any files. + +- If a task manager is lost, the local state from all its task is lost. + +### Configuring task-local recovery + +Task-local recovery is *deactivated by default* and can be activated through Flink's configuration with the key `state.backend.local-recovery` as specified +in `CheckpointingOptions.LOCAL_RECOVERY`. Users have currently two choices: + +- `DISABLED`: Local recovery is disabled (default). +- `ENABLE_FILE_BASED`: Local recovery is activated, based on writing a secondary copy of the task state on local disk. + +### Details on task-local recovery for different state backends + +***Limitation**: Currently, task-local recovery only covers keyed state backends. Keyed state is typically by far the largest part of the state. In the near future, we will +also cover operator state and timers.* + +The following state backends can support task-local recovery. + +- FsStateBackend: task-local recovery is supported for keyed state. The implementation will duplicate the state to a local file. This can introduce additional write costs +and occupies local disk space. In the future, we might also offer an implementation that keeps task-local state in memory. + +- RocksDBStateBackend: task-local recovery is supported for keyed state. For *full checkpoints*, state is duplicated to a local file. This can introduce additional write costs +and occupies local disk space. For *incremental snapshots*, the local state is based on RocksDB's native checkpointing mechanism. This mechanism is also used as the first step --- End diff -- In the way we use the terms snapshot and checkpoint in Flink, it is not wrong: a snapshot is what one subtask produces from its state. A checkpoint is the collection of all task's snapshots.
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