Github user bowenli86 commented on a diff in the pull request:
https://github.com/apache/flink/pull/5239#discussion_r167356160
--- 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
--- End diff --
life cycle**s**
primary cop**ies**
---
