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From bowenli86 <...@git.apache.org>
Subject [GitHub] flink pull request #5239: [FLINK-8360] Implement task-local state recovery
Date Sat, 10 Feb 2018 05:05:25 GMT
Github user bowenli86 commented on a diff in the pull request:

    --- 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
    --- End diff --
    abd -> and 


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