Github user arunmahadevan commented on the issue:
https://github.com/apache/storm/pull/2218
This builds on top of the existing state checkpointing mechanism
(documented here -
https://github.com/apache/storm/blob/master/docs/State-checkpointing.md).
Theres nothing extra added to the underlying checkpointing mechanism itself and
its pretty straightforward.
The tuples in window (think a FIFO queue) are split into multiple
partitions so that they are more manageable and can be distributed/sharded via
the underlying key-value state (redis/hbase etc). The modified partitions are
saved during a checkpoint. During iteration the partition are loaded on demand
from the underlying state backend as they are accessed. A subset of the most
recently used partitions are cached in memory (backed by a guava Loading Cache).
During checkpoint, the following are saved :
1. Any modified or newly created window partitions.
2. Any state needed to recover the Trigger/Eviction policies.
3. State thats exposed to the user where the user may have saved some
values.
Since the KV state does not guarantee any specific ordering of the keys
during iteration, a separate structure is maintained to store the ordered
partition Ids which is used during iteration to retrieve the partitions in
order. This is also saved during the checkpoint.
The above mechanism kicks in only if user choses to use the windowed state
persistence, otherwise the current behavior (keeping the tuples in an in-memory
queue) is retained.
I would like to capture the design details in the PR/Documentation itself
as far as possible than adding it in separate docs which tend to get lost.
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