Many users implement a queue service using RocksDB. In these services, from each queue, new items are added with a higher sequence ID and removed from the smallest sequence ID. Users usually read from a queue in sequence ID increasing order.
You can simply encode it as
<queue_id, sequence_id>, where queue_id is fixed length encoded and sequence_id is encoded as big endian.
While iterating keys, a user can create an iterator and seek to
<queue_id, target_sequence_id> and iterate from there.
Old Item Deletion Problem
Since the oldest items are deleted, there can be a large amount of “tombstones” in the beginning of each
queue_id. As a result, the two queries might be exceptionally slow:
- While you are in the last sequence ID of a
queue_idand try to call Next()
To mitigate a problem, you can remember the first and last sequence ID of each queue_id, and never iterate over the range.
As another way to solve the second problem, you can set an end key of your iterate when you iterate inside a queue_id, by letting
ReadOptions.iterate_upper_bound point to
<queue_id + 1>. We encourage you always set it no matter whether you see the slowness problem caused by deletions.
Checking new sequence IDs of a queue_id
If a user finishes processing the last
sequence_id of a
queue_id, and keep polling new item to be created, just Seek(
<queue_id, last_processed_id>) and call Next() and see whether the next key is still for the same
<queue_id>. Make sure
ReadOptions.iterate_upper_bound points to
<queue_id + 1> to avoid slowness for the item deletion problem.
If you want to further optimize this use case, to avoid binary search of the whole LSM tree each time, consider using
ForwardIterator called in some parts of the codes) (https://github.com/facebook/rocksdb/blob/master/include/rocksdb/options.h#L1235-L1241).
Reclaiming space of deleted items faster
The queue service is a good use case of
CompactOnDeletionCollector, which prioritizes ranges with more deletes when scheduling compactions. Set ImmutableCFOptions::table_properties_collector_factories to the factory defined here: https://github.com/facebook/rocksdb/blob/master/include/rocksdb/utilities/table_properties_collectors.h#L23-L27