Details
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Improvement
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Status: Resolved
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Major
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Resolution: Fixed
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None
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None
Description
When reading a stream of data from Meta Storage (e.g. using MetaStorageManager#range method) a Cursor is created on the server side (see MetaStorageListener), which is stored in memory for resource management purposes. This approach allows Cursor resources to be replicated and no new Cursors need to be created if a leader changes during an ongoing range operation. On the other hand, it requires additional resources to be consumed on Raft Followers and requires all Cursor management commands to be Write commands, which doesn't allow them to be executed not on a Leader.
It is proposed to replace this approach with the following: make Range commands Read commands and pass the last received key as a parameter. This will allow reading the next batch of data without re-using a previous cursor, a new cursor will simply be created. On every batch request, a cursor will be opened, the batch will be collected into a list and the cursor will be closed. We will lose RocksDB snapshot isolation though (since a new iterator will be created every time), but this should not be a problem as every request should be bound to a particular Meta Storage revision.
This approach will also help with migrating Meta Storage to ReplicaService mechanism instead of using Raft for reading data, which will allow reads from any node. However, this should be done in a different ticket.
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