"
The multishard combined reader provides a convenient
flat_mutation_reader implementation that takes care of efficiently
reading a range from all shards that own data belonging to the range.
All this happens transparently, the user of the reader need only pass a
factory function to the multishard reader which it uses to create
remote readers when needed. These remote readers will then be managed
through foreign reader which abstracts away the fact that the reader is
located on a remote shard.
Sub readers are created for the entire read range, meaning they are free
to cross shard-range limits to fill their buffer. The output of these
sub readers is merged in a round-robin manner, the same way data is
distributed among shards. The multishard reader will move to the next
shard's reader whenever it encounters a partition whose token is after
the delimiter token.
To improve throughput and latency two levels of read-ahead is employed.
One in foreign_reader, which will try to fill the remote shard reader's
buffer in the background, in parallel to processing the results on the
local shard. And one in the multishard reader itself which will
exponentially increase concurrency whenever a sub-reader's buffer
becomes empty. But only if this happened after crossing a shard
boundary. This is important because there is no point in increasing
concurrency if a single sub reader can fill the multishard readers'
buffer.
"
* 'multishard-reader/v3' of https://github.com/denesb/scylla:
Add unit tests for multishard_combined_reader
Add multishard_combined_reader
flat_mutation_reader: add peek_buffer()
Add unit tests for foreign_reader
forwardable reader: implement fast_forward_to(position_in_partition)
Add foreign_reader
flat_mutation_reader: add detach_buffer()
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