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Benedict commented on CASSANDRA-9975:
-------------------------------------
On writing the follow up, CASSANDRA-9976, it became apparent it was easier and
cleaner to do both at once, since they ultimately share quite a bit of
functionality and approach. This is now available
[here|https://github.com/belliottsmith/cassandra/tree/9975].
This version also goes a step further to completely eliminate the wrapping
partition iterators, and all but a handful of wrapping row iterators (the
remainder of which can be eliminated following CASSANDRA-9986). This is
achieved by supporting concatenation / short read protection via a
transformation function that "refils" the contents of an iterator with one or
more new iterators. i.e.m once the input we're transforming is exhausted, we
ask for a new one, which is swapped in, and the function stacks updated as
necessary. This all but eliminates the jumping around of control flow for the
outer execution of a query, although the inner execution (memtable, sstable)
still has some cleaning up to be done to make it easy to debug.
> Flatten Iterator call hierarchy with a shared Transformer
> ---------------------------------------------------------
>
> Key: CASSANDRA-9975
> URL: https://issues.apache.org/jira/browse/CASSANDRA-9975
> Project: Cassandra
> Issue Type: Sub-task
> Components: Core
> Reporter: Benedict
> Assignee: Benedict
> Fix For: 3.0.x
>
>
> Stepping through a read response is made exceedingly difficult by the sheer
> depth of the call hierarchy, and how rapidly your context jumps around. This
> ticket intend to partially address that, by flattening one of the main causes
> of this: iterator transformations.
> I have a patch that attempts to mitigate (but not entirely eliminate) this,
> through the introduction of a {{RowTransformer}} class that all
> transformations are applied through. If a transformation has already been
> applied, the {{RowTransformer}} class does not wrap a new iterator, but
> instead returns a new {{RowTransformer}} that wraps the original underlying
> (untransformed) iterator and both transformations. This can accumulate an
> arbitrary number of transformations and, quite importantly, can apply the
> filtration step {{Unfiltered -> Row}} in the same instance as well. The
> intention being that a majority of control flow happens inside this
> {{RowTransformer}}, so there is far less context jumping to cope with.
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