make system iterators thread-safer
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Key: ACCUMULO-533
URL: https://issues.apache.org/jira/browse/ACCUMULO-533
Project: Accumulo
Issue Type: Bug
Reporter: Adam Fuchs
Assignee: Adam Fuchs
It is possible that at some point in the future users will want to write
multi-threaded iterators. This could possibly introduce thread safety problems
in the system iterators, especially in the case where multiple threads might be
calling "next", "getTopKey", etc. at the same time. In order to maintain some
semblance of sanity, we need to synchronize these methods at the top of the
system iterator tree. Eventually, we might also want to change the iterator API
to make it useful to call multiple of these methods concurrently, but that will
be in another ticket.
The immediate solution calls for a simple iterator that is similar to the
WrappingIterator called the SynchronizedIterator, all of whose public methods
are synchronized on the object's monitor. In single-node performance testing we
found the read performance cost of this solution to be below the noise
threshold, approximately less than a 1% cost.
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