In general, it's not safe to keep the iterator open, because when
process() is executed in-between two punctuator calls, it might modify
the store and invalidate the iterator. There is no guarantee that the
returned iterator supports concurrency.
Hence, even if it happens that the currently used iterator is
concurrent, there is no API contract about it.
-Matthias
On 11/7/22 7:41 AM, Joshua Suskalo wrote:
Hello Matthias, thanks for the response!
"Matthias J. Sax" <mj...@apache.org> writes:
Spanning your own thread and calling context.forward() is _not_ safe, and there
is currently no way for you to make is safe. The runtime code makes certain
assumptions about being single threaded which would break if you call
context.forward() from a different thread. (The runtime _always_ assume that
context.forward() is called inside process() or inside the punctuation
callback.)
This is how I expected, so I'm not too concerned here.
The only way forward I can see, would be trying to make the punctuation call
shorter, eg, not scanning the full store but only a small part of it, such that
the thread can go back to execute process() quicker (it's of course an
additional challenge to keep track where you stopped the scan and to resume
it...), and to make the punctuation interval shorter.
This is where I have another question about safety. Is it safe to use a
KVStoreIterator that was retrieved during a punctuator after that punctuator
call has exited, perhaps using it to store offset information across multiple
runs? This seems to me to be the most obvious way to handle this.
A related question if that one has an affirmative answer would be asking if the
KVStoreIterator can be safely sent to another thread for use (akin to Rust's
Send trait, in that I do not wish to have concurrent access, only to use it from
another thread), as if that were possible I could use the j.u.c.ConcurrentQueue
to allow a thread created on the punctuator to compute the messages that need to
be sent and enqueue them, and then in further punctuator runs I could then send
messages which have been computed.
Hope this helps.
What you've said already is quite helpful and I will begin pursuing methods
based on what you've said, but I also hope that someone can answer my further
questions since they will be helpful to my implementation as well.
Thanks so much,
Joshua
