[External] : Re: JEP draft: Scope Locals

[Mike Rettig]

On Mon, May 17, 2021 at 2:55 PM Brian Goetz <brian.goetz at oracle.com> wrote:

>
> Let's try again: why is that important?   What decisions would you make
>> differently if you had this information?  What benefit would come from
>> those decisions?
>>
>
> Threading is hard.
>
>
> Gee, thanks MIke for pointing that out to me, obviously I'm new to the
> topic :)
>

Sorry if that sounded condescending to anyone. I was stating the difficulty
of threading as a basis for my decision making. I wasn't implying that the
various experts on this list don't know about threading.


>
> Scoped variables are also hard to get right. Combining the two without an
> explicit api ignores the complexity that will arise as scoped variables are
> passed to other threads.
>
>
> OK, now we're getting to the actual question; you obviously have some
> assumptions about the degree of sharing expected, that might be skipping a
> few steps (and then skipping some more steps by jumping to a solution).  So
> to ask again: what patterns of access / sharing are you assuming, what
> risks are you anticipating, etc?
>
>
I'm assuming values that are shared are not always simple immutable data
structures. Without life cycles mutable data structures are difficult to
share without a well defined lifecycle and concurrency approach.  I also
think there are many potential benefits beyond just sharing variables. For
example, accumulating request information and setting log4j's NDC would be
trivial.

https://logging.apache.org/log4j/1.2/apidocs/org/apache/log4j/NDC.html

Scope locals may be called upon in order to reuse an expensive resource
like a database connection or output stream.  These resources need to be
flushed and closed at the end of scope. An ideal solution would pass the
required data structures directly through the api and to any subtasks.
However thread locals and scoped variables are used when that isn't
realistic.  With the current proposal I can't simply flush and close at the
end of the scope because there might be a child scope that is still active.
Unless the code I'm invoking gives me some way to await that scope, then I
don't know when I can proceed.

Mike