Thank you again for the feedback Sophie!
As I tried to point out in my previous e-mail, removing a stream thread
from a Kafka Streams client that does not have alive stream threads is
nothing exceptional for the client per se. However, it can become
exceptional within the context of the user. For example, if users want
to remove a stream thread from a client without alive stream threads
because one if their metrics say so, then this is exceptional in the
context of that user metric not in the context of the Kafka Streams
client. In that case, users should throw an exception and handle it.
Regarding returning null, I do not like to return null because from a
development point of view there is no distinction between returning null
because we have a bug in the code or returning null because there are no
alive stream threads. Additionally, Optional<String> makes it more
explicit that the result could also be empty.
Thank you for the alternative method names! However, with the names you
propose it is not immediately clear that the method returns an amount of
stream threads. They rather suggest that the method returns a list of
handles to the stream threads. I chose to use "aliveStreamThreads" to be
consistent with the client-level metric "alive-stream-threads" which
reports the same number of stream threads that
numberOfAliveStreamThreads() should report. If others also think that
the proposed name in the KIP is too clumsy, I am open to rename it, though.
Best,
Bruno
On 08.09.20 20:12, Sophie Blee-Goldman wrote:
it's never a good sign when the discussion moves into the vote thread
Hah, sorry, the gmail consolidation of [VOTE] and [DISCUSS] threads strikes
again.
Thanks for redirecting me Bruno
I suppose it's unfair to expect the callers to keep perfect track of the
current
number of stream threads, but it also seems like you shouldn't be calling
removeStreamThread() when there are no threads left. Either you're just
haphazardly removing threads and could unintentionally slip into a state of
no
running threads without realizing it, or more realistically, you're
carefully
removing threads based on some metric(s) that convey whether the system is
over or under-provisioned. If your metrics say you're over-provisioned but
there's
not one thread running, well, that certainly sounds exceptional to me. Or
you might
be right in that the cluster is over-provisioned but have just been
directing the
removeStreamThread() and addStreamThread() calls to instances at random, and
end up with one massive instance and one with no threads at all. Again,
this
probably merits some human intervention (or system redesign)
That said, I don't think there's any real harm to just returning null in
this case, but I hope
that users would pay attention to this since it seems likely to indicate
something has gone
seriously wrong. I suppose Optional<String> would be a reasonable
compromise.
As for the method name, what about activeStreamThreads() or
liveStreamThreads() ?
On Mon, Sep 7, 2020 at 1:45 AM Bruno Cadonna <br...@confluent.io> wrote:
Hi John,
I agree with you except for checking null. I would rather prefer to use
Optional<String> as the return type to both methods.
I changed the subject from [VOTE] to [DISCUSS] so that we can follow up
in the discussion thread.
Best,
Bruno
On 04.09.20 23:12, John Roesler wrote:
Hi Sophie,
Uh, oh, it's never a good sign when the discussion moves
into the vote thread :)
I agree with you, it seems like a good touch for
removeStreamThread() to return the name of the thread that
got removed, rather than a boolean flag. Maybe the return
value would be `null` if there is no thread to remove.
If we go that way, I'd suggest that addStreamThread() also
return the name of the newly created thread, or null if no
thread can be created right now.
I'm not completely sure if I think that callers of this
method would know exactly how many threads there are. Sure,
if a human being is sitting there looking at the metrics or
logs and decides to call the method, it would work out, but
I'd expect this kind of method to find its way into
automated tooling that reacts to things like current system
load or resource saturation. Those kinds of toolchains often
are part of a distributed system, and it's probably not that
easy to guarantee that the thread count they observe is
fully consistent with the number of threads that are
actually running. Therefore, an in-situ `int
numStreamThreads()` method might not be a bad idea. Then
again, it seems sort of optional. A caller can catch an
exception or react to a `null` return value just the same
either way. Having both add/remove methods behave similarly
is probably more valuable.
Thanks,
-John
On Thu, 2020-09-03 at 12:15 -0700, Sophie Blee-Goldman
wrote:
Hey, sorry for the late reply, I just have one minor suggestion. Since
we
don't
make any guarantees about which thread gets removed or allow the user to
specify, I think we should return either the index or full name of the
thread
that does get removed by removeThread().
I know you just updated the KIP to return true/false if there
are/aren't any
threads to be removed, but I think this would be more appropriate as an
exception than as a return type. I think it's reasonable to expect
users to
have some sense to how many threads are remaining, and not try to remove
a thread when there is none left. To me, that indicates something wrong
with the user application code and should be treated as an exceptional
case.
I don't think the same code clarify argument applies here as to the
addStreamThread() case, as there's no reason for an application to be
looping and retrying removeStreamThread() since if that fails, it's
because
there are no threads left and thus it will continue to always fail. And
if
the
user actually wants to shut down all threads, they should just close the
whole application rather than call removeStreamThread() in a loop.
While I generally think it should be straightforward for users to track
how
many stream threads they have running, maybe it would be nice to add
a small utility method that does this for them. Something like
// Returns the number of currently alive threads
boolean runningStreamThreads();
On Thu, Sep 3, 2020 at 7:41 AM Matthias J. Sax <mj...@apache.org>
wrote:
+1 (binding)
On 9/3/20 6:16 AM, Bruno Cadonna wrote:
Hi,
I would like to start the voting on KIP-663 that proposes to add
methods
to the Kafka Streams client to add and remove stream threads during
execution.
https://cwiki.apache.org/confluence/display/KAFKA/KIP-663%3A+API+to+Start+and+Shut+Down+Stream+Threads
Best,
Bruno
