Gwen, Randall thank you for your very insightful observations. I'm glad you
find this first draft to be an adequate platform for discussion.
I'll attempt replying to your comments in order.
Gwen, I also debated exactly the same two options: a) interpreting absence
of module path as a user's intention to turn off isolation and b)
explicitly using an additional boolean property. A few reasons why I went
with b) in this first draft are:
1) As Randall mentions, to leave the option of using a default value open.
If not immediately in the first version of isolation, maybe in the future.
2) I didn't like the implicit character of the choice of interpreting an
empty string as a clear intention to turn isolation off by the user. Half
the time could be just that users forget to set a location, although they'd
like to use class loading isolation.
3) There's a slim possibility that in rare occasions a user might want to
avoid even the slightest increase in memory consumption due to class
loading duplication. I admit this should be very rare, but given the other
concerns and that we would really like to keep the isolation implementation
simple, the option to turn off this feature by using only one additional
config property might not seem too excessive. At least at the start of this
discussion.
4) Debugging during development might be simpler in some cases.
5) Finally, as you mention, this could allow for smoother upgrades.
Randall, regarding your comments:
1) To keep its focus narrow, this KIP, as well as the first implementation
of isolation in Connect, assume filesystem based discovery. With careful
implementation, transitioning to discovery schemes that support broader
URIs I believe should be easy in the future.
2) The example you give makes a good point. However I'm inclined to say
that such cases should be addressed more as exceptions rather than as being
the common case. Therefore, I wouldn't see all dependencies imported by the
framework as required to be filtered out, because in that case we lose the
advantage of isolation between the framework and the connectors (and we are
left only with isolation between connectors).
3) I tried to abstract implementation details in this the KIP, but you are
right. Even though filtering here is mainly used semantically rather than
literally, it gives an implementation hint that we could avoid.
4) In the same spirit as in 3) I believe we should reserve enough
flexibility to the implementation to discover and load classes, when they
appear in multiple locations under the general module location.
Thanks again! Let me know what you think.
Konstantine
On Mon, May 1, 2017 at 10:12 AM, Randall Hauch <rha...@gmail.com> wrote:
> Very nice work, Konstantine. Conflicting dependencies of connectors is
> indeed a big issue that makes it hard to manage installed connectors.
>
> I do like Gwen's idea about removing the 'module.isolation.enabled'
> property. However, I would have anticipated always using classpath
> isolation for *only* those components registered under the module path and
> not really for anything else already on the normal classpath. So, people
> could continue to place custom connector JARs onto the classpath, though
> this would become deprecated in favor of installing custom connector JARs /
> modules via the module path. This keeps configuration simple, gives people
> time to migrate, but let's people that need classpath isolation get it to
> install a variety of connectors each with their dependencies that
> potentially conflict with other components.
>
> The challenge is whether there should be a default for 'module.path'.
> Ideally there would be so that users know where they can install their
> connectors. However, I suspect that this might be difficult to do unless it
> can make use of system properties such as "${kafka.home}" so that relative
> directories can be specified.
>
> A few other questions/comments:
>
> 1) Does the KIP have to specify how are components / modules installed,
> discovered, or recognized by Kafka Connect? Or perhaps the KIP needs to
> just specify the semantics of the file system module path (e.g., the
> directories below those specified in the module path are to be unique and
> identify an installed component).
>
> 2) Will the module classloader filtering also have to exclude Kafka Connect
> dependencies? The only one that I can think of is the SLF4J API, which
> can't be loaded from the module's classloader if the connector is to send
> its log messages to the same logging system.
>
> 3) Rather than specify filtering, would be it a bit more flexible to simply
> say that the implementation will need to ensure that Java, Kafka Connect,
> and other third party APIs (e.g., SLF4J API) will not be loaded from the
> module classloaders? It'd be better to avoid specifying how it will be
> done, just in case the implementation needs to evolve or use a different
> technique (e.g., load the Java and public Kafka Connect APIs via one
> classloader that is reused and that always appears before the module
> classloader, while Kafka Connect implementation JARs appear after the
> component's classloader.
>
> 4) Perhaps to address #2 and #3 above, perhaps the KIP could explicitly
> specify the classloader order for a deployed connector. For example,
> 'java', 'kafka-connect-apis', 'connector-module', 'smt-module-1', ...,
> 'kafka-connect-impls', where 'connector-module' is the classloader for the
> (first) module where the connector is found, 'smt-module-1' is the
> classloader for the (first) module where the first SMT class is found (if
> specified and found in a separate module), 'smt-module-2' is the
> classloader .... Might also need to say that the KIP does not specify how
> the implementation will pick the module if a specified class if found in
> more than one module.
>
> Thoughts?
>
> Randall
>
> On Mon, May 1, 2017 at 6:43 AM, Gwen Shapira <g...@confluent.io> wrote:
>
> > Hi Konstantine,
> >
> > Thank you so much for driving this! The connector classpath mess is
> driving
> > me nuts (or worse, driving me to use Docker).
> >
> > I like the proposal for micro-benchmarks to test the context switching
> > overhead.
> >
> > I have a difficult time figuring out the module.isolation.enabled.
> > Especially with a default to false. I can't think of a reason that anyone
> > will not want classpath isolation. "No! I want my connectors to mess up
> > each other's dependencies" said no one ever.
> >
> > So it looks like this is mostly for upgrade purpose? Because the initial
> > upgrade will not have the module.path set and therefore classpath
> isolation
> > will simply not work by default?
> >
> > In that case, why don't we simply use the existence of non-empty
> > module.path as an indicator of whether isolation should work or not? seem
> > simpler and intuitive to me.
> >
> > Thanks!
> >
> > Gwen
> >
> >
> >
> >
> >
> > On Sat, Apr 29, 2017 at 9:16 AM, Konstantine Karantasis <
> > konstant...@confluent.io> wrote:
> >
> > > * Because of KIP number collision, please disregard my previous KIP
> > > announcement and use this thread for discussion instead *
> > >
> > >
> > > Hi everyone,
> > >
> > > we aim to address dependency conflicts in Kafka Connect soon by
> applying
> > > class loading isolation.
> > >
> > > Feel free to take a look at KIP-146 here:
> > >
> > > *https://cwiki.apache.org/confluence/display/KAFKA/KIP-
> > > 146+-+Classloading+Isolation+in+Connect
> > > <https://cwiki.apache.org/confluence/display/KAFKA/KIP-
> > > 146+-+Classloading+Isolation+in+Connect>*
> > >
> > > which describes minimal required changes to public interfaces and the
> > > general implementation approach.
> > >
> > > This is a much wanted feature for Kafka Connect. Your feedback is
> highly
> > > appreciated.
> > >
> > > -Konstantine
> > >
> >
> >
> >
> > --
> > *Gwen Shapira*
> > Product Manager | Confluent
> > 650.450.2760 | @gwenshap
> > Follow us: Twitter <https://twitter.com/ConfluentInc> | blog
> > <http://www.confluent.io/blog>
> >
>
