Thank you Stephane,
your comments bring interesting and useful subjects to the discussion. I'm
adding my replies below Ewen's comments.
On Tue, May 2, 2017 at 10:15 PM, Ewen Cheslack-Postava <e...@confluent.io>
wrote:
> On Tue, May 2, 2017 at 10:01 PM, Stephane Maarek <
> steph...@simplemachines.com.au> wrote:
>
> Excellent feedback, Stephane!
>
> Thanks for the work, it’s definitely needed!
> > I’d like to suggest to take it one step further.
> >
> > To me, I’d like to see Kafka Connect the same way we have Docker and
> > Docker repositories.
> >
> > Here’s how I would envision the flow:
> > - Kafka Connect workers are just workers. They come with no jars
> whatsoever
> > - The REST API allow you to add a config to the connect cluster
> > - The workers, seeing the config, pull the jars from the available
> > (maven?) repositories (public or private)
> >
>
> I think supporting this mode is really valuable. It seems *really*
> attractive if you have some easily accessible, scalable, centralized
> storage for connectors (i.e. some central distributed FS).
>
> But having to jump through these hoops (which presumably include some extra
> URLs to point to the connectors, some definition of what that URL points to
> e.g. zip of jars? uberjar? something with a more complex spec?) just to do
> a quickstart seems like quite a bit of overhead. I think we should consider
> both a) incremental progress that opens up new options but doesn't prevent
> us from the ideal end state and b) all local testing/dev/prod use cases
> (which is also why I still like having the plain old CLASSPATH option
> available).
>
> I think the proposal leaves open the scope for this -- it doesn't specify
> connector-specific overrides, but that's obviously something that could be
> added easily.
>
>
Regarding 1) the workers carrying no modules, I believe we are pretty close
to this situation today, at least in terms of connectors. Still, I feel
that whether we bundle a few basic modules with the framework is orthogonal
to class loading isolation. I agree with you that Connectors, Converters
and Transformations should be external modules that are loaded by the
framework. But having a few basic ones shipped with Connect simplifies
on-boarding and quickstarts significantly.
Regarding 2) and 3) these are very good to have, and definitely belong to
the near term vision for Kafka Connect. Many interesting things to do here,
from programmatically fetching connectors and their dependencies from maven
repos (using something like Aether maybe), to deciding to support certain
types of module bundling such as zip, uberjars etc. However dealing with
the issue of extended discoverability at this point seems to broaden
significantly the scope of this KIP. I think it's more practical (and
probably faster) to proceed in phases. I estimate that after this KIP,
subsequent KIPs will be intuitive and quite transparent.
> > - Classpath isolation comes into play so that the pulled jar doesn’t
> > interact with other connectors
> > - Additionally, I believe the config should have a “tag” or “version”
> > (like docker really), so that
> > o you can run different versions of the same connector on your connect
> > cluster
> > o configurations are strongly linked to a connector (right now if I
> update
> > my connector jars, I may break my configuration)
> >
>
> We have versions on Connectors. We don't have them on transformations or
> converters, which was definitely an oversight -- we should figure out how
> to get them in there.
>
> I think none of the proposals here rule out taking advantage of versioning
> in the future -- would it be helpful to add a "Future Work" section that
> gives an idea of how we'd extend this in the future to handle versions as
> well?
>
>
Dealing with versioning was left out of the scope of this KIP because
Transformations and Converters currently are not versioned. I like the idea
of adding a Future Work section. I'll add one to highlight the intention to
support versioning.
>
> > I know this is a bit out of scope, but if major changes are coming to
> > connect then these are my two cents.
> >
>
> :) I think we can address most of these incrementally, but keep the scope
> here a bit smaller and more manageable (and hopefully get it into
> 0.11.0.0!). I just don't want perfect to be the enemy of good -- getting
> the first step in as long as it doesn't cause problems down the line seems
> like a good step.
>
>
> >
> > Finally, maybe extend that construct to Transformers. The ability to
> > externalise transformers as jars would democratize their usage IMO
> >
>
> This should definitely happen! It's important for pretty much any pluggable
> component, though I think Transformations will, on average, have the fewest
> extra dependencies. Connectors have the most, followed by Converters, then
> Transformations. But I think Konstantine has mentioned these in the KIP --
> if it could be clearer in the proposed changes, perhaps you could suggest
> where to add details?
>
> -Ewen
>
Thanks again for the comments, let me know if this plan sounds good.
-Konstantine
>
>
> >
> > On 3/5/17, 4:24 am, "Ewen Cheslack-Postava" <e...@confluent.io> wrote:
> >
> > Thanks for the KIP.
> >
> > A few responses inline, followed by additional comments.
> >
> > On Mon, May 1, 2017 at 9:50 PM, Konstantine Karantasis <
> > konstant...@confluent.io> wrote:
> >
> > > 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.
> > >
> >
> > I'm not sure any of these keep you from removing the extra config. Is
> > there
> > any reason you couldn't have clean support for relying on the
> CLASSPATH
> > while still supporting the classloaders? Then getting people onto the
> > new
> > classloaders does require documentation for how to install
> connectors,
> > but
> > that's pretty minimal. And we don't break existing installations
> where
> > people are just adding to the CLASSPATH. It seems like this:
> >
> > 1. Allows you to set a default. Isolation is always enabled, but we
> > won't
> > include any paths/directories we already use. Setting a default just
> > requires specifying a new location where we'd hold these directories.
> > 2. It doesn't require the implicit choice -- you actually never turn
> > off
> > isolation, but still support the regular CLASSPATH with an empty list
> > of
> > isolated loaders
> > 3. The user can still use CLASSPATH if they want to minimize
> > classloader
> > overhead
> > 4. Debugging can still use CLASSPATH
> > 5. Upgrades just work.
> >
> >
> > >
> > > 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.
> > >
> >
> > Maybe just mention a couple of quick examples in the KIP. When
> > described
> > inline it might be more obvious that it will extend cleanly.
> >
> >
> > > 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.
> > >
> >
> > I think we're missing another option -- don't do filtering and
> require
> > that
> > those dependencies are correctly filtered out of the modules. If we
> > want to
> > be nicer about this, we could also detect maybe 2 or 3 classes while
> > scanning for Connectors/Converters/Transformations that indicate the
> > classloader has jars that it shouldn't and warn about it. I can't
> > think of
> > that many that would be an issue -- basically connect-api,
> > connect-runtime
> > if they really mess it up, and maybe slf4j.
> >
> >
> > > 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.
> > >
> > >
> > And a couple of addition comments:
> >
> > - module.path should be module.paths if it accepts multiple paths
> > - I think the description of the module paths is a bit confusing
> > because I
> > think for simpler configuration we actually want to specify the
> > *parent*
> > directory of module paths. I definitely prefer this since it's
> simpler
> > although I am not certain how it will mix with future extensions to
> > other
> > URL handlers (where a zip file or http URL wouldn't do the same
> > discovery
> > within subdirectories)
> >
> > -Ewen
> >
> >
> > > 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>
> > > > >
> > > >
> > >
> >
> >
> >
> >
>
