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From Colin McCabe <cmcc...@apache.org>
Subject Re: [DISCUSS] KIP-117: Add a public AdministrativeClient API for Kafka admin operations
Date Fri, 03 Feb 2017 18:20:38 GMT
On Thu, Feb 2, 2017, at 17:54, Dong Lin wrote:
> Hey Colin,
> Thanks for the KIP. I have a few comments below:
> - I share similar view with Ismael that a Future-based API is better.
> PurgeDataFrom() is an example API that user may want to do it
> asynchronously even though there is only one request in flight at a time.
> In the future we may also have some admin operation that allows user to
> move replica from one broker to another, which also needs to work in both
> sync and async style. It seems more generic to return Future<?> for any
> that requires both mode.
> - I am not sure if it is the cleanest way to have enum classes
> CreateTopicsFlags and DeleteTopicsFlags. Are we going to create such
> class
> for every future API that requires configuration? It may be more generic
> to
> provide Map<String, ?> to any admin API that operates on multiple
> entries.
> For example, deleteTopic(Map<String, Boolean>). And it can be Map<String,
> Properties> for those API that requires multiple configs per entry. And
> we
> can provide default value, doc, config name for those API as we do
> with AbstractConfig.

Thanks for the comments, Dong L.

EnumSet<CreateTopicsFlags>, EnumSet<DeleteTopicsFlags>, and so forth are
type-safe ways for the user to pass a set of boolean flags to the
function.  It is basically a replacement for having an api like
createTopics(...., boolean nonblocking, boolean validateOnly).  It is
preferrable to having the boolean flags because it's immediately clear
when reading the code what createTopics(...,
CreateTopicsFlags.VALIDATE_ONLY) means, whereas it is not immediately
clear what createTopics(..., false, true) means.  It also prevents
having lots of function overloads over time, which becomes confusing for
users.  The EnumSet is not intended as a replacement for all possible
future arguments we might add, but just an easy way to add more boolean
arguments later without adding messy function overloads or type
signatures with many booleans.

Map<String, ?> is not type-safe, and I don't think we should use it in
our APIs.  Instead, we should just add function overloads if necessary.

> - I not sure if "Try" is very intuitive to Java developer. Is there any
> counterpart of scala's "Try" in java

Unfortunately, there is no equivalent to Try in the standard Java
library.  That is the first place I checked, and I spent a while
searching.  The closest is probably Java 8's Optional.  However,
Optional just allows us to express Some(thing) or None, so callers would
not be able to determine what the error was.  

> We actually have quite a few
> existing
> classes in Kafka that address the same problem, such as
> ProduceRequestResult, LogAppendResult etc. Maybe we can follow the same
> conversion and use *Result as this class name.

Hmm.  ProduceRequestResult and LogAppendResult just store an exception
alongside the data, and make the caller responsible for checking whether
the exception is null (or None) before looking at the data.  I don't
think this is a good solution, because if the user forgets to do this,
they will interpret whatever is in the data fields as valid, even when
it is not.  ProduceRequestResult and LogAppendResult are also internal
classes, not user-facing APIs, so we did not spend time thinking about
how to make them easy to use for end-users.

> - How are we going to allow user to specify timeout for blocking APIs
> such
> as deleteTopic? Is this configured per AdminClient, or should it be
> specified in the API parameter?

Right now, it is specified by the configuration of the AdminClient.

> - Are we going to have this class initiate its own thread, as we do with
> Sender class, to send/receive requests? If yes, it will be useful to have
> have a class that extends AbstractConfig and specifies config and their
> default values.

Yes, I agree.  I will add this to the KIP.


> Thanks,
> Dong
> On Thu, Feb 2, 2017 at 3:02 PM, Ismael Juma <ismael@juma.me.uk> wrote:
> > Hi Colin,
> >
> > Thanks for the KIP, great to make progress on this. I have some initial
> > comments, will post more later:
> >
> > 1. We have KafkaProducer that implements the Producer interface and
> > KafkaConsumer that implements the Consumer interface. Maybe we could have
> > KafkaAdminClient that implements the AdminClient interface? Or maybe just
> > KafkaAdmin. Not sure, some ideas for consideration. Also, I don't think we
> > should worry about a name clash with the internal AdminClient written in
> > Scala. That will go away soon enough and choosing a good name for the
> > public class is what matters.
> >
> > 2. We should include the proposed package name in the KIP
> > (probably org.apache.kafka.clients.admin?).
> >
> > 3. It would be good to list the supported configs.
> >
> > 4. KIP-107, which passed the vote, specifies the introduction of a method
> > to AdminClient with the following signature. We should figure out how it
> > would look given this proposal.
> >
> > Future<Map<TopicPartition, PurgeDataResult>>
> > purgeDataBefore(Map<TopicPartition, Long> offsetForPartition)
> >
> > 5. I am not sure about rejecting the Futures-based API. I think I would
> > prefer that, personally. Grant had an interesting idea of not exposing the
> > batch methods in the AdminClient to start with to keep it simple and
> > relying on a Future based API to make it easy for users to run things
> > concurrently. This is consistent with the producer and Java 8 makes things
> > a lot nicer with CompletableFuture (similar to Scala Futures). I will think
> > more about this and other details of the proposal and send a follow-up.
> >
> > Ismael
> >
> > On Thu, Feb 2, 2017 at 6:54 PM, Colin McCabe <cmccabe@apache.org> wrote:
> >
> > > Hi all,
> > >
> > > I wrote up a Kafka improvement proposal for adding an
> > > AdministrativeClient interface.  This is a continuation of the work on
> > > KIP-4 towards centralized administrative operations.  Please check out
> > > https://cwiki.apache.org/confluence/display/KAFKA/KIP-117%
> > 3A+Add+a+public+
> > > AdministrativeClient+API+for+Kafka+admin+operations
> > >
> > > regards,
> > > Colin
> > >
> >

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