Hi all,
I made some major revisions to the proposal on the wiki, so please check
it out.
The new API is based on Ismael's suggestion of grouping related APIs.
There is only one layer of grouping. I think that it's actually pretty
intuitive. It's also based on the idea of using Futures, which several
people commented that they'd like to see.
Here's a simple example:
> AdminClient client = new AdminClientImpl(myConfig);
> try {
> client.topics().create("foo", 3, (short) 2, false).get();
> Collection<String> topicNames = client.topics().list(false).get();
> log.info("Found topics: {}", Utils.mkString(topicNames, ", "));
> Collection<Node> nodes = client.nodes().list().get();
> log.info("Found cluster nodes: {}", Utils.mkString(nodes, ", "));
> } finally {
> client.close();
> }
The good thing is, there is no Try, no 'get' prefixes, no messing with
batch APIs. If there is an error, then Future#get() throws an
ExecutionException which wraps the relevant exception in the standard
Java way.
Here's a slightly less simple example:
> AdminClient client = new AdminClientImpl(myConfig);
> try {
> List<Future<Void>> futures = new LinkedList<>();
> for (String topicName: myNewTopicNames) {
> creations.add(client.topics().
> setClientTimeout(30000).setCreationConfig(myTopicConfig).
> create(topicName, 3, (short) 2, false));
> }
> Futures.waitForAll(futures);
> } finally {
> client.close();
> }
I went with Futures because I feel like ought to have some option for
doing async. It's a style of programming that has become a lot more
popular with the rise of Node.js, Twisted python, etc. etc. Also, as
Ismael commented, Java 8 CompletableFuture is going to make Java's
support for fluent async programming a lot stronger by allowing call
chaining and much more.
If we are going to support async, the simplest thing is just to make
everything return a future and let people call get() if they want to run
synchronously. Having a mix of async and sync APIs is just going to be
confusing and redundant.
I think we should try to avoid creating single functions that start
multiple requests if we can. It makes things much uglier. It means
that you have to have some kind of request class that wraps up the
request the user is trying to create, so that you can handle an array of
those requests. The return value has to be something like Map<Node,
Try<Value>> to represent which nodes failed and succeeded. This is the
kind of stuff that, in my opinion, makes people scratch their heads.
If we need to, we can still get some of the efficiency benefits of batch
APIs by waiting for a millisecond or two before sending out a topic
create() request to see if other create() requests arrive. If so, we
can coalesce them. It might be better to figure out if this is an
actual performance issue before implementing it, though.
I think it would be good to get something out there, annotate it as
@Unstable, and get feedback from people building against trunk and using
it. We have removed or changed @Unstable APIs in streams before, so I
don't think we should worry that it will get set in stone prematurely.
The AdminClient API should get much less developer use than anything in
streams, so changing an unstable API should be much easier.
best,
Colin
On Wed, Feb 8, 2017, at 07:49, Ismael Juma wrote:
> Thanks for elaborating Jay. I totally agree that we have to be very
> careful
> in how we use our complexity budget. Easier said than done when people
> don't agree on what is complex and what is simple. :) For example, I
> think
> batch APIs are a significant source of complexity as you have to do a
> bunch
> of ceremony to group things before sending the request and error handling
> becomes more complex due to partial failures (things like `Try` or other
> mechanisms that serve a similar role are then needed).
>
> Maybe a way forward is to write API usage examples to help validate that
> the suggested API is indeed easy to use.
>
> Ismael
>
> On Wed, Feb 8, 2017 at 4:40 AM, Jay Kreps <j...@confluent.io> wrote:
>
> > Totally agree on CompletableFuture. Also agree with some of the rough edges
> > on the Consumer.
> >
> > I don't have much of a leg to stand on with the splitting vs not splitting
> > thing, really hard to argue one or the other is better. I guess the one
> > observation in watching us try to make good public apis over the years is I
> > am kind of in favor of a particular kind of simple. In particular I think
> > since the bar is sooo high in support and docs and the community of users
> > so broad in the range of their capabilities, it makes it so there is a lot
> > of value in dead simple interfaces that don't have a lot of conceptual
> > weight, don't introduce a lot of new classes or concepts or general
> > patterns that must be understood to use them correctly. So things like
> > nesting, or the Try class, or async apis, or even just a complex set of
> > classes representing arguments or return values kind of all stack
> > conceptual burdens on the user to figure out correct usage. So like, for
> > example, the Try class is very elegant and represents a whole generalized
> > class of possibly completed actions, but the flip side is maybe I'm just a
> > working guy who needs to list his kafka topics but doesn't know Rust, take
> > pity on me! :-)
> >
> > Nit picking aside, super excited to see us progress on this.
> >
> > -Jay
> >
> > On Tue, Feb 7, 2017 at 3:46 PM Ismael Juma <ism...@juma.me.uk> wrote:
> >
> > > Hi Jay,
> > >
> > > Thanks for the feedback. Comments inline.
> > >
> > > On Tue, Feb 7, 2017 at 8:18 PM, Jay Kreps <j...@confluent.io> wrote:
> > > >
> > > > - I think it would be good to not use "get" as the prefix for things
> > > > making remote calls. We've tried to avoid the java getter convention
> > > > entirely (see code style guide), but for remote calls in particular
> > it
> > > > kind
> > > > of blurs the line between field access and remote RPC in a way that
> > > > leads
> > > > people to trouble. What about, e.g., fetchAllGroups() vs
> > > getAllGroups().
> > > >
> > >
> > > Agreed that we should avoid the `get` prefix for remote calls. There are
> > a
> > > few possible prefixes for the read operations: list, fetch, describe.
> > >
> > >
> > > > - I think futures and callbacks are a bit of a pain to use. I'd
> > second
> > > > Becket's comment: let's ensure there a common use case motivating
> > > these
> > > > that wouldn't be just as easily satisfied with batch operations
> > (which
> > > > we
> > > > seem to have at least for some things). In terms of flexibility
> > > > Callbacks >
> > > > Futures > Batch Ops but I think in terms of usability it is the
> > exact
> > > > opposite so let's make sure we have worked out how the API will be
> > > used
> > > > before deciding. In particular I think java Futures are often an
> > > > uncomfortable half-way point since calling get() and blocking the
> > > > thread is
> > > > often not what you want for chaining sequences of operations in a
> > > truly
> > > > async way, so 99% of people just use the future as a way to batch
> > > calls.
> > > >
> > >
> > > We should definitely figure out how the APIs are going to be used before
> > > deciding. I agree that callbacks are definitely painful and there's
> > little
> > > reason to expose them in a modern API unless it's meant to be very low
> > > level. When it comes to Futures, I think it's important to distinguish
> > what
> > > is available in Java 7 and below versus what is available from Java 8
> > > onwards. CompletableFuture makes it much easier to compose/chain
> > operations
> > > (in a similar vein to java.util.Stream, our own Streams API, etc.) and it
> > > gives you the ability to register callbacks if you really want to
> > (avoiding
> > > the somewhat odd situation in the Producer where we return a Future _and_
> > > allow you to pass a callback).
> > >
> > >
> > > > - Personally I don't think splitting the admin methods up actually
> > > makes
> > > > things more usable. It just makes you have to dig through our
> > > > hierarchy. I
> > > > think a flat class with a bunch of operations (like the consumer
> > api)
> > > is
> > > > probably the easiest for people to grok and find things on. I am
> > kind
> > > > of a
> > > > dumb PHP programmer at heart, though.
> > > >
> > >
> > > I am not sure it's fair to compare the AdminClient with the Consumer. The
> > > former has APIs for a bunch of unrelated APIs (topics, ACLs, configs,
> > > consumer groups, delegation tokens, preferred leader election, partition
> > > reassignment, etc.) where the latter is pretty specialised. For each of
> > the
> > > resources, you may have 3-4 operations, it will get confusing fast. Also,
> > > do you really think an API that has one level of grouping will mean that
> > > users have to "dig through our hierarchy"? Or are you concerned that once
> > > we go in that direction, there is a danger of making the hierarchy more
> > > complicated?
> > >
> > > Finally, I am not sure I would use the consumer as an example of
> > something
> > > that is easy to grok. :) The fact that methods behave pretty differently
> > > (some are blocking while others only have an effect after poll) with no
> > > indication from the type signature or naming convention makes it harder,
> > > not easier, to understand.
> > >
> > > Ismael
> > >
> >
