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From Jordan Zimmerman <jor...@jordanzimmerman.com>
Subject Re: Exception throwing
Date Mon, 04 Aug 2014 14:36:06 GMT
Just 
protect me from that exception. 
I agree with this.

I’d like to see Curator move to unchecked exceptions everywhere. For “recoverable exceptions”,
I’d like to see something like this (BTW - I SO wish I did this initially): have the various
forPath() methods return a class (rather than byte[], etc.) like:

public interface CuratorResult
{
	public byte[] getBytes();

	public CuratorResultType getResultType();
}

public enum CuratorResultType
{
	SUCCESS,
	NO_NODE,
	NODE_EXISTS,
	BAD_VERSION
	… etc
}

To me, this is FAR superior than the recoverable exceptions. It would also make the APIs a
lot easier to use. You could do:

CuratorResult  result = client.create().inBackground().forPath(myPath);
if ( result.getResultType() == SUCCESS )
{
	// etc.
}
else 
{
	// etc.
}

From: Mike Drob <madrob@cloudera.com>
Reply: dev@curator.apache.org <dev@curator.apache.org>>
Date: August 4, 2014 at 9:29:14 AM
To: Cameron McKenzie <mckenzie.cam@gmail.com>>
Cc: dev@curator.apache.org <dev@curator.apache.org>>, vines@apache.org <vines@apache.org>>
Subject:  Re: Exception throwing  

I can see how it may be unappealing modifying everything in the library to  
accommodate this. And I think I figured out why this matters (before I was  
suggesting it from mostly intuition).  

Jordan, you posit that Curator assumes familiarity with ZooKeeper. In some  
cases, that's warranted, but in others, I'm not so sure. I understand the  
basic ZooKeeper semantics - you can put and get data, it provides quorum  
guarantees, data is stored hierarchically, etc... However, I don't know the  
ins and outs of the ZooKeeper API. I know that KeeperException is a thing,  
but I couldn't tell you which methods throw it, why they throw it, and what  
all the (enum? int? string?) codes are. Dealing directly with ZooKeeper is  
thorny!  

Enter Curator! It gives me so much for free - retry policies, sanity  
checking, a testing server for unit tests, it's great. Until I run into a  
KeeperException. That's a very jarring experience because I thought that  
Curator protected me from such nonsense. And it never advertised that  
KeeperException was a possibility! My only indication was the 'throws  
Exception' clause on most methods, which I have been taught by the library  
to dutifully ignore it and just fail fast. Had I seen a 'throws  
KeeperException' declaration, I might have thought about what that means,  
but that information was not available to me.  

Yes, it is unfortunate that ZooKeeper uses exceptions to return result  
codes. That doesn't mean that we have to do the same. I would love to see  
CreateBuilder implement BackgroundPathAndBytesable<Boolean> instead of  
String and for it to eat the KeeperException. Or maybe return a Stat. Just  
protect me from that exception.  

Mike  



On Sat, Aug 2, 2014 at 5:47 PM, Cameron McKenzie <mckenzie.cam@gmail.com>  
wrote:  

> I don't have a super strong view either way. If we were designing from  
> scratch I'd go with checked exceptions, just because that's how I prefer to  
> code.  
>  
> Having said that, I'm not convinced that modifying the whole code base at  
> this point to add a CuratorException is worth the effort.  
>  
>  
> On Sat, Aug 2, 2014 at 8:09 AM, Jordan Zimmerman <  
> jordan@jordanzimmerman.com> wrote:  
>  
>> Curator assumes familiarity with ZooKeeper. Of course, the docs should be  
>> improved where needed. Curator doesn’t hide any KeeperExceptions except  
>> where it advertises that it handles them - e.g. creatingParentsIfNeeded().  
>> The fact that ZooKeeper uses exceptions to return result codes for correct  
>> behavior is the problem here.  
>>  
>> That said, I wrote nearly all of the Curator recipes and have never had  
>> an occasion where Curator’s use of Exception was a problem.  
>>  
>> -JZ  
>>  
>> From: Mike Drob <madrob@cloudera.com>  
>> Reply: dev@curator.apache.org <dev@curator.apache.org>>  
>> Date: August 1, 2014 at 5:05:04 PM  
>> To: vines@apache.org <vines@apache.org>>  
>> Cc: dev@curator.apache.org <dev@curator.apache.org>>  
>> Subject: Re: Exception throwing  
>>  
>> The set with version is basically a compareAndSet. java.util chooses to  
>> implement this also with a 'return false' value for when some other thread  
>> got there first. I'll have to go back and look at the Curator API and see  
>> how these failures are currently communicated.  
>>  
>>  
>> On Fri, Aug 1, 2014 at 4:08 PM, John Vines <vines@apache.org> wrote:  
>>  
>> > There's KeeperException.BADVERSION, which is when you  
>> > setData().withVersion and the version in ZK changed vs. the version seen  
>> > prior. That one is pretty critical to support, IMO. The other cases  
>> around  
>> > node existance can definitely be handled by the user, but given the  
>> > possibilities for races in distributed systems you still can't be  
>> certain.  
>> > But there could be user cases where they want to create a node and not  
>> fail  
>> > if it exists or have a delete pass if the node was already deleted by  
>> > something else (not sure if a flag was added for that case, I vaguely  
>> > recall a discussion).  
>> >  
>> >  
>> > On Fri, Aug 1, 2014 at 5:04 PM, Mike Drob <madrob@cloudera.com> wrote:
 
>> >  
>> >> John, thanks for your input. So some of this is improper use of the  
>> API,  
>> >> right? If you are attempting to create a node and it already exists,  
>> then  
>> >> that can be an exceptional case. If you just want to make sure that a  
>> node  
>> >> exists, regardless of who created it, then that's a case for a  
>> different  
>> >> API. Asserting that you created the node could be important - see the  
>> >> distinction in ConcurrentMap between put() and putIfAbsent(). Then  
>> again,  
>> >> failing to create the node because it already exists might not need to 

>> be  
>> >> an exceptional case and simply warrants a 'return false' on the  
>> method? Do  
>> >> the other cases you mentioned have similar analogues? Maybe the end  
>> result  
>> >> of what comes out of this is better docs.  
>> >>  
>> >> Mike  
>> >>  
>> >>  
>> >> On Fri, Aug 1, 2014 at 3:39 PM, John Vines <vines@apache.org> wrote:
 
>> >>  
>> >>> It's not a matter of it being a bug, it's a matter of usability.  
>> Because  
>> >>> every single method just throws Exception it gives me, as a user,  
>> >>> absolutely zero inclination at writing time to figure out what sort
of  
>> >>> failures can happen. And the complete lack of javadocs compound this
 
>> >>> issue.  
>> >>> This has been my biggest issue with Curator.  
>> >>>  
>> >>> Yes, there are some unrecoverable errors. But not all of them are, 

>> such  
>> >>> as  
>> >>> a subset of the KeeperExceptions around node state, security, and  
>> >>> versions.  
>> >>> I could be sold on a split, where those type of items are exposed and
 
>> >>> then  
>> >>> the critical ones you keep mentioning are Runtime. But as much as I
 
>> >>> dislike  
>> >>> generic Exceptions for everything, forcing users to catch  
>> >>> RuntimeExceptions  
>> >>> to do proper exception handling for known and well defined exceptions
 
>> is  
>> >>> an  
>> >>> awful practice to put people though.  
>> >>>  
>> >>>  
>> >>> On Fri, Aug 1, 2014 at 3:49 PM, Jordan Zimmerman <  
>> >>> jordan@jordanzimmerman.com  
>> >>> > wrote:  
>> >>>  
>> >>> > -1 (binding)  
>> >>> >  
>> >>> > If I could go back I’d remove all checked exceptions entirely.
I  
>> don’t  
>> >>> > think there’s an objective answer here - it comes down to personal
 
>> >>> > preference, etc. I don’t see much value in touching nearly every
 
>> file  
>> >>> in  
>> >>> > the library in order to do this. We’ve had maybe 2 or 3 requests
in  
>> the  
>> >>> > many years that Curator has exists. This suggests that the  
>> overwhelming  
>> >>> > majority accept the current exception semantics. If you can point
 
>> to an  
>> >>> > actual bug that this causes that would be helpful.  
>> >>> >  
>> >>> > -Jordan  
>> >>> >  
>> >>> > From: Mike Drob <madrob@cloudera.com>  
>> >>> > Reply: dev@curator.apache.org <dev@curator.apache.org>>
 
>> >>> > Date: August 1, 2014 at 2:32:07 PM  
>> >>> > To: dev@curator.apache.org <dev@curator.apache.org>> 

>> >>> > Subject: Exception throwing  
>> >>> >  
>> >>> > I'd like to revisit the discussion around always throwing Exception
 
>> in  
>> >>> the  
>> >>> > API. There were two JIRA issues - CURATOR-135 and CURATOR-29 -
that  
>> >>> touch  
>> >>> > on this subject, but I think there is a good conversation to be
had.  
>> >>> >  
>> >>> > I understand the suggestions that if an exception is thrown, we
are  
>> in  
>> >>> a  
>> >>> > bad state, regardless of the type of exception. However, throwing
 
>> >>> Exception  
>> >>> > comes with some unfortunate Java baggage...  
>> >>> >  
>> >>> > By declaring thrown Exception, we force consumers to also catch
 
>> >>> > RuntimeExceptions instead of letting them propagate as they normally
 
>> >>> would.  
>> >>> > In some cases, the calling code may need to attempt roll-back 

>> >>> semantics, or  
>> >>> > retry outside of what Curator provides, or something else that
we  
>> >>> haven't  
>> >>> > thought of.  
>> >>> >  
>> >>> > I'd like to propose replacing much of the thrown Exception methods
 
>> with  
>> >>> > CuratorException. This will still carry the connotation that it
 
>> doesn't  
>> >>> > matter what kind of exception we encounter, they're all bad. It
will  
>> >>> also  
>> >>> > be backwards compatible with the previous API, since users will
 
>> still  
>> >>> be  
>> >>> > able to catch Exception in their calling code. And it has the 

>> >>> advantage of  
>> >>> > separating checked exceptions from unchecked ones, so that users
 
>> don't  
>> >>> > unintentionally catch something unrelated.  
>> >>> >  
>> >>> > Thoughts?  
>> >>> >  
>> >>> > I tried looking for more details behind the design decision to
 
>> always  
>> >>> throw  
>> >>> > Exception, but wasn't able to find them. If they're already  
>> >>> documented, I'd  
>> >>> > love to be pointed at the wiki or site, or a mailing list thread
 
>> will  
>> >>> do in  
>> >>> > a pinch.  
>> >>> >  
>> >>> > Thanks,  
>> >>> > Mike  
>> >>> >  
>> >>>  
>> >>  
>> >>  
>> >  
>>  
>  
>  

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