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From Rafael Schloming <rafa...@redhat.com>
Subject Re: Use of AMQShortString in client side code
Date Wed, 19 Sep 2007 01:52:07 GMT
John O'Hara wrote:
> I agree with Rob that the lower levels of the stack should be implemented in
> AMQPShortString *where it occurs in the protocol* for the following reasons:
> 1) It provides the opportunity to validate the semantics; just because we're
> not checking length today doesn't mean we shouldn't

AMQShortString really isn't the appropriate place to validate domain 
level semantics. Different uses of shortstr have different domain level 
constraints. Also, any validation we put in AMQShortString is forced to 
run for every single shortstr field that passes through a broker. This 
isn't particularly useful because when decoding fields off the wire, 
such validation is unnecessary as it is already performed by the codec 
in a more efficient manner that is specialized to the wire format.

> 2) We may introduce AMQPShortStrong Tokenisation in the protocol in the
> future (has been discussed often, I think it's quite likely).  Doing this we
> can collapse a shortstring to 2 bytes and reduce garbage.

I presume you're referring to some scheme for caching commonly used 
strings? If so this is a decoding optimization that would equally well 
apply when decoding directly to Strings, or any other type for that 
matter. In fact such an optimization would likely nullify any 
performance advantage rendered by AMQShortString since decoding/encoding 
of anything would only be necessary when there is a cache miss.

> 3) I'm unsure of the memory ownership semantics but I believe the JMS spec
> explicitly requires a copy of the message to be take to prevent grim race
> conditions on message reuse.  Some products have the option to turn this
> off, but that's not the spec.  It's like not DMA'ing from userspace without
> extreme care.

I'm unsure how this relates to the use of AMQShortString. Any such 
copying would happen well past the point where raw types are decoded off 
the wire.

> Also, Rob has said it has been proven to be faster in the past.
> In the absence of a measured, demonstrable issue why change this arguably
> more correct implementation?

As it stands today AMQShortString is really just an optimization for the 
broker, and one that comes at a pretty high cost to the client. So if 
there is a better way to solve the performance issue for the broker 
without encumbering the client, it's certainly worth investigating.

That's why I asked about the original problem being solved. For example 
I'd guess that in the critical path the broker really never needs to 
decode much more than the exchange name and routing key in order to 
deliver a message, so it might be possible to limit the use of 
AMQShortString to just those fields (or decode to specific Exchange and 
RoutingKey classes) and get the necessary performance benefit in the 
broker, with much less impact on the client.


> Cheers
> John
> On 19/09/2007, Rafael Schloming <rafaels@redhat.com> wrote:
>> Robert Godfrey wrote:
>>> On 13/09/2007, Rajith Attapattu <rajith77@gmail.com> wrote:
>>>> I am wondering why we are using AMQShortString indiscriminately all
>> over
>>>> the
>>>> client side code?
>>>> There is no performance benefit of using AMQShortString (based on the
>> way
>>>> it
>>>> is used) on the client side and is purely used for encoding.
>>> Rajith,
>>> as we have discussed before - there *is* a significant performance
>> benefit
>>> which we have tested and proved previously.
>> Can you point me to the previous discussion? I'd like to learn more
>> about the original issue.
>>    Many short strings are re-used
>>> frequently within the client library, and by using our own type we can
>>> exploit this.
>> Unless we're excessively copying them I don't see how this matters. For
>> both an AMQShortString and a String we should just be passing around
>> pointers when they are reused.
>>    Further, the domain for many parameters in AMQP is *not* a
>>> unicode string, but is tightly defined as upto 255 bytes of data with a
>>> particular encoding.  Java Strings are not the appropriate type to use
>> for
>>> this.  Encoding and decoding Java Strings is expensive, and also prone
>> to
>>> error (i.e. you need to make sure that you *always* use the correct
>> explicit
>>> encoding).
>> Despite the name AMQShortString, I don't think the AMQShortString class
>> actually represents the AMQP type short-string, for example there is no
>> length limit for an AMQShortString. It's really just a generic
>> implementation of CharSequence that is optimized specifically for rapid
>> decoding from a ByteBuffer. From a domain restriction perspective, using
>> an ordinary String is just as correct.
>>> It makes sense to use it on Broker side as you deal at bytes level and I
>> can
>>>> understand the performance benefit of not having convert back and forth
>>>> into
>>>> a String.
>>> The low level API should be using correct AMQ domains.  High level APIs
>>> (such as JMS) will obviously want to present these parameters as java
>>> Strings.
>>> On the client side we just merely wrap/unwrap a String using
>> AMQShortString.
>>>> Why can't we do that at the encoding/decoding level for the client side
>> ?
>>> In some cases this may be true, but in others certainly not.  When
>>> converting into JMS Destinations on receipt of a message, for instance,
>> one
>>> never needs to convert to a String... it is *much* faster to simply use
>> the
>>> correct type of AMQShortString/
>> Unfortunately using AMQShortString imposes additional overhead whenever
>> we need to en/decode to/from an ordinary String. It basically requires
>> an additional copy when compared with directly encoding/decoding to/from
>>   a String. As the common case on the client side is dealing with
>> Strings, I'm not at all convinced that ubiquitous use of AMQShortString
>> is a net win for the client.
>> I believe what would be optimal is to use the CharSequence interface
>> everywhere. This way String values passed to us by an application could
>> be directly passed all the way down the stack and encoded directly onto
>> the wire without an additional copy, and incoming data could be
>> efficiently decoded into a private impl of CharSequence that could be
>> converted to a String on demand.
>> --Rafael

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