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From Emmanuel LŽcharny <elecha...@gmail.com>
Subject Re: [DN] Existing API review
Date Thu, 14 Jan 2010 22:33:45 GMT
Matthew Swift a écrit :
>
>
> On 13/01/10 13:03, Francois wrote:
>> Le 13/01/2010 12:24, Matthew Swift a écrit :
>>> Hi Emmanuel,
>>
>> I'm just giveng my point of view about the following, I don't think I 
>> can be relevant elsewhere:
>>
>>> Also, I strongly believe that DNs and RDNs and AVAs should be immutable
>>> objects (as well as any other low level API type). What do you think?
>>
>> I will go further : DN, RDN and AVA should be immutable, but 
>> Attribute should be immutable too, and Entry should have at least an 
>> immutable DN (and facilities to copy an entry with a new DN or 
>> ParentDN at factory level).
>>
>> For attribute, the rationnal is that one replace attribute by a new 
>> one most of the time, and it's much easier to deal with immutable 
>> attribute - that's one of the aspect of UnboundId SDK that I prefer.
>
> I understand very well why you think Attribute could be immutable. In 
> fact, I went to great lengths to convert our Attribute API in OpenDS 
> server so that it was immutable in order to be able to optimize for 
> the common case (single valued attributes) and to avoid defensive 
> copies. The result was a 4x performance improvement in the server (!). 
> However, this did introduce some pain - any code in the server which 
> tried to modify an attribute became more complex. For example, I had 
> to implement an AttributeBuilder class to facilitate incremental 
> construction of Attributes.
>
> In fact, ironically, I heard on the grape vine that Neil Wilson was 
> pretty impressed with the improvements that we got and this is why 
> UnboundID's Attribute class is immutable (except that Neil, being 
> notoriously bad at API design, has not made it fully immutable - look 
> at all those arrays with Javadoc warnings saying that you are not 
> allowed to modify the content. Very suspect!).
>
> I thought about this long and hard for the OpenDS SDK and decided that 
> a client API with an immutable Attribute would not be very easy to 
> use. Many client apps will want to create and modify attributes and we 
> should make it as easy as possible. In addition, I think that it is 
> very likely that some users of the SDK (including the server itself) 
> will need to have different types of Attribute - in other words, 
> they'll need to sub-class Attribute (e.g. in order to create lazy 
> decoded Attributes, virtual Attributes, copy on write, synchronized, 
> etc). By implication any class which is non-final is not immutable 
> since a sub-class could be implemented which breaks the contract (I 
> realized that I had made this mistake in the server since our 
> Attribute class is in fact an interface).
>
> Therefore the approach I took in the OpenDS SDK was to copy the 
> Collections API approach. Provide a high-level interface providing 
> read/write operations (side note: an Attribute can be thought of as a 
> Set of values) and then provide an "unmodifiable" adapter as per 
> Collections.unmodifiableSet. Then we get the best of all worlds - 
> including the ability to avoid defensive copies by implementing 
> getters which return "unmodifiable" Attributes (the OpenDS SDK 
> implementation also optimizes for the case where an attribute is 
> single valued even though it is mutable).
I must say that we went through the same journey on ADS : from Immutable 
Attributes to mutable ones, for almost the same reason. People have to 
understand that once an Attribute comes into the server it *will* be 
modified (modifiersName and timestamp, probably the addition of missing 
ObjectClass, UUID and entryCSN for addition...). From the client POV, 
well, as soon as you can clone the entry, I would say it's probably enough.



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