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From Phil Steitz <phil.ste...@gmail.com>
Subject Re: [configuration] Thoughts about multi-threading
Date Tue, 18 Sep 2012 18:09:27 GMT
On 9/17/12 12:39 PM, Oliver Heger wrote:
> Hi Jörg,
>
> many thanks for your input!
>
> Am 17.09.2012 10:01, schrieb Jörg Schaible:
>> Hi Oliver,
>>
>> Oliver Heger wrote:
>>
>>> Hi,
>>>
>>> one limitation of the 1.x versions of [configuration] is the
>>> incomplete
>>> support for concurrent access to Configuration objects. In
>>> version 2.0
>>> we should try to improve this.
>>>
>>> I have some ideas about this topic - not fully thought out - and
>>> would
>>> like to start a discussion. Here they are (in no specific order):
>>>
>>> - Thread-safety is not always required. Therefore, I would like
>>> to take
>>> an approach similar to the JDK Collections framework: having basic,
>>> unsynchronized configurations which can be turned into
>>> thread-safe ones.
>>
>> Fair approach.
>>
>>> - Many Configuration implementations are based on a hash map.
>>> Access to
>>> their content could be made thread-safe by replacing the map by a
>>> ConcurrentHashMap.
>>
>> You could use a protected method to instantiate the underlaying
>> HashMap.
>> Then you're free in overloaded
>> Configurations.synchronizedConfiguration
>> methods to use a derived class or a wrapper. It has also
>> implications on
>> subset().
> Or pass the map in at construction time. Using a protected method
> to create the map would either mean that the constructor has to
> invoke this method (problematic for subclasses which are not yet
> fully initialized at this time) or the field cannot be made final.
> Alternatively, there could be an abstract method getMap()
> returning the reference to the map.
>
>>
>>> - For hierarchical configurations situation is more complex.
>>> Here we
>>> will probably need something like a ReadWriteLock to protect their
>>> content. (There could be different lock implementations including a
>>> dummy one used by unsynchronized configurations).
>>>
>>> - Reloading is a major problem; at least in the way it is
>>> implemented
>>> now, it is very hard to get synchronization correct and efficient.
>>> Therefore, I would like to use a different strategy here. One
>>> option
>>> could be to not handle reloading in Configuration objects, but
>>> on an
>>> additional layer which creates such objects.
>>>
>>> - Other properties of configuration objects (e.g. the
>>> throwExceptionOnMissing flag or the file name) must be taken into
>>> account, too. In a typical use case those should not be accessed
>>> frequently, so it is probably not an issue to always synchronize
>>> them or
>>> make them volatile.
>>>
>>> Looking forward to your input
>>
>> Another option would be immutability (well, apart probably from
>> reloading).
>> Personally I have often the use case that I do not want to offer my
>> clients/consumers to write into the configuration. One approach
>> can also be
>> the JDK approach creating Collections.unmodifiableConfiguration.
>>
>> However, what also bugs me in the meantime is the current hard
>> relation
>> between the configuration object and its format. Why should I
>> care at all in
>> what format the configuration had been saved when I access its
>> values?
>>
>> For some time I am thinking now of something in the line of:
>>
>> - interface Configuration: Core interfaces, only getters
>> - interface ReloadableConfiguration extends Configuration,
>> Reloadable
>> - class BaseConfiguration: In memory, implements all the stuff for
>> interpolation and the setters
>>
>> - interface ConfigurationSource: Core interface to load (and
>> probably save a
>> configuration)
>> - class PropertiesConfigurationSource: Concrete implementation
>> that loads a
>> properties file and creates a BaseConfiguration
>>
>> This approach offers immutability for the Configuration itself
>> and also
>> allows Serializability. Format is separated completely from the
>> configuration functionality.
>>
>> I know, this looks more like Configuration 3.0 ... ;-)
>>
> I really like this approach. I was also thinking about separating
> loading and saving from core Configuration classes. However, I
> fear such an approach will make it difficult to preserve the
> format of a configuration. E.g. XMLConfiguration currently stores
> the XML document it was loaded from. So when saved to disk, result
> looks much like the original document.
>
> Read-only configurations is also an interesting topic.

This obviously makes the concurrency problem easier :)

Apart from this case, it would be good to agree on exactly what it
means for [configuration] to be threadsafe.  Is it basically the
semantics of ConcurrentHashmap?  Or are there sequencing / event
serialization constraints?  For example, suppose the sequence below
happens
Thread A start add property
Thread B start clear
Thread A notify property change
Thread B notify clear
Thread B clear map
Thread A update map

Is it OK for this sequence to happen?  Is it OK for A's add to trump
B's clear even though B's activation started later and B's
notification was later?

Phil

>
> But I think you are right, we have to start with smaller steps
> first. Not sure whether we can manage this - but I would really
> like to get something out in the not-too-far future.
>
> Oliver
>
>> - Jörg
>>
>>
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>
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