singleton v/s pooling

[nicetohaveyou]

What are the differences between singleton and pooling? If singleton design pattern is used to have just one connection. Also, we know that the connection pooling helps to organise the connections. How are hese two things similar and different? Many Thanks....

[Curtis Schlak]

A Singleton implementation should guarantee that there is ONE AND ONLY ONE instance of a particular object in a system. A pool is a management object that contains resources. It is generally used to maintain high-cost connections to resources and limit the number of the resources generated by the system.

"we must lose precision to make significant statements about complex systems." -deKorvin on uncertainty

[led mike]

nicetohaveyou wrote:

What are the differences between singleton and pooling?

nicetohaveyou wrote:

How are hese two things similar and different?

Well, if you have a Singleton Pool they are one thing and so not different nor similar. ;)

[Curtis Schlak]

Is there a lifeguard on duty there? 'Cause I'm not that strong of a swimmer....

"we must lose precision to make significant statements about complex systems." -deKorvin on uncertainty

[Luc Pattyn]

You might ask that question in the get-together forum. My best guess is the guard is in one of the lounges though. :)

Luc Pattyn [Forum Guidelines] [My Articles]

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[nicetohaveyou]

Can we have a practical explanation of that? More elaborate... t.a.

[Curtis Schlak]

Sure.

Singleton Example

Let's say that you're implementing an interpreter for a scripting language that contains some mathematical operators. Now, you design your language so that none of the objects that implement the operators (such as AddOperator, MultOperator) have state, just a collection of methods. To reduce the memory consumption of the interpreter, you can choose to have each of those operators as singletons, one instance of the AddOperator will exist in any AppDomain. This is in contrast to parsing the input and creating an AddOperator into an abstract syntax tree every time the parser encounters the + symbol.

Pool Example

Quite often in multi-threaded applications that access data from a persistent store, the connections that the applications use to connect to the persistent store are pooled. For example, it's a common practice to pool a certain number of connections to a database because it's so darn expensive to create the connection. Then, when an application wants to get data from the database, it asks the pool for a connection instead of creating its own.

I think that your confusion may be coming from the thought that "a resource pool is a singleton." That can be true and quite often is. In general, why would you create multiple pools of resource connections when that kind of defeats the purpose of having that limited number of expensive connections?

"we must lose precision to make significant statements about complex systems." -deKorvin on uncertainty

[nicetohaveyou]

Thanks for explanation. However, could you please select another example. Call me a dumbo, but i am in a loss of differentiating between the two. For example, if i have a web application where there are 7 odd connections strings. Do i hav to use the Singleton or just use Pooling to take care of that? You are already writing good and helping my understanding. Appreciate Much!

[supercat9]

In general, why would you create multiple pools of resource connections when that kind of defeats the purpose of having that limited number of expensive connections? Designing an architecture around singleton objects that hold state may lead to problems if requirements change in unexpected ways. Even if it's expected that the singleton will be associated with a hardware resource of which there will never be more than one (e.g. the current display monitor) it's possible that there may in future be a need to support more than one such resource, no matter how unlikely it may seem today. That having been said, the proper type of object is something like a singleton, in that at most one such object should be associated with any particular hardware resource, and the object should persist as long as anyone has use for it. There are other pooling situations, like thread pools, where sharing one pool between different parts of an application may lead to resource starvation. While there may be ways of having a single pool implement a suitable prioritization strategy to ensure that every separately-designated part of an application is allowed to have at least one thread running at a time from a shared pool, having different parts use different pools may be simpler.

[Curtis Schlak]

Right you are! That's why I started it with "In general." :) Special cases for anything exist.

"we must lose precision to make significant statements about complex systems." -deKorvin on uncertainty

[PIEBALDconsult]

Visions of Pamela Anderson running in slow motion... :cool: