Design patterns: Object pool

What do car rentals and libraries have in common? They temporarily lend resources only as long as someone needs to use them and then take it back so that someone else can use them. In object-oriented programming, such as system is called the ‘object pool’.

Object pooling in real life

Let’s explore the philosophy of a library with regards to a book named ‘The Art of Computer Programming’.

  1. No one needs to read this book 24 hours a day for 7 days a week all his/her life. People need books only for three purposes: learning, reference, review. A person most likely needs a book only for 40 hours in his/her entire life. He/she may need it for 20-25 hours during the learning phase, upto 8-9 hours per day. During reference/review phases, the book is needed for at most half an hour to skim over topics that are forgotten.
  2. Instead of buying his/her own copy of the book, a person might as well borrow the book for the duration needed and return it.
  3. A good library solves exactly this need. They stock up multiple copies of the most popular books and some more. Let’s call our library, the ‘Friend of Readers’.
  4. ‘Friends of Readers’ library estimates that they should stock up 5 copies of ‘The Art of Computer Programming’ on their shelves to start with. This is what they will call the ‘initial’ or ‘default’ level. If no one has borrowed this book right now, you will see 5 books on their shelves.
  5. As readers start borrowing this book, the library waits and watches. However, they have set themselves a minimum. At any point of time, readers need to see at least 2 copies of the book on the shelf. So as soon as the number of copies goes down to 1, they will take action. They may either have more copies stored in a basement warehouse or they may get in touch with the publisher. Either way, new copies are arranged. How much should they replenish? Well, the library always strives to revert to their ‘default’ level.
    Here is a scenario for clarity. The library has 5 books right now. One person borrows and the number of copies goes down to 4. The library simply waits. As more readers borrow, the number goes down to 3 and 2. The library still waits. Now if one more book is borrowed, the number of copies goes down to 1. The minimum level is breached and the library has to take action. They will arrange for 4 more books, so that they return to their default level of 5.
  6. But about peak season? Perhaps there is an upcoming graduation examination for computer programming majors. Or some leading companies have announced a huge hiring camps. Candidates need to prepare for interviews. In such cases, the library will up the ante and stock more than 5 books. However, a library is a physical space and the number of shelves are limited. They have to take a call and set a ‘maximum’. At no point will they have more than 15 copies of the book in circulation. This include both the borrowed and in-shelf books put together.
  7. As the peak season fades, the library will remove the extra books to free up space and scale back to their default level of 5.

So what exactly is object pooling?

Let’s use the library example to describe an ‘object pool’ system. In the object pool, there is a pool of objects that clients want to use. These objects are called ‘resource objects’. This is similar to the book ‘The Art of Programming’.

Resource pooling in action with a library. Please click to enlarge.
Resource pooling in action with a library. Please click to enlarge.

A pool manager object overseas the pool. The ‘Friends of Readers’ library is the pool manager. Just like readers do not buy their own copy of the book, the clients objects do not directly create a copy of the resource object. Similar to the readers approaching the library, the client object approaches the pool manager.

The pool manager maintains the default, minimum and maximum levels. The pool manager starts by creating the default level of resource objects. It does not allow the level to fall below the minimum. If the level is breached, the pool manager quickly scales back to the default level. During peak usage, the pool manager has to create more objects, but it never creates more than the maximum. At any point of time, there can be no more resource objects in circulation than the level mandated by the maximum.

Object pooling in software

Database connections take time to set up. Once set up, they consume memory and processor to stay put. In other words, database connections are expensive resources. Hence, in a big application, it does not make sense for every corner of the program to set up its own database connection and maintain it. Instead, there is a database connection pool. The pool manager sets up a fixed number of connections and allows other parts of the application to borrow a connection for use whenever the database needs to be used. There is a default number of connections to start with, a minimum below which the number of unoccupied connections cannot fall and a maximum number of connections to the database, both occupied and unoccupied combined.

On one hand, the effort and time required to set up database connections is taken away from the client and managed by the connection pool. On the other hand, the client doesn’t have to worry about disconnecting when done. The pool takes back the returned connection and gives it to another part of the application that needs it.


Pooling resources and the system of borrowing and lending them is an efficient way to own and use resources. The ones who want to use it do not have to worry about how to get it in the first place. They just need to walk into a place they trust will have it available for them. Also, since the users do not own the resources and keep them permanently, the pool gets back the resources and others can use them. This way, a less number of resources, consuming less space and requiring lower maintenance, can make the world a less crowded place for everyone.