{"id":26930,"date":"2019-11-07T16:22:48","date_gmt":"2019-11-07T22:22:48","guid":{"rendered":"https:\/\/stackify.com\/?p=26930"},"modified":"2024-03-19T11:04:59","modified_gmt":"2024-03-19T11:04:59","slug":"what-is-an-iis-application-pool","status":"publish","type":"post","link":"https:\/\/stackify.com\/what-is-an-iis-application-pool\/","title":{"rendered":"What Is an IIS Application Pool"},"content":{"rendered":"\n

People who are new to hosting web apps on IIS (Internet Information Services) sometimes struggle with the concept of application pools. What is an IIS application pool? What purpose does it serve? In this post, we\u2019ll answer these questions\u2014and more.<\/p>\n\n\n\n

We\u2019ll begin by offering a brief introduction to IIS itself. If you\u2019re already well acquainted with this piece of software, then the first section isn\u2019t for you; feel free to skip it. Those of you who choose to read it will learn a bit about IIS and the role it plays in the Microsoft stack.<\/p>\n\n\n\n

After that, we\u2019ll proceed to cover the concept of \u201cpools\u201d in the tech world. If you\u2019re an IT professional\u2014especially in the field of software development\u2014you likely come across a reasonable number of \u201cpools\u201d in your day-to-day work, so it makes sense to define those as well.<\/p>\n\n\n\n

Then we get to the main point of the article: IIS application pools. You\u2019ll see the definition of the IIS application pool, understand its purpose, and learn details about the different types of pools.<\/p>\n\n\n\n

A Brief Introduction to IIS<\/strong><\/h2>\n\n\n\n

IIS stands for Internet Information Services, as you\u2019ve seen in the introduction. Formerly known as \u201cInternet Information Server,\u201d IIS is a web server created by Microsoft.<\/p>\n\n\n\n

The paragraph above summarizes what IIS is. We could leave it at that and call it a day, if not for the \u201cweb server\u201d part. You see, \u201cweb server\u201d is a somewhat ambiguous term, because it can mean two different\u2014yet related\u2014things.<\/p>\n\n\n\n

\u201cWeb server\u201d can mean a computer (which can be both a physical machine or a virtualized computer. ) It can also mean a type of software that runs on said computer. Both the hardware and software varieties of the web servers are there for the same purpose: they take and reply to requests.<\/p>\n\n\n\n

Since Microsoft is\u2014or at least has been, for the most part of its history\u2014a software company, it shouldn\u2019t take you long to understand that IIS belongs to the \u201csoftware\u201d category of web servers. So, IIS is an application used to host and manage web applications\/websites on Windows. It was introduced back in 1995 as a free add-on for Windows NT 3.51, and it\u2019s been part of the Windows NT family since then.<\/p>\n\n\n\n

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The Concept of \u201cPools\u201d in Tech<\/strong><\/h3>\n\n\n\n

Tech workers come across the concept of pools all the time in their works; this is especially true for software development. Since it\u2019s such a pervasive concept, it\u2019d make sense to dedicate a little time to define it.<\/p>\n\n\n\n

According to Wikipedia<\/a>:<\/p>\n\n\n\n

In computer science, a pool is a collection of resources that are kept ready to use, rather than acquired on use and released afterwards.<\/em><\/p>\n\n\n\n

You might think that the definition above doesn\u2019t sound that far from caching, and you wouldn\u2019t be far from the truth. Sure, there are differences between the concepts<\/a>, but they also have a lot in common. Think of caching as storing values for later reuse, while pooling deals with resources.<\/p>\n\n\n\n

The justification for adopting pools is usually performance; for instance, think about scenarios when acquiring some resources might present a high-cost. In situations like that, pooling might come in handy.<\/p>\n\n\n\n

A common example would be a pool of database connections. Creating connections presents relevant costs, so it\u2019s advantageous to keep a pool of existing connections from which programming routines can obtain them, instead of creating new connections every time one is required.<\/p>\n\n\n\n

IIS Application Pool \u2013 What It Is, What It\u2019s Used for<\/strong><\/h3>\n\n\n\n

We started the post by giving an overview of IIS, proceeding to define the concept of \u201cpools\u201d in tech. Now we\u2019re ready to put it all together and start answering the title question.<\/p>\n\n\n\n

Defining the IIS Application Pool<\/strong><\/h4>\n\n\n\n

After learning about IIS and understanding the meaning of the pool in Computer Science, defining the \u201cIIS application pool\u201d should be almost trivial. An IIS application pool is a pool\u2014i.e., a collection\u2014that houses applications on IIS. Each application pool consists of a process called w3wp.exe that runs on the server machine. That\u2019s pretty much it.<\/p>\n\n\n\n

The question then obviously becomes: why? What is the purpose of application pools on IIS? What do you use them for? Are there implications for your apps?<\/p>\n\n\n\n

IIS Application Pool: Understanding the Need<\/strong><\/h4>\n\n\n\n

As the previous section just covered, you can think of an application pool as a collection or compartment of applications. An application pool can contain n apps, and it allows you to create isolation levels for your applications. Why would that be useful?<\/p>\n\n\n\n

One reason to adopt application pools might be to ensure availability for your apps<\/strong>. Since each pool runs in its dedicated process, an error in one app won\u2019t take down applications in other application pools.<\/p>\n\n\n\n

Additionally, you might want to use application pools to configure different levels of security for different applications<\/strong>. For instance, you have five applications. One of them requires a higher level of security than the other four. In this case, you could create two separate application pools for the two different groups of applications. You can configure each pool to run under a different user account, by specifying the identity for the Application Pool<\/a>.<\/p>\n\n\n\n

Finally, you might also want to run applications on different versions of the .NET framework. This is yet another case where the application pool might come in handy.<\/p>\n\n\n\n

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Going Deeper: Shared vs. Dedicated Application Pools<\/strong><\/h3>\n\n\n\n

You can classify IIS application pools into two main categories: shared application pools and dedicated application pools.<\/p>\n\n\n\n

Shared Application Pools<\/strong><\/h4>\n\n\n\n

You call an application pool \u201cshared\u201d if it hosts several web applications running under it. You\u2019d typically combine many applications into a single application pool if you want to preserve the memory of the server. As we have already mentioned, each pool consists of a w3wp.exe process. If you have ten applications, each one in its own pool, that means you have ten processes in execution. On the other hand, if you host all ten applications under a single application pool, you end up with a single w3wp.exe process running.<\/p>\n\n\n\n

What about a dedicated application pool? Well, that\u2019s exactly the opposite: you say a pool is dedicated when it only has a single application running on it. Isolating each app in its own pool enables you to have a more rigid separation between them. Also, as we\u2019ve already said, this compartmentalization prevents the crash of an app that brings other apps down as well.<\/p>\n\n\n\n

Dedicated Application Pools<\/strong><\/h4>\n\n\n\n

Here are some of the most important advantages of having dedicated application pools over shared ones:<\/p>\n\n\n\n