What is Plesk application pool in IIS?

This article describes when, why, and how many application pools are created by Plesk in IIS.

There are two types of application pools in Plesk: shared and dedicated. Shared application pools are assigned to serve sites that do not have dedicated pools. Dedicated pools are used to serve only the sites of the customer or reseller to whom the pool is assigned. Dedicated pools isolate sites of different customers or resellers from each other. It increases the stability of different sites if one of the customer’s or reseller’s sites crashes. For more information about application pools and their management see the Plesk Administrator’s Guide.

I am not going to describe dedicated and shared application pools, instead my aim is to write about how Plesk application pools are related to IIS application pools. Generally, it does not matter what type of application pool (shared or dedicated) Plesk creates – they are all provisioned in the same way. Let’s introduce two terms:

• Plesk application pool – a pool that a Plesk user is allowed to manage. The pool’s settings can be managed in Plesk UI, and it can be either shared or dedicated.
• IIS application pool – a pool that is created directly in IIS (one you can see in the IIS Manager).

It is important to understand that Plesk application pools are represented in IIS as groups of IIS application pools. It means that an application pool in Plesk (whether shared or dedicated) includes several IIS application pools. The number of IIS application pools in the group depends on the number of sites with different ASP.NET versions that are served by the corresponding Plesk application pool.

For example, consider a subscriber that has a dedicated Plesk application pool and two sites, “domain1.com” and “domain2.com”. “domain1.com” uses .NET v4.0, while “domain2.com” uses .NET v2.0. In this case, two application pools are created in IIS: “domain1.com(domain)(2.0)(pool)” for “domain2.com”, and “domain1.com(domain)(4.0)(pool)” for “domain1.com”. Note that in this case the IIS application pool serving “domain2.com” is named “domain1.com(domain)(2.0)(pool)” because the site belongs to the “domain1.com” web space.

Configuration in Plesk:

Configuration of pools in IIS:

The same is true for shared application pools. The maximum number of IIS application pools assigned to a single Plesk application pool is three (one pool for ASP.NET 1, for ASP.NET 2, and one for ASP.NET 4). When a user switches ASP.NET version for a site in Plesk, if an IIS application pool with the chosen ASP.NET version does not exist, one is created.

To sum it up, it is not possible to explicitly control the number of application pools in IIS using Plesk. Keep in mind that whenever you switch on the dedicated Plesk application pool for a user, the real number of dedicated pools in IIS could increase by more than one.

Why is a Plesk application pool a group of IIS application pools?

An application pool in IIS can only serve sites with only one version of ASP.NET. So, if a subscriber has sites with different ASP.NET versions, several application pools in IIS are required, one for each version of ASP.NET. A Plesk application pool is a set of application pool settings that should be applied for all the sites of the subscriber. Consequently, if there are sites with different ASP.NET versions, then the set of application pool settings should be applied for all application pools in IIS.

How do Plesk application pool settings work?

When application pool settings are changed in Plesk, they are applied to all corresponding application pools in IIS. For instance, if a Plesk application pool consists of three IIS application pools, and a user changes “The maximum number of worker processes” setting via the Plesk GUI, then the value of this setting will be applied to all IIS application pools included in the Plesk application pool. Consequently, the real maximum number of worker processes in the system is increased by three times the number set in Plesk.