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VDI Connection Broker Defined, Part 2 – Assignment Policies

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This is the second article in my series on VDI (desktop virtualization) connection brokers, in which I explain what they are and describe their functionality. In the first part of this series I provided a high-level overview of the role and functionality of the connection broker. I explained that the fundamental role of the connection broker is to pair client devices with desktops running on virtual machines, and to provide the client devices with the information required to establish connections to the virtual desktops. In this article I will delve deeper into the policies and configuration settings that govern this process and show why you would want to use different policies and settings in different situations.

At this point I want to disclose that I work for Ericom Software, and that our flagship product PowerTerm WebConnect is a connection broker. The information provided in this article is a result of the research we conducted while developing this product, and the features we implemented in it.

Free Sitting vs. Fixed Sitting

In order to assign a virtual desktop to a particular client, the connection broker must first identify that client. This identification can be either of the user using the client device or of the client device itself. Assigning the desktop based on the identity of the user is called free sitting because the user is free to roam between various client devices while the connection broker performs the same assignment in all cases. For example, a user can be connected to her virtual desktop from a thin client device in her office, and then drive home and reconnect to the same desktop from a PC in her house. In this case identification can be based on the user’s domain name or group associations in a directory service, such as Microsoft Active Directory.

Assignment based on the identity of the client device itself is known as fixed sitting because the same assignment will be performed regardless of which user is using that device. Identification of the device can be based on its DNS name or MAC address. A sample use case for this approach is a thin client device at a teller station in a bank. In this example the same virtual desktop will be assigned to the device regardless of which teller is currently working at the station.

It’s worth noting that while fixed sitting more closely resembles Local Computing in that the same desktop is always provided at a specific location, free sitting is more commonly used. This is because one of the main benefits of centralized computing, such as VDI, is its ability to provide a user with access to his or her desktop regardless of the user’s physical location.

Static Assignment vs. Dynamic Assignment

There are two main methods in which a virtual desktop can be assigned to a user or device: statically or dynamically. Static assignment is when an administrator explicitly assigns a particular virtual desktop to a specific user or client device. This type of assignment is appropriate, for example, when a virtual desktop was created specifically for a particular user. This can occur when a user was migrated to the VDI environment by P2Ving his or her local computer. If you require static assignment I highly recommend that you choose a solution that enables mass import of such pairings from an external data source. Otherwise you will be required to manually enter all the static assignments into the connection broker one by one.

Even with mass import static assignment of all the desktops can be a very tedious process, and one that requires constant supervision as users come and go. It’s much preferable to have the connection broker perform the desktop assignment itself, automatically and on-demand. Fortunately connection brokers that support dynamic assignment can perform this operation. When using this type of assignment, groups of users are associated with groups of virtual desktops so that when a user belonging to such a group requires a desktop an available one is automatically allocated to him or her. Because the connection broker may allocate any available virtual desktop in the group, these desktops need to be equivalent to each other. Such groups of equivalent virtual desktops are referred to as pools.

Working with pools of virtual desktops rather than with each desktop individually has several notable advantages. Usually pooled desktops are created from a common template. Using such a template enables the connection broker to create additional virtual desktops on demand if there aren’t enough available desktops left in a pool. And because desktops can be created on demand, it’s possible to remove extraneous desktop to free up resources.

Assignment Duration

When a virtual desktop is assigned to a specific client that assignment remains in effect for the duration of the session. But what happens when the session ends? One option is retain the ownership relationship – making it permanent. The next time this client (user or device or both) requests a virtual desktop, the connection broker will assign the exact same one. Permanent assignments remain in effect until an administrator explicitly terminates them. This type of assignment can be useful, for example, when you want to give users the freedom to personalize their desktop – in this scenario users will expect to connect to the same desktop every time.

On the opposite end of the spectrum assignments can be wholly transitory. In this case the virtual desktop becomes available for a different client the instant the session ends. Unless the desktop is locked down so that the user cannot modify it, you may want the connection broker to reverted it to its initial state before assigning it to a different user. A benefit of this approach is that desktops can be reused so that you will need fewer desktop instances. Also, if desktops are reverted to their initial state they become easier to manage because they remain identical to each other.

An intermediary option is to assign a desktop for a specified duration of time. This can be useful, for example, when assigning a desktop to an external consultant or contractor. The desktop can be specifically assigned to that person for the duration of a project, and be automatically recycled afterward.


A robust VDI connection broker offers several policies and configuration settings that govern the virtual desktop assignment process. Combined they yield numerous possible assignment strategies. One of the main tasks when implementing VDI is determining which assignment strategies are appropriate for which clients. Selecting the appropriate strategies is a key requirement for a successful VDI deployment. You must, therefore, verify that the connection broker you choose supports the strategies you need to implement.

Dan Shappir is responsible for all aspects of product design, software development and maintenance of the Ericom's product lines. Mr. Shappir joined Ericom in 2001 and brings over 15 years experience in computer programming, and in the design and architecture of software products. Mr. Shappir holds a B.Sc. in Computer Science (with honors) from the Hebrew University of Jerusalem, Israel and an M.Sc. in Computer Science (with honors) from Tel Aviv University, Israel. | Ericom Software
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