Introduction: the complete user experience features three phases with Citrix
The goal of Citrix XenApp/XenDesktop technology is to securely deliver virtual applications and desktops to remote end-users.
The delivery of these resources can be based on a variety of techniques, such as “hosted applications”, “VM-hosted applications”, “shared desktops”, “streamed applications”, and “remote PC access”.
From an end-user perspective—and independent of the technique being used—the complete user experience features three phases:
- Phase 1: Authenticating and gaining access to the available and allowed resources
- Phase 2: Selecting and launching the required resources
- Phase 3: Using the resources
At each phase of the Citrix user experience, different Citrix components (illustrated in the figure below) are involved.
Historically, shared applications and shared desktops resources were provided by two separate Citrix products, respectively XenApp and XenDesktop. Since their – version 7.6, these have been merged into a single product. The corresponding global solution is known as Citrix “Flexcast Management
Considering that there are so many components involved in a Citrix environment, it is important tounderstand the communication flows that occur during the different phases of a complete Citrix user experience before trying to troubleshoot potential performance degradation issues.
Phase 1: Authenticating and gaining access to the resources
When working in a Citrix environment, the first phase consists of viewing the catalog of available resources according to the user’s credentials.
In practice, the user connects to the Netscaler—or to the StoreFront directly in case there is no Netscaler component in place or for local connections—which consists of a web interface that displays the different virtual desktops and applications that the user is authorized to access.
This process involves communications between different servers to first authenticate the user and then validate the resources that are accessible according to his/her credentials.
The authentication process typically implies communications between the StoreFront server and the Active Directory server (or any LDAP server for that matter). The StoreFront server receives the user’s credentials and forwards them to the Active Directory server for validation.
Presenting the available and allowed resources to the user involves additional Citrix components: StoreFront, Delivery Controller, Active Directory, and the SQL database. The Citrix Delivery Controller plays the role of conductor. It receives the request from the StoreFront server, validates the user’s credentials with the Active Directory server and then requests the end-user’s corresponding available resources from the SQL database server.
Any problems encountered during these different communication steps (e.g., bad network conditions or problems on the servers themselves) may cause the resource access process to fail.
Phase 2: Selecting and launching the required resources
At this stage, the end-user views the different resources that he/she is allowed to access.
The next step is to select and launch one of these resources. This is done by clicking on the corresponding resource icon and then receiving the .ica file, which is then opened to launch the resource.
In this phase, the Delivery Controller determines the best Worker server to be used by requesting the SQL database. (Note: Worker servers are dedicated for hosting published applications and desktops.)
The Delivery Controller then informs this Worker server about this new connection. The Citrix Licensing server is also called upon to check for available licenses before delivering the resource.
Phase 3: Using the resources
The final step in the Citrix user experience is to use the resource (published desktop or application) through the ICA protocol and corresponding virtual channels.
During a session, the end-user experience depends on a variety of factors such as network conditions and Citrix policies. These policies have a significant impact on the different system components that are involved and the traffic characteristics that are generated.
The real Citrix experience for end-users does not start when using the network resource. It actually begins when accessing the service catalog from the Citrix StoreFront / Netscaler web interface.
Unfortunately, the standard key performance indicators (KPIs) proposed by the leading application performance monitoring solutions available on the market today do not take all of these steps into account.
In the following articles, discover how SkyLIGHT™ PVX performs complete, end-to- end Citrix management and delivers superior end-user experience: