A Citrix product goes through a three-step process to check out licenses:
- At startup, a computer running a Citrix product checks out a startup license.
- A client device connects to the product server.
- The product requests a license from the License Server.
Citrix products, such as XenDesktop, store a replica of the licensing information from the License Server, including the number and type of licenses. If a Citrix product server loses its connection to the License Server, or the License Server does not respond in a timely manner, the product enters a grace period. During the grace period, the product uses its local record of the licenses to continue licensing the product. Citrix products update this record every hour.
If product servers lose communication with the License Server, the users and the products are protected by a grace period. The grace period allows the product servers to continue operations as if they were still in communication with the License Server. After the Citrix product checks out a startup license, the product and the License Server exchange "heartbeat" messages every five minutes. The heartbeat indicates to each that they are still up and running. If the product and the License Server don't send or receive heartbeats, the product lapses into the licensing grace period and licenses itself through cached information.
Citrix sets the grace period. It is typically 30 days but can vary depending upon the product. The Windows Event Log, and other in-product messages, indicate if the product has entered the grace period, the number of hours remaining in the grace period. If the grace period runs out, the product stops accepting connections. After communication is re-established between the product and the License Server, the grace period is reset.
The grace period takes place only if the product has successfully communicated with the License Server at least once.
Grace period example - two sites, both using the same License Server
The connection between Site 1 and the License Server goes down causing Site 1 to go into the grace period, continuing operation and making connections. For concurrent licenses, they can connect up to the maximum concurrent licenses installed. For user/device licenses, they have unlimited connections. When Site 1 reestablishes communication with the License Server, connections are reconciled and no new connections are allowed until they are within normal license limits. Site2 is unaffected and operates as normal.
If the License Server goes down, both sites go into the grace period. Each site allows up to the maximum number of licenses installed. As above, the user/device licenses have no limit.
Supplemental grace period
For the supplemental grace period to be available, you must use a minimum of XenApp 7.6 or XenDesktop 7.6.
If all licenses are in use, supplemental grace period enables unlimited connections to a product, allowing you to understand and address the issue. This period lasts until 15 days elapses or you install more retail licenses, whichever comes first. We recommend that after the supplemental grace period starts, you allow it to run out rather than adding licenses immediately to clear the warning condition.
After the period expires, normal connection limits are enforced. Users are not disconnected, but as they disconnect, no new connections can occur until license levels return to normal.
Once the supplemental grace period completes, it isn't automatically reenabled. To reenable the supplemental grace period, install another retail license, and then normal license limits are enforced. When you reeneable the supplemental grace period by installing the new retail license, a new 15-day supplemental grace period starts the next time you exceed maximum installed licenses.