Compatibility mode is a temporary state of Appliances V6.6 (or higher) that is previous to federation. In compatibility mode, Appliances work in much the same way as in V6.5 (or previous), hence its name. Starting from V6.17, federation of the Appliances is mandatory for proper communication between Portal and Engines.
Appliances enter compatibility mode right after their installation or after being updated from V6.5 (or previous). In this mode, there are no centralized configuration settings or coordinated updates. Unless you have a particular problem when federating your Appliances, it is recommended to switch from compatibility mode to federated mode as soon as possible.
An Appliance in primary / secondary configuration, that is, an Appliance with both the Portal and the Engine installed, is automatically federated. Therefore, it can never enter compatibility mode.
On the other hand, a secondary Appliance which runs in compatibility mode shows two configuration settings in the Web Console that disappear when the Appliance is federated:
The Update section
The Portal address field
The Update section is found in the left-hand side menu of the Web Console, when the Appliance tab is selected. It allows for secondary Appliances to be updated individually. After federation, the Update section is only available in the Web Console of the primary Appliance.
When working in compatibility mode, the Engine is able to send real-time information to the Portal only if you specify the address of the Portal in the Web Console of the secondary Appliance. The Portal address setting is found by selecting the Engine tab, the General section of the left-hand side menu, and looking under Parameters. After federation, the address of the Portal is known to the Engine and this parameter is no longer needed; thus, it is removed from the Web Console.
Since V6.17, the automatic hardening of new installations of the Appliance prevents the real-time communication of data between the Portal and the Engines if they are not federated. Therefore, federation becomes mandatory from V6.17 on.