Strictly speaking, Session Manager is a backend API system that stores and processes session data in JSON objects. Each VTEX account has settings that indicate which installed apps have a session dependency and how they intend to process this information.

Apps with a session dependency monitor change to their inputs and modify session parameters through their outputs. When a session parameter changes, Session Manager notifies all apps monitoring it and listens to their response, which indicates whether any other parameters must be updated as a result. Session Manager then patches the session data by compiling the responses sent by all apps, if necessary. We refer to this notification / response / update cycle as transform call or transform.

Transforms often trigger other transforms, repeating until apps send no further parameter updates. This operation is, naturally, carefully monitored for loop conditions. The diagram below illustrates one such transform cycle:

1. A change was made to session parameter X
2. App A was monitoring session parameter X, so that triggered Transform 1
3. Transform 1 led App A to change session parameter Y
4. Apps B and C were monitoring session parameter Y, so that triggered Transform 2
5. Transform 2 led App B to change session parameter Z
6. App C was monitoring session parameter Z, so that triggered Transform 3
7. Transform 3 had no side effects on parameters monitored by apps, so the cycle ended, and the session was saved. In the future, other cycles may start if other changes are made.

Transform calls are made to all apps simultaneously for the sake of performance. That's why App C was affected both by Transform 2 and Transform 3 - it had no way of knowing that the result of Transform 2 on App B would lead to further changes. When a new session is created, a simpler version of this cycle is executed, which we call create call. All apps that have set their configuration with RunOnCreate: true will be notified to run simultaneously with an empty input. If these apps modify any parameters that are monitored by other apps, that triggers a transform cycle that will run until the input dependency is resolved.