Let's start at the beginning.
What is Just in Time Deployment?
Simplistically, Just in Time (JIT) Deployment provides a centralized, automated update facility for any application deployed using the Deployment Tool.
All the Packages associated with an Application are installed on a centralized Application Server, that is, the initial Version and any subsequent major upgrades or Patches. After the initial installation, each time a user launches the Application from a Target PC, the Target PC connects to the Application Server to see if new upgrades are available to install. Any new upgrades will be automatically installed before the Application is launched.
How does it work?
To get started the initial Package to install the application must include:
A variety of deployment templates are available to assist in the construction of these Packages.
The initial Package may be delivered in various ways including:
It is only after this initial installation that Just in Time Deployment really comes into play although we have now identified the Application Server and set up the Just in Time configuration to access the Application Server whenever the Application is started on the Target PC.
Subsequent updates to the application are either a major upgrade Version or a Patch.
When you are using Package Upgrades (the most common option) and you deploy a Package upgrade to the Application Server you no longer create an entry for the Package in the associated APPLPKG.DAT file. It's a simple matter of just copying the MSI or MSP into the Application directory
Each time a user launches the Application on a Target PC a connection is established with the Application Server. Before the Application is started the Just in Time processing checks if there are any upgrades which have not already been applied locally. If any Packages are found they are installed before the Application is launched.
For more detailed information on the Just in Time set up refer to: