The logic

The environment that you have set up places a number of virtual machines on your computer.

The virtual machines play the role of the servers of different cohorts; they have Opal and R installed and each contain some simulated test data.

Your machine, the host, plays the role of the client; that is, the computer that is conducting the analysis.

The DataSHIELD code is also split into client and server packages. The DataSHIELD client packages are installed in R on your machine (so that it can act as a client), and the DataSHIELD server packages are installed in R on each of the virtual machines.*

• Modifying the client packages therefore involves downloading the DataSHIELD source code to your machine, making your changes, and the installing the changed client packages on your machine.

• Modifying the server packages also involves downloading the DataSHIELD source code to your machine and making your changes. However the changed server packages must be installed on each of your virtual machines.