The opportunistic usage pattern

Like before the ESM analyzes the dependencies and builds a packed version that contains all software needed. The packaging can be modular, i.e. each needed package can be packaged independently so that the installation script can download from the software server (the SE in the proposal) only those packages which are missing.
During the installation a more sophisticated discovery phase is added that tries to locate common used components. This could be general packages and libs, but can include HEP specific software.
In the following installation process only those parts missing are installed and the environment variables are set accordingly for the experiments software to run.
The efficiency of this pattern can vary depending on the effort put by the ESM in the discovery scripts and by the amount of software installed on the site.
The chance that software can be located correctly will be increased by adhering to use standard locations for packages (like /opt/$<$packageName$>$)
In the case of a shared file system the load on the shared filesystem and the network will be reduced and the efficiency improved since more software is accessed locally.
In case of the local installation the number of packages needed to be installed is smaller which reduces the time spent on this task and reduces the network and local disk capacity needed.
The "opportunistic usage pattern" really works if there is some kind of agreement between the ESM and the site manager. If this is not the case, a site manager could decide to remove/update/modify any of the packges needed by a given version of the experiment sw AFTER the exploration/installation phase is over and WITHOUT notifying the ESM. This would result in sites that publish a tag but where the corresponding sw version does not work.