At the moment, Ingenta registers DOIs once, and that's it. However, we have some cases where data changes after the DOI has been registered. Most notably, that can be the iLibrary URL alias. This case is particularly problematic as the DOI resolver will continue to send requests to the old, no longer functioning iLibrary URL alias.
We should set up a regular process that tests all our DOIs (via a HEAD request) to see whether they return a 404 and list them somewhere, together with the iLibrary URL alias. We should then spend some quality time to understand and fix each case.
In the future, the "fixing" part should not be necessary, if we can trigger a resubmission with Crossref when certain metadata fields change (again, most notably the iLibrary URL alias).
[not sure this "product" is the right one]