Hi Rafael,
Welcome to the AtoM community!
The short answer: for what you describe, you probably need different installations per institution. With some development you could create a shared public front end, but AtoM's existing functionality may not give you exactly what you're looking for without additional work on your end.
Some further details:
AtoM supports basic multi-repository access - you can have multiple archival institution records in a single installation, and are able to link descriptions and authority records to each. In Canada, many of our provincial networks use AtoM in this way as a portal site for regional archives. See for example:
However, at this time AtoM does not currently offer true multi-repository support for all entities. The focus on AtoM's multi-repository support right now is on access, and not in acting as multiple separate installations in a single application. I'll provide some examples of what I mean below.
While AtoM supports
basic per-repository theming and includes some settings that can make it easier for users to browse the records of one specific institution (
see here for example), you cannot apply different themes or settings per institution. The settings, plugins, menus, and static pages are global to the installation, and creating a custom theme for AtoM would also apply to the whole site.
There are also several modules that do not support multi-repository usage, such as accessions, donors, rightsholders, functions, terms, and physical storage. What I mean is, there's no way to associate records in these modules with a specific institution, and there's no way to limit access to some of these records to only one institution. You would need to share these modules, and be comfortable with the other archives seeing your records.
Finally, related to this: AtoM's permissions module has some known scalability issues when users attempt to apply complex permissions, such as limiting a user from one institution from seeing the descriptions of another.There's much more that can be said about this (for more details see for example
this forum post, and
this one) AtoM's permissions module was first created well over a decade ago, when AtoM's primary use case was small and medium-sized archives. No one has sponsored the work required to overhaul the permissions module and make it more performant - as you may know, Artefactual's method of maintaining AtoM so far relies on community support for major development. You can learn more about how we maintain and develop AtoM here:
There are potential ways that multiple separate AtoM installations could be combined into a single public-facing front end, but none that are ready to use without local development to make them happen. This is a long-term goal for the AtoM project, but for now you would have to create the solution yourself.
One very simple example I can think of is St. John's College at the University of Cambridge. There, the institutional archives use a separate AtoM instance from the personal collections they maintain - but a member of their IT staff created a search interface that the public can use to find results in either site. See:
I believe that they are using the API endpoints provided by AtoM's Elasticsearch search index to do this, but something similar could be done using AtoM's
REST API endpoints. I also know of at least one service provider in South Africa who provides clients with Wordpress sites linked to AtoM instance to act as a front end in a similar way.
If you don't have the capacity to create something like this for yourself, then for now you may need to make do with three separate installations. Perhaps other members of the community will have other suggestions and workarounds for you, though!
Hope this helps!