Let mirrors specify multiple countries

[mdomsch]

#267
requests the ability for a mirror (really, a globally load balanced
set of mirrors behind a single DNS name) to be able to specify the
countries of their actual mirrors.

This patch lets Host.country be a comma-separated list of countries.

[ott]

I think this is a useful feature, in particular for smaller countries and for people who live in a border region. Several use cases in Europe come to mind. However, I think it would be preferred in the relational model to make the country column a table instead of a comma separated string. Only smaller changes would be required for it to add a model definition and migration the current data.

@mdomsch What do you think about this idea? And are you still interested in this pull request? If not, I would try to adopt it.

[mdomsch]

I'm not directly involved in mirrormanager maintenance anymore. The current maintainers would be in a better position to comment as to the direction they want to see here (if any).

[ott]

@abompard I would volunteer to update this pull request. Can you briefly comment on how you would like the database schema to be for multiple countries, so that I can work in the right direction?

As mentioned, I would suggest a separate n:m relation between hosts and countries.

[abompard]

Hey! It looks like hosts and countries are already linked by a relationship table: HostCountry. So, hosts can already have multiple countries. Am I missing something? (I only did maintenance on the code a few months ago, I'm not aware of all use cases).

[abompard]

Ah, I see, there's a confusion between HostCountry and HostCountryAllowed in the UI. The list of allowed countries in the host form populates the .countries attribute, which is for HostCountry, not HostCountryAllowed. This needs to be fixed.

[ott]

I'm not sure whether I understand the original intent of the data model but I think there is a bigger mismatch between the data model and its usage:

I agree that the form seems to be intended to add entries to HostCountryAllowed but adds them to HostCountry. Most likely the current entries would need to be migrated, perhaps even by hand as there might already be entries in HostCountryAllowed or entries in HostCountry were created in mirrormanager v1 with different semantics.

In addition this pull request points out and tries to address defect that the ByCountry and ByCountryInternet2 caches are created from the attribute country of Host. It seems that the intention of HostCountry was that a host could be designated to serve multiple countries and not just the country in which it is located. This would be useful in Europe, for example, in Luxembourg, which has three neighbouring countries and is well connected to Frankfurt am Main, or in Southeast Europe.

In addition, there might be anycast, DNS load balancing or Geo DNS mirror servers which are located in multiple countries as the description of this pull request points out. So the country attribute cannot represent the multiple locations of such mirror servers as they would need a multiple-valued attribute which would have to be implemented as a weak entity.