Xapian is a library but is licensed under GPL, so you can't build on it without making your whole app GPL.

You can get around that by having the search happen in a separate process or something, maybe. But this is a huge issue for something that one might want to embed.

that is a slight misunderstanding of how open source licensing works.

the GPL bleed only happens if you distribute your application, meaning to sell or give away binary packages for customers to install. if your product is a hosted api that you do not distribute, you do not invoke that clause.

also, a lot of open source projects handle this by having things like the core engine licensed on a copy-left friendly license (GPL,AGPL). however, the language connectors and bindings are licensed under the slightly less restrictive apache license. unless you are offering a saas service of the product itself, it is more likely you are actually interacting with the connectors anyways. mongodb is a classic example of this model.

Yes, if you don't distribute it, the license doesn't matter. That is more a flaw than the intention, but that is correct.

The point is that I can use SQLite, Tantivy, RocksDB, ... in my app no problem. I can make it open core, I can make it AGPL, BSD, MIT, not problem. Because those things are meant to be embedded. But I almost definitely can't use Xapian.

Let's be honest, if I want a search solution for use in my SaaS, I will grab Elasticsearch or an equivalent, I have no need for a library. It seems to me that the only use case where Xapian could really shine is crippled by their license. That is a shame.

It can be a problem if you intended to make an embeddable search engine within applications meant to be executed by your end-users (as is the case with milli-py above).