Hi Martí,
We review at http://vschart.com/compare/basex/vs/exist-db that it is horizontal scalable but, on BaseX documentation portal we can not find it.
I don’t know who edited the info on this page, but I’ve just revised the entry. We actually have some internal projects on horizontally scaling (and replicating) data to other nodes, but, unfortunately, there is nothing yet that we can provide out of the box.
We are analysing BaseX as an option to store over 100 million xml documents, and one of our main concerns is, which scalability features it offers.
In our Wiki, you can find some information on larger data instances we have experimentally worked with in BaseX [1,2]. There are commercial projects on Big Data with BaseX, too, but currently none of them (as far as I know) are publicly documented.
As a general note, I can tell that XQuery is powerful enough to address multiple databases and local & remote sources in single queries, so the limits given in the Wiki article are not to be interpreted as absolute limits. However, it’s mostly up to the developer to care about the distribution of data and the infra structure, which means that (in contrast to other distributed document stores) some knowledge is required to make your project fly.
Hope this helps, Christian
[1] http://docs.basex.org/wiki/Twitter [2] http://docs.basex.org/wiki/Statistics