It seems it has neither the OSI's blessing as an Open Source licence, nor the FSF's blessing as a Free Software licence. Can anyone comment on why not, given that the AGPL has the blessing of both organisations?
edit Here's an informative StackExchange comment. [0] Apparently it's a good deal stricter than the AGPL, and introduces much more legal uncertainty.
One of the core tenets of free software licensing is that there is a fair, symmetrical relationship for all parties involved.
Elastic obviously does not publish their own management infrastructure code under SSLP, so the reason for this license to exist is to make the playing field uneven as opposed to all the other free software licenses.
Basically, they can benefit from your code on top of and around ES, while you can't from theirs. This is actually the result of dual licensing but with other free licenses, at least the symmetry is maintained for the core product.
I am not making a judgement call here, just explaining what the distinction is.
Yes, I could see someone putting forward the argument that the SSPL is so unusable that ElasticSearch isn't actually expecting anyone to use it, and therefore it's more of a marketing gimmic than a license.
edit Here's an informative StackExchange comment. [0] Apparently it's a good deal stricter than the AGPL, and introduces much more legal uncertainty.
[0] https://opensource.stackexchange.com/a/7523/