[ZBmath](https://zbmath.org/) (formerly Zentralblatt für Mathematik) will become "Open access" in 2021 (see for instance at [EMS site](https://euro-math-soc.eu/news/19/12/17/zbmath-become-open-access) and at [FIZ Karlsruhe site](https://www.fiz-karlsruhe.de/en/nachricht/zbmath-open-informationen-fuer-die-mathematik-werden-frei-zugaenglich)). It used to be under paywall, although some partial access was allowed.


My question (which was suggested in the comments of [this answer](https://mathoverflow.net/a/352213/14094) to [Is a free alternative to MathSciNet possible?](https://mathoverflow.net/352213/)):



> 
> What are guarantees (technical and/or legal) that the reviews will **remain** in unrestricted universal access?
> 
> 
> 


(Concretely, I'd be reluctant to write reviews if ZBmath has the possibility to suddenly turn back to partial paywall in 2030 for whatever reasons, especially if this paywall affects searches for reviews made during the so-called open-access period.)



Sustainability of ZBmath unrestricted access
Thanks a lot for the question! On behalf of Klaus Hulek (as zbMATH Editor-in-Chief) we can confirm that it is correct that we will go Open Access as of 1st January 2021. This will mean that the database is freely accessible by everybody worldwide. It also means that most of the data will become open via a CC-BY-SA license. Such a license cannot legally be revoked. This applies in particular to the reviews written by our reviewers (only excluding the very rare case of reviews where the copyright is owned by a third party, which amounts to less than 0.5% of the reviews). The other main exception is third-party content such as author summaries, which may be provided under a different license. We will further have an API which will allow users to download parts (especially, the CC-BY-SA-licensed parts) of the database.


