There is scheduled database maintenance on Wednesday April 17th from 18:00 - 20:00 UTC (11am - 1pm Pacific) for Wikidata and the English and German Wikipedias. During this window there may be brief (30 seconds - 2 minutes) moments where affected wikis are read-only.
The purpose for the maintenance is to update our "master" database servers for those wikis to MariaDB (https://mariadb.org/). This is part of our on-going migration to MariaDB from MySQL at the Wikimedia Foundation (blog post forth-coming!).
Please report any issues after 20:00 UTC/1pm Pacific to #wikimedia-operations or https://bugzilla.wikimedia.org/ (see this[1] page for tips on reporting bugs).
Best,
Greg
[1] https://blog.wikimedia.org/2013/03/18/how-to-create-a-good-first-bug-report/