The switchover of the master datacenter in mediawiki requires the following steps:
# Set the databases in the master datacenter in read-only mode
# Switch the `wmfMasterDatacenter` variable to the new datacenter
# Switch the databases in the new master datacenter
each one of these changes requires traditionally a git commit and a deploy. Our goal is to make switching easier, and - per T149617 - we've chosen to use Etcd to that end.
We have now `EtcdConfig` that can do that inside mediawiki-config, we just need to thoroughly test it and bring that to production.
Our objectives are thus as follows
[x] Test EtcdConfig in different failure scenarios T185078
[x] Prepare conftool for safely editing mediawiki-config values (additional validation, separate etcd credentials)
[x] Deploy EtcdConfig to production
[ ] Allow use of EtcdConfig to configure slave databases (stretch)