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→Inter-DC Redundancy
We'll replicate the entire stack into a second data-center, which will maintain a full backup copy of all shards. In concrete AWS terms, this means a second AWS Region.
One DC will be the active master for all shards, and the other is purely a backup. We could have it accepting web traffic and forwarding all DB queries across to the master, or we could remove it from DNS entirely unless we need it. Every shard has a designated warm-standby host in this DC, configured for asynchronous WAN replication from the hot standby (so that the master doesn't have additional load from this replication). Likewise, the internal DB Router state is replicated into the second DC:
'''TODO:''' We could potentially fail over to the second DC for individual shards, if we happen to lose all DBs for that shard in the master DC. At the cost of sending DB queries to a separate region. Worth it?
== Database Snapshots ==
