1. A method comprising: receiving, at a first server of a plurality of servers, a first request to sign-in to a particular account associated with account data that is replicated across the plurality of servers; responsive to the first request, determining that no other server in the plurality of servers stores session data describing an active session involving the particular account; responsive to said determining that no other server in the plurality of servers stores session data describing an active session involving the particular account: generating new session data describing a new session involving the first server and the particular account; and while the new session remains active, the first server servicing additional requests involving the particular account by manipulating the account data associated with the particular account within a copy of the account data that is local to the first server; receiving, at the first server of the plurality of servers, a second request to sign-in to the particular account; responsive to the second request, determining that a second server of the plurality of servers stores session data describing an active session involving the second server and the particular account; responsive to said determining that the second server stores session data describing an active session involving the second server and the particular account, the first server redirecting the second request to the second server; wherein the method is performed by one or more computing devices.