The replication of Megastore gives a unified picture of the data kept in its dependent copies. No matter which replica a client accesses to begin an operation, read or write operations can be started from any replica, and ACID semantics will be retained. Replication is performed per entity group by synchronously replicating the transaction log of the group to a quorum of replicas. Usually, writes need one round of communication between data centers, while reads, in a healthy case, run locally. Megastore guarantees the following for reads:

  • The read operation will always be based on the write that was acknowledged at last.
  • Following the completion of a write operation, all future read operations will reflect the modifications made by the write.

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