  By introducing intermediate states for metadata changes and ensuring that at
most two versions of metadata exist in the cluster at the same time,
shared-nothing databases are capable of making online, asynchronous schema
changes. However, this method leads to delays in the deployment of new schemas
since it requires waiting for massive data backfill. To shorten the service
vacuum period before the new schema is available, this paper proposes a
strategy named SLSM for zero-downtime schema migration on shared-nothing
databases. Based on the lazy migration of stand-alone databases, SLSM keeps the
old and new schemas with the same data distribution, reducing the node
communication overhead of executing migration transactions for shared-nothing
databases. Further, SLSM combines migration transactions with user transactions
by extending the distributed execution plan to allow the data involved in
migration transactions to directly serve user transactions, greatly reducing
the waiting time of user transactions. Experiments demonstrate that our
strategy can greatly reduce the latency of user transactions and improve the
efficiency of data migration compared to existing schemes.
