Commits a transaction. The request includes the mutations to be applied to rows in the database. Commit might return an ABORTED error. This can occur at any time; commonly, the cause is conflicts with concurrent transactions. However, it can also happen for a variety of other reasons. If Commit returns ABORTED, the caller should retry the transaction from the beginning, reusing the same session. On very rare occasions, Commit might return UNKNOWN. This can happen, for example, if the client job experiences a 1+ hour networking failure. At that point, Cloud Spanner has lost track of the transaction outcome and we recommend that you perform another read from the database to see the state of things as they are now.