Sharding and Transactions with MySQL

With sharding, how can you maintain a reliable transaction across multiple database servers?

For example, if I had a table named AccountLedger on one database server (MySQL instance) and a table named User on another database server, is it possible to run a transaction across both database instances that will both reliably commit, or rollback on failure?

Example transaction:

AccountLedger database server:

START TRANSACTION;
INSERT INTO AccountLedger SET
    UserID = @UserID,
    Date = @Date,
    Debit = @Debit,
    Balance = @Balance;

User database server:

START TRANSACTION;
UPDATE User SET
    Balance = @Balance
WHERE UserID = @UserID;

AccountLedger database server:

COMMIT;

User database server:

COMMIT; -- What happens if the COMMIT fails here (power goes out or whatever)

I've read a quite a lot about sharding, but I can't seem to find any information on using transactions with sharding. Can someone point me in the right direction?