A distributed transaction consists of subtransactions that execute at different sites and access local DBMSs at those sites. For example, a distributed transaction that transfers money from a bank account at site A to a bank account at site B executes a subtransaction at A that does the withdrawal and then a subtransaction at B that does the deposit.
Such a distributed transaction must satisfy the ACID properties in a global sense: it must be globally atomic, isolated, consistent, and durable. The issue is, if the subtransactions at each site individually satisfy the ACID properties, does the distributed transaction necessarily satisfy the ACID properties? Certainly if the subtransactions at each site are individually durable, the distributed transaction is durable. Give examples of situations in which
(a) The subtransactions at each site are atomic, but the distributed transaction is not atomic.
(b) The subtransactions at each site are consistent, but the distributed transaction is not consistent.
(c) The subtransactions at each site are isolated, but the distributed transaction is not isolated.