transaction schedule (idea)

A transaction schedule is an ordering of several transactions or - more often - of the operations of several transactions. It is needed in database theory to define serializability, an important concept used to maintain two of the ACID principles of transactions: isolation and consistency.

In most cases the operations of the transactions shown in the schedule are only reads and writes, since in the standard case only those are necessary to determine the serializeability of a schedule (i.e. the existence of a reordering of all operations so that in the end all transactions are executed one after another and not interwoven). But some non-standard approaches also take the actual calculations into account to improve serializability. Have e.g. a look at the following schedule, in which two transactions concurrently change the content of datum x :

        T1               T2
    V1:=Read(x)
    V1:=V1*2
    Write(x,V1)
                     V3:=Read(x)
                     V3:=V3+1
                     Write(x,V3)
    V2:=Read(x)
    V2=V2+1
    Write(x,V2)

If just the read and write operations are observed, there is a conflict between T₁ and T₂ since T₂ reads and writes intermediate results of T₁ violating isolation. But knowing the actual calculations, one can take advantage of the commutativity of the addition. The serial schedule T₁-T₂ (i.e. first complete execution of T₁ then of T₂) is then equivalent and the above schedule therefore serializeable.