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Isolation Levels

Levels of granularity defining the extent to which concurrent transactions are shielded from each other's effects.
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The statement of the theorem

Let T1\mathcal{T}_1 and T2\mathcal{T}_2 be concurrent transactions. Isolation levels define the permissible visibility of intermediate states. \n\n1. **Read Uncommitted:** T1\mathcal{T}_1 may read uncommitted writes from T2\mathcal{T}_2 (Dirty Reads allowed).\n2. **Read Committed:** T1\mathcal{T}_1 only reads values committed by T2\mathcal{T}_2. Read(T1,X)  ⟹  X committed before Read(T1,X)\text{Read}(\mathcal{T}_1, X) \implies X \text{ committed before } \text{Read}(\mathcal{T}_1, X).\n3. **Repeatable Read:** T1\mathcal{T}_1 reads the same committed value of XX throughout its execution, preventing Non-Repeatable Reads. Read(T1,X) at time t1=Read(T1,X) at time t2\text{Read}(\mathcal{T}_1, X) \text{ at time } t_1 = \text{Read}(\mathcal{T}_1, X) \text{ at time } t_2.\n4. **Serializable:** T1\mathcal{T}_1 must behave as if it executed entirely before or entirely after T2\mathcal{T}_2, preventing Phantom Reads and all other anomalies.