2. WHAT IS A SCHEDULE?
• When the transactions are executing concurrently in an
interleaved fashion , then the order of execution of the
operations from various transactions is known as a
schedule
• In recovery and concurrency we deal with read_item,
write_item operations and commit and abort operations
• We’ll denote read_item, write_item, commit and abort by
R, W, Com and Ab
3. REPRESENTATION OF A SCHEDULE
D = R1(X); W(X); Com1; R2(X); W2(X); Com2; R3(X); W3(X); Com3;
4. HOW THE OPERATIONS
CONFLICT IN A SCHEDULE?
• They belong to a different transaction
• They access the same item X
• At least one of operation is a write_item(X)
Example:
S=R1(X); R2(X); W1(X); R1(Y) ; W2(X); W1(Y) ;
5. COMPLETE SCHEDULE
• A complete schedule is:
• The operations in Schedule are those including
a commit or abort as the last operation for
each transaction
• For any pair of operations from the same
transactions Ti, their order of appearance in
schedule is same as in Ti
• Set of conflicting pairs of one schedule are
same to that in other
8. HOW TO AVOID CASCADING
ROLLBACK
• Also named cascadeless. A single transaction
abort leads to a series of transaction rollback.
• Strategy to prevent cascading aborts is to
disallow a transaction from reading
uncommitted changes from another transaction
in the same schedule.
9. STRICT ACHEDULE
• Here transactions can neither read nor write an
item X until the last transaction that wrote X has
commited(or aborted).