Week 3 - Lock-Based Concurrency Control

Question 1

What is the most the most widely used protocol Lock-Based Concurrency Control

Answer 1

Strict Two-phase Locking, or Strict 2PL

Question 2

What is a lock-based concurrency control?

Answer 2

he lock-based concurrency control,
the basic idea is to use locks to lock data items, that we kind of suspecting if there is a conflict that might lead to inconsistency
if these data items are read or written

Question 3

What happen in a Strict 2PL

Answer 3

1) If T wants to read an object, first obtains an S lock.
2) If T wants to modify an object, first obtains X lock.
3) Hold all locks until end of transaction.

Question 4

What are the advantages of Strict 2PL

Answer 4

Guarantees serializability, and avoids cascading aborts, too!

also avoids WW problems!

Question 5

3Items of Strict 2PL

Answer 5

1) Each transaction must obtain a Shared lock on object before reading, and an exclusive lock on object before writing.
2) All locks held by transaction are released when the transaction completes
3) Only allows serializable schedules

Question 6

What controls all the locking

Answer 6

Lock manager -

Keeps track of what kind of lock this item

Question 7

What does a lock manager do?

Answer 7

1) Handled lock and unlock requests
2) Locking and unlocking have to be attomic operations
3) Lock upgrade

Question 8

What does a Lock upgrade do?

Answer 8

Transactions that holds a shared lock can be upgraded to hold an exclusive lock

Question 9

Strict two-phase locking gets is name from:

1) The two phases, lock acquisition and lock release
2) The two types of locks used
3) The two phases, locking/unlocking and cleanup
4) The two phases, lock acquisition and data write

Answer 9

1) The two phases, lock acquisition and lock release

Question 10

What is a deadlock

Answer 10

Cycle of transactions waiting for locks to be released by each other.

A deadlock means that
one transaction wants to acquire a lock on a data item, and this data item is already
locked by another transaction.

On the other hand, this other transaction also want to acquire a lock on a data item
that is already also locked by the other transaction.

They’re both waiting on each other,

Question 11

Two ways to avoid Deadlocks

Answer 11

1) here are deadlock prevention algorithms. So you add extra logic
into your concurrency control so that
you account for deadlocks. - Overhead

2) let the deadlock happen and take steps to actually break that deadlock.

Question 12

One way to detect Deadlocks

Answer 12

Waits-for graph

Question 13

What is a Waits-for graph

Answer 13

A waits-for graph is you
have the nodes in the graph or the transactions, and there is an edge between two transactions.
If one transaction is waiting for the other transaction to release a lock.

Question 14

What kind of lock must a transaction acquire in order to read from a database?

1) Concurrency lock
2) Read lock
3) Shared lock
4) Exclusive lock

Answer 14

3) Shared lock

Question 15

What kind of lock must a transaction acquire in order to write to a database?

1) Write lock
2) Atomicity lock
3) Exclusive lock
4) Shared lock

Answer 15

3) Exclusive lock

Question 16

4 types of Database Failures

Answer 16

1) Transaction Failures
2) System Failures (hardware)
3) Media failures (hard-drive)
4) Communication failures (network)

Question 17

4 types of Database Failures

Answer 17

1) Transaction Failures - (abort transaction)
2) System Failures - (hardware)
3) Media failures - (hard-drive)
4) Communication failures - (network)

Question 18

When do databases enter a recovery state?

1) When there is a failure during a transaction.
2) When there is a failure of any kind.
3) When there is a failure due to loss of power.
4) When deleted data needs to be restored.

Answer 18

2) When there is a failure of any kind.

Question 19

2 Places that database info is stored

Answer 19

1) Hard drive

2) Database Buffer Manager (main memory buffer)

Question 20

Recovery Manager uses what

Answer 20

Log file, just appends (old value,and new value)

Question 21

5 Items that are in the log (write-ahead log)

Answer 21

1) Transaction identifier
2) Type of operation (action)
3) items accessed by the transaction to perform the action
4) Before image (old value)
5) After image (new value)

Question 22

Recovery - Redo

Answer 22

Performing actions from the log that might have been done, but not sure (committed)

Question 23

Recovery - Undo

Answer 23

Not committed before the crash, undo those.