Ludwig Huang

4. Concurrency Control

CMU 15-445 Project Review: Concurrency Control

In this series, I will show you how to build a disk-oriented DBMS (Database Management System) roughly. This article is about Concurrency Control, contains: Transaction, Isolation Levels, Two-Phase Locking, Deadlock Prevention.

This article mainly describes: How the DBMS seiralizes execution of multiple transactions.

N. Logging & Recovery  ^ Up (high level)
4. Concurrency Control |  <- Today
N. Query Planning      |
3. Operator Execution  |
2. Access Methods      |
1. Buffer Pool Manager |
0. Disk Manager        | Bottom (low level)


A transaction is the execution of a sequence of one or more operations (e.g., SQL queries) on a database to perform some higher-level function. It is the basic unit of change in DBMS.

Transaction has 4 critical properties called "ACID":

  1. A (Atomic): all or nothing
  2. C (Consistency): it looks correct to me
  3. I (Isolation): as if alone
  4. D (Duration): survive failures

In most DBMSs, "A" is guaranteed by both Concurrency Control and Logging & Recovery "C" is not as practical for one-node DBMSs. "I" is guaranteed by Concurrency Control. "D" is guaranteed by Logging & Recovery. (This series will only cover Concurrency Control)

Isolation Levels

To guaranteed Atomic, there are some conflicts we need to handle:

  • Overwriting Uncommitted Data: T1 writes O, then T2 writes it, but T2 commits earlier than T1, so finally O is of the T1 modified value.
  • Reading Uncommitted: T1 writes O, then T2 reads O, but T1 aborts finally, T2 reads O of the wrong value.
  • Unrepeatable Reads: T1 reads object O, then T2 modified it, T1 cannot reads O of the same value.
  • Phantom Reads: T1 reads some tuples from a table, T2 inserts a new tuple, then T1 does the same operation again but gets different results.

To solve conflicts above, we need a protocol between applications and DBMSs. Applications are responsible for prevent particular query combinations in multiple transactions. The protocol is Isolation Level:

Isolation Levels

Two-Phase Locking

  • Lock: High-level, guarantee multiple transactions works well
  • Latch: Low-level, protect internal data structures

Lock manager is the DBMS component for concurrency control. It is responsible for forcing transaction following particular scheme to get and release latches based on the isolation levels. In bustub, we use 2PL & Wound-Wait as the low level architecture.

2PL (Two-Phase Locking) is a concurrency control protocol that determines whether a txn can access an object in the database on the fly. 2PL is a good way to guarantee conflict serializability. Conflict serializability means that the conflicting action in transactions is the same order as serial schedule.

As the name says, 2PL has 2 two phases Growing Phase & Shrinking Phase. If release a lock in growing phase, then enters shrinking phase. In shrinking phase, application cannot grant a lock. In strong strict 2PL (SS2PL, rigorous 2PL), a transaction holds all granted locks until commit.

For different isolation level, 2PL does different things.

  • Serializable: Obtain all locks first; plus index locks, plus strict 2PL.
  • Repeatable Reads: Same as above, but no index locks.
  • Read Committed: Same as above, but S locks are released immediately.
  • Read Uncommitted: Same as above but allows dirty reads (no S locks).

Deadlock Prevention

2PL may lead to deadlock, so we need a way to handle with it.

In bustub, lock manager uses deadlock prevention. When there might be a deadlock, kill one of the transactions according strategy below.

Wound-Wait ("Young Waits for Old"):

  • If requesting txn has higher priority than holding txn (i.e., requesting_txn_id < holding_txn_id), then holding txn aborts and releases lock.
  • Otherwise requesting txn waits.


In the end of this series, we implement a lock manager for parallel execution of multiple transactions. The lock manager uses 2PL & Wound-Wait scheme to grant and release locks based on isolation levels.

So we have built a one-node disk-oriented DBMS without log & recovery. If add a SQL level and a shell, it is a DBMS that can manage database well (though it cannot survive failures).

Published by Tech Blog - Huang Blog.

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