ACID properties in MySQL - Step-by-Step Execution

Concept Flow - ACID properties

Start Transaction

↓

Perform Operations

↓

Check Atomicity

↓

Check Consistency

↓

Check Isolation

↓

Check Durability

↓

Commit or Rollback

↓

End

This flow shows how a database transaction follows ACID properties step-by-step to ensure reliable data handling.

Execution Sample

MySQL

START TRANSACTION;
UPDATE accounts SET balance = balance - 100 WHERE id = 1;
UPDATE accounts SET balance = balance + 100 WHERE id = 2;
COMMIT;

This transaction transfers 100 units from account 1 to account 2, ensuring ACID properties.

Execution Table

Step	Action	Atomicity	Consistency	Isolation	Durability	Result
1	START TRANSACTION	Transaction started as a single unit	Database state unchanged yet	Transaction isolated from others	Changes not yet durable	Transaction open
2	UPDATE accounts SET balance = balance - 100 WHERE id = 1	Partial update done, still part of transaction	Balance remains consistent (no negative if checked)	Other transactions cannot see this change yet	Change not durable until commit	Balance of account 1 decreased by 100 in transaction
3	UPDATE accounts SET balance = balance + 100 WHERE id = 2	Still atomic, both updates part of one transaction	Balances consistent after update	Isolation maintained	Not durable yet	Balance of account 2 increased by 100 in transaction
4	COMMIT	All changes applied together or none	Database state consistent after commit	Changes visible to other transactions now	Changes saved permanently	Transaction committed successfully
5	END	Transaction finished	Database consistent	Isolation no longer applies	Durability ensured	Transaction complete

💡 Transaction ends after COMMIT, ensuring all ACID properties are met.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4 (Commit)
balance_account_1	1000	900	900	900
balance_account_2	500	500	600	600
transaction_state	none	open	open	committed

Key Moments - 3 Insights

Why does the balance of account 1 not permanently change after step 2?

Because the transaction is still open (see execution_table step 2), changes are not durable or visible outside until COMMIT.

How does isolation protect the transaction during updates?

Isolation ensures other transactions cannot see partial changes (execution_table steps 2 and 3), preventing inconsistent reads.

What happens if the transaction is rolled back instead of committed?

All changes are undone together, maintaining atomicity and consistency by not applying partial updates.

Visual Quiz - 3 Questions

Look at the execution table, what is the state of 'transaction_state' after step 3?

A committed

B none

C open

D rolled back

💡 Hint

Check variable_tracker column 'After Step 3' for 'transaction_state'

At which step do changes become permanent and visible to other transactions?

A Step 3

B Step 4

C Step 2

D Step 5

💡 Hint

Look at execution_table step 4 under Durability and Isolation columns

If the transaction was rolled back after step 3, what would happen to the balances?

A Balances revert to start values

B Only account 1 balance reverts

C Balances remain changed as in step 3

D Only account 2 balance reverts

💡 Hint

Atomicity means all or nothing; see key_moments explanation about rollback

Concept Snapshot

ACID properties ensure reliable transactions:
Atomicity: all or nothing changes
Consistency: valid data state
Isolation: transactions don't interfere
Durability: changes saved permanently
Use START TRANSACTION, COMMIT, ROLLBACK in SQL

Full Transcript

This visual execution traces a simple money transfer transaction in MySQL demonstrating ACID properties. The transaction starts, performs two updates, then commits. Atomicity ensures both updates succeed or fail together. Consistency keeps balances valid. Isolation hides partial changes from others. Durability saves changes permanently after commit. Variable tracking shows balances and transaction state at each step. Key moments clarify why changes are not permanent before commit and how rollback would undo all changes. The quiz tests understanding of transaction states and ACID effects.