Transactions in NestJS - Step-by-Step Execution

Concept Flow - Transactions

Start Transaction

↓

Execute DB Operations

↓

Check for Errors

↓

Commit

↓

End Transaction

This flow shows how a transaction starts, runs database operations, then commits if all is good or rolls back if errors occur.

Execution Sample

NestJS

await this.dataSource.transaction(async manager => {
  await manager.save(user);
  await manager.save(profile);
});

This code runs two save operations inside a transaction that commits if both succeed or rolls back if any fail.

Execution Table

Step	Action	Operation	Result	Transaction State
1	Start transaction	Begin transaction	Transaction started	Active
2	Save user	Insert user record	User saved	Active
3	Save profile	Insert profile record	Profile saved	Active
4	No errors detected	Check operations	All successful	Active
5	Commit transaction	Commit changes	Transaction committed	Committed
6	End transaction	Close transaction	Transaction ended	Committed

💡 Transaction ends after commit because all operations succeeded

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
transactionState	None	Active	Active	Committed
userSaved	False	True	True	True
profileSaved	False	False	True	True

Key Moments - 2 Insights

Why does the transaction rollback instead of commit if an error happens?

If any operation inside the transaction fails (not shown in this trace), the flow moves to rollback instead of commit to undo all changes, ensuring data stays consistent. See step 4 where errors are checked.

What does 'transactionState' mean in the variable tracker?

'transactionState' shows if the transaction is active, committed, or rolled back. It starts None, becomes Active when transaction begins, and ends Committed if successful (see execution_table steps 1 and 5).

Visual Quiz - 3 Questions

Look at the execution table, what is the transaction state after saving the user?

A Committed

B None

C Active

D Rolled back

💡 Hint

Check the 'Transaction State' column at step 2 in the execution_table

At which step does the transaction commit?

A Step 5

B Step 4

C Step 3

D Step 6

💡 Hint

Look for the 'Commit transaction' action in the execution_table

If saving the profile failed, what would happen to the transaction state?

A It would stay Active

B It would rollback

C It would commit anyway

D It would end without commit or rollback

💡 Hint

Recall the key moment about rollback on errors and the flow after step 3

Concept Snapshot

Transactions in NestJS:
- Use dataSource.transaction(async manager => {...})
- All DB ops inside run atomically
- If all succeed, transaction commits
- If any fail, transaction rolls back
- Keeps data consistent and safe

Full Transcript

In NestJS, transactions let you group database operations so they all succeed or fail together. The flow starts a transaction, runs operations like saving user and profile, then checks for errors. If no errors, it commits the transaction to save changes permanently. If errors occur, it rolls back to undo all changes. The transaction state changes from None to Active when started, then to Committed if successful. This ensures your data stays consistent and avoids partial updates.