Transaction basics in Firebase - Step-by-Step Execution

Process Flow - Transaction basics

Start Transaction

↓

Read Data Snapshot

↓

Apply Update Logic

↓

Attempt Commit

↓

Commit

A transaction reads data, applies changes, tries to commit, and retries if conflicts occur.

Execution Sample

Firebase

db.runTransaction(transaction => {
  return transaction.get(docRef).then(doc => {
    const newCount = doc.data().count + 1;
    transaction.update(docRef, { count: newCount });
  });
});

This code reads a document's count, increments it by one, and updates it atomically.

Process Table

Step	Action	Data Read	Update Applied	Commit Attempt	Result
1	Start transaction	N/A	N/A	No	Transaction started
2	Read document snapshot	{"count": 5}	N/A	No	Data read successfully
3	Calculate new count	5	count = 6	No	Update prepared
4	Apply update	5	count = 6	No	Update staged
5	Attempt commit	5	count = 6	Yes	Commit successful
6	Transaction ends	N/A	N/A	No	Transaction complete

💡 Commit successful, transaction ends without retry

Status Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
count	undefined	5	6	6	6
transactionState	not started	started	update prepared	update staged	committed

Key Moments - 3 Insights

Why does the transaction retry sometimes?

If the commit fails due to data changes by others, the transaction retries from reading data again (see concept_flow and step 5 in execution_table).

Is the data read inside the transaction always the latest?

Yes, the transaction reads the latest data snapshot at step 2 before applying updates, ensuring consistency.

What happens if the update logic depends on stale data?

The transaction will detect conflicts at commit and retry, so the update logic always runs on fresh data (see retry path in concept_flow).

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'count' after step 3?

A 5

B 6

C undefined

D 7

💡 Hint

Check the 'Data Read' and 'Update Applied' columns at step 3 in execution_table.

At which step does the transaction try to save changes to the database?

A Step 5

B Step 4

C Step 2

D Step 6

💡 Hint

Look for 'Attempt commit' column in execution_table.

If the commit fails, what happens next according to the concept_flow?

A Transaction ends immediately

B Update is discarded without retry

C Transaction retries from reading data again

D Transaction commits anyway

💡 Hint

See the retry loop in the concept_flow ASCII diagram.

Concept Snapshot

Transaction basics in Firebase:
- Start transaction
- Read latest data snapshot
- Apply update logic
- Attempt commit
- If commit fails, retry transaction
- Ensures atomic, consistent updates

Full Transcript

A Firebase transaction starts by reading the latest data snapshot. Then it applies update logic based on that data. Next, it attempts to commit the changes. If the commit succeeds, the transaction ends. If the commit fails due to concurrent changes, the transaction retries from reading data again. This process ensures atomic and consistent updates to the database.