SCAN for safe key iteration in Redis - Step-by-Step Execution

Concept Flow - SCAN for safe key iteration

Start with cursor = 0

↓

Call SCAN cursor MATCH pattern COUNT n

↓

Receive new cursor and keys batch

↓

Process keys batch

↓

Is cursor == 0?

No→Repeat SCAN with new cursor

Yes↓

End iteration

SCAN starts with cursor 0, returns a batch of keys and a new cursor. Repeat until cursor returns to 0, safely iterating keys without blocking.

Execution Sample

Redis

SCAN 0 MATCH user:* COUNT 2
SCAN 3 MATCH user:* COUNT 2
SCAN 0 MATCH user:* COUNT 2

This example shows scanning keys matching 'user:*' in batches of 2, iterating safely until cursor returns to 0.

Execution Table

Step	Input Cursor	Command	Returned Cursor	Keys Returned	Action
1	0	SCAN 0 MATCH user:* COUNT 2	3	[user:1, user:2]	Process keys user:1, user:2
2	3	SCAN 3 MATCH user:* COUNT 2	0	[user:3]	Process key user:3
3	0	End	-	-	Cursor 0 means iteration complete

💡 Cursor returned 0 at step 2, so iteration ends safely without blocking.

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
cursor	0	3	0	0
keys_batch	[]	[user:1, user:2]	[user:3]	[]

Key Moments - 3 Insights

Why do we start SCAN with cursor 0?

SCAN always starts with cursor 0 to begin iteration from the start. See execution_table step 1 where input cursor is 0.

What does it mean when SCAN returns cursor 0?

Cursor 0 means the iteration is complete and no more keys remain. See execution_table step 2 where returned cursor is 0, ending iteration.

Why is SCAN safer than KEYS for large datasets?

SCAN returns keys in small batches without blocking Redis, unlike KEYS which blocks. This is shown by multiple steps returning partial keys in execution_table.

Visual Quiz - 3 Questions

Look at the execution_table, what is the cursor value after step 1?

A 2

B 3

C 0

D 1

💡 Hint

Check the 'Returned Cursor' column in execution_table row for step 1.

At which step does the SCAN command indicate the iteration is complete?

A Step 3

B Step 1

C Step 2

D Step 4

💡 Hint

Look for the step where 'Returned Cursor' is 0 in execution_table.

If COUNT was increased to 5, how would the keys returned per step change?

A More keys per batch, fewer steps

B Fewer keys per batch, more steps

C No change in keys per batch

D SCAN would return all keys at once

💡 Hint

COUNT controls batch size, see how keys returned relate to COUNT in execution_table.

Concept Snapshot

SCAN command iterates keys safely without blocking.
Start with cursor 0.
Each SCAN returns a new cursor and a batch of keys.
Repeat SCAN with new cursor until cursor returns 0.
Use MATCH to filter keys and COUNT to control batch size.

Full Transcript

The SCAN command in Redis allows safe iteration over keys without blocking the server. It starts with cursor 0 and returns a batch of keys plus a new cursor. You process the keys returned, then call SCAN again with the new cursor. When SCAN returns cursor 0, the iteration is complete. This method is safer than using KEYS because it does not block Redis and handles large datasets efficiently. You can use MATCH to filter keys and COUNT to control how many keys are returned per batch.