SCAN for safe key iteration in Redis

Introduction

SCAN helps you look through all keys in Redis without freezing the database. It is safer than getting all keys at once.

When you want to find keys matching a pattern without stopping Redis.

When your database has many keys and you want to avoid slow commands.

When you want to process keys in small parts instead of all at once.

When you want to build a tool that checks or cleans keys safely.

When you want to avoid blocking other users while searching keys.

Syntax

Redis

SCAN cursor [MATCH pattern] [COUNT count]

The cursor starts at 0 and moves forward with each call until it returns 0 again.

MATCH lets you filter keys by pattern, COUNT suggests how many keys to return per call.

Examples

Start scanning all keys from the beginning.

Redis

SCAN 0

Scan keys that start with 'user:'.

Redis

SCAN 0 MATCH user:*

Try to return about 100 keys in this scan call.

Redis

SCAN 0 COUNT 100

Sample Program

This command scans keys starting with 'session:' in small groups of 10 to avoid blocking.

Redis

SCAN 0 MATCH session:* COUNT 10

Important Notes

SCAN does not guarantee to return all keys in one call; you must repeat calls with the new cursor.

The cursor returned is a string number; when it returns '0', the scan is complete.

COUNT is a hint, not a guarantee; Redis may return more or fewer keys.

Summary

SCAN lets you safely look through keys without blocking Redis.

Use MATCH to filter keys by pattern and COUNT to control batch size.

Repeat SCAN calls with the returned cursor until it returns 0.

Practice

1. What is the main advantage of using SCAN over KEYS in Redis for iterating keys?

easy

A. SCAN returns all keys at once for faster access.

B. SCAN only works with sorted sets.

C. SCAN deletes keys while iterating.

D. SCAN does not block Redis and returns keys in batches.

Solution

1. Step 1: Understand KEYS behavior

   KEYS returns all matching keys at once and blocks Redis during execution.

2. Step 2: Understand SCAN behavior

   SCAN returns keys in small batches without blocking Redis, allowing safe iteration.

3. Final Answer:

   SCAN does not block Redis and returns keys in batches. -> Option D

4. Quick Check:

   Safe iteration = SCAN non-blocking [OK]

Hint: Remember: SCAN is non-blocking, KEYS blocks [OK]

Common Mistakes:

• Thinking SCAN returns all keys at once
• Confusing SCAN with key deletion
• Assuming SCAN only works on sorted sets

2. Which of the following is the correct syntax to start a SCAN iteration from the beginning in Redis CLI?

easy

A. SCAN 1

B. SCAN 0

C. SCAN *

D. SCAN start

Solution

1. Step 1: Identify the starting cursor for SCAN

   The SCAN command requires a cursor argument; starting cursor is always 0.

2. Step 2: Check options for correct cursor

   SCAN 0 starts iteration from the beginning; other options are invalid.

3. Final Answer:

   SCAN 0 -> Option B

4. Quick Check:

   Start cursor = 0 [OK]

Hint: Always start SCAN with cursor 0 [OK]

Common Mistakes:

• Starting with cursor 1 instead of 0
• Using wildcard * as cursor
• Using invalid strings like 'start'

3. Given the following Redis commands executed sequentially, what will be the output of the last SCAN command?

SET user:1 Alice
SET user:2 Bob
SET admin:1 Carol
SCAN 0 MATCH user:*

medium

A. [cursor, ["user:1", "user:2"]]

B. [0, ["user:1", "user:2"]]

C. [cursor, ["user:1", "user:2", "admin:1"]]

D. [0, ["admin:1"]]

Solution

1. Step 1: Understand SCAN with MATCH

   The MATCH user:* filters keys starting with 'user:'. So keys 'user:1' and 'user:2' match; 'admin:1' does not.

2. Step 2: Understand SCAN output format

   SCAN returns a two-element array: a cursor (string) and a list of keys matching the pattern in this batch. Cursor is not necessarily 0 unless iteration is complete.

3. Final Answer:

   [cursor, ["user:1", "user:2"]] -> Option A

4. Quick Check:

   Filtered keys = user keys only [OK]

Hint: MATCH filters keys; cursor is a string, not always 0 [OK]

Common Mistakes:

• Expecting cursor 0 on first SCAN call
• Including keys not matching pattern
• Confusing cursor with count

4. You wrote this Redis command to iterate keys safely:

SCAN 0 MATCH user:*

But you notice it returns only a partial list of keys matching the pattern. What is the likely mistake?

medium

A. Not repeating SCAN with the returned cursor until it returns 0.

B. Using MATCH with an incorrect pattern.

C. Starting SCAN with cursor 1 instead of 0.

D. Using COUNT option with a value too high.

Solution

1. Step 1: Understand SCAN iteration process

   SCAN returns a cursor and a batch of keys; to get all keys, you must repeat SCAN with the new cursor until it returns 0.

2. Step 2: Identify the mistake

   Running SCAN 0 MATCH user:* only once returns partial keys; not looping causes incomplete results.

3. Final Answer:

   Not repeating SCAN with the returned cursor until it returns 0. -> Option A

4. Quick Check:

   Repeat SCAN until cursor 0 [OK]

Hint: Always loop SCAN calls until cursor returns 0 [OK]

Common Mistakes:

• Expecting all keys in one SCAN call
• Misusing MATCH pattern syntax
• Starting with wrong cursor

5. You want to safely iterate all keys starting with session: in Redis but avoid blocking the server. You also want to limit each batch to about 10 keys. Which command sequence correctly achieves this?

hard

A. Use SCAN 0 COUNT 10 without MATCH.

B. Run KEYS session:* once to get all keys.

C. Repeat SCAN cursor MATCH session:* COUNT 10 until cursor returns 0.

D. Use SCAN 0 MATCH session:* once without COUNT.

Solution

1. Step 1: Use SCAN with MATCH and COUNT

   To filter keys starting with 'session:' and limit batch size, use MATCH session:* and COUNT 10.

2. Step 2: Repeat SCAN calls until cursor is 0

   Iterate by passing the returned cursor each time until it returns 0 to get all keys safely without blocking.

3. Final Answer:

   Repeat SCAN cursor MATCH session:* COUNT 10 until cursor returns 0. -> Option C

4. Quick Check:

   Safe iteration + filter + batch size = Repeat SCAN cursor MATCH session:* COUNT 10 until cursor returns 0. [OK]

Hint: Combine MATCH and COUNT, loop until cursor 0 [OK]

Common Mistakes:

• Using KEYS which blocks Redis
• Not looping SCAN calls
• Omitting MATCH or COUNT when needed