OBJECT ENCODING for internal encoding in Redis - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is OBJECT ENCODING in Redis?

OBJECT ENCODING is how Redis stores data internally in different formats to save memory and improve speed.

beginner

Name two common internal encodings Redis uses for strings.

Redis uses RAW (simple string) and EMBSTR (embedded string) encodings for storing strings internally.

intermediate

Why does Redis use different encodings for the same data type?

Redis chooses encoding based on data size and type to save memory and speed up operations.

intermediate

What encoding does Redis use internally for small lists?

Redis uses ZIPLIST encoding for small lists to save memory by storing elements in a compact way.

beginner

How can you check the internal encoding of a Redis key?

Use the command OBJECT ENCODING <key> to see how Redis stores that key internally.

Which Redis encoding is used for small strings to optimize memory?

A ZIPLIST

B EMBSTR

C RAW

D HASHTABLE

EMBSTR encoding stores small strings efficiently by embedding metadata and string in one allocation.

What encoding does Redis use for large strings?

A RAW

B EMBSTR

C INTSET

D ZIPLIST

RAW encoding stores large strings as simple dynamic strings without embedding.

Which encoding is NOT used by Redis for internal storage?

A ZIPLIST

B EMBSTR

C INTSET

D BINARYTREE

Redis does not use BINARYTREE encoding internally.

How does Redis store small sets internally?

A ZIPLIST

B HASHTABLE

C INTSET

D RAW

Small sets with integer elements are stored as INTSET for memory efficiency.

Which command shows the internal encoding of a Redis key?

A OBJECT ENCODING <key>

B INFO ENCODING

C GET ENCODING <key>

D ENCODING SHOW <key>

The command OBJECT ENCODING reveals the internal encoding used by Redis for that key.

Explain why Redis uses different internal encodings for the same data type.

Think about how storing small vs large data differently can help Redis.

Describe how you can find out the internal encoding of a key in Redis and why this might be useful.

Consider commands that reveal internal details about keys.

Practice

1. What does the Redis OBJECT ENCODING command show about a key?

easy

A. The number of clients connected to Redis

B. The expiration time set on the key

C. The total memory used by the Redis server

D. The internal format Redis uses to store the key's value

Solution

1. Step 1: Understand the purpose of OBJECT ENCODING

   The command reveals how Redis internally stores the data of a given key.

2. Step 2: Differentiate from other Redis info commands

   Expiration time, client count, and total memory are shown by other commands, not OBJECT ENCODING.

3. Final Answer:

   The internal format Redis uses to store the key's value -> Option D

4. Quick Check:

   OBJECT ENCODING = internal format [OK]

Hint: OBJECT ENCODING shows how data is stored inside Redis [OK]

Common Mistakes:

• Confusing encoding with expiration time
• Thinking it shows server-wide stats
• Mixing it up with client info commands

2. Which of the following is the correct syntax to check the encoding of a key named user:1 in Redis?

easy

A. GET ENCODING user:1

B. ENCODING OBJECT user:1

C. OBJECT ENCODING user:1

D. SHOW ENCODING user:1

Solution

1. Step 1: Recall Redis command syntax for OBJECT ENCODING

   The correct command is OBJECT ENCODING <key>.

2. Step 2: Verify options against correct syntax

   Only OBJECT ENCODING user:1 matches the correct command structure.

3. Final Answer:

   OBJECT ENCODING user:1 -> Option C

4. Quick Check:

   Correct command syntax = OBJECT ENCODING user:1 [OK]

Hint: Remember: OBJECT ENCODING followed by key name [OK]

Common Mistakes:

• Swapping command words order
• Using GET or SHOW which don't exist for encoding
• Adding extra keywords not in Redis syntax

3. Given a Redis key mylist storing a small list, what encoding is most likely returned by OBJECT ENCODING mylist?

medium

A. raw

B. ziplist

C. hashtable

D. intset

Solution

1. Step 1: Identify encoding for small lists in Redis

   Redis uses ziplist encoding for small lists to save memory.

2. Step 2: Differentiate from other encodings

   intset is for small sets, hashtable for larger collections, and raw for strings.

3. Final Answer:

   ziplist -> Option B

4. Quick Check:

   Small list encoding = ziplist [OK]

Hint: Small lists use ziplist encoding in Redis [OK]

Common Mistakes:

• Confusing intset with list encoding
• Assuming hashtable for small collections
• Thinking raw applies to lists

4. You run OBJECT ENCODING myset but get an error saying the key does not exist. What is the most likely cause?

medium

A. The key myset was deleted or never created

B. The OBJECT ENCODING command is misspelled

C. Redis server is down

D. The key myset is a string, not a set

Solution

1. Step 1: Understand error meaning

   An error about key not existing means Redis cannot find myset.

2. Step 2: Eliminate other causes

   If the command was misspelled, Redis would show unknown command error. Server down would prevent any response. Key type mismatch does not cause key missing error.

3. Final Answer:

   The key myset was deleted or never created -> Option A

4. Quick Check:

   Key missing error = key not present [OK]

Hint: Key missing error means key doesn't exist in Redis [OK]

Common Mistakes:

• Assuming command syntax error
• Thinking server is down without checking
• Confusing key type errors with missing key

5. You have a Redis sorted set myzset with 1000 elements. Initially, OBJECT ENCODING myzset returns ziplist. After adding 10,000 more elements, what encoding will Redis most likely use?

hard

A. skiplist

B. ziplist

C. intset

D. hashtable

Solution

1. Step 1: Understand encoding for sorted sets in Redis

   Small sorted sets use ziplist encoding for memory efficiency.

2. Step 2: Recognize encoding change on size increase

   When sorted sets grow large, Redis switches to skiplist encoding for better performance.

3. Step 3: Eliminate other options

   intset is for small sets, hashtable is not used for sorted sets, and ziplist is only for small sizes.

4. Final Answer:

   skiplist -> Option A

5. Quick Check:

   Large sorted set encoding = skiplist [OK]

Hint: Large sorted sets switch from ziplist to skiplist encoding [OK]

Common Mistakes:

• Assuming ziplist stays for large sets
• Confusing intset with sorted set encoding
• Thinking hashtable applies to sorted sets