Object storage with hashes in Redis - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a Redis hash?

A Redis hash is a data structure that stores a mapping between string fields and string values, similar to a dictionary or object. It is useful for storing objects with multiple attributes.

beginner

How do you set a field in a Redis hash?

Use the HSET command with the hash key, field name, and value. For example: HSET user:1 name "Alice" sets the 'name' field to 'Alice' in the 'user:1' hash.

beginner

How can you retrieve all fields and values from a Redis hash?

Use the HGETALL command with the hash key. It returns all fields and their values as a list.

intermediate

Why use hashes for object storage in Redis instead of strings?

Hashes allow you to store multiple related fields under one key efficiently. You can update or retrieve individual fields without rewriting the entire object, saving memory and bandwidth.

intermediate

What happens if you HSET a field that already exists in a Redis hash?

The existing field's value is overwritten with the new value. HSET returns 0 if the field was updated, and 1 if a new field was created.

Which Redis command sets a field in a hash?

A HSET

B SET

C GET

D LPUSH

HSET sets a field in a Redis hash. SET is for strings, GET retrieves values, and LPUSH is for lists.

What does HGETALL return?

A Only the values of a hash

B Only the keys of a hash

C All fields and values in a hash

D The number of fields in a hash

HGETALL returns all fields and their values in the specified hash.

If you want to update just one attribute of an object stored in Redis, which data structure is best?

A String

B Hash

C List

D Set

Hashes allow updating individual fields without rewriting the whole object.

What does HSET return when updating an existing field?

A 0

B 1

C -1

D Error

HSET returns 0 if the field was updated (already existed). It returns 1 if a new field was created.

Which of these is NOT a benefit of using Redis hashes for object storage?

A Reduced bandwidth for partial updates

B Ability to update individual fields

C Efficient memory usage

D Automatic data encryption

Redis hashes do not provide automatic encryption; that must be handled separately.

Explain how Redis hashes help in storing objects with multiple attributes.

Think about how you store a person's details like name, age, and email in one place.

Describe the difference between using a Redis string and a Redis hash for storing user profile data.

Consider updating just one piece of information like the email.

Practice

1. What does a Redis hash store?

easy

A. A set of unique keys without duplicates

B. Only a single string value per key

C. A list of keys without values

D. Multiple fields and their values under one key

Solution

1. Step 1: Understand Redis hash structure

   Redis hashes store multiple field-value pairs grouped under a single key.

2. Step 2: Compare options with hash definition

   Only Multiple fields and their values under one key correctly describes storing multiple fields and values under one key.

3. Final Answer:

   Multiple fields and their values under one key -> Option D

4. Quick Check:

   Hashes = multiple fields + values [OK]

Hint: Hashes group fields and values under one key [OK]

Common Mistakes:

• Thinking hashes store only one value
• Confusing hashes with lists or sets
• Assuming keys are stored without values

2. Which Redis command correctly adds a field 'name' with value 'Alice' to a hash key 'user:1'?

easy

A. HGET user:1 name Alice

B. HSET user:1 name Alice

C. SET user:1 name Alice

D. HMGET user:1 name Alice

Solution

1. Step 1: Identify command to add/update hash field

   HSET is the Redis command to set a field in a hash.

2. Step 2: Check command syntax

   HSET key field value is correct; so HSET user:1 name Alice is valid.

3. Final Answer:

   HSET user:1 name Alice -> Option B

4. Quick Check:

   HSET adds/updates hash fields [OK]

Hint: Use HSET to add or update hash fields [OK]

Common Mistakes:

• Using HGET instead of HSET to add data
• Using SET which is for strings, not hashes
• Confusing HMGET (retrieves multiple fields) with HSET

3. Given the following Redis commands executed in order:
HSET user:2 age 30
HSET user:2 name Bob
HGETALL user:2
What is the output of HGETALL user:2?

medium

A. ["age", 30, "name", "Bob"]

B. ["user:2", "age", "30", "name", "Bob"]

C. ["age", "30", "name", "Bob"]

D. ["name", "Bob"]

Solution

1. Step 1: Understand HSET commands

   HSET adds fields 'age' with '30' and 'name' with 'Bob' to hash 'user:2'.

2. Step 2: Understand HGETALL output format

   HGETALL returns an array of field-value pairs as strings in order added.

3. Final Answer:

   ["age", "30", "name", "Bob"] -> Option C

4. Quick Check:

   HGETALL returns all fields and values as string pairs [OK]

Hint: HGETALL returns flat list of field-value strings [OK]

Common Mistakes:

• Expecting key name in output
• Mixing data types (number vs string)
• Only getting one field instead of all

4. You run the command HGET user:3 email but get a null response. What could be the reason?

medium

A. The field 'email' does not exist in the hash 'user:3'

B. The key 'user:3' is a string, not a hash

C. The Redis server is down

D. The command syntax is incorrect

Solution

1. Step 1: Understand HGET behavior

   HGET returns null if the field does not exist in the hash.

2. Step 2: Check other options

   Key being a string would cause an error, not null; server down or syntax error would cause different errors.

3. Final Answer:

   The field 'email' does not exist in the hash 'user:3' -> Option A

4. Quick Check:

   Missing field in hash returns null [OK]

Hint: Null means field missing, not syntax or server error [OK]

Common Mistakes:

• Assuming null means server or syntax error
• Confusing key type errors with missing field
• Expecting error instead of null for missing field

5. You want to store user profiles with fields 'name', 'age', and 'email' in Redis hashes. Which approach is best to retrieve all user data efficiently?

hard

A. Store each user as a separate hash with key 'user:<id>' and use HGETALL to get all fields

B. Store all users in one hash with fields like 'user1_name', 'user1_age', 'user2_name', etc.

C. Store each field as a separate key like 'name:<id>', 'age:<id>', 'email:<id>'

D. Store user data as JSON strings in simple keys

Solution

1. Step 1: Analyze storage options

   Storing each user as a separate hash groups related fields logically and allows efficient access.

2. Step 2: Evaluate retrieval efficiency

   Using HGETALL on 'user:<id>' fetches all fields at once, simpler and faster than scattered keys or complex parsing.

3. Final Answer:

   Store each user as a separate hash with key 'user:<id>' and use HGETALL to get all fields -> Option A

4. Quick Check:

   Separate hashes per user = organized and efficient [OK]

Hint: Use one hash per user for grouped fields and easy retrieval [OK]

Common Mistakes:

• Mixing all users in one hash causing complex keys
• Using separate keys for each field complicates retrieval
• Storing JSON loses Redis hash benefits