Redis Hashes

10. Redis Hashes

What is a Redis Hash?

A Redis Hash stores a mapping of fields to values within a single key. It is perfect for storing object-like data.

Analogy: A Hash is like a row in a spreadsheet — the key is the row ID, and the fields are column names with their values.

How Hashes Structure Data

flowchart TB
    Key["Key: user:42<br/>One top-level key"] --> Fields["Fields & Values<br/>(stored efficiently together)"]

    Fields --> F1["name → 'Prathamesh'"]
    Fields --> F2["age → '26'"]
    Fields --> F3["city → 'Pune'"]
    Fields --> F4["role → 'engineer'"]
    Fields --> F5["login_count → '0'"]

    HSET["HSET user:42 name 'Alice' age '30'<br/>Sets multiple fields in one command"] --> Key
    HGET["HGET user:42 name<br/>→ Returns: 'Alice'"] -.-> F1
    HGETALL["HGETALL user:42<br/>→ Returns all fields"] -.-> Fields

    style Key fill:#7c3aed,color:#fff
    style Fields fill:#3b82f6,color:#fff
    style F1 fill:#f59e0b,color:#fff
    style F2 fill:#f59e0b,color:#fff
    style F3 fill:#f59e0b,color:#fff
    style F4 fill:#f59e0b,color:#fff
    style F5 fill:#f59e0b,color:#fff
    style HSET fill:#059669,color:#fff
    style HGET fill:#3b82f6,color:#fff
    style HGETALL fill:#ec4899,color:#fff

Core Commands

# Set multiple fields at once
HSET user:42 name "Prathamesh" age "26" city "Pune" role "engineer"

# Get a single field
HGET user:42 name
# Output: "Prathamesh"

# Get all fields and values
HGETALL user:42
# Output:
# 1) "name"
# 2) "Prathamesh"
# 3) "age"
# 4) "26"
# 5) "city"
# 6) "Pune"
# 7) "role"
# 8) "engineer"

# Get multiple fields at once
HMGET user:42 name city
# Output: 1) "Prathamesh" 2) "Pune"

# Get just field names
HKEYS user:42
# Output: name, age, city, role

# Get just values
HVALS user:42

# Check if a field exists
HEXISTS user:42 email
# Output: (integer) 0

# Delete a field
HDEL user:42 age

# Increment a numeric field
HSET product:10 stock 100
HINCRBY product:10 stock -5
# Output: (integer) 95

Real-World Examples

Example 1: User Profile

HSET user:profile:101 \
  username "john_doe" \
  email "john@example.com" \
  plan "pro" \
  login_count 0

# Update login count on each login
HINCRBY user:profile:101 login_count 1

Example 2: Product Catalog

HSET product:SKU-9001 \
  name "Mechanical Keyboard" \
  price "4500" \
  stock "50" \
  category "electronics"

# Update stock after sale
HINCRBY product:SKU-9001 stock -1

Hash vs Storing JSON as a String

Approach	Pros	Cons
Hash (`HSET`)	Update individual fields without reading all data	More commands to manage
JSON String (`SET`)	Single command to store/retrieve	Must deserialize entire object to update one field

Best practice: Use Hashes when you need to update individual fields frequently. Use JSON strings when you always read/write the entire object.