Redis Strings

6. Redis Strings

What is a String?

A String in Redis is the most basic type. Despite the name, it can hold:

Plain text: "Hello, World!"
Integers: 42
Floats: 3.14
Binary data: images, serialized objects (up to 512 MB)

Analogy: A String is like a labeled jar — the label is the key, and whatever you put inside is the value.

How Strings Work

flowchart LR
    subgraph SET[SET key value]
        K1["Key: username"] --> V1["Value: 'prathamesh'"]
    end

    subgraph GET[GET key]
        K2["Key: username"] --> V2["Returns: 'prathamesh'"]
    end

    subgraph Counter[INCR key — Atomic Counter]
        K3["Key: page:views"] --> V3["0 → INCR → 1 → INCR → 2"]
    end

    subgraph TTL[SET key value EX seconds]
        K4["Key: session:token"] --> V4["Stored with 3600s expiry
        Auto-deletes after 1 hour ⏰"]
    end

    style SET fill:#7c3aed,color:#fff
    style GET fill:#3b82f6,color:#fff
    style Counter fill:#059669,color:#fff
    style TTL fill:#f59e0b,color:#fff

Core Commands

# Set a key
SET username "prathamesh"

# Get a key
GET username
# Output: "prathamesh"

# Set with expiry (in seconds)
SET session:user123 "token_abc" EX 3600

# Set multiple keys at once
MSET name "Alice" age "30" city "Pune"

# Get multiple keys at once
MGET name age city
# Output: 1) "Alice" 2) "30" 3) "Pune"

# Check if a key exists
EXISTS username
# Output: 1 (exists) or 0 (not found)

# Delete a key
DEL username

# Append to existing value
SET greeting "Hello"
APPEND greeting ", World!"
GET greeting
# Output: "Hello, World!"

Counters — Increment & Decrement

Redis Strings can store integers and perform atomic increment/decrement operations.

# Initialize a counter
SET page:views:homepage 0

# Increment by 1 (atomic operation)
INCR page:views:homepage
# Output: (integer) 1

INCR page:views:homepage
# Output: (integer) 2

# Increment by a specific amount
INCRBY page:views:homepage 100
# Output: (integer) 102

# Decrement
DECR page:views:homepage
# Output: (integer) 101

DECRBY page:views:homepage 50
# Output: (integer) 51

Why is INCR useful? Because INCR is atomic. Even if 1,000 concurrent clients call INCR at the same time, each increment is applied correctly. No race conditions.

Real-World Examples

Example 1: Page Visit Counter

# Every time a user visits the homepage
INCR visits:homepage

# Get total visit count
GET visits:homepage

Example 2: User Session Storage

# Store session token with 30-minute expiry
SET session:user:42 "eyJhbGciOiJIUzI1NiJ9..." EX 1800

# Validate session
GET session:user:42
# Returns token if valid, nil if expired

Example 3: Rate Limiting

# Track API calls per minute per user
SET ratelimit:user:42 0 EX 60
INCR ratelimit:user:42

# If value > 100, reject the request
GET ratelimit:user:42

Advantages of Redis Strings

Simple and fast
Atomic counter operations
Supports TTL natively
Binary-safe (can store any data)

Limitations

Maximum value size is 512 MB
Not suitable for structured or relational data