Redis Strings Skill

Overview

Production-grade String operations for Redis. Master atomic counters, caching patterns, and binary-safe string manipulation.

Core Commands

Basic Operations

SET key value [EX seconds] [PX milliseconds] [NX|XX] [KEEPTTL]
GET key
GETEX key [EX seconds|PX ms|EXAT ts|PXAT ts|PERSIST]  # Get + set TTL
DEL key [key ...]
EXISTS key [key ...]

Atomic Counters

INCR key                      # +1 (creates if missing, starts at 0)
DECR key                      # -1
INCRBY key increment          # +N
DECRBY key decrement          # -N
INCRBYFLOAT key increment     # Float increment

String Manipulation

APPEND key value              # Append to value
STRLEN key                    # Get length
GETRANGE key start end        # Substring (0-indexed, inclusive)
SETRANGE key offset value     # Replace from offset

Batch Operations

MSET key value [key value ...]      # Set multiple
MGET key [key ...]                  # Get multiple
MSETNX key value [key value ...]    # Set multiple if none exist

Advanced Options (Redis 6.2+)

SET key value GET                   # Set and return old value
SET key value IFEQ oldvalue         # Set if current equals (Redis 7.4+)
GETDEL key                          # Get and delete
SETEX key seconds value             # Set with TTL (deprecated, use SET EX)

Production Patterns

Pattern 1: Cache with TTL

# Set with 1-hour TTL
SET cache:user:123 '{"name":"John","email":"john@example.com"}' EX 3600

# Get (returns nil if expired)
GET cache:user:123

# Refresh TTL on access
GETEX cache:user:123 EX 3600

Pattern 2: Rate Limiter (Fixed Window)

# Increment counter, set TTL on first request
INCR ratelimit:user:123:minute
EXPIRE ratelimit:user:123:minute 60 NX

# Check count
GET ratelimit:user:123:minute

Pattern 3: Distributed Lock

# Acquire lock (NX = only if not exists)
SET lock:resource:123 "holder-uuid" NX EX 30

# Release lock (only if we own it) - use Lua
EVAL "if redis.call('get',KEYS[1])==ARGV[1] then return redis.call('del',KEYS[1]) else return 0 end" 1 lock:resource:123 "holder-uuid"

Pattern 4: Session Storage

# Create session
SET session:abc123 '{"user_id":1,"created":1704067200}' EX 86400

# Touch session (reset TTL)
GETEX session:abc123 EX 86400

# Invalidate session
DEL session:abc123

Command Complexity

Command	Complexity	Notes
SET/GET	O(1)	Constant time
MSET/MGET	O(N)	N = number of keys
INCR/DECR	O(1)	Atomic
APPEND	O(1)	Amortized
GETRANGE	O(N)	N = returned length
STRLEN	O(1)	Stored metadata

Assets

• rate-limiter.lua - Atomic rate limiting script
• config.yaml - Pattern configuration

Scripts

• string-benchmark.sh - Performance testing script
• helper.py - Python utilities

References

• STRING_PATTERNS.md - Common patterns guide
• GUIDE.md - Complete reference

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Troubleshooting Guide

Common Issues & Solutions

1. WRONGTYPE Error

WRONGTYPE Operation against a key holding the wrong kind of value

Cause: Trying string operation on non-string key

Diagnosis:

TYPE mykey  # Check actual type

Prevention:

• Use consistent key naming conventions
• Check type before operations in scripts

2. Value Too Large

string exceeds maximum allowed size (512MB)

Fix:

• Compress data before storing
• Split into multiple keys
• Use appropriate data structure (Hash for objects)

3. INCR on Non-Integer

ERR value is not an integer or out of range

Cause: INCR/DECR on string that isn't a valid integer

Fix:

# Check current value
GET counter

# Reset if corrupted
SET counter 0

4. NX/XX Not Working as Expected

# NX only sets if key doesn't exist
SET key value NX  # Returns nil if key exists

# XX only sets if key exists
SET key value XX  # Returns nil if key missing

Debug Checklist

□ Key exists? (EXISTS key)
□ Correct type? (TYPE key)
□ Value is valid integer for INCR? (GET key)
□ TTL set correctly? (TTL key)
□ Key pattern consistent?
□ Value size reasonable?

Performance Considerations

Scenario	Issue	Solution
Large values	High latency	Compress or split
Many small keys	Memory overhead	Use Hash for related data
No TTL on cache	Memory leak	Always set TTL
MGET 1000+ keys	Blocking	Batch into smaller chunks

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Error Codes Reference

Code	Name	Description	Recovery
S001	WRONGTYPE	Type mismatch	Check TYPE, fix key
S002	OVERFLOW	Integer overflow	Use INCRBYFLOAT or reset
S003	NAN	Not a number	Validate before INCR
S004	TOOLARGE	Value > 512MB	Compress or split
S005	SYNTAX	Invalid command	Check syntax

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Test Template

# test_redis_strings.py
import redis
import pytest

@pytest.fixture
def r():
    return redis.Redis(decode_responses=True)

def test_set_get(r):
    r.set("test:string:1", "hello")
    assert r.get("test:string:1") == "hello"
    r.delete("test:string:1")

def test_incr(r):
    r.set("test:counter", "10")
    r.incr("test:counter")
    assert r.get("test:counter") == "11"
    r.delete("test:counter")

def test_set_nx(r):
    r.delete("test:nx")
    assert r.set("test:nx", "first", nx=True) == True
    assert r.set("test:nx", "second", nx=True) == None
    r.delete("test:nx")

def test_ttl(r):
    r.set("test:ttl", "value", ex=60)
    ttl = r.ttl("test:ttl")
    assert 55 < ttl <= 60
    r.delete("test:ttl")