skills/redis-development/rules/json-vs-hash.md

title	impact	impactDescription	tags	description	alwaysApply
Choose JSON vs Hash vs String Appropriately	MEDIUM	Optimal data model for your use case	json, hash, string, data-structures, documents	Choose JSON vs Hash vs String Appropriately	true

Choose JSON vs Hash vs String Appropriately

Redis offers three ways to store structured data: JSON, Hash, and serialized strings. Each has distinct trade-offs around atomic partial operations and indexability.

Feature	JSON	Hash	String (serialized JSON)
Structure	Nested objects and arrays	Flat key-value pairs	Any structure
Atomic partial reads	Yes ($.field)	Yes (HGET)	No (must fetch entire value)
Atomic partial writes	Yes (JSON.SET $.field)	Yes (HSET)	No (must rewrite entire value)
RQE indexing	Yes	Yes	No
Geospatial indexing	Yes	Yes	No
Memory efficiency	Higher overhead	More efficient	Most compact
Field-level expiration	No	Yes (HEXPIRE)	No

When to use each:

• JSON: Nested structures with atomic partial updates and indexing needs
• Hash: Flat objects with atomic field access, field-level expiration, or memory efficiency
• String: Simple caching where you always read/write the entire object and don't need indexing

Correct: Use JSON for nested structures with atomic partial updates.

Python (redis-py):

# JSON supports nested structures and atomic deep updates
redis.json().set("user:1001", "$", {
    "name": "Alice",
    "preferences": {"theme": "dark", "notifications": True}
})

# Atomic update of nested field - no read-modify-write needed
redis.json().set("user:1001", "$.preferences.theme", "light")

Java (Jedis):

import redis.clients.jedis.UnifiedJedis;
import redis.clients.jedis.json.Path2;
import org.json.JSONObject;

try (UnifiedJedis jedis = new UnifiedJedis("redis://localhost:6379")) {
    JSONObject user = new JSONObject();
    user.put("name", "Alice");
    user.put("preferences", new JSONObject().put("theme", "dark"));

    jedis.jsonSet("user:1001", new Path2("$"), user);

    // Atomic update of nested field
    jedis.jsonSet("user:1001", new Path2("$.preferences.theme"), "light");
}

Correct: Use Hash for flat objects with atomic field access.

Python (redis-py):

# Hash is efficient for flat data with atomic field operations
redis.hset("session:abc", mapping={
    "user_id": "1001",
    "created_at": "2024-01-01",
    "ip": "192.168.1.1"
})

# Atomic field read and update
ip = redis.hget("session:abc", "ip")
redis.hset("session:abc", "ip", "10.0.0.1")

Correct: Use String for simple caching without partial updates.

Python (redis-py):

import json

# String is fine when you always read/write the entire object
# and don't need indexing or partial updates
config = {"feature_flags": {"dark_mode": True}, "version": "1.0"}
redis.set("config:app", json.dumps(config), ex=3600)

# Must fetch and parse entire object
config = json.loads(redis.get("config:app"))

Incorrect: Using String when you need atomic partial updates.

Python (redis-py):

import json

# BAD: Must fetch, parse, modify, serialize, and rewrite entire object
data = json.loads(redis.get("user:1001"))
data["preferences"]["theme"] = "light"  # Not atomic!
redis.set("user:1001", json.dumps(data))
# Another client could have modified the object between GET and SET

Reference: Data Type Comparison