Concepts/3. Data & Storage/SQL Relational vs NoSQL Engines
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3. Data & StorageBeginner

SQL Relational vs NoSQL Engines

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SQL databases enforce structured schemas and ACID transactions with relations. NoSQL systems optimize for flexible schemas, horizontal partition scale, and eventual consistency.

STAFF ENGINEER TAKEAWAY: Use SQL for relational integrity (financial ledgers); use NoSQL for massive key-value or document scale (clickstreams, user profiles).
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Storage Paradigm Selection Flow

1
Incoming Write Request
Order checkout transaction
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2
Relational SQL Node
Multi-table JOIN & ACID lock
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3
NoSQL Document Store
Single-document write <2ms
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✓ ARCHITECTURAL ADVANTAGES

SQL provides robust JOINs and ACID; NoSQL provides seamless horizontal sharding.

✗ TRADE-OFFS & COMPLEXITIES

SQL sharding is complex; NoSQL lacks cross-table ACID transactions without distributed locks.

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Production Systems Leveraging This Pattern

⚡PostgreSQL⚡MySQL⚡MongoDB⚡DynamoDB⚡Cassandra
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More Topics in 3. Data & Storage

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🏛️ACID Transactions & Isolation Levels

Guarantees Atomicity (all-or-nothing), Consistency (rules preserved), Isolation (concurrent safety), and Durability (WAL commits survive power loss).

🔺CAP Theorem (Consistency vs Availability)

In any distributed data store, network partitions (P) are inevitable. When a partition occurs, the system must choose between Consistency (CP) or Availability (AP).

👯Database Replication (Leader-Follower)

A primary leader node accepts all write transactions and streams change logs to secondary read replicas to scale read QPS and enable high availability.