Consistency, transactions, availability

 

Consistency, Transactions, and Availability

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1. Consistency

Meaning

Consistency means all users see the same data at the same time after an update.

In NoSQL systems:

• Consistency can be strong or eventual

• Many NoSQL databases follow eventual consistency

Consistency in MongoDB

• Primary node handles writes

• Secondary nodes replicate data

• There may be a small delay in replication

Types:

• Strong consistency – Read from primary

• Eventual consistency – Read from secondary

Advantages

• Better performance

• High scalability

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2. Transactions

Meaning

A transaction is a group of operations executed as a single unit.

ACID Properties

• Atomicity – All or nothing

• Consistency – Valid data state

• Isolation – No interference

• Durability – Data is saved permanently

Transactions in MongoDB

• Earlier: single-document atomicity

• Now: supports multi-document ACID transactions

• Works across collections and databases

Use Cases

• Banking systems

• Order processing

• Inventory management

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3. Availability

Meaning

Availability means the database remains accessible even during failures.

Availability in MongoDB

• Achieved using replica sets

• If primary fails:

  • Secondary becomes primary automatically

• Minimal downtime

Benefits

• Fault tolerance

• High reliability

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4. CAP Theorem (Related Concept)

CAP states a distributed system can provide only two of the three:

• C – Consistency

• A – Availability

• P – Partition tolerance

MongoDB focuses on:

• Consistency + Partition tolerance (CP)

• Availability may reduce during network failure