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ACID compliance is a cornerstone of reliable database systems, ensuring that transactions are processed accurately and reliably. This becomes even more critical in distributed databases, where data spans across multiple nodes and geographical locations. In this article, we will explore the four elements of ACID compliance, its importance, methods to achieve it, and its implementation in TiDB.

What are the Four Elements of ACID Compliance?


Atomicity guarantees that each transaction is all-or-nothing. If one part of the transaction fails, the entire transaction fails, and the database state remains unchanged. This is analogous to the all-or-nothing principle; either all steps are completed successfully, or no steps are executed at all.


Consistency ensures that a transaction brings the database from one valid state to another, maintaining the database rules such as unique constraints and foreign keys. It checks that any transaction will leave the database in a legitimate state. For example, if a transaction takes the database from one consistent state, it cannot violate any defined database constraints as it processes.


Isolation ensures that transactions are independent and do not interfere with each other. Even when multiple transactions are executed concurrently, each transaction should be unaware of the other transactions running simultaneously. This property is crucial to prevent unexpected behaviors that could compromise the database’s integrity.


Durability guarantees that once a transaction is committed, it remains so even in the event of a system crash. This means that the changes made by the transaction are stored permanently, typically by writing to non-volatile storage such as hard drives or SSDs, ensuring data persistence.

Why Does ACID Compliance Matter?

ACID compliance is critical because it maintains data integrity and reliability. Without these guarantees, databases might return inconsistent results, violate business rules, or lose data. Here are a few scenarios that illustrate the importance of ACID compliance:

Financial Transactions: In banking systems, it’s crucial that money transfer processes either complete fully or not at all to prevent discrepancies in account balances.

E-commerce Platforms: Order processing systems must ensure that orders are correctly placed, paid for, and shipped without leaving the system in an inconsistent state.

Inventory Management: In systems tracking stock levels, transactions must accurately reflect the available inventory to avoid issues like over-selling or stockouts.

Methods to Achieve ACID Compliance on Distributed Systems

Two-Phase Commit

Two-phase commit (2PC) is a protocol that ensures all nodes in a distributed system agree on a transaction’s outcome, making it an essential method for achieving atomicity and consistency. It operates in two phases:

  1. Prepare Phase: The coordinator asks every participant if they can commit the transaction. Each participant performs local checks and writes to their transaction log but doesn’t commit.
  2. Commit Phase: If all participants agree, the coordinator asks them to commit. If any participant votes to abort, the coordinator instructs all participants to roll back.

Multi-Version Concurrency Control

Multi-Version Concurrency Control (MVCC) allows multiple versions of data to exist simultaneously, enabling transactions to read from a consistent snapshot without blocking writes. This method helps in achieving isolation by ensuring read operations do not interfere with write operations, thus maintaining transactional consistency.

Persistency Methods

Persistent methods involve writing changes to non-volatile storage to ensure durability. Techniques such as Write-Ahead Logging (WAL) where changes are logged before being applied, ensure the database can recover and maintain a consistent state even after a crash.

Is ACID Compliance Always Necessary?

While ACID compliance is crucial for many use cases, there are scenarios where the strict enforcement of ACID might not be necessary or could impact performance significantly. For example, in applications like social media feeds or log data where eventual consistency suffices, sacrificing some aspects of ACID can lead to significant performance improvements.

In such systems, BASE (Basically Available, Soft state, Eventual consistency) properties are often preferred. BASE systems provide flexibility and scalability at the cost of immediate consistency, suitable for cases where high availability and partition tolerance are more critical than immediate consistency.

ACID Compliant Databases

Several databases ensure full ACID compliance to guarantee robust transaction processing:

  • PostgreSQL: A renowned open-source RDBMS that supports advanced features and ensures ACID through MVCC and other methods.
  • Oracle Database: Provides strong ACID guarantees, often used in enterprise environments requiring high reliability.
  • Microsoft SQL Server: Another enterprise-grade RDBMS ensuring ACID properties through its transaction system and recovery mechanisms.
  • TiDB: A distributed SQL database that ensures ACID compliance across distributed nodes, making it a robust choice for scalable and highly available systems.

ACID Compliance in TiDB

TiDB, an open-source distributed SQL database, excels at providing ACID compliance within a distributed architecture. Here’s how TiDB achieves each aspect of ACID:

  • Atomicity: TiDB uses the Percolator transaction model, which ensures that transactions are atomic. It employs a Two-Phase Commit (2PC) protocol to guarantee that all nodes involved in a transaction either commit or rollback entirely. This approach ensures that partial transactions do not leave the database in an inconsistent state.
  • Consistency: TiDB uses the Raft consensus algorithm to replicate data across multiple nodes, ensuring that all changes meet the database constraints before being applied.
  • Isolation: TiDB allows concurrent transactions to read and write without stepping on each other’s toes by maintaining multiple versions of data. This ensures that transactions operate correctly even when they are executed simultaneously.
  • Durability: Durability in TiDB is achieved through its use of persistent storage mechanisms. The changes made by a transaction are logged to stable storage before the transaction is considered committed. In the event of a system failure, TiDB can recover the transaction’s state from the logs, ensuring no data loss.

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Optimization and Scalability

TiDB not only ensures ACID compliance but also offers elastic scalability. This allows the system to scale out horizontally by adding more nodes, thus maintaining performance as the load increases. The separation of compute and storage allows independent scaling of each layer, making TiDB highly efficient for large-scale deployments.


ACID compliance is fundamental to ensuring the reliability, integrity, and consistency of database transactions. While the challenges increase in distributed systems, methods such as Two-Phase Commit, MVCC, and robust persistency measures enable maintaining ACID properties. TiDB stands out as a distributed database that provides full ACID compliance along with the flexibility and scalability required by modern applications. Whether dealing with financial transactions, e-commerce systems, or large-scale data applications, TiDB ensures that your data remains consistent, isolated from concurrent operations, and durable across system failures.

Last updated June 7, 2024

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