MongoDB ACID Transactions

Last Updated : 01 May, 2024

MongoDB ACID transactions are fundamental for ensuring data integrity in database transactions. In MongoDB, ACID properties play a crucial role in maintaining the reliability and consistency of data operations.

In this article, We will learn about ACID transactions in MongoDB, understand their importance with examples, and see how the transaction works in MongoDB along with their use cases.

What Are ACID Transactions?

A transaction is a set of operations that are executed as a single, atomic unit. Transactions provide data consistency by ensuring that either all the operations within the transaction are committed to the database or none of them are.

Visual Representation of Transactions States:

Visual Representation of Transaction states

Example of ACID Transactions

For example, let’s say Mridul, wants to transfer 1000 rupees from his savings account (account A) to his checking account (account B). Here’s how the transaction could proceed:

Begin Transaction: The system starts a transaction for Mridul’s fund transfer.
Read Data: It reads the current balances of Mridul’s savings and checking accounts. Let’s say the balance of account A is 5000 rupees, and the balance of account B is 3000 rupees.
Perform Operations: The system deducts 1000 rupees from account A and adds 1000 rupees to account B.
Commit Transaction: If the deduction and addition are successful without any issues, the transaction is committed. Account A now has a balance of 4000 rupees, and account B has a balance of 4000 rupees.
Rollback Transaction (if necessary): If any error occurs during the transaction (e.g., insufficient funds in account A), the changes are rolled back. Neither account A nor account B is affected, maintaining data consistency.

ACID Properties in MongoDB

ACID Properties ensures the integrity and reliability of the database. The term ACID stands for Atomicity, Consistency, Isolation, and Durability. Let’s understand each of these ACID transaction properties with their examples.

1. Atomicity

It states that all the operations of the transaction take place at once and if not then the transaction is aborted. Transaction can not occur partially. Either all of the operations transactions are executed properly in a database or none of them will execute.

Example: Ensuring Atomicity in Bank Transactions

Consider a bank transaction where money is transferred from one account to another. Atomicity ensures that if the debit operation from one account succeeds, the corresponding credit operation to the other account also succeeds. If any failure occurs during the transfer (for example, a system crash), the transaction is rolled back, and both accounts retain their original balances.

2. Consistency

The integrity constraints must be maintained before and after the transaction. The consistency in transactions ensures that before and after the transaction operations the database remains in a valid state. The database must be consistent after the transaction happens.

Example: Maintaining Grade Integrity

In such a database, when we want to update a student’s grade using some transaction, the consistency concept guarantees that the updated grade value is always within the valid range (for instance, A, B, C, D, F) before and after the update. If the update violates any constraints (for example, assigning an invalid grade), the transaction is aborted to maintain consistency.

3. Isolation

Isolation is one of the ACID properties that ensures that transactions are executed independently of each other without interference. Changes occurring in a particular transaction will not be visible to any other transaction. Isolation guarantees that the changes made by one transaction are not visible to other transactions until the transaction is committed.

Example: Ensuring Transaction Integrity

Suppose there are two transactions: Transaction A transfers money from Account X to Account Y, while Transaction B queries the balance of Account Y. Isolation causes the state of the committed Account Y either before or after transaction A to be visible to Transaction B, without consideration of whether transaction A has already completed. It prevents Transaction B from seeing any intermediate, uncommitted state of Account Y caused by Transaction A.

4. Durability

This property is used to indicate the performance of the database’s consistent state. It states that after the transaction completes successfully the changes it has made to the database are permanent even if there are system failures.

Example: Transaction Persistence and Recovery

Once the transaction of fund transfer is effected smoothly on a banking system interface, the resilient feature makes it possible for the transaction input data to be stored and saved permanently in the system database regardless of the system crash that follows immediately after the transaction. When the system restarts, it should be able to recover the committed changes and restore the database to its last consistent state.

Why are ACID Transactions important?

ACID transactions are important in database systems for several reasons:

Data Integrity: ACID transactions ensure that database operations maintain data integrity, meaning that data remains accurate, consistent, and valid before and after transactions. It is important for applications where data accuracy is important, such as financial systems or e-commerce platforms.
Concurrency Control: ACID transactions provide mechanisms for managing concurrent access to the database by multiple users or processes. This ensures that transactions executed simultaneously do not interfere with each other, preventing issues like data corruption or inconsistent query results.
Reliability: ACID transactions guarantee that if a transaction is committed, its changes will persist even in the event of system failures or crashes. This reliability is essential for applications that cannot afford to lose data or suffer from incomplete transactions.
Simplicity: ACID transactions offer a straightforward and standardized approach to managing database operations, making it easier for developers to design and maintain complex applications without worrying about low-level data consistency issues.

How Do Multi-Document ACID Transactions Work In MongoDB?

The multi-document ACID (Atomicity, Consistency, Isolation, and Durability) transactions model in MongoDB allows developers to implement transactions over several documents within a MongoDB database, the resulting transactions are guaranteed to be atomic and consistent across documents.

Start Transaction: To start a multi-document transaction the developer first gets the session started with the help of the method startSession() of the MongoDB driver.
Define Operations: Within the transaction session, it is possible to determine what read and write operations the developer defines which can also go across several documents or collections. The operations can be such as insertion, updating, removal, and querying.
Execute Operations: Once the operations are specified, the developer executes them within the transaction session. All processes conducted in the session hold the same transaction.
Isolation: MongoDB provides the isolation of data by keeping it for separate transaction processes separately. This implies that the changes incorporated in one transaction are not obtained by other transactions until they are also committed.
Consistency: MongoDB allows you to maintain consistency by validating or enforcing any data constraints or rules that have been defined in the database schema. Transactions, which guarantee that the database remains in a consistent state throughout running and after stopping, ensure this.
Atomicity: In the case that any one operation within the transaction fails or encounters an error, MongoDB automatically undoes all the changes made by the same transaction.
Commit Transaction: When, at last, the developer is certain that all operations within the transaction session have been successfully executed without any errors, he/she can commit the transaction to the remote database by using the commitTransaction() method.
Rollback Transaction: In case any operation of the transaction generates an error or if the transaction is intentionally ended up by the developer, MongoDB takes care of the rollback of all the changes made by the transaction and maintains the data consistency.
Durability: Once a transaction is committed, MongoDB ensures durability by persisting the changes made by the transaction to disk. This ensures that the changes are durable and remain intact even in the event of a system failure or crash.

When Should We Use MongoDB Multi-Document Transactions?

We should consider using multi-document transactions in MongoDB when:

Transactional Requirements: Your application demands a transactional aspect in which the guarantees are across various documents including consistency between related data entities and data integrity maintenance during complex operations.
Complex Data Operations: The application executes data operations that involve multiple documents and has a Boolean logic which entails that either all operations succeed or no operations are implemented.
Data Consistency: Your application should oblige the data consistency constraints and make sure that changes made to the database by the transactions are both atomic and consistent even in the presence of concurrent access.

ACID Transactions Use Cases and Best Practices

ACID transactions in MongoDB make application development more promising than previously, especially where data availability is on a real-time step and consistency is a must. Such examples as business activities, internet shopping, and organization software involving dynamic data aggregation among different files are often the most widespread applications.

While ACID transactions provide robust guarantees, developers should be mindful of certain best practices to optimize their usage:

Transaction Granularity: Design transactions to encapsulate related operations within a single transaction to minimize the risk of conflicts and improve performance.
Transaction Duration: Keep transactions short-lived and minimize their length so that both contention and concurrency is low.
Error Handling: Error-protection mechanisms should be robust enough to handle transaction failures and protect data integrity.
Indexing Strategy: Secure database indexes following several criteria to answer transactional queries efficiently and minimize lock contention.

Conclusion

ACID transactions are important in MongoDB for maintaining data integrity and reliability. By understanding these principles, developers can ensure that database operations are executed consistently and accurately, even in complex scenarios involving multiple documents.

Understanding and implementing ACID properties in MongoDB not only enhances data consistency but also provides a robust foundation for building reliable and efficient applications.