Unit of Work Design Pattern in Software Development

The Unit of Work pattern stands out for its effectiveness in managing database transactions, and is a core feature of persistence frameworks like Entity Framework, Hibernate, and MartenDB.

Characteristics

The essence of the Unit of Work pattern lies in its approach to tracking changes that need to be applied to a database within a defined scope. This scope ensures that when a unit of work is committed, it initiates a database transaction where all changes are either fully applied or entirely rolled back, depending on the success of the transaction. This all-or-nothing approach is crucial for maintaining data integrity and consistency.

Furthermore, when combined with optimistic concurrency controls, the Unit of Work pattern resolves concurrent writes while minimising database locking, enhancing performance. Despite the occasional need for retrying failed transactions, this trade-off is generally beneficial, ensuring smoother database operations.

One of the strengths of the Unit of Work pattern is its adaptability to various scenarios that align to unit of work semantics such as API calls, message processing from service buses or Kafka topics, and the execution of background jobs. This versatility makes it a preferred choice for many developers.

var customer = session.get<Customer>('id');
var loyalty = session.get<Loyalty>(customer.id);
customer.status = 'deleted'
loyalty.status = 'deleted';

// Saves changes to both records in the same db transaction 
// and checking the records have not been modified by another
// process since they were loaded
session.saveChanges();

Common Issues

Successful implementation of this pattern requires awareness of common pitfalls.

Multiple commits within an atomic operation

Data inconsistencies can arise from committing changes at multiple stages within a single operation that is expected to be atomic. This can happen when trying to wrap a unit of work persistence provider in a repository pattern because they follow different semantics (unit of work queues changes / repository pattern abstracts updates).

It can also happen simply as a mistake by calling shared code from a Service class or event handler that performs an update and persists them atomically.

Such mistakes raise questions about the resultant database state if only a subset of changes is committed.

Tracking too many entities.

Because the unit of work pattern tracks entities as they are loaded, processes that iterate over large collections of entities performing updates sequentially can have unintended performance problems

foreach(var id in orders) {
  var order = loadOrder(id);
  order.status = completed;
  unitOfWork.SaveChanges()
}

Database Contention

Even though this is not an issue specific to unit of work semantics, because persistence implementations tend to use optimistic concurrency locking and update multiple entities in the same database transaction, this increases the chance of update failures that need to be retried. Even small amounts of contention can have negative performance impacts.

Summary

Despite the challenges and additional complexity this pattern can introduce, the Unit of Work pattern remains a popular pattern in application development. Its ease of adoption, thanks to the mature frameworks that implement it, offers a seamless way to enhance application reliability and performance.