0013-protect-business-invariants-using-exceptions.md

13. Protect business invariants using exceptions

Date: 2019-07-01

Log date: 2019-11-05

Status

Accepted

Context

Aggregates should check business invariants. When the invariant is broken, we should stop processing and return an error immediately to the client.

Possible solutions

1. Use exceptions

Pros

• we can stop processing immediately (fail-fast)
• popular approach in C#
• we don't need to check the result of each method (if-else statements)
• we can catch all Business Exceptions in one place and translate them (for example in the API layer to some HTTP response code).

Cons

• indirection
• little performance impact
• for special cases, we need to add a specific catch.

2. Return Result object

Pros

• no indirection
• no performance impact
• signature method is more descriptive.

Cons

• we need to add checks result of each method (if-else statements)
• approach is less-known in the C# world
• it needs a library or more coding to support Results

Decision

Solution number 1 - Use exceptions.

Performance cost of throwing an exception is irrelevant, we don't want too many if/else statements in entities, more familiar with exceptions approach.

Consequences

• We need to add special BusinessException class to separate business rules validation exceptions from other exceptions
• We need to create different business exceptions for each business rule
• We will have a small performance impact (throwing exceptions)
• We will have generic mechanism which catches BusinessException
• We will not have a lot of if/else statements in Entities/Value Objects to check method results
• Some monitoring tools logs automatically each exception. If we want to use one of this tool we should be aware of this and figure it out proper solution