Why is it not allowed to make an HTTP request inside a database transaction?

Making an HTTP request inside a database transaction is not recommended for several reasons:

1. Resource Locking: A transaction holds locks in the database until it completes. If an HTTP request that might be long or hang is made inside a transaction, it can lead to prolonged resource locking, reducing performance and causing deadlocks.

2. Violation of Atomicity: A transaction should be atomic and complete quickly. An external HTTP request is an operation outside the control of the database, which can fail or be delayed, complicating transaction management.

3. Rollback Issues: If the HTTP request changes external systems and the transaction is rolled back, external changes will not be reverted, leading to data inconsistency.

4. Violation of Separation of Concerns: A transaction should only manage changes within the database. External calls are better made before or after the transaction.

The correct approach is to perform the HTTP request first, check the result, and then start the transaction, or vice versa — start the transaction first and perform the HTTP request after its successful completion to avoid consistency issues.