What order must DML statements and callouts follow in a transaction?

In Salesforce, when executing DML statements and callouts within a transaction, a specific order must be adhered to in order to ensure that the operations proceed correctly. The correct order is that DML statements can come before callouts.

This is because Salesforce enforces a restriction where a callout (for example, an HTTP request to an external web service) cannot be made once any DML operations have been performed within the transaction. The reasoning behind this limitation is to maintain data integrity and ensure that the execution context remains stable. By allowing DML operations to occur before the callout, Salesforce guarantees that once the callouts are made, the data changes have been applied and there are no conflicts that could arise from further data manipulation.

If callouts were allowed to precede DML statements, it could create a situation where external systems might operate on outdated or uncommitted data, leading to inconsistencies and potential data loss. Thus, enforcing the requirement that DML must come first helps maintain a coherent transactional state.