Apex Trigger Exceptions

Custom Apex Triggers execute on standard CRM objects (Account, Contact, Lead etc.) and custom objects in response to all data modifications applied via the Salesforce web application, API transactions and custom Apex script. As such it is imperative that trigger code adheres to patterns that promote performance and maintainability, guards against recursive behaviour and most critically protects data integrity.

Apex code outside of triggers (Batch Apex, Visualforce Controllers etc.) is completely isolated to the specific context, trigger code is not; the effect of errant code is felt across the standard web app, mobile apps, API-based integration flows, data loads and so on. As such triggers must be used judiciously and avoided wherever possible on the core standard objects. I often come across multiple complex triggers defined on Account, where the logic applied is non-time critical and could have been handled by Batch Apex, or indeed by declarative methods. In certain circumstances this idealistic view is impractical and triggers are a necessary friend. When writing triggers always stick to a proven pattern, good examples provided by community experts are easy to find, and read the relevant sections in the Apex Language Reference if you haven’t done so for a while.

This post aims to provide some practical insight related to exception behaviour within Apex Triggers that may not be obvious from the referenced resources.

Key Concepts
1. DML Processing Mode
All bulk DML transactions on the platform run in either allOrNothing or partial processing mode. In some cases the behaviour is implicit and fixed, in other cases flags can be set to override the default behaviour.

allOrNothing
– Apex Database statements (e.g. update contactsToUpdate;)

partial processing
– Apex Database methods (Database.update(contactsToUpdate);. The default behaviour can be overridden via DmlOptions.
– API (default behaviour – can be overridden)

2. .addError()
The .addError(custom message) method can be invoked at the record or field level within the trigger context to flag a record as invalid. This is particularly useful in asserting logical errors. A best practice in regard to the custom messages is to use Custom Labels with a defined Apex Trigger Error category.

3. Runtime Exceptions
If an unhandled runtime exception is thrown then the Apex Transaction will immediately rollback and Apex exception notifications will occur. Structured exception handling can be added to the trigger code to catch runtime exceptions and to apply custom handling logic.

Apex Trigger – Exception Test Cases
The following test cases highlight the impact of the .addError() method and runtime exceptions in the context of the 2 processing modes. Tests were conducted using Execute Anonymous and the Apex Data Loader at API version 30.0.

Test Case 1 – Does code continue after .addError()?
Result: Yes (this is necessary in both allOrNothing and partial processing cases to gather a complete set of errors).

Test Case 2.1 – all_or_none – Does .addError() rollback the transaction if any subset of records has .addError() applied?
Result: Yes – Full rollback.

Test Case 2.2 – all_or_none – Does .addError() rollback the transaction if all records have .addError() applied?
Result: Yes – Full rollback.

Test Case 2.3 – all_or_none – Does an un-handled runtime exception rollback the transaction if no records have .addError() applied?
Result: Yes – Full rollback.

Test Case 2.4 – all_or_none – Does a handled runtime exception rollback the transaction if no records have .addError() applied?
Result: No – Records are committed (complete trigger context – regardless of which record caused the runtime exception).

Test Case 3.1 – partial – Does .addError() rollback the transaction if any subset of records has .addError() applied?
Result: Yes – Full rollback of all uncommitted changes, the trigger fires again for the subset of records that have not had addError() applied. Up to 2 retries are performed, after which the “Too many batch retries in the presence of Apex triggers and partial failures.” exception is thrown.

Test Case 3.2 – partial – Does .addError() rollback the transaction if all records have .addError() applied?
Result: Yes – Full rollback.

Test Case 3.3 – partial – Does an un-handled runtime exception rollback the transaction if no records have .addError() applied?
Result: Yes – Full rollback.

Test Case 3.4 – partial – Does a handled runtime exception rollback the transaction if no records have .addError() applied?
Result: No – Records are committed (complete trigger context – regardless of which record caused the runtime exception).

Test Case 4 – partial – In a partial failure case are static variables set within the original trigger invocation reset before retry invocations?
Result No – Static variables are not reset between trigger invocations. This means that statics guard variables defined to protect against recursive behaviour will stop trigger logic being applied to records processed by retry invocations.

Conclusions
1) In all cases .addError() results in the full Apex Transaction being rolled back. Custom logging schemes such as writing exceptions to a Custom Object (including via @future) or sending notification emails are futile gestures when employed in conjunction with .addError().

2) Handled runtime exceptions result in the Apex Transaction being committed, unless .addError() is applied to at least one record.

3) In the partial processing case, governor limits are reset to their original state for each retry trigger invocation, static variables are not.

4) Where static guard variables are used to protect against re-execution of the trigger logic via field updates within update DML transactions, partial processing retry trigger invocations will also be blocked. Such invocations are consistent with those initiated by workflow field updates later in the order of execution.

In order to distinguish between a field update trigger execution and a partial processing retry it becomes difficult to use static variables – batch sizes can be equivalent, sequencing is unpredictable etc.

One possible approach is to use a custom field on the target object (Is Workflow Processing?), a field update action would be added to set this to “True” alongside the existing actions. In the trigger code we can now reliably test this field state to identify a workflow field update initiated trigger execution. The trigger code must always reset the field to “False” before returning, such that any partial processing retry executions safely proceed. Where trigger logic resides in an after update trigger, a before update trigger could be added simply for convenience of reseting the field – in this case a static could be set in the before context and referenced in the after context.

It’s possible there are more efficient ways to provide a robust solution to this. A complete Apex based solution would definitely be preferable to the custom field based solution.

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