It was to my understanding that when you throw an exception all previous operations are rolled back.

When an exception occurs, code execution halts and any DML
operations that were processed prior to the exception are rolled back
and aren’t committed to the database. Exceptions get logged in debug
logs.

I am running into a situation where I am able to insert a record in a logger class in the catch and throw an exception after it. What gives?

The flow is: Class A calls Class B -> Class B calls Class C -> Class C has a webservice and a try/catch

public HttpResponse postTokenizer(String securityToken, String payloadJSON) 
 try 
 if (payloadJSON == null) 
 throw new GenericException('Tokenizer was called with no IDs. The ID must be either a Contact or Lead.');
 
 HttpRequest tokenizerRequest = new HttpRequest();
 tokenizerRequest.setMethod('POST');
 tokenizerRequest.setEndpoint(tokensAPIEndpoint);
 tokenizerRequest.setHeader('Authorization', 'token ' + securityToken);
 tokenizerRequest.setHeader('Content-Type', 'application/json');
 tokenizerRequest.setTimeout(30000);
 tokenizerRequest.setBody(payloadJSON);
 Http http = new Http();
 HttpResponse response = http.send(tokenizerRequest);
 if (!WebHelper.isSuccessStatus(response)) 
 throw new GenericException('There has been an error calling the tokenizer service.');
 
 return response;
 catch(Exception e) 
 NFLogger.logError('TokenizerHelper, tokenizer', 'Call to postTokenizer() failed.', e); //todo:: how is this inserting????? shouldn't this be rolled back?
 throw e;
 

NFLOGGER:

global with sharing class NFLogger 
private enum LogLevel Error, Info

public static void logError(String tags, String errorMessage)
 logMessage(logLevel.Error, tags, errorMessage, null);


public static void logError(String tags, String errorMessage, System.Exception ex)
 logMessage(logLevel.Error, tags, errorMessage, ex);


public static void logInfo(String tags, String infoMessage)
 logMessage(logLevel.Info, tags, infoMessage, null);


private static void logMessage(LogLevel level, String tags, String message, System.Exception ex)
 try 
 LogMessage__c newMessage = new LogMessage__c(level__c = level.name(), tags__c = tags, message__c = message, exception__c = ex.getMessage());
 insert newMessage;
 
 catch (Exception e)
 //intentionally we are ignoring "we suppress" any exception
 //we need to guarantee that logMessage will never throw any exceptions
 

The transaction will only be rolled back automatically if the thrown exception isn't caught.

Your example showed a call stack like:

1. Class A calls Class B
   


2. Class B calls Class C
   


3. Class C attempts a callout to a webservice.
   
   
   • It catches any resulting exceptions,
   
   
   • does DML to log the exception details,
   
   
   • and then throws the exception again.
   
   

Since the resulting LogMessage__c record exists after the transaction, there must be a catch in either class B or A that is handling the exception thrown by class C. This prevents the exception terminating the transaction and causing a rollback of any DML that had occured.

It looks like your quote came from Exceptions in Apex, What Happens When an Exception Occurs?. The last part of that paragraph clarifies that the transaction rollback is just for unhandled exceptions. It is probably a bit misleading to imply that a handled/caught exception would also cause a rollback.

What Happens When an Exception Occurs?

When an exception occurs, code execution halts. Any DML operations that were processed before the exception are rolled back and aren’t committed to the database. Exceptions get logged in debug logs. For unhandled exceptions, that is, exceptions that the code doesn’t catch, Salesforce sends an email that includes the exception information. The end user sees an error message in the Salesforce user interface.