Apex SOQL FOR UPDATE

Let’s say that you want sole access to a record and no other thread can access it.

This is implemented by adding a FOR UPDATE clause in the SELECT statement.

While a sObject record is locked, no other client or user is allowed to make updates either through code or the Salesforce user interface.

The client locking the records can perform logic on the records and make updates with the guarantee that the locked records won’t be changed by another client during the lock period.

The lock gets released when the transaction completes.

Account [] accts = [SELECT Id FROM Account LIMIT 2 FOR UPDATE];

Please note that you can’t use the ORDER BY keywords in any SOQL query that uses locking

A note of caution is that while you use FOR UPDATE it could lead to errors if the records are not accessible to other threads for a prolonged time so be careful in using them.

Please go through the locking considerations which are as follows:

• While the records are locked by a client, the locking client can modify their field values in the database in the same transaction. Other clients have to wait until the transaction completes and the records are no longer locked before being able to update the same records. Other clients can still query the same records while they’re locked.
  
• If you attempt to lock a record currently locked by another client, your process waits a maximum of 10 seconds for the lock to be released before acquiring a new lock. If the wait time exceeds 10 seconds, a QueryException is thrown. Similarly, if you attempt to update a record currently locked by another client and the lock isn’t released within a maximum of 10 seconds, a DmlException is thrown.
  
• If a client attempts to modify a locked record, the update operation might succeed if the lock gets released within a short amount of time after the update call was made. In this case, it is possible that the updates will overwrite those made by the locking client if the second client obtained an old copy of the record. To prevent this from happening, the second client must lock the record first. The locking process returns a fresh copy of the record from the database through the SELECT statement. The second client can use this copy to make new updates.
  
• The record locks that are obtained in Apex via FOR UPDATE clause are automatically released when making callouts. Use caution while making callouts in contexts where FOR UPDATE queries may have been previously executed.
  
• When you perform a DML operation on one record, related records are locked in addition to the record in question.

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