Virtualization-based transaction handling in an on-demand network code execution system

Systems and methods are described for providing virtualized transaction retries in an on-demand network code execution system. A user may generate a task on the system by submitting code. The task may include a transaction that can succeed or fail depending on external factors, such as accessing a resource that other processes are also trying to access. The system may obtain success criteria for evaluating whether the transaction was successful, and may configure a virtual machine instance to execute the submitted code. The system may generate a "snapshot" or otherwise capture information regarding the state of the virtual machine instance prior to executing the code, and may then execute the code or a portion of it to attempt the transaction. The system may then apply the success criteria to determine whether the transaction was successful, and if not may retry the transaction by restoring the snapshot and re-executing the code.