What happens when an atomic block throws an exception? There is a debate in the STM community about whether the transaction should be rolled back or committed. ScalaSTM uses a hybrid approach that tries to do the right thing.

Exception –> rollback + rethrow

If an atomic block throws an exception, ScalaSTM rolls it back and then rethrows the exception. The atomic block will be left rolled back when the exception is rethrown. For exceptions that represent real errors this is a good default behavior, because it prevents corruption of any shared data structures.

Control-flow exception –> commit + rethrow

Sometimes exceptions represent a non-local control transfer, rather than an unexpected error. In this case, the transaction should be committed. ScalaSTM tests each exception that escapes an atomic block to determine which behavior is appropriate (look at the ScalaDoc for TxnExecutor.isControlFlow for more). By default all exceptions that extend scala.util.control.ControlThrowable are considered to be control flow.

Exceptions and nesting

The previous rules about exception handling apply to nested transactions. This means that a nested transaction might be rolled back while the outer transaction is committed. For example, after the following code runs last will hold the value "outer":

val last = Ref("none")
atomic { implicit txn =>
  last() = "outer"
  try {
    atomic { implicit txn =>
      last() = "inner"
      throw new RuntimeException
  } catch {
    case _: RuntimeException =>

To make nesting very cheap, ScalaSTM tries to flatten all of the nesting levels together into a single top-level transaction. If an inner transaction throws an exception then there isn’t enough information to perform the partial rollback, so ScalaSTM restarts the entire transaction in a mode that does exact nesting. This optimization is called subsumption.