Dynamo - Revit Transaction Modes

I'm sitting at an airport bar waiting for my flight back to Boston to start boarding, so I figure now is as good a time as any to discuss how Dynamo interacts with the Revit API via transactions.

All Dynamo nodes know whether or not they require a transaction in order to evaluate. This is necessary in order to make sure that they are run within a transaction, but also so that they aren't run in a transaction if they don't need to be.

Dynamo has three internal "transaction modes" which are set based on various factors.

Automatic

The default mode. If any node in the workflow requires a transaction to evaluate then the whole Dynamo expression is run in a transaction in the Revit idle thread. Otherwise, it is run in its own thread.

Manual

This mode is active if the workflow contains a Transaction node. In this mode, it is up to the user to make sure that all nodes which require a transaction are connected to a Transaction node (either directly or through one or more "parent" nodes). This is desired if you would like to see intermediate results in Revit or if you need a complete Transaction before continuing to evaluate. A good example is the Solar Radiation Optimizer sample, which has to end a Transaction in order for Solar Radiation to update and feed data back into Dynamo.

Debug

Activated when the Debug box is checked in the UI, every node which requires a transaction is evaluated in its own transaction. This allows you to see every node's effect on the document, but is also really slow. Fun fact: this was the first (and for a while, the only) transaction mode.