Phlex program startup

[knoepfel]

This is the central hub for discussing design issues for program startup, and related aspects of Phlex.

[knoepfel]

@marcpaterno, @beojan, and @knoepfel met April 9, 2026 to discuss program startup. My summary of the meeting:

1. The "Program Startup" section of the TDR will be adjusted so that output ports and input ports are connected by an edge when the product query of the input port matches the data product of the output port.
2. We had some discussion about the terms computational node, pure node, IO node, etc. That is not yet settled.
   • There is a similarity between Haskell's IO Monad and Phlex's driver and providers
3. A product query with an absent layer specification implies that any data products that match the product query will be presented to the node.
   • This is in contrast to a formerly suggested behavior, where a product query that resolves to multiple data products is an ambiguity.
4. We must be able to detect cycles among computational nodes (likely involves a third-party technology)
5. Forming the graph in Python may be easier than doing it in C++.
   • It may be possible to delay the construction of TBB flow-graph nodes until after the graph-formation mechanics are largely complete.
6. We must provide user-friendly error messages for users to easily understand how a program has been misconfigured.