Re: SNL question to the community

[Ralph Lange via Tech-talk <tech-talk at aps.anl.gov>]

Hi Pierrick,

This has been discussed in the past... on Tech-Talk and at meetings.

Ben, the current author/maintainer of SNL/SNC, has stated on tech-talk that he doesn't see an easy straightforward way to integrate SNL with PVA without fundamentally changing the concepts of SNL.

But. SNL/SNC is the best-tested and best-documented EPICS module. (Most of that is Ben's work. Kudos!) It is in use at almost every installation and nothing could replace it any time soon. It will not go away.

I see the sequencer's strength in implementing small state machines, close to the device, running on an IOC. Ramps, switch-on/switch-off procedures, slow feedback-type applications, that genre.

Complicated procedure sequencers, driving whole sub-systems using 1000s of PVs across many IOCs - that's the scope for high-level operations sequencers like oac-tree. For these applications, SNL/SNC doesn't scale well enough and has conceptual limitations.

With Channel Access slowly being sunset, the current SNL/SNC as a pure CA client will not be maintainable forever.

The more I think about it, the more I like Michael Davidsaver's idea: Make SNC/SNL directly work with the EPICS process database, with no Channel Access involved. That would cut down the code base, significantly reduce complexity, and put the sequencer exactly where it belongs - inside the IOC, complementing the process database as a local tool to implement finite state machines. If state machines need access to resources outside the IOC, local proxy records can do the remote connection - using standard PVA or CA provided by Base for cross-IOC links.

Cheers,
~Ralph