3. Use of substitution files to generate EPICS Databases

Date: 2023-11-30

Status

Accepted

Context

There are two proposals for how EPICS Databases should be generated:

1. At IOC startup ibek should generate a substitution file that describes the required Databases.

   The IOC instance yaml combined with the definitions from support module yaml controls what the generated substitution file will look like.

   ibek will then execute msi to generate the Databases from the substitution file.

2. The dbLoadRecord calls in the startup script will pass all macro substitutions in-line. Removing the need for a substitution file.

Decision

Proposal 1 is accepted.

Some template files such as those in the pmac support module use the following pattern:

substitute "P=$(PMAC):, M=CS$(CS):M1, ADDR=1, DESC=CS Motor A"
include "pmacDirectMotor.template"

This pattern is supported by msi but not by the EPICS dbLoadRecord command which does not recognise the substitute command.

Consequences

An extra file ioc.subst is seen in the runtime directory. In reality this is easier to read than a full Database file. So can be useful for debugging.

Finally those developers who are unable to use ibek yaml for some reason can supply their own substitution file and ibek will expand it at runtime. This is much more compact that supplying a full Database file and important due to the 1MB limit on K8S ConfigMaps.