# 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.