add some Architectural Decisions

Author: gilesknap

docs/developer/explanations/decisions/0002-switched-to-pip-skeleton.rst

@@ -1,5 +1,5 @@
-2. Adopt epics-containers.github.io for project structure
-=========================================================
+2. Adopt python3-pip-skeleton for project structure
+===================================================
 
 Date: 2022-02-18
 
@@ -11,7 +11,7 @@ Accepted
 Context
 -------
 
-We should use the following `pip-skeleton <https://github.com/epics-containers/epics-containers.github.io>`_.
+We should use the following `pip-skeleton <https://github.com/DiamondLightSource/python3-pip-skeleton>`_.
 The skeleton will ensure consistency in developer
 environments and package management.
 

docs/developer/explanations/decisions/0003-use-substitution-files.rst

@@ -0,0 +1,55 @@
+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:
+
+.. code-block::
+
+    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.

docs/developer/explanations/decisions/0004-autosave-req-files.rst

@@ -0,0 +1,45 @@
+4. How to configure autosave for IOCs
+=====================================
+
+Date: 2023-11-30
+
+Status
+------
+
+Accepted
+
+Context
+-------
+
+There is a choice of supplying the list of PVs to autosave by:
+
+- adding info tags to the Database Templates
+- supplying a raw req file with list of PVs to autosave
+
+Decision
+--------
+
+We will go with req files for the following reasons:
+
+- https://epics.anl.gov/tech-talk/2019/msg01600.php
+- adding info tags would require upstream changes to most support modules
+- default req files are already supplied in many support modules
+- req files are in common use and many facilities may already have their own
+  req files for support modules.
+
+We expect to autogenerate the list of PVs to autosave from the IOC's. We could
+therefore generate a Database override file which adds info tags. But it is
+simpler to just generate a req file.
+
+The mechanism for using req files is that defaults will come from the support
+module or from the generic IOC if the support module does not supply a req file.
+
+Then override files can exist at the beamline level and / or at the IOC
+instance level. These will simply take the form of a req file with the same
+name as the one it is overriding.
+
+Consequences
+------------
+
+Everything is nice and simple.
+

docs/developer/explanations/decisions/0005-python-scripting.rst

@@ -0,0 +1,44 @@
+2. Use Python for scripting inside and outside containers
+=========================================================
+
+Date: 2022-11-30
+
+Status
+------
+
+Accepted
+
+Context
+-------
+
+Inside the container, we use the ``ibek`` tool for scripting. Outside we
+use ``ec`` from ``epics-containers-cli``.
+
+Much of what these tools do is
+call command line tools like ``docker``, ``helm``, ``kubectl``, compilers,
+etc. This seems like a natural fit for bash scripts.
+
+These features were originally implemented in bash but were converted to
+python for the following reasons:
+
+- python provides for much richer command line arguments
+- it is also much easier to provide rich help
+- managing errors is vastly improved with exception handling
+- the unit testing framework allows for 85% coverage in continuous integration
+- complex string handling is a common requirement and is much easier in python
+- there is a clear versioning strategy and packages can be installed with pip,
+  meaning that you can check which version of the script you are running and
+  report bugs against a specific version
+- the code is much easier to read and maintain
+- because the packages can be pip installed they can be used in CI and inside
+  multiple containers without having to copy the scripts around
+
+Decision
+--------
+
+We always prefer Python and keep bash scripts to a minimum
+
+Consequences
+------------
+
+Scripting is much easier to maintain and is more reliable.