Merge pull request #21 from FAST-HEP/kreczko-variations

docs: add description of variations

docs/configuration/index.md

@@ -13,6 +13,7 @@ a local machine or on a cluster using
 global_settings.md
 register.md
 environments.md
+variations.md
 ```
 
 ## Basic usage

docs/configuration/variations.md

@@ -0,0 +1,93 @@
+# Variations of existing stages
+
+Often it will be necessary to clone a stage or re-run the whole workflow with a
+to one variable. These might be due to new calibration constants, new
+systematics, or updated or new algorithms. In these cases it might be useful to
+have a way to define a new stage that is almost identical to an existing stage,
+but with a few changes.
+
+`fasthep-flow` provides a way to do this by using the `variations` key in the
+YAML file. This key is a dictionary, where the keys are the names of the
+variations, and the values are the changes to the stage. The changes are defined
+in the same way as the stage itself, but only the changes are needed. The
+changes are applied to the stage, and the new stage is then added to the
+workflow.
+
+Here's an example of a variation:
+
+```yaml
+
+stages:
+  - name: my_stage
+    type: fasthep_flow.operators.BashOperator
+    kwargs:
+      bash_command: echo "Hello World!"
+variations:
+    - name: my_stage [variation]
+        changes:
+          kwargs:
+            bash_command: echo "Hello Universe!"
+```
+
+In this example, the `my_stage [variation]` stage is a variation of the
+`my_stage` stage. The only change is that the `bash_command` argument is changed
+from `echo "Hello World!"` to `echo "Hello Universe!"`. Since the workflow has
+only one stage, the new stage will be added to the workflow in parallel to the
+original stage.
+
+## Changing data
+
+Let's say you are measuring the invariant mass of two particles, and you have a
+stage that calculates the mass. Since the invariant mass depends on the momenta
+and energies of the two particles, they are likely to have their own
+calibrations. Luckily, these up- and down-systematics are already included in
+the data, but under different names.
+
+let's take the example from the CMS Public Tutorial, where we have a stage that
+calculates the invariant mass of two muons. The stage looks like this:
+
+```yaml
+- name: Muon Invariant Mass
+  type: fasthep_carpenter.operators.DiObjectMass
+  kwargs:
+    four_momenta: ["Muon_Px", "Muon_Py", "Muon_Pz", "Muon_E"]
+    output: "DiMuonMass"
+    when:
+      all:
+        - "NIsoMuon >= 2"
+        - "Muon_Charge[0] == -Muon_Charge[1]"
+```
+
+The systematics are included in the data as `Muon_Px_up` and `Muon_Px_down`,
+`Muon_Py_up` and `Muon_Py_down`, and so on. We can use the `variations` key to
+define a new stage that uses the up-systematics:
+
+```yaml
+variations:
+  - name: Muon Invariant Mass [up]
+    changes:
+      kwargs:
+        four_momenta: ["Muon_Px_up", "Muon_Py_up", "Muon_Pz_up", "Muon_E_up"]
+  - name: Muon Invariant Mass [down]
+    changes:
+      kwargs:
+        four_momenta:
+          ["Muon_Px_down", "Muon_Py_down", "Muon_Pz_down", "Muon_E_down"]
+```
+
+Since this is a full analysis example, this stage is not in isolation. Before
+this stage we have the `Input data` and `Create variables` stages, and after it
+we have the `Creating histograms` stages, and after it we have the
+`Creating histograms`, `Select events`, `Creating histograms after selection`,
+and `Output data` stages. As previously, a new stage will be added to the
+workflow in parallel to the original stage, but fasthep-flow will also create
+new stages for all subsequent stages, and add them to the workflow in parallel
+to the original stages.
+
+Alternatively, if you have lots of variations, you might want to use the
+`source` key to define the location of the variations:
+
+```yaml
+variations:
+  source: /path/to/variation_*.yaml
+```