Argo Workflow to continue processing during fan-out - dynamic

Question

General question here, wondering if anyone has any ideas or experience trying to achieve something I am right now. I'm not entirely sure if its even possible in the argo workflow system...
I'm wondering if it is possible to continue a workflow regardless if a dynamic fanout has finished. By dynamic fanout I mean that B1/B2/B3 can go to B30 potentially.
I want to see if C1 can start when B1 has finished. The B stage is creating a small file which then in C stage I need to run an api request that it has finished and upload said file. But in this scenario B2/B3 still are processing.
And finally, D1 would have to wait for all of C1/2/3-C# to finish to complete
Diagram what I'm trying to achieve
# *
# |
# A1 (generates a dynamic list that can change depending on the inputs)
# |
# / | \
# B1 B2 B3 +++ B#
# | | |
# C1 +++ C#
# * * *
# \ | /
# \ | /
# D1
I was viewing https://github.com/argoproj/argo-workflows/blob/master/docs/enhanced-depends-logic.md but I cant wrap my head around if this is what I need to achieve this. Especially if the fan-out steps are dynamic.
Seems to me that it would bind C stage to the entirety of B stage and require for for B to finish

Answer 1

Something like this should work:
apiVersion: argoproj.io/v1alpha1
kind: Workflow
spec:
templates:
- name: main
steps:
- - name: A
template: A
- - name: B_C
template: B_C
arguments:
parameters:
- name: item
value: "{{item}}"
withParam: "{{steps.A.outputs.parameters.items}}"
- - name: D
template: D
- name: A
# container or script spec here
outputs:
parameters:
- name: items
valueFrom:
path: /tmp/items.json
- name: B_C
inputs:
parameters:
- name: item
steps:
- - name: B
template: B
arguments:
parameters:
- name: item
value: "{{inputs.parameters.item}}"
- - name: C
template: C
arguments:
artifacts:
- name: file
from: "{{steps.B.outputs.artifacts.file}}"
- name: B
inputs:
parameters:
- name: item
# container or script spec here
outputs:
artifacts:
- name: file
path: /tmp/file
- name: C
inputs:
artifacts:
- name: file
# container or script spec here
- name: D
# container or script spec here
Step B_C in the main template runs instances of the B_C template in parallel.
Template B_C runs B and C in series. Once template B_C starts, it runs as quickly as possible, completely unaware of any concurrent executions of the B_C template. So C1 blocks only on B1, never on B2 or B3 or any other B#.
Once all instances of B_C are finished, the main template finally invokes the D template.