Add `before/after_pipelines_registered` hook OR `before/after_pipeline_filtered`

[marrrcin]

Description

It would be great to have a hook / a good place to attach some code after all pipelines have been registered but before the execution starts. I would like to do this for the sake of performing some validations on the pipeline definitions (w.r.t node tags).

Context

When doing a "Kedro-pipeline" to an "orchestrator-pipeline" translation, e.g. Airflow, there is a great pefromance optimization opportunity to "group" multiple Kedro nodes together and map multiple Kedro nodes as a single orchestrator level task instead of doing 1:1 mapping as it's done e.g. by Kedro-AzureML / Kedro-VertexAI / Kedro-Airflow. Grouping the nodes by any mechanism (e.g. by Kedro tags) requires some additional validation of pipeline structure - checking whether after grouping the pipeline will still be a DAG.

Possible Implementation

Add new hook specs like: after_pipelines_registered and before_pipelines_registered to access the pipelines before they are executed. For starters they could be a read-only, but it would be extra nice to be able to modify them on the fly too - especially when there would be a access e.g. do Kedro Context (this slightly connects to dynamic pipelines case; maybe it's too much for this issue anyway).

Possible Alternatives

Right now, we're plugging the custom grouping/validation code in the register_pipelines which has some downsides:

• not portable to other projects
• not deliverable by a plugin
• no access to context / settings of any sort (without abusing Kedro and loading the config file manually, which is pure evil)

Updated on 2023-09-12: I think even better alternative would be to have before/after_pipeline_filtered hooks as shown below: I would expect those hooks to access:

• all information in the record_data,
• all information passed to pipeline.filter
• Kedro Context + those hooks should allow to return the modified pipeline object

[datajoely]

I think it would have be injected here - but I'm not sure https://github.com/kedro-org/kedro/blob/0293dc15812b27330bba31a01c7b332b3165af2a/kedro/framework/project/__init__.py#L142

[marrrcin]

I think it would have be injected here - but I'm not sure

https://github.com/kedro-org/kedro/blob/0293dc15812b27330bba31a01c7b332b3165af2a/kedro/framework/project/__init__.py#L142

This place unfortunately has no KedroContext 😞 But yeah, it's the initial place to have this.

[noklam]

Looks reasonable to me, what will be the minimal signatures for these two hooks? Do you just need to executing some code before/after or you need to mutate the pipeline itself?

[marrrcin]

Mutating the pipeline while having KedroContext at hand would be great :D Even if no mutation is possible, then having KedroContext and/or ConfigLoader at this point would do the job - not sure whether it fits Kedro's lifecycle though...

[noklam]

This place unfortunately has no KedroContext 😞 But yeah, it's the initial place to have this. In case these diagram could helps. https://docs.kedro.org/en/0.18.3/faq/architecture_overview.html

x-ref #1718

It's unclear to me how injecting this in _ProjectPipelines will work. I don't want to bring in the Dynamic pipeline topic here yet since this will be a huge scope, it's definitely related though. Maybe it helps if you can show some example/snippets how your current workaround works.

[marrrcin]

The current workaround is just "execute the code in register_pipelines() of a project" with some hard-coded values (e.g. tag prefix strings by which we group the nodes) :)

[marrrcin]

New insights, see Updated on 2023-09-12 section in the first post.

[datajoely]

@marrrcin we have the click context at this point so I think we should include this in the hook in some workable form

[Lasica]

I've had the need for such functionality, as I was implementing grouping node execution by tags using kedro-airflow plugin. I wanted to validate whether the tagging was correct (so no disjoint, co-dependent nodes were joined and create a cycle dependency), so I had hard coded special prefix for tag to treat it as a grouping tag i.e "airflow:". With that tag "airflow:preprocessing" would translate to an airflow node "preprocessing" containing all nodes tagged "airflow:processing".

I wished that this tag validation were done at the step of pipeline creation/validation and issue warning about incorrect tagging, so I implemented it in pipeline registry. Then I wanted to source this hard coded prefix with other parts of code that were doing the grouping and translation, but there was no mechanism for that. I couldn't refer to parameters in pipeline registry in any way. This hook would help with that.

TL;DR: I had a use case to refer to parameters during registering pipelines and validating custom behavior and was unable to do that, while this feature would solve that.