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conn_id breaking change; once more with feeling

The short of this email is: can we please name all the connection id named parameters to all hooks and operators as similarly as possible. EG just `conn_id`?

So, when we started using Airflow I _had_ thought that minor versions would be compatible for a user's DAG, assuming no use of anything marked beta or deprecated, such that v1.7.1, 1.8.0, 1.8.1, 1.8.2 and 1.9.0 etc would all run dags from prior versions in that linage, each with possible stability and feature improvements and each with possibly more operators, hooks, executors (even) etc.

This is (now) obviously not the case, and it's the group's choice about what should and what should not break on a release-by-release basis. I think a clear policy would be appropriate for full Apache status, but then I may have missed where the policy is defined.  Though, if defined as not giving stability to the user's dags for most version changes isn't in my opinion going to grow confidence for Airflow being something you can grow with.

Not to be overly dramatic, but currently the tiny change that the `s3Hook(…)` takes `aws_conn_id='any_string'` vs `s3_conn_id='still_any_string'` means that basically I have to maintain a 1.8.2 setup in perpetuity, because no one (here) wants to briefly code freeze before during and after an update so that we can update all the uses and be ready to roll back the update if something else breaks (also some people access the not-actually-private `_a_key and _s_key` and would need to switch to still-not-private `_get_credentials()[…]`). Instead we'll just run a second airflow setup at 1.9.0 (in each and every staged environment) and move the 8k dags piecemeal whe[never] people get the spare time and inclination. I mean, we might. It looks like 1.10.0 changes some config around s3 logging one more time… so maybe it's better to skip it?

I saw the couple of PRs where the project itself had to make the changes to usages of the named field. There was momentary and passing concern that users' dags would need to do the same. In the PRs, of the options discussed (shims, supporting the deprecated name as deprecated, doing a hard rename), it wasn't brought up if the rename to aws_conn_id was the best name. [Was this discussed elsewhere?]

And so I wondered why is there this virtual Hungarian notation on all the `conn_id`s?
A hook generally takes one `conn_id`, most operators take only one. In these cases couldn't the named parameter have been `conn_id`? When an operator needs a couple conn_ids, could it not have `src_conn_id` and `dest_conn_id` instead of locking in either s3 or aws, mysql or maria, dataflow or beam etc? Those are hypotheticals, I believe.

Could I propose to plan to break absolutely all uses of `{named}_conn_id`s, before or by version 2.0, with an eye towards never again having to break a named parameter for the rest of 2.x's life? There's probably more named parameters that should be fixed per major release, and if you agree, some work should be done to identify them all, but this just happens to be the one that seems most likely to change often, and has so recently.


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