Analyse and create a "dry run" command in the if feasible

[marquesVF]

It’s desired to be able to validate a migration plan before we actually execute it. With that we could:

Make sure that the migration will do what is expected.

[code-shambles]

I have an idea/question:
Would it be possible to freeze a current state of content? When doing a dry-run, the result may be okay. But in the meantime, until the real migration is triggered, a change could happen (i. e. by another dev) and the result would be different from what the dry-run had promised. Just thinking... 😉

[n4bb12]

Usually, changes outside a migration are expected. Not just developers who might be working on a separate migration in parallel, but also content managers continuously working on the main branch. You don't want some of that content to do a time leap because someone ran a migration with dry run.

[pmbanugo]

I checked to see if there's a way to implement this but the contentful-migrations doesn't expose many internal functions that we could probably re-use to implement something like this. There's an open issue from 4 years ago asking for a similar thing. I think we could implement this if some functions used internally are also exposed by the library. From what I checked, only the runMigrations function is exposed as a public API.

For code context, here’s the part of the file that renders the description of changes to the content model: https://github.com/contentful/contentful-migration/blob/a90a8329af0c87a046feae07d78ced1835e69eb0/src/bin/cli.ts#L141

And this is used when the --yes flag is applied: https://github.com/contentful/contentful-migration/blob/a90a8329af0c87a046feae07d78ced1835e69eb0/src/bin/cli.ts#L197 . It would either run the migrations without a prompt if true, or prompt the user if they want the migration to be applied.

If we decide to fork and extend the library or send a PR, we could add another dry-run option and used that to render a description of the planned migration.