Add core_version to server api query

[zanieb]

Summary

Adds a human readable version string to the server api query indicating the Prefect Core version.

Importance

Displaying the core version the Server is using in the UI is helpful.

Gets going on #120 -- Will need UI update

Checklist

This PR:

• adds new tests (if appropriate)
• adds a change file in the changes/ directory (if appropriate)

[cicdw]

Code wise this looks good; before we move forward with this implementation though, I want to raise a UX issue I foresee: I suspect most users use prefect server start to stand up Server. The images that this uses are always built prior to tagging prefect for release, meaning that if I'm on 0.13.4 of Prefect (for example), I'll see something like 0.13.3+63.g4c764ddb8 as my core_version which I suspect will confuse people.

I'm not 100% sure how to avoid this chicken / egg problem

[znicholasbrown]

Could we add an env variable to the build step (something like target_release) to use for official releases but fall back to the commit hash for non-tagged spinups?

[zanieb]

Code wise this looks good; before we move forward with this implementation though, I want to raise a UX issue I foresee: I suspect most users use prefect server start to stand up Server. The images that this uses are always built prior to tagging prefect for release, meaning that if I'm on 0.13.4 of Prefect (for example), I'll see something like 0.13.3+63.g4c764ddb8 as my core_version which I suspect will confuse people.

I'm not 100% sure how to avoid this chicken / egg problem

I see two options:

• Rebuild the docker images with the correct version of Prefect Core, it seems a little weird that we are retagging when the core code could be out of date anyway
• Pass an environment variable in the prefect server start docker-compose and use that if present e.g.

        "version": os.getenv("PREFECT_SERVER_VERSION", prefect_server.__version__),
        "core_version": os.getenv("PREFECT_CORE_VERSION", prefect.__version__),

Could we add an env variable to the build step (something like target_release) to use for official releases but fall back to the commit hash for non-tagged spinups?

The build step for the docker image? This would still be out of date like Chris mentioned.

[cicdw]

I vote we take the environment variable route:

"core_version": os.getenv("PREFECT_CORE_VERSION", prefect.__version__),

Purely as FYI: the only thing that Server relies on from Prefect Core is serialization logic for state objects and some Flow objects, so more often than not we don't need to touch the Server images when we make core changes. Given that, I think this core_version field is more of a "schema compatibility" signal, so you know what version of Core "sanctioned" this particular server image.

[codecov-io]

Codecov Report

Merging #126 (47253db) into master (4c8e562) will decrease coverage by 0.03%.
The diff coverage is 100.00%.

[cicdw]

really minor request to re-categorize as enhancement, otherwise this LGTM!

[zanieb]

Note this will require a PR in prefect to pass this variable and a PR in ui to display it.