Publish to Heroku is broken: "WARNING: You must pass the application as an import string to enable 'reload' or 'workers"

[simonw]

2019-11-13T16:27:59.821483+00:00 heroku[web.1]: Starting process with command `datasette serve --host 0.0.0.0 -i fixtures.db --cors --port 36817 --inspect-file inspect-data.json`
2019-11-13T16:28:01.856471+00:00 heroku[web.1]: State changed from starting to crashed
2019-11-13T16:28:01.750253+00:00 app[web.1]: Serve! files=() (immutables=('fixtures.db',)) on port 36817
2019-11-13T16:28:01.771524+00:00 app[web.1]: WARNING:  You must pass the application as an import string to enable 'reload' or 'workers'.
2019-11-13T16:28:01.837839+00:00 heroku[web.1]: Process exited with status 1

[simonw]

The problem is this code in uvicorn: https://github.com/encode/uvicorn/blob/020dd085a0a5b609f14ddd2b36fc0a97ba4df019/uvicorn/main.py#L297-L302

    if (config.reload or config.workers > 1) and not isinstance(app, str):
        logger = logging.getLogger("uvicorn.error")
        logger.warn(
            "You must pass the application as an import string to enable 'reload' or 'workers'."
        )
        sys.exit(1)

From this commit 4 days ago: encode/uvicorn@b034f7a - see Uvicorn encode/uvicorn#478 and encode/uvicorn#481

This is breaking because --workers "Defaults to the $WEB_CONCURRENCY environment variable if available" - and it turns out on Heroku WEB_CONCURRENCY is set to 2 by default!

[simonw]

Short term fix: set --workers=1 when publishing with Heroku. Or even easier - set WEB_CONCURRENCY to 1.

[nryberg]

Using the Heroku web interface, you can set the WEB_CONCURRENCY = 1