slowroll

Let's say you want to roll out a feature flag, or something that selects a canary version. Put a json config up somewhere, decide how you're hashing your users, and use SlowrollConfig:

from slowroll import SlowrollConfig

sc = SlowrollConfig("https://example.com/config.json", "app_features")
if sc.get_value("features", "install_to_opt_foo", socket.gethostname()):
    ...

The config for this would look like

{
    "features": {
        "install_to_opt_foo": {
            "end_time": "2025-01-01 00:00Z",
            "duration": "14d",
            "stable": false,
            "new": true,
            "early_adopters": ["canary-host"],
        }
    }
}

Note that the values for "stable" and "new" can be any valid json. Here they are bools, but could just as easily be strings that represent versions of a Python project. start_time is not explicitly specified, but calculated from the duration.

There are few restrictions on the http server, namely that it must provide etags using the default implementation, but you can provide your own that takes an opaque url and an opaque value that you can use to store any etag-esque metadata.

As in semver, expect no compatibility guarantees until this reaches 1.0. Both the API and config format are expected to change.

Extensibility

• What if I want to store the config in zookeeper? Sure, provide a custom callback that returns the latest version you know of.

Future Work

• Ability to locally lock a value (either old or new)
• Example using a requests session
• Retries/fallback
• More testing on _pct values
• Config validator
• Interpolation of numeric values
• Regex early_adopters
• Rollbacks that count down

License

slowroll is copyright Tim Hatch, and licensed under the MIT license. I am providing code in this repository to you under an open source license. This is my personal repository; the license you receive to my code is from me and not from my employer. See the LICENSE file for details.