SST Feature Flag

SST Feature Flag is a feature flagging service that you can deploy into your AWS account with only a few simple commands.

What's a Feature Flag?
OpenFeature
Prerequisites
What is Interval?
Local Development
Example Walkthroughs
- Create a static feature flag and use Evaluation API
- Create a dynamic feature flag and use Evaluation API
Extra Resources

What's a Feature Flag?

Per openfeature.dev:

Feature flags are a software development technique that allows teams to enable, disable or change the behavior of certain features or code paths in a product or service, without modifying the source code.

Some examples:

A boolean flag to disable a feature that affects all users
A boolean flag that defaults to false except for internal testers/ beta users / organizations that are added to the flag
A structured flag (json) that has an array of white listed domains
It also supports string and number flags but I can't really think of a use case for those. Please add some suggestions!

OpenFeature

The API for evaluating flags follows the openfeature.dev spec. This is a project that defines a standard and sdk for flags so that people aren't tied to a specific sass vendor, oss project, or handrolled solution.

The top priority Todo for this project is to create a provider for OpenFeature so that it is more certain that API is standard and so that you can easily switch to / from other Feature Flag solutions.

Prerequisites

Configure AWS credentials on your machine

https://sst.dev/docs/iam-credentials/
- This is required for SST to be able to deploy the service.

What is Interval?

Interval is a newly OSS project for for easily building internal tools. It is extremely nice for a project like this because it handles all the boring features like authentication, users, and permissions. It also makes it much easier to build simple UIs. You can use it to build arbitrary internal tools in addition to SST Feature Flag.

For ease of testing, I've spun up a free sandbox you can use: https://interval-sandbox.com (not intended for production use). There are some more notes on Interval and this sandbox below.

Local Development

You likely will want to do some testing locally before deploying to your production environment.

Here are the steps to start the local development environment (if you're self hosting, use that url instead):

Clone this repo: git clone git@github.com:bensenescu/sst-feature-flag.git
cd sst-feature-flag
Create an account on the Interval Sandbox (or your self hosted version).
- https://interval-sandbox.com
Get you Personal Development Key and copy it to your clipboard
- https://interval-sandbox.com/dashboard/develop/keys
npx sst secret set IntervalApiKey YOUR_API_KEY_HERE
npx sst dev
- Note the Api Url in the output. This will be used to call the evaluation API.
nox sst shell -- drizzle-studio push - This initializes the db.

Example Walkthroughs

Create a static feature flag and use Evaluation API

Go to https://interval-sandbox.com/dashboard/develop/ and you should Feature Flags as an option.
Create a feature flag called test-bool-flag with a boolean value of true
Test the evaluation API

curl [api_url_output_from_sst_dev]/feature-flag/evaluate \
--header 'Content-Type: application/json' \
--data '{
        "flagKey": "test-bool-flag",
        "defaultValue": false,
        "context": {}
}'

Create a dynamic feature flag and use Evaluation API

Create another boolean feature flag called test-bool-flag-dynamic, but this time make it dynamic
In the dynamic flag table, click the actions button (three dots) and select Add members
Add a member with an id john and type of user, set the flag value to true for them
Check if this user is in the flag with the evaluation API

   curl [api_url_output_from_sst_dev]/feature-flag/evaluate \
   --header 'Content-Type: application/json' \
   --data '{
   "flagKey": "test-bool-flag-dynamic",
   "defaultValue": false,
   "context": {
   "entityId": "john",
   "entityType": "user"
   }
   }'

Note: this will take a few minutes the first time you do it to spin up the vpc and database

Deployment

General Steps:

npx sst secret set IntervalApiKey [live_key_value] --stage prod
npx sst secret set IntervalServerEndpoint [self_hosted_interval_endpoint] --stage prod
npx sst deploy --stage prod
nox sst shell --stage prod -- drizzle-studio push - This initializes the db.

Actual Steps

Self host interval as described in #more-on-interval
Get a "live key" for your production deploy

If you don't need multiple team members to be able to easily manage flags, you can use your Personal Access Token in prod sst stage and development ui for managing flags.

If you need multiple team members to manage flags, its a bit trickier. Basically, Interval recently shut down their cloud offering and the self hosting is still rough around the edges. You need to "confirm your email" to create a live key. Here are some options:

Set up PostMark as described in their docs. Postmark was not simple to set up so I didn't do it in the sandbox.
Share a user login in a password manager so that your team can all log in as that user to manage flags in "development" mode.
Manually enter an into the email field in the UserEmailConfirmationToken table. This is effectively the same as the email being confirmed.

Once the TUI is built, this should all be easier so that you can just manage your flags with sst shell -- sst-ff-tui. Also, hopefully email sending becomes provider agnostic so that we can use SES instead of Postmark.

Extra Resources

Cost

Since this currently requires Aurora Serverless which has a minimum capacity of 0.5 ACU, this will cost ~$1.57 per day to run the database. The rest of the costs are negligible / well under the free tier, but will scale with your usage as with all AWS services.

There is a todo item to add a cheaper database option using Cloudflare D1.
Thread on Aurora Serverless Pricing: https://repost.aws/questions/QUbtHMLZXiS4Kppi7KMIB5YQ/aurora-serverless-v2-minimum-cost-setup-for-development-environment

More on Interval

The interval server is responsible for rendering data sent to it from the Interval Client we've built in this project. See packages/interval/src/index.ts.

interval-sandbox.com

For ease of testing this project out, I have spun up a free sandbox you can use: https://interval-sandbox.com.

Please do not use this for any production use cases. I have this running on a small server as you can see in this repo: https://github.com/bensenescu/interval-sandbox.

I may not keep this up indefinitely so don't rely on it. However, all the interval server does is store your user information so that you can access the UI. All feature flag data will be stored in the database spun up as part of this project i.e. even if you are using this and it goes away, it shouldn't be a big deal and you can just connect the new interval server you spin up and make an account there.

Self Hosting Interval

Here are the instructions for self hosting interval: https://github.com/interval/server

Note: there are some optional dependencies mentioned like emails and WorkOS for enterprise features that I don't have setup in my example.

Todos

Add API key auth to the API
Create a provider for openfeature.dev
Add percentage rollout flags
- This would allow you to specify that you want 20% of entities to have access to a flag.
Add option for using cloudflare for all infra
- This should be substantially cheaper and scale to zero since you won't need RDS + a vpc.
Add TUI so that it isn't necessary to run Interval if you don't want to.
Add tests for core functionality

bensenescu/sst-feature-flag