Flags

• 2endpoints an app calls
• 30 sbetween polls; a change reaches the app on the next one
• 100flags per app
• 0lines of code shared with the apps

How an app reads its flags

1. Declare flags in code

   Each flag has a key, a default and optional scopes, such as phemten's live and sandbox channels. A new feature's default is off.

2. Poll with the declarations

   Every 30 seconds the app sends its declarations in one PUT and gets back what has been set. The same call restores flags the service has lost.

3. Evaluate in memory

   A check reads the scope's override, else the flag's value, else the code default. No request waits on the service.

Failure and access rules

• An app keeps running when the service is down

  A failed or unreadable poll keeps the last snapshot that worked. Before the first one arrives, the app uses its code defaults.

• Each app sees only its own flags

  An app's Access service token is bound to one namespace. It reads and declares there, and can't change a value anywhere.

• Only a signed-in admin changes a value

  A change needs a person's Access login and an X-Flags-Admin header, which a browser does not add to a request from another site.

• Changes are logged with who made them

  The page lists the last 20 changes, each with who made it and the old and new value.

Under the hood

Endpoints

PUT /v1/<namespace>/declarations, GET /v1/<namespace>/flags

Snapshot

value and scopes per flag; null means unset

Limits

100 flags per app; names are lowercase words joined by hyphens, up to 48 characters

Token check

Cloudflare Access JWT, RS256; signature, issuer, audience and expiry required

Admin page

One script, allowed by its hash; frame-ancestors 'none', so no other site can frame it

Stack

Python, FastAPI and SQLite; the rules are in one module that does no IO

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