Sauci/docs/mobile-ota.md

Mobile over-the-air updates

Sauci ships JavaScript-only mobile changes over the air with expo-updates against a self-hosted XPREM update server (formerly expo-open-ota) at https://ota.apps.jbrunton.co.uk. XPREM speaks the official Expo Updates protocol, so the client is stock expo-updates; only the endpoint, the request headers, and the code-signing certificate are Sauci specific.

Store releases still follow Mobile release process. OTA never replaces a store build; it only refreshes the JavaScript bundle and assets of an already installed build.

Status

The client is wired but not yet live. Two values from the server deployment are still outstanding; see Pending server values.

How it works

1. A build embeds a manifest URL, a release channel, an app id, and a public code-signing certificate.
2. On launch the app requests https://ota.apps.jbrunton.co.uk/manifest, sending expo-channel-name, expo-app-id, xprem-branch, and the standard expo-runtime-version and expo-platform headers.
3. XPREM resolves the channel to a branch, finds the newest update published for that branch and runtime version, and returns a signed manifest.
4. The client verifies the signature against the embedded certificate, downloads the bundle, and launches it on the next start.

Branches and channels

XPREM separates storage from delivery. A branch groups updates for a release line. A release channel is bound to a build at build time and points at a branch. Repointing a channel at a different branch promotes or reverts an update without a rebuild.

EAS build profile	RELEASE_CHANNEL	Publish branch
development	development	development
preview	staging	staging
production	production	production

Channels and their branch mappings are created in the XPREM dashboard under Channels. The client cannot create them.

Runtime version

apps/mobile/app.config.js sets:

runtimeVersion: { policy: 'appVersion' }

The runtime version is therefore the expo.version field in app.json, for example 1.0.6. An update is only offered to builds whose runtime version matches exactly.

The practical rule: any change that touches native code, native dependencies, or the ios//android/ projects requires a new store build with a bumped expo.version. Bumping version starts a fresh runtime version on the server, so old builds stop receiving updates that assume the new native surface. JavaScript, asset, and configuration changes that do not alter the native binary can ship over the air against the current version.

appVersion is chosen over fingerprint deliberately. Sauci commits its native projects and ships several config plugins and an Apple widget target, so a fingerprint would churn on native edits that never reach a device, and it would be harder to reason about from a store release. appVersion ties the runtime version to the number already visible in App Store Connect and Google Play.

Client configuration

apps/mobile/app.config.js is a dynamic Expo config that spreads app.json and layers on the updates block. app.json remains the static source of truth for version, ios.buildNumber, and android.versionCode, which EAS rewrites during a release build.

Resolved configuration:

Key	Value
updates.url	https://ota.apps.jbrunton.co.uk/manifest
updates.enabled	true
updates.checkAutomatically	ON_LOAD
updates.fallbackToCacheTimeout	5000
updates.codeSigningCertificate	./certs/certificate.pem
updates.codeSigningMetadata	{ keyid: 'main', alg: 'rsa-v1_5-sha256' }
updates.requestHeaders['expo-channel-name']	process.env.RELEASE_CHANNEL, defaulting to development
updates.requestHeaders['expo-app-id']	process.env.XPREM_APP_ID
updates.requestHeaders['xprem-branch']	''
runtimeVersion	{ policy: 'appVersion' }

expo-updates only sends request headers that were declared at build time, so xprem-branch is declared even though it is empty. Removing it would not disable branch surfing, it would strip the header from every poll for the life of the install.

RELEASE_CHANNEL is set per profile in apps/mobile/eas.json. The config throws a descriptive error whenever RELEASE_CHANNEL is set but the app id or the certificate is still a placeholder, so a misconfigured build fails at config resolution rather than shipping a client that can never validate a manifest.

Local development

The dev server serves bundles directly and no signed manifest exists, so disable code signing when running a dev client:

cd apps/mobile
DISABLE_CODE_SIGNING=1 npx expo start --dev-client

Code signing

XPREM signs every manifest with an RSA private key held by the server. The app embeds only the matching public certificate.

• apps/mobile/certs/certificate.pem is the public certificate. It is public key material and is committed.
• private-key.pem belongs to the update server only. apps/mobile/certs/.gitignore blocks it. It must never be committed or kept in a developer checkout.

The keypair is generated once, on the machine that configures the server:

npx eoas generate-certs

The certificate can also be downloaded from the XPREM dashboard App Info page for the Sauci app.

Native projects

apps/mobile/ios and apps/mobile/android are committed but generated by Expo CNG, and docs/releasing.md treats expo prebuild as a source change rather than a release step. The native projects currently still carry the updates-disabled defaults:

• ios/Sauci/Supporting/Expo.plist: EXUpdatesEnabled is false.
• android/app/src/main/AndroidManifest.xml: expo.modules.updates.ENABLED is false.

The expo-updates config plugin writes the URL, the resolved runtime version, the request headers, the code-signing metadata, and the certificate contents into these files. Those values embed the real app id and certificate, so the native projects are deliberately left untouched until both land.

Once the pending values below are in place, regenerate and commit the native projects as a reviewed source change:

cd apps/mobile
RELEASE_CHANNEL=production npx expo prebuild
git diff   # expect only expo-updates related native changes

A build produced before that prebuild will not check for updates.

Publishing an update

Publishing uses the XPREM CLI, eoas, pinned as a dev dependency of apps/mobile. Authentication is an app-scoped API token generated in the XPREM dashboard and supplied as EOO_TOKEN; it is a secret and must not be committed.

cd apps/mobile

# Staging: consumed by builds made with the EAS `preview` profile.
EOO_TOKEN=<token> npm run ota:publish:staging

# Production.
EOO_TOKEN=<token> npm run ota:publish:production

Both scripts wrap eoas publish --branch <branch> with the matching RELEASE_CHANNEL. Useful flags to pass through:

Flag	Purpose
--platform ios|android|all	Defaults to all.
-m "<text>"	Update description shown in the dashboard.
--rollout-percentage <1-99>	Progressive rollout to a fraction of devices.
--nonInteractive	Required in CI.

eoas publish refuses to run against a dirty git working tree. Commit first rather than reaching for --disableRepositoryCheck or EAS_NO_VCS=1; the published update records the commit it came from.

Publish only from a commit that has passed npm run verify:full. An OTA update reaches installed devices without store review, so it carries the same risk as a production deploy and none of the review latency.

Verifying a published update

curl -sD - "https://ota.apps.jbrunton.co.uk/manifest" \
  -H "expo-app-id: <xprem-app-id>" \
  -H "expo-channel-name: production" \
  -H "expo-runtime-version: 1.0.6" \
  -H "expo-platform: ios"

Rollback

Two mechanisms, in order of preference:

1. Repoint the channel. In the XPREM dashboard, map the affected channel to a branch holding the last known good update. This takes effect at each app's next update check and needs no rebuild and no republish.

2. Roll back from the CLI.

   cd apps/mobile
   EOO_TOKEN=<token> npm run ota:rollback
   

   eoas rollback publishes a rollback directive for a branch and runtime version, sending clients back to the bundle embedded in the store build. eoas republish can instead re-serve a specific earlier update.

A bad update that prevents the app from starting cannot be fixed over the air on devices that never complete a launch. Keep fallbackToCacheTimeout short and treat a startup-path change as store-release risk, not OTA risk.

Pending server values

The client wiring is complete apart from two values owned by the XPREM server deployment. Both are non-secret.

Value	Where it goes	How to obtain it
XPREM app id (UUID)	OTA_APP_ID_PLACEHOLDER in apps/mobile/app.config.js, or the XPREM_APP_ID env var	XPREM dashboard, App Info page for the Sauci app
Code-signing certificate (PEM)	Replace the whole of apps/mobile/certs/certificate.pem	XPREM dashboard App Info download, or npx eoas generate-certs

Also required, outside this repository:

• An EOO_TOKEN publish token per environment, stored as a secret.
• development, staging, and production channels created in the dashboard and mapped to branches of the same names.
• The private-key.pem counterpart of the certificate installed in the server key store.

After both values land: replace them, run the prebuild described in Native projects, commit the native diff, and cut a new store build. Only builds produced after that point can receive updates.