#Fingerprint Update Strategy

The "Fingerprint" update strategy in hot-updater leverages Expo's @expo/fingerprint library to track and manage changes in your project's native code. This mechanism significantly enhances the stability of app updates by preventing the delivery of JavaScript bundles incompatible with the native code.

#Core Concepts

• Fingerprint: This is a hash value (e.g., 8b47da71b3b7cf7fa7fd0ad4938207d01d584430) representing the current state of your project's native code. It's calculated every time the CLI is run. It changes whenever there are modifications to native dependencies, direct native code edits, or Expo SDK updates.
• Role: When you run the hot-updater deploy command, hot-updater compares the stored fingerprint hash. It then ensures that updates are only sent to clients whose native version is compatible with the JavaScript bundle being deployed.
• Automatic Injection: Each time hot-updater builds and deploys a JavaScript bundle, it automatically injects the current project's fingerprint hash into that bundle. The client application uses this injected fingerprint to verify compatibility with its own native code.

#Configuration

You can enable this feature by setting the updateStrategy option to fingerprint in your hot-updater.config.ts file. If this option is not explicitly set, fingerprint is used by default.

import { defineConfig } from "hot-updater";

export default defineConfig({
  // ... other configurations
  updateStrategy: "fingerprint", // Or omit this line to use "fingerprint" by default
});

The fingerprint configuration also helps determine when builds should be cached and invalidated in non-standard settings, such as when you have git submodules in your project:

import { defineConfig } from "hot-updater";

export default defineConfig({
  // ... other configurations
  updateStrategy: "fingerprint",
  fingerprint: {
    extraSources: ["./git-submodule"], // Add extra sources to include in fingerprint calculation
    ignorePaths: ["./temp"], // Ignore paths during fingerprint calculation
  },
});

#Managing and Updating Fingerprints

It's crucial to check and, if necessary, update the fingerprint whenever changes are made to the native code.

1. Check Current Fingerprint:

   npm

   pnpm

   yarn

   npx hot-updater fingerprint

   This command calculates the current project's fingerprint and shows whether it matches the stored value.

2. Create a New Fingerprint: If the command above indicates a mismatch, you must generate a new fingerprint using:

   npm

   pnpm

   yarn

   npx hot-updater fingerprint create

   Important: After creating a new fingerprint, you must rebuild your app. This ensures that the updated native environment and the new fingerprint are correctly incorporated into the app binary, preventing compatibility issues with apps built using the previous fingerprint.

#Fingerprint File Example (fingerprint.json)

{
  "ios": {
    "hash": "11142b9062165fa48665f5efa095dd94e9e45eb0",
    // ... other details
  },
  "android": {
    "hash": "c763ed5729a0bcccf23248ee0183ddf9016c2e6e",
    // ... other details
  }
}

#Endpoint Testing

You can test if the update check endpoint, configured via the hot-updater init command, is working correctly. When using the fingerprint strategy, the endpoint format is typically:

• GET /check-update/fingerprint/:platform/:fingerprintHash/:channel/:minBundleId/:bundleId

Example of testing with curl:

curl "https://your-update-endpoint.com/check-update/fingerprint/ios/11142b9062165fa48665f5efa095dd94e9e45eb0/production/00000000-0000-0000-0000-000000000000/00000000-0000-0000-0000-000000000001"

Request Parameter Descriptions:

• :platform: The platform, such as ios or android.
• :fingerprintHash: The current fingerprint hash of the client app.
• :channel: The channel name you want to check for updates (e.g., default, staging).
• :minBundleId: The minimum bundle ID supported by the client (this value is updated when the native app is rebuilt. If unknown for testing, you can use 00000000-0000-0000-0000-000000000000).
• :bundleId: The client's current bundle ID.

This request allows you to verify if an update is available for a specific fingerprint hash and channel.