Bundle Diffing

What it is

Bundle diffing helps OTA updates download less data.

Instead of always installing the full archive, Hot Updater can reuse files that already exist in the currently installed OTA bundle and download only the files that changed.

You do not need to change your update-check code or deploy command. After your app is using a diff-enabled Hot Updater runtime, new deployments include the data needed for bundle diffing automatically.

How to use it

No special command is required. Deploy as usual:

npx hot-updater deploy -p ios
npx hot-updater deploy -p android

For apps running a bundle-diffing capable runtime, that is enough for manifest-based diffing.

Binary patch optimization for Hermes bundle changes is enabled by default:

import { defineConfig } from "hot-updater";

export default defineConfig({
  patch: {
    enabled: true,
    maxBaseBundles: 3,
  },
});

This is the default behavior. Add the patch block only when you want to override it, for example to disable patch generation or change the number of base bundles.

maxBaseBundles controls how many previous compatible OTA bundles are used as patch bases when deploying a new OTA update.

For example:

Base bundle -> OTA A -> OTA B -> OTA C

When deploying OTA C with maxBaseBundles: 2, Hot Updater can prepare patches from:

• OTA B to OTA C
• OTA A to OTA C

This means devices currently on OTA A or OTA B can download a small patch to OTA C instead of the full changed Hermes bundle. Devices that only have the base bundle still use the compressed archive for their first OTA update. Devices on an older OTA outside the selected patch bases download the full changed Hermes bundle file instead of a patch.

Notes:

• Defaults to 3.
• Uses recent compatible bundles from the same channel and platform.
• Higher values increase the range of OTA versions covered by patches, but can make deployment time grow exponentially because more patch artifacts must be generated.

If a patch cannot be created or applied, Hot Updater downloads the full changed Hermes bundle file instead. If the bundle-diffing path itself cannot be used, it falls back to the compressed archive.

How it works

The first OTA update after installing a native app uses the normal compressed archive. At that point, the app does not have a previous OTA bundle directory that can be reused.

From the second OTA update onward, the app can compare the new update with the currently installed OTA bundle:

1. Files that did not change are copied from the current OTA bundle.
2. Files that changed are downloaded from the new update.
3. If a compatible patch exists for the Hermes bundle, the app downloads and applies the patch instead of downloading the full changed Hermes bundle.
4. If reuse or patching fails, the app falls back to the normal compressed archive.

Signing

Bundle diffing works without signing, but signing is recommended. When signing is enabled, the archive, manifest, and manifest assets are verified before the update is installed.

See Bundle Signing for setup.