Glossary

Patch borrows vocabulary from three places: mobile release engineering, the WebAssembly toolchain, and Apple’s developer agreements. Each term below is defined once, standalone, in the sense Patch uses it. Short answers to product questions are on the FAQ.

What is an OTA update?

An OTA (over-the-air) update is a code change delivered to an app already installed on a device, without submitting a new binary to the App Store. With Patch the delivered artifact is a WebAssembly module compiled from the Swift you changed; the signed App Store binary stays exactly as Apple reviewed it, and the module runs inside it.

What is a hotfix?

A hotfix is a small, urgent correction to code already in production — a crash, a wrong price, a broken eligibility rule — shipped on its own rather than waiting for the next scheduled release. Patch’s purpose is to make a hotfix to a native Swift app a release you run from a terminal instead of a submission you wait on.

What is a patch, and how is it different from a release?

A patch is the WebAssembly module that carries your changed code. A release is the act of publishing one to a channel with a rollout percentage, targeting and release notes — what patchcli release performs, and the row you see in the console’s Rollouts view. One build can be released more than once: to staging first, then to production.

What is a channel?

A channel is a named update stream — production, staging, beta, or any string you choose. A build subscribes to one channel in PatchConfiguration and receives only releases pushed to it, so an internal beta and production run in parallel from one codebase. status and rollback are channel-scoped too. See Deployment channels.

What is a staged rollout?

A staged rollout is a release served to a percentage of eligible devices rather than all of them, so a problem is found on a slice of the fleet. Patch buckets each device deterministically from its device id and the release, so a device that is in stays in as you raise the percentage. See Staged rollouts & A/B.

What is a rollback?

A rollback re-activates the previous module for a channel, undoing a release across the fleet. Devices pick it up on their next update check, and a device already running the withdrawn release is told to deactivate and fall back. It is one command — patchcli rollback — and needs no App Store submission.

What is a cohort, and what is targeting?

Targeting decides which devices are even eligible for a release: minimum and maximum app version, minimum OS version, and a named cohort. A cohort is a label a device reports — set by your app, or derived by the backend from a stable hash of the device id. Targeting composes with the rollout percentage: both must pass. See Release targeting.

What is a force update?

A force update is a release marked mandatory, so your app can require it before the user continues. The flag rides through the update check and surfaces as UpdateInfo.isMandatory; you decide what UI to show. It applies to the OTA module only — a native change still needs an App Store release. See Force updates.

What is the native shell?

The native shell is the compiled Swift inside your signed App Store binary: the code the engine kept native, plus the SDK, the linked frameworks, the entitlements and the Info.plist. A patch is not a whole app — it is a fragment that calls into symbols the shell already contains, which is why the shell’s identity has to be pinned.

What is a native-shell fingerprint?

A native-shell fingerprint is a deterministic SHA-256 hash over everything that fixes the signed binary’s layout — native Swift sources, the SDK and WasmKit versions, bridge definitions, Info.plist, entitlements, linked frameworks, the deployment target and the compiler version. A device reports its build’s fingerprint on every update check, and the backend serves a patch only when the fingerprints match. See The compatibility fingerprint.

What is WebAssembly?

WebAssembly (Wasm) is a portable binary instruction format with a defined sandbox: a module has its own linear memory and can reach the outside world only through imports its host explicitly provides. Patch compiles the OTA-eligible parts of your Swift to a WebAssembly module, which is why a patch cannot invent a capability your app does not already have.

What is WasmKit?

WasmKit is a WebAssembly runtime written in Swift. PatchSDK embeds it, so the module a patch delivers is executed by an interpreter compiled into your app rather than by anything downloaded or dynamically loaded. It is the reason a patch is interpreted code rather than native machine code.

What is Embedded Swift?

Embedded Swift is a language subset that compiles without the Swift runtime’s reflection and metadata machinery, producing very small binaries. Patch’s default packaging tier uses it: the module carries no Foundation or ICU, and Foundation values such as Decimal, Date and JSON are satisfied by host bridges into the native shell’s real Foundation instead.

What is a PMOD container?

A PMOD container is the file format a Patch build ships: one default WebAssembly sub-module plus optional additive sub-modules, packaged together. The SDK instantiates each sub-module as its own WasmKit instance and routes calls to the right one. Sub-modules are kept separate rather than merged because two Swift modules cannot share one linear memory.

What is dynamic replacement, and what is a thunk?

Dynamic replacement is the Swift mechanism — the one Xcode Previews uses — that lets a marked dynamic function be substituted at runtime. patchcli prepare marks each var body: some View dynamic and generates a thunk: a small @_dynamicReplacement(for: body) function that asks the SDK whether a patched body exists and renders it, with the original native body as the fallback.

What is a mixed view?

A mixed view is a SwiftUI view whose body is only partly lowerable. Rather than demoting the whole view, the engine lowers the parts it can and leaves the rest as native slots — a custom child view or an unsupported modifier is passed whole to the renderer by the build-time thunk. The structure ships over the air; the slotted leaves render natively.

What is demote-to-native?

Demote-to-native is the engine’s default answer whenever it cannot prove a change is safe to run as WebAssembly: the function or view body stays in the signed binary and runs from there. It happens at build time — with a per-view diagnostic naming what blocked it — and again on-device if a module fails to verify or activate. Coverage is what is at stake, never correctness.

What is a host bridge?

A host bridge is a host function the SDK exposes to the WebAssembly module so guest code can use a capability the module itself does not contain — networking, UserDefaults, the keychain, date and locale formatting, regular expressions. The bridge calls code already linked into your signed binary, so it can never widen what the app is able to do.

What is the binary-symbol wall?

The binary-symbol wall is the one hard limit on what an OTA patch can reach: it cannot introduce a native symbol, framework or entitlement absent from the signed binary, and it cannot rewrite the compiled body of native code that is not a patchable View.body. This is a property of code signing, not a backlog item. See What Patch can & can’t update.

What is interpreted code under DPLA §3.3.1(B)?

Interpreted code, in the sense of Apple’s Developer Program License Agreement §3.3.1(B) (formerly §3.3.2), is downloaded code an app runs through an interpreter it already contains — permitted provided it does not change the app’s primary purpose, does not create a store or storefront for other code, and does not bypass the sandbox or code signing. See Apple compliance.

What is App Review Guideline 2.5.2?

App Review Guideline 2.5.2 is the review rule that apps “should be self-contained in their bundles, and may not read or write data outside the designated container area, nor may they download, install, or execute code which introduces or changes features or functionality of the app, including other apps.” Read verbatim and in context on Apple compliance.

What is a phased release?

A phased release is App Store Connect’s own staged delivery: an approved version is released to a growing share of users who have automatic updates switched on. It happens after review, applies only to a version Apple already approved, and cannot be used to ship a change that has not been reviewed. A Patch rollout percentage is the equivalent control for an OTA module.

What is an expedited review?

An expedited review is a request to Apple to prioritise a submission ahead of the normal queue, reserved for critical situations such as a serious bug in a released app. It is a request, not a setting, and Apple decides. An OTA patch addresses the same urgency without a submission — for changes that stay inside the interpreted-code boundary.

What is a feature flag?

A feature flag is a runtime switch that turns behaviour already shipped in the binary on or off. It changes configuration, not code, so anything the flag can reach had to be written and reviewed in advance. An OTA patch changes the code itself, which is why it can fix a bug a flag can only hide.

What is remote config?

Remote config is server-delivered values — strings, numbers, colours, JSON — that an installed app reads at runtime. Like a feature flag it cannot introduce logic the binary does not already contain. Patch ships the logic, so it covers the cases where the fix is a code change rather than a value change.

What is server-driven UI?

Server-driven UI is an architecture where the server sends a description of the interface and the app renders it from a fixed catalogue of components you built in advance. It requires designing the app around that catalogue up front. Patch lowers the SwiftUI you already wrote, so no architectural commitment is required to change a view over the air.

What is CodePush?

CodePush is Microsoft’s over-the-air update service for React Native, which kept an app’s JavaScript and images in sync with releases and could not distribute changes that touched native code. The hosted App Center service was retired on 31 March 2025 and the client repository was archived on 20 May 2025; a standalone self-hosted server was published and is archived too. See How Patch compares.

What is EAS Update?

EAS Update is Expo’s over-the-air update service — in Expo’s words, “a cloud service that serves updates for projects using the expo-updates library”. It updates a React Native app’s non-native pieces, such as JavaScript, styling and images; native changes require a new build. See How Patch compares.

What is Shorebird?

Shorebird is the over-the-air code-push service for Flutter. It patches Dart code, running it on iOS through a custom Dart interpreter, and states that it “does not support changing native code (e.g. Java/Kotlin on Android or Objective-C/Swift on iOS)”. See How Patch compares.

Related

• FAQ — short answers to the questions behind these terms
• How it works — the pipeline these terms describe
• What Patch can & can’t update — the measured coverage map
• Apple compliance — DPLA §3.3.1(B) and Guideline 2.5.2