@leanjs/runtime v0.0.7
@leanjs/runtime
This runtime
enables micro-frontends to share state or execution context in a controlled manner, keeping your micro-frontends performant and maintainable. By default nothing is shared. You can read more about the why of this package in this post.
The runtime
is created in two steps:
configureRuntime
. In a distributed architecture there are many places where a runtime could be created. For instance, each micro-app will create aruntime
if they run in isolation. However, when micro-apps are composed into a single app, only oneruntime
should be created and shared across all of them. Theruntime
can be created in more than one place but the configuration of it should be the same for all of them. Don't invokeconfigureRuntime
more than once in a project. By project I mean in your whole micro-frontends architecture.createRuntime
. InvokingconfigureRuntime
returns a function calledcreateRuntime
which creates aruntime
when invoked. You can usecreateRuntime
in each distributed micro-app. Remember, there should be only one sharedruntime
in the runtime program.createRuntime
is not a singleton so you are responsible for making sure no more than oneruntime
is created.
There are two things that you can share in a runtime
:
- State. By design, we don't facilitate creating complex data structures. The
runtime
shared state is a flatten data structure, it doesn't support nested states unlike Redux for instance. However, you can add any object in a given state property. You can think of theruntime
state as a read-write hash table. - Execution context. It contains instances of code that we want to share. E.g. a WebSocket client that holds WS connections. You can think of it as a read-only hash table.
Installation
yarn add @leanjs/runtime
Usage
Guiding principles
When designing your shared runtime follow these recommendations:
- Less is more. The more things shared between micro-frontends the higher coupling. Use this
runtime
sparingly. - Make the
runtime
type-safety. Only types defined in the configuration of theruntime
are allowed. This way developers in different teams know what can be shared and what can't be shared. Use TypeScript. - Centralise the configuration of the
runtime
. Anyone can use theruntime
but only a few people should be able to change what can be shared. ExecuteconfigureRuntime
in its own repo with restricted access, or use CODEOWNERS if in a monorepo, then export it for anyone to use.
Basic
const defaultState = {
locale: "en",
};
const { createRuntime } = configureRuntime(defaultState)({
onError: (error) => {}, // required, log the error properly, e.g. Sentry, Datadog, etc
});
With execution context
const defaultState = {
locale: "en",
};
const { createRuntime } = configureRuntime(defaultState)({
onError: () => {}, // required, log the error properly, e.g. Sentry, Datadog, etc
context: {
eventEmitter: new MyEventEmitter(),
},
});
API
configureRuntime
It's a function with two curried arguments. The argument of the first function receives the default state. The argument of the second function is the configuration of the runtime.
The default state must be an object. The keys of the objects are used at runtime to validate access to the shared state. For instance, given the following default state:
const defaultState = {
locale: "en",
};
if a consumer of the runtime
tries to read or write a shared state property named foo
, the runtime
will throw an error. Only locale
is a valid shared state property. In other words, the default state is also used as a runtime validator. This behaviour can't be disabled.
If you use TypeScript, the runtime
will infer the types of the shared state from the default state. For instance, in the previous defaultState
TypeScript will only allow consumers of your shared state to read and write a state property called locale
and its only possible value will be a string.
configureRuntime
is a generic function so you can pass a TS type definition for your shared state. This is useful if your default state values don't match all the possible values of your shared state, e.g.
interface SharedState {
locale?: string;
}
const defaultState = {
locale: undefined,
};
// without passing a concrete type to the generic `configureRuntime`,
// locale could only be assigned to undefined because of the defaultState value
const { createRuntime } = configureRuntime<SharedState>(defaultState)({
onError: () => {}, // required, log the error properly, e.g. Sentry, Datadog, etc
});
onError - required function
The runtime
makes any asynchronous code internally look synchronous externally. This means that you won't be able to catch all the promises that might be generated. The onError
function will be invoked whenever there is an errors in the runtime, either sync or async.
context - optional object
Similarly to defaultState
each property in this context object argument is used to validate access at runtime to the shared context. Context is read-only. If you use TypeScript, since context can't change, the types of the context values will be inferred by TypeScript as follows:
configureRuntime(defaultState)({
onError,
context: {
// wsClient1 type is WsClient
wsClient1: new WsClient(),
// wsClient2 type is WsClient
wsClient2: () => new WsClient(),
// wsClient3 type is WsClient
wsClient3: async () => new WsClient()),
},
});
createRuntime
It creates a runtime
. Example:
const defaultState = {
locale: "en",
};
const { createRuntime } = configureRuntime(defaultState)({
onError,
});
const runtime = createRuntime();
booted
Async method that resolves true when all the async context resolves. If any of the async context properties is rejected it resolves false.
await runtime.booted();
state
It holds the current shared state.
// you can read state
const locale = runtime.state.locale;
// you can write state
runtime.state.locale = "es";
// subscribers to state.locale are notified
subscribe
It's used to subscribe to state changes. It receives a state property and a callback. When the state property changes the callback is invoked. It returns an unsubscribe
function. Example:
const unsubscribe = runtime.subscribe("locale", (locale) =>
console.log(`locale changed ${locale}`)
);
context
It holds the current shared context. Example:
const wsClient = runtime.context.wsClient;
// context is read only, the following line throws an error
// ❌ runtime.context.wsClient = new WsClient()
It's not recommended to access the context directly whenever possible. If you want to update some shared state based on an event from the shared context then use the on
method underneath.
on
This method is used to update shared state based on events from the context.
It has two arguments, the context property that you want to use, and a callback function that will receive the context instance and the current state. The callback must return a clean-up function. on
returns an off
function which calls the callback clean-up function upon invocation. Example:
const off = runtime.on("wsClient", (wsClient, state) => {
function updateLocale(value) {
state.locale = value;
}
wsClient?.on("locale-changed", updateLocale);
return () => {
wsClient.off("locale-changed", updateLocale);
};
});
off(); // wsClient.off("locale-changed", updateLocale); is called
load
It loads some value in a given state property. Once a state property is loaded with a value, no other loader will have effect on the given state property. load
is async. Example:
const locale = await runtime.load("locale", fetchLocale);
// locale equals runtime.state.locale
When calling load
many times for the same state property, the runtime
will only execute the first loader.
// ✅ fetchLocale is executed
runtime.load("locale", fetchLocale);
// ❌ fetchLocale is skipped
runtime.load("locale", fetchLocale);
// ❌ fetchLocale is skipped
runtime.load("locale", fetchLocale);
loaded
It's an async method that will await while a given state property is being loaded. If the state property is not being loaded it resolves immediately. Example:
runtime.load("locale", () => Promise.resolve("es"));
// in real-world project the next line would not be after the previous `load` call but in a different part of the codebase
const locale = await runtime.loaded("locale"); // locale equals "es"
The previous code has the same effect as the following code. The reason for having loaded
is that in a distributed UI, the code that needs to await
might not be the same as the code that load
s the value, unlike the following example:
const locale = await runtime.load("locale", () => Promise.resolve("es"));
If loaded
is called with no state property then it awaits for all the loaders to resolve.
runtime.load("locale", fetchLocale);
runtime.load("token", fetchToken);
await runtime.loaded();
// both locale and token have been loaded
loader
It returns the state of a loader: loading: boolean
and error?: string
. Example:
runtime.load("locale", fetchLocale);
// runtime.loader.locale.loading is true
await runtime.loaded("locale");
// runtime.loader.locale.loading is false
// Heads up, make sure to await runtime.loaded("state_property") before checking if there is an error
const didError = runtime.loader.locale.error;
// didError has an error message if the load method failed.