Live Queries
Our todo list app can have multiple users using it at the same time. However, changes made by one user are not seen by others unless they manually refresh the browser.
Let's add realtime multiplayer capabilities to this app.
Realtime updated todo list
Let's switch from fetching Tasks once when the React component is loaded, and manually maintaining state for CRUD operations, to using a realtime updated live query subscription for both initial data fetching and subsequent state changes.
- Modify the contents of the
useEffect
hook in theApp
component:
// src/App.tsx
useEffect(() => {
return taskRepo
.liveQuery({
limit: 20,
orderBy: { createdAt: "asc" }
//where: { completed: true },
})
.subscribe(info => setTasks(info.applyChanges))
}, [])
Let's review the change:
- Instead of calling the
repository
'sfind
method we now call theliveQuery
method to define the query, and then call itssubscribe
method to establish a subscription which will update the Tasks state in realtime. - The
subscribe
method accepts a callback with aninfo
object that has 3 members:items
- an up to date list of items representing the current result - it's useful for readonly use cases.applyChanges
- a method that receives an array and applies the changes to it - we send that method to thesetTasks
state function, to apply the changes to the existingtasks
state.changes
- a detailed list of changes that were received
- The
subscribe
method returns anunsubscribe
function which we use as a return value for theuseEffect
hook, so that it'll be called when the component unmounts.
- As all relevant CRUD operations (made by all users) will immediately update the component's state, we should remove the manual adding of new Tasks to the component's state:
// src/App.tsx
const addTask = async (e: FormEvent) => {
e.preventDefault()
try {
// const newTask = await taskRepo.insert({ title: newTaskTitle }) <- Delete this line
await taskRepo.insert({ title: newTaskTitle }) // <- replace with this line
// setTasks([...tasks, newTask]) <-- this line is no longer needed
setNewTaskTitle("")
} catch (error: unknown) {
alert((error as { message: string }).message)
}
}
- Optionally remove other redundant state changing code:
// src/App.tsx
//...
{
tasks.map(task => {
const setTask = (value: Task) =>
setTasks(tasks => tasks.map(t => (t === task ? value : t)))
const setCompleted = async (completed: boolean) =>
// setTask(await taskRepo.save({ ...task, completed })) <- Delete this line
await taskRepo.save({ ...task, completed }) // <- replace with this line
const setTitle = (title: string) => setTask({ ...task, title })
const saveTask = async () => {
try {
// setTask(await taskRepo.save(task)) <- Delete this line
await taskRepo.save(task) // <- replace with this line
} catch (error: unknown) {
alert((error as { message: string }).message)
}
}
const deleteTask = async () => {
try {
await taskRepo.delete(task)
// setTasks(tasks.filter(t => t !== task)) <- Delete this line
} catch (error: unknown) {
alert((error as { message: string }).message)
}
}
//...
})
}
Open the todo app in two (or more) browser windows/tabs, make some changes in one window and notice how the others are updated in realtime.
Under the hood
The default implementation of live-queries uses HTTP Server-Sent Events (SSE) to push realtime updates to clients, and stores live-query information in-memory.
For scalable production / serverless environments, live-query updates can be pushed using integration with third-party realtime providers, such as Ably, and live-query information can be stored to any database supported by Remult.