When building apps that rely on data from an API, two things are essential: we want our data to be fresh, and we want it fast. The stale-while-revalidate caching pattern helps us to strike a balance between both.

If our cache already contains a copy of the requested information, we can immediately show the (potentially stale) data. But at the same time, we revalidate our cache and fetch a new version. This can make navigating our apps feel instantaneously while also making sure that the user sees the latest data eventually.

In this video, you can see two components which both fetch their data from the same API endpoint. Thanks to the SWR composable, the data is always in sync. Even if a new component is added, it immediately has access to the cached data while fetching a fresh copy in the background.

If you want to try it yourself, you can check out the demo hosted on Netlify and take a closer look at the code on GitHub.

Inspiration in the React world

In the last couple of weeks, I played around with Preact, and I learned a thing or two about how to do things in the React ecosystem. I discovered that because of how React Hooks work, there are a few special requirements on how to fetch data in React components.

This is why since the introduction of Hooks, two quite similar data fetching libraries saw the light of day: React Query and SWR. What we build today is inspired by those libraries.

SWR Data Fetching Composable with Vue 3

I kept an eye on the development of the Vue 3 Composition API since the first RFC was published, but I never got around actually using it extensively. After experimenting with it for a couple of days, I can say this: it is fantastic! Now that I have (minimal) knowledge about how things are done in React, I can even more appreciate how powerful and straightforward to use the Composition API is.

So without further ado, let's take a look at a naive implementation of a stale-while-revalidate data fetching composable for Vue 3.

// src/composables/swr-cache.js
import { reactive, readonly, toRefs } from 'vue';
import LRU from 'lru-cache';

import { asArray } from '../utils/as-array';

const CACHE = new LRU({ max: 1024 });

const DEFAULT_OPTIONS = {
  dedupingInterval: 2000,
};

// We use `Symbol` for the state properties to prevent
// consumers of this package to use raw strings.
// See: https://bit.ly/2Lh2lEM
export const STATE = {
  error: Symbol('error'),
  idle: Symbol('idle'),
  loading: Symbol('loading'),
  revalidating: Symbol('revalidating'),
};

export function useSwrCache(parameter, callback, customOptions) {
  const options = {
    ...DEFAULT_OPTIONS,
    ...customOptions,
  };

  // Wrap `parameter` in an array if it is not an array already.
  const parameters = asArray(parameter);
  // Naive way of creating a unique cache key.
  const cacheKey = `${JSON.stringify(parameters)}${callback.toString()}`;
  const cacheKeyDedupe = `${cacheKey}_dedupe`;
  const cachedResponse = CACHE.get(cacheKey);

  // Use the reactive object from the cache or create a new one.
  const response = cachedResponse || reactive({
    data: null,
    error: null,
    reload: undefined,
    state: undefined,
  });

  if (!cachedResponse) CACHE.set(cacheKey, response);

  const load = async () => {
    try {
      // Dedupe requests during the given interval.
      if (CACHE.get(cacheKeyDedupe)) return;

      CACHE.set(cacheKeyDedupe, true, options.dedupingInterval);

      response.state = response.data ? STATE.revalidating : STATE.loading;
      // Wait for the result of the callback and set
      // the reactive `data` property.
      response.data = Object.freeze(await callback(...parameters));
      response.state = STATE.idle;
    } catch (error) {
      console.error(error);

      CACHE.del(cacheKeyDedupe);

      response.error = error;
      response.state = STATE.error;
    }
  };

  response.reload = load;
  load();

  // Using `toRefs()` makes it possible to use
  // spreading in the consuming component.
  // Making the return value `readonly()` prevents
  // users from mutating global state.
  return toRefs(readonly(response));
}

One beautiful thing about this approach but also a potential gotcha is that every instance that resolves to the same cache key shares the same reactive object. So if one component triggers a reload, the data is updated everywhere. It almost works like a global Vuex store.

One thing to mention is that the way how the cacheKey is created is not very reliable. Because: JSON.stringify({ a: 'a', b: 'b' }) !== JSON.stringify({ b: 'b', a: 'a' }). Property order does matter! But as long as you only use strings as parameters, that's no problem.

Usage

Now let's take a look at how we can use our newly created useSwrCache() composable.

<template>
  <div>
    <h2>Profile</h2>

    <template v-if="user">
      Name: {{ user.name }}
      <!-- ... -->
    </template>
    <template v-else-if="state === STATE.loading">
      LOADING ...
    </template>
    <template v-else-if="state === STATE.error">
      {{ error }}
    </template>

    <!-- Stale data is shown while revalidating! -->
    <template v-if="state === STATE.revalidating">
      <small>REVALIDATING ...</small>
    </template>
  </div>
</template>

<script>
import { fetcher } from '../utils/fetcher';
import { useSwrCache, STATE } from '../composables/swr-cache';

export default {
  name: 'UserProfile',
  setup() {
    const {
      data: user,
      error,
      state,
    } = useSwrCache('https://jsonplaceholder.typicode.com/users/1', fetcher);

    return {
      STATE,
      error,
      state,
      user,
    };
  },
};
</script>

The fetcher utility is a simple wrapper around fetch(). useSwrCache returns a reactive object with the data, the current state and, if applicable, an error.

We have two different states for loading (no cached data) and revalidating.

Wrapping it up

Although it works perfectly fine, this is a quite raw implementation and in no way as comprehensive as React Query or SWR. If you consider using something like this in production, you might want to use a more bulletproof approach for generating unique cache keys.