How to cache data publicly from firebase function? - firebase

Question

I'm a little bit lost on how firebase functions work with authentication,
Suppose I have a function that pulls 100 documents and sets the cache header for 24 hours.
res.set('Cache-Control', 'public, max-age=0, s-maxage=86400' // 24 * 60 * 60
By default, does that apply to all users or is it cached per user? There's some instances where the 100 documents are unique to the user - while other functions where the 100 documents are available to any user that's authenticated.
I see in the docs that you can set a __session which implies it's for individual users data, however there isn't much documentation for how to set that (or where). Is it set by default?
My goal is to have a function that requires the user be authenticated, then return 100 documents from a non-user specific collection - aka not have to read 100 documents per user. However, I don't think thats feasible because it would need to check if each user is authorized (not cacheable). So is there a way to just make a publicly available cache?
Any light that can be shared on this is greatly appreciated!

Answer 1

The Cache-Control header is used to instruct a user's browser and any CDN edge server on how to cache the request.
For requests requiring authentication, making use of the CDN is not really possible for this as you should be using Cache-Control: private for these responses (the default for Cloud Functions).
While you could check that your users are authenticated and then redirect them to a publically cached resource (like https://example.com/api/docs?sig=<somesignature>), this URL would still be accessible if someone got hold of that URL/cached data.
Arguably the best approach would be to store your "cached" responses in a single Cloud Firestore document (if it is less than 1MB in size and is JSON-compatible) or store it in Cloud Storage.
The code included below is an example of how you could do this with a Cloud Firestore cache. I've used posts where the authenticated user is the author as an example, but for this specific use case, you would be better off using the Firebase SDK to make such a query (realtime updates, finer control, query API). A similar approach could be applied for "all user" resources.
If attempting to cache HTML or some other not JSON friendly format, I would recommend changing the caching layer to Cloud Storage. Instead of storing the post's data in the cache entry, store the path and bucket to the cached file in storage (like below). Then if it hasn't expired, get a stream of that file from storage and pipe it through to the client.
{
data: {
fullPath: `/_serverCache/apiCache/${uid}/posts.html`,
bucket: "myBucket"
},
/* ... */
}
Common Example Code
import functions from "firebase-functions";
import { HttpsError } from "firebase-functions/lib/providers/https";
import admin from "firebase-admin";
import hash from "object-hash";
admin.initializeApp();
interface AttachmentData {
/** May contain a URL to the resource */
url?: string;
/** May contain Base64 encoded data of resource */
data?: string;
/** Type of this resource */
type: "image" | "video" | "social" | "web";
}
interface PostData {
author: string;
title: string;
content: string;
attachments: Record<string, AttachmentData>;
postId: string;
}
interface CacheEntry<T = admin.firestore.DocumentData> {
/** Time data was cached, as a Cloud Firestore Timestamp object */
cachedAt: admin.firestore.Timestamp;
/** Time data was cached, as a Cloud Firestore Timestamp object */
expiresAt: admin.firestore.Timestamp;
/** The ETag signature of the cached resource */
eTag: string;
/** The cached resource */
data: T;
}
/**
* Returns posts authored by this user as an array, from Firestore
*/
async function getLivePostsForAuthor(uid: string) {
// fetch the data
const posts = await admin.firestore()
.collection('posts')
.where('author', '==', uid)
.limit(100)
.get();
// flatten the results into an array, including the post's document ID in the data
const results: PostData[] = [];
posts.forEach((postDoc) => {
results.push({ postId: postDoc.id, ...postDoc.data() } as PostData);
});
return results;
}
/**
* Returns posts authored by this user as an array, caching the result from Firestore
*/
async function getCachedPostsForAuthor(uid: string) {
// Get the reference to the data's location
const cachedPostsRef = admin.firestore()
.doc(`_server/apiCache/${uid}/posts`) as admin.firestore.DocumentReference<CacheEntry<PostData[]>>;
// Get the cache entry's data
const cachedPostsSnapshot = await cachedPostsRef.get();
if (cachedPostsSnapshot.exists) {
// get the expiresAt property on it's own
// this allows us to skip processing the entire document until needed
const expiresAt = cachedPostsSnapshot.get("expiresAt") as CacheEntry["expiresAt"] | undefined;
if (expiresAt !== undefined && expiresAt.toMillis() > Date.now() - 60000) {
// return the entire cache entry as-is
return cachedPostsSnapshot.data()!;
}
}
// if here, the cache entry doesn't exist or has expired
// get the live results from Firestore
const results = await getLivePostsForAuthor(uid);
// etag, cachedAt and expiresAt are used for the HTTP cache-related headers
// only expiresAt is used when determining expiry
const cacheEntry: CacheEntry<PostData[]> = {
data: results,
eTag: hash(results),
cachedAt: admin.firestore.Timestamp.now(),
// set expiry as 1 day from now
expiresAt: admin.firestore.Timestamp.fromMillis(Date.now() + 86400000),
};
// save the cached data and it's metadata for future calls
await cachedPostsRef.set(cacheEntry);
// return the cached data
return cacheEntry;
}
HTTPS Request Function
This is the request type you would use for serving Cloud Functions behind Firebase Hosting. Unfortunately the implementation details aren't as straightforward as using a Callable Function (see below) but is provided as an official project sample. You will need to insert validateFirebaseIdToken() from that example for this code to work.
import express from "express";
import cookieParserLib from "cookie-parser";
import corsLib from "cors";
interface AuthenticatedRequest extends express.Request {
user: admin.auth.DecodedIdToken
}
const cookieParser = cookieParserLib();
const cors = corsLib({origin: true});
const app = express();
// insert from https://github.com/firebase/functions-samples/blob/2531d6d1bd6b16927acbe3ec54d40369ce7488a6/authorized-https-endpoint/functions/index.js#L26-L69
const validateFirebaseIdToken = /* ... */
app.use(cors);
app.use(cookieParser);
app.use(validateFirebaseIdToken);
app.get('/', async (req, res) => {
// if here, user has already been validated, decoded and attached as req.user
const user = (req as AuthenticatedRequest).user;
try {
const cacheEntry = await getCachedPostsForAuthor(user.uid);
// set caching headers
res
.header("Cache-Control", "private")
.header("ETag", cacheEntry.eTag)
.header("Expires", cacheEntry.expiresAt.toDate().toUTCString());
if (req.header("If-None-Match") === cacheEntry.eTag) {
// cached data is the same, just return empty 304 response
res.status(304).send();
} else {
// send the data back to the client as JSON
res.json(cacheEntry.data);
}
} catch (err) {
if (err instanceof HttpsError) {
throw err;
} else {
throw new HttpsError("unknown", err && err.message, err);
}
}
});
export const getMyPosts = functions.https.onRequest(app);
Callable HTTPS Function
If you are making use of the client SDKs, you can also request the cached data using Callable Functions.
This allows you to export the function like this:
export const getMyPosts = functions.https.onCall(async (data, context) => {
if (!context.auth) {
throw new functions.https.HttpsError(
'failed-precondition',
'The function must be called while authenticated.'
);
}
try {
const cacheEntry = await getCachedPostsForAuthor(context.auth.uid);
return cacheEntry.data;
} catch (err) {
if (err instanceof HttpsError) {
throw err;
} else {
throw new HttpsError("unknown", err && err.message, err);
}
}
});
and call it from the client using:
const getMyPosts = firebase.functions().httpsCallable('getMyPosts');
getMyPosts()
.then((postsArray) => {
// do something
})
.catch((error) => {
// handle errors
})