This answer will summarize the options for getting a download URL when uploading a file to Google/Firebase Cloud Storage. There are three types of download URLS:
- Token download URLs, which are persistent and have security features
- Signed download URLs, which are temporary and have security features
- Public download URLs, which are persistent and lack security
There are two ways to get a token download URL. Signed and public download URLs each have only one way to get them.
Token URL method #1: From the Firebase Storage Console
You can get the download URL from Firebase Storage console:
The download URL looks like this:
https://firebasestorage.googleapis.com/v0/b/languagetwo-cd94d.appspot.com/o/Audio%2FEnglish%2FUnited_States-OED-0%2Fabout.mp3?alt=media&token=489c48b3-23fb-4270-bd85-0a328d2808e5
The first part is a standard path to your file. At the end is the token. This download URL is permanent, i.e., it won't expire, although you can revoke it.
Token URL method #2: From the Front End
The documentation tells us to use getDownloadURL():
let url = await firebase.storage().ref('Audio/English/United_States-OED-' + i +'/' + $scope.word.word + ".mp3").getDownloadURL();
This gets the same download URL that you can get from your Firebase Storage console. This method is easy but requires that you know the path to your file, which in my app is difficult. You could upload files from the front end, but this would expose your credentials to anyone who downloads your app. So for most projects you'll want to upload your files from your Cloud Functions, then get the download URL and save it to your database along with other data about your file.
I can't find a way to get the token download URL when I write a file to Storage from a Cloud Function (because I can't find a way to tell the front end that a file has written to Storage), but what works for me is to write a file to a publicly available URL, write the publicly available URL to Firebase, then when my Angular front end gets the download URL from Firebase it also runs getDownloadURL(), which has the token, then compares the download URL in Firestore to the token download URL, and if they don't match then it updates the token download URL in place of the publicly available URL in Firestore. This exposes your file to the public only once.
This is easier than it sounds. The following code iterates through an array of Storage download URLs and replaces publicly available download URLs with token download URLs.
const storage = getStorage();
var audioFiles: string[] = [];
if (this.pronunciationArray[0].pronunciation != undefined) {
for (const audioFile of this.pronunciationArray[0].audioFiles) { // for each audio file in array
let url = await getDownloadURL(ref(storage, audioFile)); // get the download url with token
if (audioFile !== url) { // download URLs don't match
audioFiles.push(url);
} // end inner if
}; // end for of loop
if (audioFiles.length > 0) { // update Firestore only if we have new download URLs
await updateDoc(doc(this.firestore, 'Dictionaries/' + this.l2Language.long + '/Words/' + word + '/Pronunciations/', this.pronunciationArray[0].pronunciation), {
audioFiles: audioFiles
});
}
} // end outer if
You're thinking, "I'll return the Storage location from my Cloud Function to my front end and then use the location with getDownloadURL() to write the token download URL to Firestore." That won't work because Cloud Functions can only return synchronous results. Async operations return null.
"No problem," you say. "I'll set up an Observer on Storage, get the location from the Observer, and then use the location with getDownloadURL() to write the token download URL to Firestore." No dice. Firestore has Observers. Storage doesn't have Observers.
"How about," you say, "calling listAll() from my front end, getting a list of all my Storage files, then calling the metadata for each file, and extracting the download URL and token for each file, and then writing these to Firestore?" Good try, but Storage metadata doesn't include the download URL or token.
Signed URL method #1: getSignedUrl() for Temporary Download URLs
getSignedUrl() is easy to use from a Cloud Function:
function oedPromise() {
return new Promise(function(resolve, reject) {
http.get(oedAudioURL, function(response) {
response.pipe(file.createWriteStream(options))
.on('error', function(error) {
console.error(error);
reject(error);
})
.on('finish', function() {
file.getSignedUrl(config, function(err, url) {
if (err) {
console.error(err);
return;
} else {
resolve(url);
}
});
});
});
});
}
A signed download URL looks like this:
https://storage.googleapis.com/languagetwo-cd94d.appspot.com/Audio%2FSpanish%2FLatin_America-Sofia-Female-IBM%2Faqu%C3%AD.mp3?GoogleAccessId=languagetwo-cd94d%40appspot.gserviceaccount.com&Expires=4711305600&Signature=WUmABCZIlUp6eg7dKaBFycuO%2Baz5vOGTl29Je%2BNpselq8JSl7%2BIGG1LnCl0AlrHpxVZLxhk0iiqIejj4Qa6pSMx%2FhuBfZLT2Z%2FQhIzEAoyiZFn8xy%2FrhtymjDcpbDKGZYjmWNONFezMgYekNYHi05EPMoHtiUDsP47xHm3XwW9BcbuW6DaWh2UKrCxERy6cJTJ01H9NK1wCUZSMT0%2BUeNpwTvbRwc4aIqSD3UbXSMQlFMxxWbPvf%2B8Q0nEcaAB1qMKwNhw1ofAxSSaJvUdXeLFNVxsjm2V9HX4Y7OIuWwAxtGedLhgSleOP4ErByvGQCZsoO4nljjF97veil62ilaQ%3D%3D
The signed URL has an expiration date and long signature. The documentation for the command line gsutil signurl -d says that signed URLs are temporary: the default expiration is one hour and the maximum expiration is seven days.
I'm going to rant here that the getSignedUrl documentation never says that your signed URL will expire in a week. The documentation code has 3-17-2025 as the expiration date, suggesting that you can set the expiration years in the future. My app worked perfectly, and then crashed a week later. The error message said that the signatures didn't match, not that the download URL had expired. I made various changes to my code, and everything worked...until it all crashed a week later. This went on for more than a month of frustration. Is the 3-17-2025 date an inside joke? Like the gold coins of a leprechaun that vanish when the leprechaun is out of sight, a St. Patrick's Day expiry date years in the future vanishes in two weeks, just when you thought your code was bug-free.
Public URL #1: Make Your File Publicly Available
You can set the permissions on your file to public read, as explained in the documentation. This can be done from the Cloud Storage Browser or from your Node server. You can make one file public or a directory or your entire Storage database. Here's the Node code:
var webmPromise = new Promise(function(resolve, reject) {
var options = {
destination: ('Audio/' + longLanguage + '/' + pronunciation + '/' + word + '.mp3'),
predefinedAcl: 'publicRead',
contentType: 'audio/' + audioType,
};
synthesizeParams.accept = 'audio/webm';
var file = bucket.file('Audio/' + longLanguage + '/' + pronunciation + '/' + word + '.webm');
textToSpeech.synthesize(synthesizeParams)
.then(function(audio) {
audio.pipe(file.createWriteStream(options));
})
.then(function() {
console.log("webm audio file written.");
resolve();
})
.catch(error => console.error(error));
});
The result will look like this in your Cloud Storage Browser:
Anyone can then use the standard path to download your file:
https://storage.googleapis.com/languagetwo-cd94d.appspot.com/Audio/English/United_States-OED-0/system.mp3
Another way to make a file public is to use the method makePublic(). I haven't been able to get this to work, it's tricky to get the bucket and file paths right.
An interesting alternative is to use Access Control Lists. You can make a file available only to users whom you put on a list, or use authenticatedRead to make the file available to anyone who is logged in from a Google account. If there were an option "anyone who logged into my app using Firebase Auth" I would use this, as it would limit access to only my users.
Deprecated: Build Your Own Download URL with firebaseStorageDownloadTokens
Several answers describe an undocumented Google Storage object property firebaseStorageDownloadTokens. This was never an official Google Cloud Storage feature and it no longer works. Here's how it used to work.
You told Storage the token you wanted to use. You then generated a token with the uuid Node module. Four lines of code and you could build your own download URL, the same download URL you get from the console or getDownloadURL(). The four lines of code are:
const uuidv4 = require('uuid/v4');
const uuid = uuidv4();
metadata: { firebaseStorageDownloadTokens: uuid }
https://firebasestorage.googleapis.com/v0/b/" + bucket.name + "/o/" + encodeURIComponent('Audio/' + longLanguage + '/' + pronunciation + '/' + word + '.webm') + "?alt=media&token=" + uuid);
Here's the code in context:
var webmPromise = new Promise(function(resolve, reject) {
var options = {
destination: ('Audio/' + longLanguage + '/' + pronunciation + '/' + word + '.mp3'),
contentType: 'audio/' + audioType,
metadata: {
metadata: {
firebaseStorageDownloadTokens: uuid,
}
}
};
synthesizeParams.accept = 'audio/webm';
var file = bucket.file('Audio/' + longLanguage + '/' + pronunciation + '/' + word + '.webm');
textToSpeech.synthesize(synthesizeParams)
.then(function(audio) {
audio.pipe(file.createWriteStream(options));
})
.then(function() {
resolve("https://firebasestorage.googleapis.com/v0/b/" + bucket.name + "/o/" + encodeURIComponent('Audio/' + longLanguage + '/' + pronunciation + '/' + word + '.webm') + "?alt=media&token=" + uuid);
})
.catch(error => console.error(error));
});
That's not a typo--you have to nest firebaseStorageDownloadTokens in double layers of metadata:!
