Firestore update on server only - firebase

Question

When retrieving data from Firestore one has the option of forcing retrieval from the server. The default option is cache and server, as determined by Firestore.
I have a certain usage where a command and control node is issuing real-time commands to remote nodes backed by Firestore. This requires the updates to be done on the server (or fail) so that the C&C node has certainty on the execution (or failure) in real-time. What I would like to do is to disable use of cache with these updates. I have not found a way to do that. Is this possible with current capabilities of Firestore?
Note that it is not desirable to disable Firestore caching at a global level as the cache is beneficial in other situations.
----EDIT-----
Based on the responses I have created this update method that attempts to force updating the server using a transaction.
A couple of notes:
This is dart code.
Utils.xyz is an internal library and in this case it is being used to log.
I have reduced the network speed for the test to simulate a bad network connection.
The timeout is set to 5 seconds.
Here is the output of my log:
I/flutter (22601): [2021-06-06 22:35:30] [LogLevel.DEBUG] [FirestoreModel] [update] [We are here!]
I/flutter (22601): [2021-06-06 22:35:47] [LogLevel.DEBUG] [FirestoreModel] [update] [We are here!]
I/flutter (22601): [2021-06-06 22:36:02] [LogLevel.DEBUG] [FirestoreModel] [update] [We are here!]
I/flutter (22601): [2021-06-06 22:37:18] [LogLevel.DEBUG] [FirestoreModel] [update] [We are here!]
I/flutter (22601): [2021-06-06 22:37:20] [LogLevel.INFO] [FirestoreModel] [update] [Transaction successful in 110929ms.]
Firebase completely ignores the timeout of 5 seconds; tries to update 4 times each time ~15 seconds apart and is finally successful after 110 seconds. I am after a real-time response within seconds (5 sec) or failure.
Future<void> update(
Map<String, dynamic> data, {
WriteBatch batch,
Transaction transaction,
bool forceServer = false,
}) async {
// If updating there must be an id.
assert(this.id != null);
// Only one of batch or transaction can be non-null.
assert(batch == null || transaction == null);
// When forcing to update on server no transaction or batch is allowed.
assert(!forceServer || (batch == null && transaction == null));
try {
if (forceServer) {
DateTime start = DateTime.now();
await FirebaseFirestore.instance.runTransaction(
(transaction) async {
await update(data, transaction: transaction);
Utils.logDebug('We are here!');
},
timeout: Duration(seconds: 5),
);
Utils.logDebug('Transaction successful in ${DateTime.now().difference(start).inMilliseconds}ms.');
} else {
DocumentReference ref =
FirebaseFirestore.instance.collection(collection).doc(this.id);
if (batch != null)
batch.update(ref, data);
else if (transaction != null)
transaction.update(ref, data);
else
await ref.update(data);
}
} catch (e, s) {
Utils.logException('Error updating document $id in $collection.', e, s);
// Propagate the error.
rethrow;
}
}

Answer 1

This requires the updates to be done on the server (or fail)
For that you could use Transactions and batched writes.
Transactions will fail when the client is offline.
Check out doc

Answer 2

To get live data from the server once, you would use:
firebase.firestore()
.doc("somecollection/docId")
.get({ source: "server" })
.then((snapshot) => {
// if here, snapshot.data() is from the server
// TODO: do something with data
})
.catch((err) => {
// if here, get() encountered an error (insufficient permissions, server not available, etc)
// TODO: handle the error
});
To get realtime live data from only the server (ignoring the cache), you would use:
const unsubscribe = firebase.firestore()
.doc("somecollection/docId")
.onSnapshot({ includeMetadataChanges: true }, {
next(snapshot) {
// ignore cache data
if (snapshot.metadata.fromCache) return;
// if here, snapshot.data() is from the server
// TODO: do something with data
},
error(err) {
// if here, onSnapshot() encountered an error (insufficient permissions, etc)
// TODO: handle the error
}
});
To write to the server, you would use the normal write operations - delete(), set(), and update(); as they all return Promises that will not resolve while the client is offline. If they have resolved, the data stored on the server has been updated.
To test if you are online or not, you can try and pull a non existant document down from the server like so:
/**
* Attempts to fetch the non-existant document `/.info/connected` to determine
* if a connection to the server is available.
* #return {Promise<boolean>} promise that resolves to a boolean indicating
* whether a server connection is available
*/
function isCurrentlyOnline() {
// unlike RTDB, this data doesn't exist and has no function
// must be made readable in security rules
return firebase.firestore()
.doc(".info/connected")
.get({ source: "server" })
.then(
() => {
// read data successfully, we must be online
return true;
}, (err) => {
// failed to read data, if code is unavailable, we are offline
// for any other error, rethrow it
if (err.code === "unavailable")
return false;
throw err;
}
);
}
/**
* A function that attaches a listener to when a connection to Firestore has
* been established or when is disconnected.
*
* This function listens to the non-existant `/.info/connected` document and
* uses it's `fromCache` metadata to **estimate** whether a connection to
* Firestore is currently available.
* **Note:** This callback will only be invoked after the first successful
* connection to Firestore
*
* #param {((error: unknown | null, isOnline: boolean) => unknown)} callback the
* callback to invoke when the isOnline state changes
* #return {(() => void)} a function that unsubscribes this listener when
* invoked
*/
function onOnline(callback) {
let hasConnected = false;
// unlike RTDB, this data doesn't exist and has no function
// must be made readable in security rules
return firebase.firestore()
.doc(".info/connected")
.onSnapshot(
{ includeMetadataChanges: "server" },
{
next(snapshot) {
const { fromCache } = snapshot.metadata;
if (!hasConnected) {
if (fromCache) return; // ignore this event
hasConnected = true;
}
callback(null, !fromCache);
},
error(err) {
callback(err);
}
}
);
}