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https://firebase.google.com/docs/firestore says:

Like Realtime Database, Cloud Firestore uses data synchronization to update data on any connected device.

I'm trying to figure out what this "data synchronization" consists of.

https://firebase.google.com/docs/firestore/manage-data/enable-offline says:

When persistence is enabled, Cloud Firestore caches every document received from the backend for offline access.

Does Cloud Firestore update that cache whenever changes happen in the back end? For example, if I have a certain document in my cache, does the document update (a) automatically when that document is changed in the back end or (b) whenever I do a query that uses that document, and I happen to be online at the time?

UPDATE

I'm going to restate my last paragraph, because it's been misunderstood by at least one person.

Imagine the following scenario:

  • I have a smartphone. To make things easy, assume the smartphone is online all the time.
  • Installed on this smartphone is a mobile app that contains the Firestore SDK.
  • The app has enabled the Firestore cache.
  • The cache contains, say, 1000 documents.

Now my question is: if one of these documents changes in the cloud (not on the smartphone), under what circumstances will the Firestore SDK update the document in the cache? Will it (a) update the document automatically, soon after the change on the cloud, or (b) update the document the next time the mobile app does a query that uses the document?

Bluebeard
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  • While I don't have any link at hand, I'm pretty sure that I have heard multiple times that B) is what happens. For example each firestore uptate on the client side counts as billable read. Thus you need to have a way to control that. I think it all makes sense that there's no magic there. Only if you do the query (or perhaps listen to the on change listener) the cache gets updated. – damienix Jun 20 '19 at 08:53

2 Answers2

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Both of the highlighted paragraphs are in my opinion straightforward explanations, and to answer your questions:

Does Firestore update that cache whenever changes happen in the back end?

Yes, if you are offline and some documents are changed, once you are back online, you get all changed documents and vice versa, if you change some documents locally (in your cache), when you're back online, all changed documents are sent to the Firebase servers.

If you have a query that returns 10 documents, it means that you are in sync with 10 documents, which basically means that if of those 10 documents is changed, your listener is triggered only for that change.

does the document update (a) automatically when that document is changed in the back end

Yes, first of all in your cache and once you are back online, also on Firebase servers.

whenever I do a query that uses that document, and I happen to be online at the time?

Whenever you are online and a document is changed, you are notified in realtime.

Edit:

if one of these documents changes in the cloud (not on the smartphone), under what circumstances will the Firestore SDK update the document in the cache? Will it (a) update the document automatically, soon after the change on the cloud, or (b) update the document the next time the mobile app does a query that uses the document?

As long as you are in sync with those documents, it happens in the second moment the document is changed. What I mean through in sync is, that you are using in your code a get() call to get those documents or if you are listening in realtime for documents changes. Simply creating a reference to a document or a query without using a listener, you aren't notified in any way.

update the document the next time the mobile app does a query that uses the document?

Is not correct since you are online and in sync with the documents, the listener is triggered instantly.

Edit2:

In short, a listener is called everytime a change in a document occurs. "Next time the app does the query" is not correct sentence because when you create a query and attach a listener, you are always in sync with the database and when a document is changed you are immediately notified. It is not like, the documents are chainging in the database and when I query (at some point of time), I get the changes, no, you are always notified as the changes occur. That's the beauty of a realtime database, to be notified as the changes are happening :)

Alex Mamo
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  • Sorry, but I think you misunderstood my question. Please see the update that I've appended to the question. – Bluebeard Jun 20 '19 at 13:37
  • Sorry, I still don't understand. (Possibly relevant: I've never written a Firestore client; I'm just evaluating Firestore for a project and I've read some of the docs.) Thanks anyway for trying. – Bluebeard Jun 20 '19 at 15:26
  • Please take a look again at my updated anwer, are you ok now? – Alex Mamo Jun 20 '19 at 15:40
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If you have the local cache enabled, Firestore automatically stores any documents it receives in that local cache.

This only happens when your app is requesting the documents by calling get() or onSnapshot. Firestore does not automatically request the documents on your behalf. So if your app doesn't request the document, the cache will not be updates.

Frank van Puffelen
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    Note that automatic caching is only on by default for Android and iOS apps. For web apps (and I'm assuming other clients) you need to call the enablePersistence() method. – cjcurrie Nov 04 '20 at 05:18