CODEWITHSUNDEEP
google-cloud-firestore
Avinta
Avinta

Reputation: 768

General question: Firestore offline persistence and synchronization

I could not find detailed information in the documentation. I have several questions regarding the offline persistence of firestore.

I understood that firestore locally caches everything and syncs back once online. My questions:

  1. If I attach an onCompleteListener to my setDocument method it only fires when the device is online and has network access. But with offline persistence enabled, how can I detect that data has successfully been written to the cache (Is it always successful?!) - I see data is immediatly there without any listener ever triggering.

  2. What if I wrote data to the cache while the device is offline then comes back online and everything gets synched. What if now any sort of error happens (So the onSuccessListener would contain an error, but the persistence cache already has the data). How do I know that offline and online data are ALWAYS in sync once network connection is restored on all devices?

  3. What about race conditions? Lets say two users update a document at the "same time" while the device is offline. What happens once it comes back online?

But the most pressing question is: right now I continue with my programflow when the onSuccessListener fires, but it never does as long as the device is offline (showing an indefinete progress bar forever). I still need to continue with my program (thats why we have offline persistence) - How do I do this?

Upvotes: 0

Views: 141

Answers (1)

Frank van Puffelen
Frank van Puffelen

Reputation: 600006

How can I detect that data has successfully been written to the cache

This is the case when the statement that write the data has completed. If writing to the local cache fails, an exception is thrown from that write statement.

You second point is hard to summarize, but:

  • Firestore keeps the pending writes separate from the snapshots it returns for local reads, and will update the cached snapshot correctly both for successful and for rejected writes.
  • If you want to know whether the snapshot you read contains any pending writes, you can check the pendingWrites field in its metadata.

What about race conditions? Let's say two users update a document at the "same time" while the device is offline. What happens once it comes back online?

The last write wins. If that's not what you need, use security rules to enforce your requirements on the server.

Upvotes: 1

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