Managing Firestore document read count in MV3 browser extension

Question

In the MV2 extension, I attach a Firestore onSnapshot listener in the persistent background page. As I understand it: 1. Firestore downloads all documents on first attaching of the listener, and afterwards, 2. only downloads the changed documents when they change. Since this listener persists over several hours, the total number of Firestore read counts (and hence the cost) is low.

But in the MV3 extension, the service worker (which houses the Firestore listener) is destroyed after five minutes. Therefore, the onSnapshot listener will be destroyed and re-attached several times in just a few hours. On every re-attachment, that listener would potentially re-download all of the user data. So, if the listener gets destroyed and attached five times, we incur five times as many document read counts (and hence the cost) in an MV3 extension as compared to an MV2 extension.

I'd like to understand:

1. Does using IndexedDB persistence help significantly reduce the document read counts? Even the when the service worker is restarted.

2. In case we are not using IDB persistence, how is the billing done? For example, the billing docs state that:

   Also, if the listener is disconnected for more than 30 minutes (for example, if the user goes offline), you will be charged for reads as if you had issued a brand-new query.

   If I re-attach the listener within 15-20 minutes, does it again incur document reads on all user data?

I tried writing a small example myself, and monitor the results on Cloud Console Monitoring to measure "Firestore Instance - Document Reads". However, I was not able to get clear results from it.

Note: For the purpose of discussion, I will avoid the workaround for making service-workers persistent, and focus on a worst case assuming that this workaround does not work.

Answer 1

1. Does using IndexedDB persistence help significantly reduce the document read counts? Even the when the service worker is restarted.

Yes. Upon a reconnect within the 30m interval, most documents can be read from the disk cache and won't have to be read from/on the server.

2. In case we are not using IDB persistence, how is the billing done? ... If I re-attach the listener within 15-20 minutes, does it again incur document reads on all user data?

If there is no data in the disk cache and no existing listener on the data, the documents will have to read from the server, and thus you will be charged for each document read there.