Firebase single value listener calls server even when local cache is available and has not changed

Question

My Firebase database has setPersistenceEnabled=true. When offline all the listeners work fine and fetch data from the local cache.

But, when online, Firebase is making a network call each time I use a addListenerForSingleValueEvent on the same node, even though I have a local cache and nothing has changed on the server.

I have two items at the node I'm attaching the listener to, and I see the following reported after setting log level to debug. This network activity is repeated every time I use the listener while online, even within sub second delays. My understanding is that if Firebase has local data, then server calls won't be made. Any sync checks might happen in the background and with efficient network usage.

conn_18 - received data message: {r=20, b={s=ok, d={}}}
conn_18 - received data message: {r=21, b={s=ok, d=}}

I am trying to aggressively reduce network usage on my app, and any suggestion to stop/reduce Firebase network activity would be very helpful.

Iskeraet · Accepted Answer · 2021-04-07 13:54:34Z

Been digging into this myself.

As far as I can tell, the second call you are experiencing is simply the network saying "Hey, there's no updates", and you're not charged for any reads.

As far as reducing the network activity, I think this is just how it works, and how you want it to work (its the actual listening part of the listener). If you don't want that, use a getDocument call, rather than a listener.

This is based on a few things:

When you look at snapshot.metadata.isFromCache (iOS SDK) for a query, the call will come twice. first from cache and then from the server. However, if you print a statement under snapshot.documentChanges, ONLY the cached call will print.
I did an experiment re-loading a view in iOS ~20 times. I also made on small change to my data. I then waited 5-10 minutes and looked at the usage on: https://console.cloud.google.com/firestore/usage. My total read count went up by 3, which means I was only charged for the update. Reloading the view a bunch of times was NOT charged.

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