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I wouldn't say the difference is exactly that clear cut. Presence is a clear example of a situation where you want to use Realtime Database over Cloud Firestore. And you're right, for something that you would otherwise do "in memory" or use memcached/redis/etc Cloud Firestore may not be the right fit.

In my experience both databases are totally appropriate for a chat app. Even though in Cloud Firestore you will pay a document write for each new chat message, that's only $1.80 for a million chat messages and you get all the rich querying from the Cloud Firestore API.

Regarding your pricing question: https://firebase.google.com/docs/firestore/pricing#operation...

> When you listen to the results of a query, you are charged for a read each time a document in the result set is added or updated. You are also charged for a read when a document is removed from the result set because the document has changed. (In constrast, when a document is deleted, you are not charged for a read.)

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