Firebase Firestore Read Costs - Clarification

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I am using Firestore DB for an e-commerce app. I have a collection of products, each product has a document that has a "title" field and "search_keywords" field. The search keyword field stores an array. For example, if the title="apple", then the "search_keywords" field would store the following array: ["a","ap","app","appl","apple"]. When the user starts typing "apple" in the search box, I want to show the user, all products where "search_keywords" contains "a", then when they type the "p", I want to show all products where search keywords contain "ap"...and so on. Here is the snippet of code that gets called each time an additional letter is typed:

firebaseFireStore.collection("Produce").whereArrayContains("search_keywords", toSearch).get()

For example, in every case, the documents that would be returned on each successive call where an additional letter was typed would be a subset of what was returned in the previous call - it would just be a smaller list of documents - documents that were read on the previous query. My question is since the documents retrieved on a successive query are a subset of those retrieved in a prior query, would I be charged reads based on how many documents each successive query returns, or would Firestore have them in the cache and read them from there since the successive result set is a subset of a prior result set. This question has been on my mind for a while and every time I search for it, I can't seem to find a clear answer. For example, based on my research, the following two posts on Stackoverflow have involved similar questions and the following are relevant quotes from there, but they seem to contradict each other because @AlexMamo says "it will always read the online version of the documents...[when online]" and @Doug Stevenson says "if the local persistence is enabled on your client (it is by default) and the documents haven't been updated in the server...[it will get them from the cache]". I would appreciate any clarification on this if anyone knows the answer. Thanks.

"If the OP has offline persistence enabled, which is by default in Cloud Firestore, then he will be able to read the cache only while offline. When the OP has internet connectivity, it will always read the online version of the documents." – Alex Mamo (https://stackoverflow.com/a/69320068/14556386)

"According to this answer by Doug Stevenson, the reads are only charged when performed upon the server, not your local cache. That is if the local persistence is enabled on your client (it is by default) and the documents haven't been updated in the server." (https://stackoverflow.com/a/61381656/14556386)

EDIT: In addition, if for each product document that was retrieved by the Firestore search, I download its corresponding image file from Firebase Storage. Would it charge me for downloading that file on successive attempts to download it or would it recognize that I had previously downloaded that image and fetch it from cache automatically?

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1 Answers

First of all, storing ["a", "ap", "app", "appl", "apple"] into an array and performing an whereArrayContains() query, doesn't sound like a feasible idea. Why? Imagine you have a really big online shop with 100k products, in which 5k start with "a". Are you willing to pay 5k reads every time a user types "a"? That's a very costly feature.

Most likely you should return the corresponding documents when the user types, for example, two, or even three characters. You'll reduce costs enormously. Or you might take into consideration using the solution I have explained in the following article:

Let's go forward.

For example, in every case, the documents that would be returned on each successive call where an additional letter was typed would be a subset of what was returned in the previous call, it would just be a smaller list of documents.

Yes, that's correct.

My question is since the documents retrieved on a successive query are a subset of those retrieved in a prior query, would I be charged reads based on how many documents each successive query returns?

Yes. You'll always be charged with a number of reads that is equal to the number of documents that are returned by your query. It doesn't matter if a query was previously performed, or not. Every time you perform a new query, you'll be charged with a number of reads that is equal to the number of documents you get.

For example, let's assume you perform this query:

.whereArrayContains("search_keywords", "a")

And you get the 100 documents, and right after that you perform:

.whereArrayContains("search_keywords", "ap")

And you get only 30 documents, you'll have to pay 130 reads, and not only 100. So it doesn't matter if the documents that are returned by the second query are a subset of the documents that are returned by the first query.

Or would Firestore have them in the cache and read them from there since the successive result set is a subset of a prior result set.

No, it won't. It will read those documents from the cache only if the user losses the internet connectivity, otherwise it will always read the online versions of the documents that exist on the Firebase servers. The cached version of the documents works only when the user is offline. I have also written an article on this topic called:

In Doug's answer:

He clearly says:

You are charged for the number of documents read on the server every time you call get().

So if you called get(), you have to pay as reads, the number of documents that are returned.

The following statement is available:

If local persistence is enabled in your client (it is by default), then the documents may come from the cache if the documents are also not changed on the server.

When you are listening for real-time updates. According to the docs:

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.

And I would add, if nothing has changed, you don't have to pay anything. Again, according to the same docs:

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.

So if the listener is active, you always read the documents from the cache. Bear in mind that a get() operation is different than listening for real-time updates.

if for each product document that was retrieved by the Firestore search, I download its corresponding image file from Firebase Storage. Would it charge me for downloading that file on successive attempts to download it or would it recognize that I had previously downloaded that image and fetch it from cache automatically?

You'll always be charged if you download the image over and over again unless you are using a library that helps you cache the images. For Android, there is a library called Glide:

Glide is a fast and efficient open-source media management and image loading framework for Android that wraps media decoding, memory and disk caching, and resource pooling into a simple and easy-to-use interface.

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