Observe Single event method returning cached data

[spandana-batchu]

[READ] Step 1: Are you in the right place?

Yes

[REQUIRED] Step 2: Describe your environment

• Xcode version: 8.3
• Firebase SDK version: 3.1
• Library version:
• Firebase Product: database

[REQUIRED] Step 3: Describe the problem

We are facing the below issue on our iOS app that uses firebase iOS SDk.
To give you some context, the users of our app are part of Groups. Each of these groups have some group information.
In the database we have user who is mapped with a list of group IDs. Using these Group Ids Further we fetch group’s info. Below is the structure.

+User1
    +GroupID1
        +timestamp
    +GroupID2
        +timestamp
+GroupID1
    +groupName:
    +groupCount:……
    +timestamp

Whenever there is a change in group’s info, we have a firebase cloud fuction that updates the timestamp at both the places. Hence any change to group’s info would notify the observers at both references database\GroupID1 and database\User1
The issue we are facing is that we have a continuous observer at reference database\User1 using the method observe event type.
Changes to group info is notifying that continuous observer as expected. After which we are fetching group info using observe single event method. But that group info returned is the cached info.
The info returned at reference database\GroupID1 is always one step old data.
Is there anything we are missing here.

Relevant Code:

observer at reference database\User1 :

observe(_ eventType: .value, with block: @escaping (FIRDataSnapshot) -> Swift.Void, withCancel cancelBlock: ((Error) -> Swift.Void)? = nil) -> UInt

observer at reference database\ GroupID1 :

observeSingleEvent(of eventType: .value, andPreviousSiblingKeyWith block: @escaping (FIRDataSnapshot, String?) -> Swift.Void)

[schmidt-sebastian]

Thanks for sending over this issue. While I understand that this can be frustrating, this is expected behavior. Single even observers will return data from the cache if there is data available. This allows us to fire them immediately. If you use .value observers, you will receive updated data in its second call.

[cbess]

I suppose this makes sense. But, is there a way to do have it pull from the server immediately/initially, without having to call it twice? Am I suppose to use the non-single event methods, then cancel the observer manually?

Also, I would say the documentation should be updated.

[MarksCode]

Any update?

[paulb777]

@MarksCode What kind of update are you looking for? This issue was closed as "expected behavior". stackoverflow is a better place to ask usage questions.

[quanvo87]

Is there a better way of doing this? I've noticed that when using a cache and attempting a singleEvent query, it can take 55-60 attempts before fresh data is returned.

[morganchen12]

@quanvo87 load time is going to depend on the size of the data you're loading and the quality of your internet connection. Are you observing one minute load times for small queries on good internet?

[quanvo87]

@morganchen12 When testing this, I was querying an endpoint that returned a small String. My connection is 15 ping 50/10 down/up. I could probably be more clear--by 55-60 attempts, what I mean is I did a ref.observeSingleEvent, and in the callback, if !snapshot.exists(), print something and repeat the function, until it finally got back data. The print statement executed 50+ times each test run. This is with persistence enabled. With persistence disabled, it just fires once.

[morganchen12]

Oh, I misread your last comment.

With the current API, observing a single event with persistence enabled doesn't let you specify whether you're trying to retrieve cached or server data. The SDK defaults to returning cached data, but this isn't useful if you're trying to retrieve server data. For the time being you should use value listeners instead like Sebastian mentioned above, but this does seem like a non-feature.

[Rahish]

Thanks for sending over this issue. While I understand that this can be frustrating, this is expected behavior. Single even observers will return data from the cache if there is data available. This allows us to fire them immediately. If you use .value observers, you will receive updated data in its second call.

Why in the Second call? As we expect refreshed data whenever we apply the observer to the Database Reference. I tried below code too, its also not refreshing the data
weak var weakSelf = self
valueRef = dbRef?.observe(.value) { (snapShotData) in

            if singleValueObserver
            {
                weakSelf?.dbRef?.removeObserver(withHandle: weakSelf?.valueRef ?? 0)
            }
            weakSelf?.dataFetched(snapShotData)
        }