How should I handle this situation in FireBase where two users go for same Database key

Question

I´m saving items called Street using the FireBase push and get a list of Street items with auto generated ids right. Lets say each Street have a node like "path" : "unique value". Let´s say two users are about to insert a new Street that has the same "path" : "unique value".

What is an appropriate way to eliminate this from happening? It´s like a web-shop where user puts an item in the basket, and the system behind now have to reserve this item until user buy it.

Should I create a separate FireBase root like "reserved_streets" and let user add his reserved Street along with user.id until he buy it? Maybe have a Servlet instance running removing entries that are 10 min old.

The Street list can be very big, maybe 100 millions in rar cases

Answer 1

The typical way to prevent duplicate values in the Firebase Database is to convert those values into keys.

So if you current model is (next time, please share such a minimal snippet in your question please):

streets: {
  "-K1234567": {
    path: "unique path"
  },
  "-K1234568": {
    path: "unique path 2"
  }
}

You would add an additional structure (or change the existing structure) that uses the paths as keys:

pathsToStreets: {
  "unique path": "-K1234567",
  "unique path 2": "-K1234568"
}

With this structure there is guaranteed to be only one street for a path and you can use security rules that the path can't be overwritten.