CODEWITHSUNDEEP
node.jsdatabasemongodb
Fun Programmer
Fun Programmer

Reputation: 1082

How to order database calls in node.js?

I have a realtime app where users are clicking on a button at the exact same time. It is a rideshare app where a ride will show up on all the users's screens, and then 2 users will basically simultaneously push the "accept ride" button. This creates problems because it saves the first user onto the ride via a database save call, but then the second user oversaves the first user. Once a user accepts the ride, another user should not be able to accept the ride. It creates major problems because the first user should just "accept the ride" while, with the second driver, it should just tell them "another driver is accepting the ride". The problem is that I can not even run a query to check if the ride already has a driver because this is all happening so quickly. The first user will hit "accept ride", then it will save them to the ride. The second user will hit "accept ride", and it will check if the ride already has a driver. It doesn't yet because the first save is still finishing. Then the second user is oversaved on the ride. It just is happening way to simultaneously where queries don't really solve the problem.

Sorry if this is a confusing explanation. I have never had to deal with this realtime of a problem, so I am not sure where to start. I think I need to build some queue or something that only lets this happen once at a time. Any direction of what to even google would be helpful. Thank you! My backend is written in node.js and I use MongoDB on Heroku.

Upvotes: 0

Views: 102

Answers (1)

jfriend00
jfriend00

Reputation: 707466

You need an atomic check and set operation in your database so in one atomic database operation, you can verify that it's not already accepted and, if not, accept it. That will only allow one person to accept it, any others will fail because it's already accepted and the API can feed that back to the user interface. The key word here is "atomic" and how you achieve it depends upon the specific database. For MongoDB, see Mongo any way to do atomic check and set.

Here's another reference: Help writing an atomic update in mongodb.

These solutions use mongodb's findandmodify so presumably, you would attempt to find a document with this id and that is also not accepted and if found, you would modify it to be accepted. Then, since the findandmodify is atomic, nobody else can get in between your find and your modify so when their findandmodify gets a turn, they won't find a document that is both the right id and is not accepted because someone else accepted it before they got in.

Upvotes: 1

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