Firestore 事务在云功能中使用时表现出意外行为

Question

I am writing an app that features an inventory in which users can reserve products. I want to ensure that 2 users cannot simultaneously reserve a product at the same time, for this, I intend on using transactions. When using transactions from the Firebase SDK, everything works as intended, but I am getting unexpected behavior when using transactions from a callable cloud function. To simulate the use case where 2 users happen to reserve the same product, I use setTimeout in my cloud function to halt the function for 3 seconds. I am launching this function from 2 different clients with different user contexts.

export const reserveProduct = functions.https.onCall(async (data,context) => {

 function testTimeout(){

     return new Promise((resolve,reject) => {
         setTimeout(()=> {
           
           return resolve(true)
        },3000)
       })

 }

if(!context.auth){
    return {
        error: `You must be logged in to reserve products`
    }
}else{


    const productRef = admin.firestore().collection('products').doc(data.productID)
    const userRef = admin.firestore().collection('users').doc(context.auth.uid)

    return admin.firestore().runTransaction((transaction) => {
        return transaction.get(productRef).then(async(doc) => {
            if(doc.get('status') == 'reserved'){
                throw "Document already reserved!"
            }else{
                console.log("Product not reserved, reserving now!")
            }

            await testTimeout()

            transaction.update(productRef, {status: 'reserved'});
            transaction.update(userRef, {reserved: admin.firestore.FieldValue.arrayUnion(data.productID)})

            
        })
    }).then(() => {
        console.log("Transaction Successfully committed !")
    }).catch((error) => {
      
        throw "Transaction failed, product already reserved"
    })

    


}

After running this function call from 2 different clients simultaneously, The function call from my first client returns successfully as expected, but only after roughly 35s (which is way too long for the simplicity of the transaction). However, the second function call times out without returning any value. I have not seen any documentation explicitly stating the use of transactions in callable cloud functions, nor should it be affected when used within the emulator.

I am expecting to simply get a return value for whichever function call is able to modify the data first, and catch the error from the function which has retried and validated the reserved state.

Any help would be appreciated, thanks!

Answer 1

One major difference between the two places is in the way the SDKs used handle transactions:

• The client-side SDKs use an optimistic compare-and-set approach for transactions, meaning that they pass the values you read in the transaction with the data you're writing. The server then only writes the new data if the documents you read haven't been updated.

• The server-side SDKs (used in your Cloud Function) use a more traditional pessimistic approach for transactions, and place a lock on each document that you read in the transaction.

You can read more about database contention in the SDKs in the documentation.

While I'm not exactly certain how this is affecting your code, I suspect it is relevant to the difference in behavior you're seeing between the client-side and server-side implementations.