php - Avoiding Double Booking / Double Spending

Solution:

You can use distributed locks to solve the double-spending /double booking issue. You need to acquire at least three locks that would update

• User's budget i.e. bidder's account balance
• Seller's account balance
• Auction

You can also create a bid ledger to verify the user's spending, seller account balance, and user's wallet balance. You can run a cron job hourly or every 10 minutes to verify any error and notify support/dev to check for potential errors.

Your logic could be like this

Procedure BidProcessor
   input: user, bid, seller
   // this should get locks for auction, user wallet balance and seller balance
   success = acquireLocks( bid.auction_id, user.id, seller.id )
   if success then
      hasSufficientFund = user.hasSufficientFund( bid.amount )
      if hasSufficientFund then
          ExecuteBid( user, bid, seller )
          releaseLocks( bid.auction_id, user.id, seller.id )
      else 
          releaseLocks( bid.auction_id, user.id, seller.id )
      endif
   else
      releaseLocks( bid.auction_id, user.id, seller.id )  
   endif
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Each executor can use their identity id as a lock value to avoid releasing someone else lock.

Procedure ExecuteBid
    input: user, bid, seller
    Description: Run a SQL transaction to update all related entities 
      START TRANSACTION;
        UPDATE user SET budget=budget-bidAmount WHERE id= user.id;
        UPDATE seller SET balance=balance+bidAmount WHERE ID = seller.id;
        UPDATE auction SET winner=bid.id, closed_at=NOW() WHERE id = bid.auction_id;
        UPDATE bid SET processed_at = NOW(), status='WON' WHERE id = bid.id;
      COMMIT;
      if commit fails then do a rollback  
        ROLLBACK;
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