/**
* Transfer resources between two stocks. This method uses compensation to recover from race
* conditions instead of locking to avoid them altogether. Arguably, this scheme provides better
* concurrency performance.
*
* @param buyer
* @param seller
* @return the amount transfered
*/
@Override
protected int transfer(ResourceStock buyer, ResourceStock seller) {
while (true) {
int wanted = buying.count(buyer);
int available = selling.count(seller);
int transfer = Math.min(available, wanted);
if (transfer == 0) {
return 0;
}
boolean procured = selling.setCount(seller, available, available - transfer);
if (procured) {
boolean sent = buying.setCount(buyer, wanted, wanted - transfer);
if (sent) {
try {
seller.remove(transfer);
} catch (RuntimeException e) {
selling.add(seller, transfer);
buying.remove(buyer, transfer);
continue;
}
try {
buyer.add(transfer);
} catch (RuntimeException e) {
// market takes care of what was sold.
// To fully solve this case, 2PCommit or 3PC is
// required. Is it worth?
buying.remove(buyer, transfer);
continue;
}
logTransfer(seller, buyer, transfer);
return transfer;
} else {
// Compensation. restore what we took from the order!
selling.add(seller, transfer);
}
}
// Reaching here mean we hit a race condition. Try again.
}
}
