Theory Of A Weird Thought - Forms Submission

Question

In theory, if you were to open two computers that were perfectly synced together on a website that has a form. This form has fields where say for example the username has to be unique. Assuming both computers have the same information on the form, and in theory let's say that the submit button was pressed at the same time, and that these two computers have the exact same build and internet speed and the same response time from the server, whose information would be submitted to the database and whose information would be denied knowing the username field is unique.

Answer 1

Given that both transactions reach the DBMS simultaneously, the database concurrency control mechanism would guarantee isolation of the transactions, in effect choosing one transaction to precede the other. Given the unique constraint of the username, the second would be rejected.

UPDATE:

In effect, avoiding / dealing with concurrency is done at the very lowest level, not even at the DBMS level, but at the CPU level. The instruction pointer is exactly that: it points at a single instruction. Even though instruction blocks (and thus, effectively, 'concurrent' database transactions) are interleaved at execution time, one will always precede the other or be committed first, causing the other to be forcefully rolled back. For completeness:

Concurrent Transaction Processing

When two transactions are being processed against a database at the same time, they are termed concurrent transactions. Although it may appear to the users that concurrent transactions are being processed simultaneously, this cannot be true because the CPU of the machine processing the database can execute only one instruction at a time. Usually, transactions are interleaved, which means that the operating system switches CPU services among tasks so that some portion of each transaction is carried out in a given interval. This switching among tasks is done so quickly that two people seated at browsers side by side, processing the same database, may believe that their two transactions are completed simultaneously; in reality, however, the two transactions are interleaved.

Kroenke & Auer, Database Processing Fundamentals: Design, Implementation (12th Ed.), Part 4 Multiuser Database Processing, p.342

Answer 2

The winner would essentially be random. Whichever request is received first will win.

The definition of "received" could mean a lot of things. If we assume that your server processes all requests in the order that the network packets were received, and there are no software bugs, then once a packet enters the server's receive queue it is considered "received".

In the way you phrased your question, the packets will take different routes to the server and one will inevitably end up first. However even if they both arrive at the same time, one will "win" and be processed first.

Read up on the OSI model for an understanding on how networks function. Once you get down to the physical layer, the winner is whichever electron get access to the network interface first.