What happens when a smart contract gets several similar calls in the same block?

Question

What happens when a smart contract gets several similar calls in the same block?

Consider for instance this greeter:

contract greeter {
    string name;

    function setName(string name_) {
        name = name_;
    }

    function getName() constant returns(string) {
        return name;
    }
}

What happens if Bob, Alice and Eve all call the Greeter at the same time and all three calls are in the same block? Does everyone get the proper response with their own name? Does the contract get run three times?

Answer 1

What happens if Bob, Alice and Eve all call the Greeter at the same time and all three calls are in the same block?

The three transactions would get called, not necessarily in sequence, and the last transaction's name will be persisted in the mined block.

Does everyone get the proper response with their own name?

No. The getName() method would return the persisted name in the mined block - this is set by the last transaction.

Does the contract get run three times?

Yes. The setName() would be run three times.

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The Details

Referring to What is the order and concurrency behavior of multiple calls to a contract in a single transaction?:

• The winning miner of the block gets to decide the order that the transactions are included in the block.
• Transactions from different addresses can be executed in an arbitrary sequence.
• Transactions from the same address will always be executed in order of their transaction nonces.

So:

• The virtual machine would call setName(...) three times, but the sequence of which transaction gets executed first is arbitrary assuming that the transactions have different From: addresses (different accounts).
• The name field would be changed three times but the name in the final transaction would be persisted in the mined block.
• Calls to getName() will return the name persisted in the mined block, after the block has been mined.
• Calls to getName() from a smart contract executed in the same block would get the getName() value from any previously run transaction of the Greeter contract. Thanks @Tjaden for clarifying this issue.

Here is an example of the last two points above:

• Transactions are sent in the order Alice, Bob and Eve
• The winning miner sequences the transaction in the order Eve, Bob and Alice
• The smart contract runs between the transaction of Bob and Alice
• The smart contract's call to getName() would return Bob.
• After the block has been mined, the name persisted in the block is Alice. Calls to getName() will now return Alice.

Answer 2

Short Answer: Both transactions will be executed in the same block. Important: The miner decide the order of the transactions within a block.

Expanded: In Ethereum, every transaction triggers a code execution that updates the entire world state. For each transaction in a block, the output of the world state of the n-th transaction becomes the input state of the n-th + 1 transactions. There is no assumption regarding the order of transaction execution within a block. When a block is mined, it's the miner that decide the order. No contract should rely on transactions ordering in it's code execution.