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What does the code have access to other than the blockchain? Can it access the internet? The node's computer?

sillysaurus3 9y ago
This is a good question, and it's one I've wondered about. There's nothing stopping you from designing and deploying a blockchain similar to Ethereum that does allow access to those things. The problem is trust. It's helpful to remember that all the participants in an Ethereum network are adversarial. It doesn't matter how many bad actors there are as long as >50% of participants agree on the outcome. (This is a simplified explanation -- the real system is transitioning to proof of stake rather than proof of work.) If you allow solidity to access the internet or the system compilers, you can't guarantee determinism anymore. For example, if you increment a counter by the result of accessing https://some-value.com https://some-value.com (which returns a JSON value), you could program it to return a different result for everyone who accesses it. None of the nodes would agree what the true value of the counter should be. It'd also result in a central point of failure. These same problems exist if you let Solidity access the system software. You could program it to give different results, so no one would agree. The innovation of Solidity is that it's a set of operations where no matter what you do, the majority of nodes will converge to the same answer.
- binofbread 9y ago
  Would it not be possible to write a contract that has intentionally random results? Could you not use the timestamp or some other piece of data to flip a coin to determine the outcome? Does the EVM prevent this in some way? I did some searching but I'm not sure I know what to look for either.
j2kun 9y ago
From my limited understanding, a smart contract essentially runs in a VM with a dedicated memory space that is unique to that contract (and copied along with the contract). However, there is a limited ability for it to interact with other contracts via their (hard-coded) addresses. So a contract can, for example, listen for events occurring with a specific contract, and invoke itself based on the results. It can do this because parts of a contract's memory space are private (I think this is how it works), and so a contract can have secret keys and manage money. Moreover, a point that was not made clear to me when I first read about Ethereum, the programs are inherently limited in the number of steps that can be run, and the VM monitors this and terminates the program prematurely if it exceeds the limit. Moreover, longer programs cost the caller (the person or other contract asking the network to run this program) more money.
xrd 9y ago
You cannot access internet information from the evm. All data must be in the blockchain. Any transaction, ie smart contract execution, must be on immutable data inside the chain.
mpeg 9y ago
You can access external data from Ethereum contracts via what they call an "Oracle" which is an intermediary between a third party service and the blockchain. If the connection to the third party service is cryptographically secure, the Oracle can even provide proof the results were not tampered with, and if multiple data sources are used it can provide a consensus answer based on them. Otherwise, the smart contract only has access to what is in the blockchain, but Oracles are pretty easy to write and use.

This is a good question, and it's one I've wondered about. There's nothing stopping you from designing and deploying a blockchain similar to Ethereum that does allow access to those things. The problem is trust. It's helpful to remember that all the participants in an Ethereum network are adversarial. It doesn't matter how many bad actors there are as long as >50% of participants agree on the outcome. (This is a simplified explanation -- the real system is transitioning to proof of stake rather than proof of work.) If you allow solidity to access the internet or the system compilers, you can't guarantee determinism anymore. For example, if you increment a counter by the result of accessing https://some-value.com https://some-value.com (which returns a JSON value), you could program it to return a different result for everyone who accesses it. None of the nodes would agree what the true value of the counter should be. It'd also result in a central point of failure. These same problems exist if you let Solidity access the system software. You could program it to give different results, so no one would agree. The innovation of Solidity is that it's a set of operations where no matter what you do, the majority of nodes will converge to the same answer.

binofbread 9y ago
Would it not be possible to write a contract that has intentionally random results? Could you not use the timestamp or some other piece of data to flip a coin to determine the outcome? Does the EVM prevent this in some way? I did some searching but I'm not sure I know what to look for either.

j2kun 9y ago

From my limited understanding, a smart contract essentially runs in a VM with a dedicated memory space that is unique to that contract (and copied along with the contract). However, there is a limited ability for it to interact with other contracts via their (hard-coded) addresses. So a contract can, for example, listen for events occurring with a specific contract, and invoke itself based on the results. It can do this because parts of a contract's memory space are private (I think this is how it works), and so a contract can have secret keys and manage money. Moreover, a point that was not made clear to me when I first read about Ethereum, the programs are inherently limited in the number of steps that can be run, and the VM monitors this and terminates the program prematurely if it exceeds the limit. Moreover, longer programs cost the caller (the person or other contract asking the network to run this program) more money.

xrd 9y ago

You cannot access internet information from the evm. All data must be in the blockchain. Any transaction, ie smart contract execution, must be on immutable data inside the chain.

mpeg 9y ago

You can access external data from Ethereum contracts via what they call an "Oracle" which is an intermediary between a third party service and the blockchain. If the connection to the third party service is cryptographically secure, the Oracle can even provide proof the results were not tampered with, and if multiple data sources are used it can provide a consensus answer based on them. Otherwise, the smart contract only has access to what is in the blockchain, but Oracles are pretty easy to write and use.