Running an external command upon transaction confirmation

Question

I was wondering if it were possible to launch an external command (system call) when a transaction sent by a node is confirmed?

For example if node A runs pyethapp (in the context of a private blockchain) as:

$ pyethapp -c eth.network_id=1337 -c node.privkey_hex=091bd6067cb4612df85d9c1ff85cc47f259ced4d4cd99816b14f35650f59c322 -b 'enode://288b97262895b1c7ec61cf314c2e2004407d0a5dc77566877aad1f2a36659c8b698f4b56fd06c4a0c0bf007b4cfb3e7122d907da3b005fa90e724441902eb19e@0.0.0.0:30303' -c eth.genesis_hash=283bd9430c5f3114872f93beefe99d6626980b3a4a18a44ddd27749cd89688f2 -m 50 run

and then node B connects to it through pyethapp:

$ pyethapp -c eth.network_id=1337 -b 'enode://288b97262895b1c7ec61cf314c2e2004407d0a5dc77566877aad1f2a36659c8b698f4b56fd06c4a0c0bf007b4cfb3e7122d907da3b005fa90e724441902eb19e@192.168.2.12:30303' -c eth.genesis_hash=283bd9430c5f3114872f93beefe99d6626980b3a4a18a44ddd27749cd89688f2 run

and sends a transaction from an account on its end to one on A's end can it take action (initiate a system call) once the transaction is confirmed?

Answer 1

I think this is a good question, but it never got an answer, so I'm answering it.

There's two ways to do this, I think.

First would be to modify the source code of the node in such a way that nothing changes except immediately after the finalization of a transaction it would fire off a system call. This would require you to download, modify, and build the node code however. That may or may not appeal to you. I wouldn't do it this way.

Another way would be to write a web3 script to listen for events (described here: https://github.com/ethereum/wiki/wiki/JSON-RPC#eth_newblockfilter) and respond to changes that you're interested in. The web3 script would be running off-chain, so you can do anything including make system calls.

There is no way to make a system call from inside a Solidity program, if that's what you're asking.