Transactions are signed messages originated by an externally owned account, transmitted by the Ethereum network, and recorded on the Ethereum blockchain. This basic definition conceals a lot of surprising and fascinating details. Another way to look at transactions is that they are the only things that can trigger a change of state, or cause a contract to execute in the EVM. Ethereum is a global singleton state machine, and transactions are what make that state machine “tick,” changing its state. Contracts don’t run on their own. Ethereum doesn’t run autonomously. Everything starts with a transaction.

In this chapter, we will dissect transactions, show how they work, and examine the details. Note that much of this chapter is addressed to those who are interested in managing their own transactions at a low level, perhaps because they are writing a wallet app; you don’t have to worry about this if you are happy using existing wallet applications, although you may find the details interesting!