Anatomy of a Smart Contract
Smart contracts are programs which are immutably stored in the chain.
The logical structure of IOTA Smart Contracts is independent of the VM type you use, be it a Wasm smart contract or any other VM type.
Identifying a Smart Contract
Each smart contract on the chain is identified by its name hashed into 4 bytes
and interpreted as
uint32 value: the
For example, the
hname of the root contract is
0xcebf5908, the unique identifier of the
root contract in every chain. The exception is the
_default contract which always has
Each smart contract instance has a program with a collection of entry points and a state. An entry point is a function of the program through which the program can be invoked.
Depending on the type of the entry point there are several ways to invoke an entry point: a request, a call and a view call
The smart contract program can access its state and account through an interface layer called the Sandbox.
The smart contract state is its data, with each update stored on the chain. The state can only be modified by the smart contract program itself. There are two parts of the state:
- A collection of key/value pairs called the
data state. Each key and value are byte arrays of arbitrary size (there are practical limits set by the database, of course). The value of the key/value pair is always retrieved by its key.
- A collection of
color: balancepairs called the
account. The account represents the balances of tokens of specific colors controlled by the smart contract. Receiving and spending tokens into/from the account means changing the account's balances.
Only the smart contract program can change its data state and spend from its account. Tokens can be sent to the smart contract account by any other agent on the ledger, be it a wallet with an address or another smart contract.
See Accounts for more info on sending and receiving tokens.
There are two types of entry points:
- Full entry points(or just entry points): These functions can modify (mutate) the smart contract's state.
- View entry points(or views): These are read-only functions. They are only used to retrieve the information from the smart contract state. They cannot modify the state, i.e. are read-only calls.
After a request to a Smart Contract is executed (a call to a "full entry point"),
receipt will be added to the
detailing the execution results of said request: whether it was successful, the block it was
included in, and other information. Any events dispatched by the Smart Contract in context of
this execution will also be added to the BlockLog.
When a smart contract execution is interrupted for some reason (exception), or it produces an
error (missing parameter, or other inconsistency), the funds will be refunded to the caller,
except the fees. Any error that resulted from the SC execution can be viewed on the contract
receipt (present in the