A smart contract is a program that is executed by the blockchain. It is similar to a stored procedure on a public distributed database. As such, it must ensure the consistency of the business logic it implements.
Smart contracts unleash the full potential of the blockchain because they enable the development of a new class of applications, called Decentralized Applications (DApps), which benefit from blockchain's strengths (decentralization, trust-less, immutability, governance by consensus in Tezos case, ...).
Structure
A smart contract possesses:
- an address
- a balance of currency
- a storage of data
- a code to implement the contract's business logic, structured as entrypoints to call
Contract storage, code and transactions (incoming and outcoming) are all publically available.
Business logic
Besides the storage data, the following information is available to the contract's business logic:
- when the contract is called
- who the contract is called by
- how much currency is transferred to the contract
- how much currency the contract owns (balance)
The contract can be programmed to send currency to an account or to another contract.
Cost
Originating (deploying) a smart contract has a cost which depends on the size of the code and the size of the initial storage.
Calling a smart contract has a cost which depends on:
- the complexity of the execution (number and nature of instructions executed by the program)
- the size of additional data it creates
- a constant fee
Currently on Tezos, the cost of origination is 0.000250 ꜩ per byte of data. The constant fee is the baker fee equal to 0.001189 ꜩ (it may be increased to increase transaction priority).
Note that once data storage is allocated to the contract, it does not decrease; if data has been removed by the contract, additional data does not require payment while total data size remains below allocated storage size.
Limits
The gas is the unit to measure code execution and storage allocation for any kind of transactions (origination, call to an entrypoint).
Currently on Tezos, the gas per transaction is limited to 1040000.