Smart contract modules

Smart contracts are deployed on the chain in smart contract modules.


A smart contract module is often referred to simply as a module.

A module can contain one or more smart contracts, allowing code to be shared among the contracts and can optionally contain contract schemas.

digraph G { subgraph cluster_0 { node [fillcolor=white, shape=note] label = "Module"; "Crowdfunding"; "Escrow"; } }

A smart contract module containing two smart contracts.

The module must be self-contained, and only have a restricted list of imports that allow for interaction with the chain. These are provided by the host environment and are available for the smart contract by importing a module named concordium.

See also

Check out Contract host functions for a complete reference.

On-chain language

On the Concordium blockchain the smart contract language is a subset of Web Assembly (Wasm in short) which is designed to be a portable compilation target and to be run in sandboxed environments. This is useful because smart contracts will be run by bakers in the network who do not necessarily trust the code.

Wasm is a low-level language and it is impractical to write by hand. Instead one can write smart contracts in a more high-level language which is then compiled to Wasm.


The blockchain environment is very particular in the sense that each node must be able to execute the contract in exactly the same way, using exactly the same amount of resources. Otherwise nodes would fail to reach consensus on the state of the chain. For this reason smart contracts need to be written in a restricted subset of Wasm.

Floating point numbers

Although Wasm does have support for floating point numbers, a smart contract is disallowed to use them. The reason for this is that Wasm floating-point numbers can have a special NaN (“not a number”) value whose treatment can result in nondeterminism.

The restriction applies statically, meaning that smart contracts cannot contain floating point types, nor can they contain any instructions that involve floating point values.


Deploying a module to the chain means submitting the module bytecode as a transaction to the Concordium network. If valid this transaction will be included in a block. This transaction, as every other transaction, has an associated cost. The cost is based on the size of the bytecode and is charged for both checking validity of the module and on-chain storage.

The deployment itself does not execute smart contract. To execute, a user must first create an instance of a contract.

See also

See Smart contract instances for more information.

Smart contract on the chain

A smart contract on the chain is a collection of functions exported from a deployed module. The concrete mechanism used for this is the Web Assembly export section. A smart contract must export one function for initializing new instances and can export zero or more functions for updating the instance.

Since a smart contract module can export functions for multiple different smart contracts, we associate the functions using a naming scheme:

  • init_<contract-name>: The function for initializing a smart contract must start with init_ followed by a name of the smart contract. The contract must consist only of ASCII alphanumeric or punctuation characters, and is not allowed to contain the . symbol.

  • <contract-name>.<receive-function-name>: Functions for interacting with a smart contract are prefixed with the contract name, followed by a . and a name for the function. Same as for the init function, the contract name is not allowed to contain the . symbol.


If you develop smart contracts using Rust and concordium-std, the procedural macros #[init(...)] and #[receive(...)] set up the correct naming scheme.