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Build a contract schema#

This guide will show you how to build a smart contract schema, how to export it to a file, and/or embed the schema into the smart contract module, all using cargo-concordium.

Note

Schemas are used by off-chain tools to represent byte data in a more human-readable manner.

Preparation#

First, ensure you have cargo-concordium installed and if not the guide Install tools for development will help you.

You also need the Rust source code of the smart contract you wish to build a schema for.

Setup the contract for a schema#

In order to build a contract schema, you first have to prepare our smart contract for building the schema.

You can choose which parts of the smart contract to include in the schema. For each init function, you can choose to include a schema for the parameter, the errors, and/or the events. And for each receive function, you can choose to include a schema for the parameter, the return value, and/or the errors.

Every type you want to include in the schema must implement the SchemaType trait. This is already done for all base types and some other types (see list of types implementing the SchemaType). For most other cases, it can also be derived automatically, using #[derive(SchemaType)]:

#[derive(SchemaType)]
struct SomeType {
    ...
}

Implementing the SchemaType trait manually only requires specifying one function (get_type()), which is a getter for a schema::Type, which essentially describes how this type is represented as bytes and how to represent it as JSON.

For example, the Cis2 library implements the SchemaType trait manually for a few types that can not derive their SchemaType from base types. The example below explores the manual SchemaType trait implementation of the Cis2Event<T, A>:

impl<T: IsTokenId, A: IsTokenAmount> schema::SchemaType for Cis2Event<T, A> {
   fn get_type() -> schema::Type {
      let mut event_map = BTreeMap::new();
      event_map.insert(
         TRANSFER_EVENT_TAG,
         (
            "Transfer".to_string(),
            schema::Fields::Named(vec![
               (String::from("token_id"), T::get_type()),
               (String::from("amount"), A::get_type()),
               (String::from("from"), Address::get_type()),
               (String::from("to"), Address::get_type()),
            ]),
         ),
      );

      ...

      schema::Type::TaggedEnum(event_map)
   }
}

Note

Event schemas can be represented by the TaggedEnum Type (see list of types available to implement a schema). The first byte of the event data logged includes a tag to distinguish the different events on-chain. For example, the TRANSFER_EVENT_TAG is 255 (u8::MAX) which is the key used in the above BTreeMap to store the Transfer event schema. The value of the Transfer event in the above BTreeMap is a tuple consisting of a String (named Transfer) and named Fields (token_id, amount, from, and to). Off-chain tools use this tuple to represent the Transfer event in a more human-readable manner. In detail, the String is used as the key in the JSON representation, and the Fields are used as the values in the JSON representation by off-chain tools.

Including schemas for init#

To generate and include the schema for the parameter, the errors, and/or the events for the init function, set the optional parameter, error, and event attributes for the #[init(..)]-macro:

#[derive(SchemaType)]
enum InitParameter { ... }

#[derive(Serial, Reject, SchemaType)]
enum InitError { ... }

#[derive(SchemaType)]
enum InitEvent { ... }

#[init(contract = "my_contract", parameter = "InitParameter",
error = "InitError", event = "InitEvent")]
fn contract_init(...) -> <..., InitError> { ... }

Note

The event schema attached to the init function is globally available. Any event logged by init or receive functions is represented by off-chain tools using this event schema.

Including schemas for receive#

To generate and include the schema for the parameter, the return value, and/or the errors for receive functions, set the optional parameter, return_value, and error attributes for the #[receive(..)]-macro:

#[derive(SchemaType)]
enum ReceiveParameter { ... }

#[derive(SchemaType)]
enum ReceiveReturnValue { ... }

#[derive(Serial, Reject, SchemaType)]
enum ReceiveError { ... }

#[receive(contract = "my_contract", name = "just_param", parameter = "String")]
fn contract_receive_just_param(...) -> ReceiveResult<String> { ... }

#[receive(contract = "my_contract", name = "just_return", return_value = "Vec<u64>")]
fn contract_receive_just_return(...) -> ReceiveResult<Vec<u64>> { ... }

#[receive(contract = "my_contract", name = "just_error", error = "ReceiveError")]
fn contract_receive_just_error(...) -> Result<Vec<u64>, ReceiveError> { ... }

#[receive(
    contract = "my_contract",
    name = "param_and_return_and_error",
    parameter = "ReceiveParameter",
    return_value = "ReceiveReturnValue",
    error = "ReceiveError"
)]
fn contract_receive(...) -> Result<ReceiveReturnValue, ReceiveError> { ... }

Building the schema#

Now, you are ready to build the actual schema using cargo-concordium, and you have the options to embed the schema and/or write the schema to a file and/or print the schema to the console.

See also

For more on which to choose see here.

Embedding the schema#

In order to embed the schema into the smart contract module, add --schema-embed to the build command by using

$cargo concordium build --schema-embed

If successful the output of the command will tell you the total size of the schema in bytes.

Outputting a schema file#

To output the schema into a file, use the --schema-out FILE where FILE is a path and filename. The path must exist and the file will be created in following command

$cargo concordium build --schema-out "/some/path/schema.bin"

If using cargo concordium version 2.6.0 or newer then the schema can be output in JSON format that can be more suitable for use in dApps. When building the contract use --schema-json-out DIR to output the schema for each contract in the module to a JSON file inside the directory DIR. The directory must exist. The files created will be named after the smart contract names that exist in the module or a counter in case of special characters in the smart contract names:

$cargo concordium build --schema-json-out "/some/path/"

If using cargo concordium version 2.7.0 or newer then the schema can be output in base64 format that is currently supported in the Concordium Wallet for Web. When building the contract use --schema-base64-out FILE, where FILE is a path and filename. The path must exist and the file will be created in following command

$cargo concordium build --schema-base64-out "/some/path/base64_schema.b64"

Instead of writing the base64 representation of the schema to the file provided with the --schema-base64-out flag, you can print it to the console by using a dash character (-) in following command

$cargo concordium build --schema-base64-out -

The --schema-out, --schema-json-out and --schema-base64-out can be used at the same time and schemas in all the formats will be output

$cargo concordium build --schema-out "/some/path/schema.bin" --schema-json-out "/some/path/" --schema-base64-out "/some/path/base64_schema.b64"

Note

Why are there so many different schema formats?

  • The flags --schema-embed/--schema-out convert the schema into bytes. This was the first schema representation used by Concordium and it is how we embed the schema into a module. This byte format is not human-readable but it is a very compact format and suitable to be used for the smart contract on-chain.

  • The command schema-base64 and the flag --schema-base64-out convert the schema into base64 format. This is the format that is currently used by the Concordium Wallet for Web and most of our front-end examples.

  • The command schema-json and the flag --schema-json-out convert the schema into JSON format. This is a human-readable format and will make it easier for developers to develop on the Concordium blockchain. Concordium is in the process of updating its tooling and implementing support for this format.

Converting a binary schema to JSON#

These commands are available in cargo concordium version 2.6.0 or newer.

To convert an existing binary schema (obtained via --schema-out) use the cargo concordium schema-json subcommand, e.g.,

$cargo concordium schema-json --schema "/some/path/schema.bin" --out "/some/path/"

Alternatively, a schema in JSON can be extracted from an embedded schema in a module (obtained via cargo concordium build --schema-embed) by using

$cargo concordium schema-json --module "module.wasm.v1" --out "/some/path/"

Converting a binary schema to base64#

These commands are available in cargo concordium version 2.7.0 or newer.

To convert an existing binary schema (obtained via --schema-out) use the cargo concordium schema-base64 subcommand, e.g.,

$cargo concordium schema-base64 --schema "/some/path/schema.bin" --out "/some/path/base64_schema.b64"

Alternatively, a schema in base64 format can be extracted from an embedded schema in a module (obtained via cargo concordium build --schema-embed) by using

$cargo concordium schema-base64 --module "module.wasm.v1" --out "/some/path/base64_schema.b64"

Instead of writing the base64 representation of the schema to the file provided with the --out flag, you can print it to the console by using a dash character (-) or by omitting the --out flag. The following four commands print the base64 representation of the schema to the console:

$cargo concordium schema-base64 --schema "schema.bin"

$cargo concordium schema-base64 --schema "schema.bin" --out -

$cargo concordium schema-base64 --module "/some/path/module.wasm.v1"

$cargo concordium schema-base64 --module "/some/path/module.wasm.v1" --out -
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