# Backend SDK overview

Fluence backend SDK consists of two crates: main and macro. The main crate is used for all memory relative operations and logging, while the macro crate contains the macro to simplify entry point functions. These crates can be used separately but the preferred way is to use the global fluence crate which combines all the others.

In Rust 2018 this can be done by adding Fluence SDK as a dependency, and then adding use fluence::sdk::* to Rust sources.

## Rust 2015

To use Fluence SDK with Rust 2015 import it like this:

#![feature(custom_attribute)]
extern crate fluence;

use fluence::sdk::*;


Example Rust 2015 application can be found here.

## Entry point function

Each WebAssembly backend application deployed to the Fluence network is expected to provide a single entry point function named invoke. The easiest way to implement this function is to use the invocation_handler macro provided by the Fluence SDK:

use fluence::sdk::*;

#[invocation_handler]
fn greeting(name: String) -> String {
format!("Hello, world! -- {}", name)
}


If anything goes wrong, cargo expand can be used for troubleshooting and macros debugging. Keep in mind that the function which the macro is attached to should:

• not have more than one input argument and always return a value
• not be unsafe, const, generic, or have custom abi linkage or variadic params
• have the input argument type (if present) and return type to be either String or Vec<u8>
• not use the invoke name, which is reserved by the Fluence SDK

The invocation_handler macro additionally allows to specify the initialization function:

use fluence::sdk::*;

fn init() {
// will be called just before the first greeting() function invocation
}

#[invocation_handler(init_fn = init)]
fn greeting(name: String) -> String {
format!("Hello, world! -- {}", name)
}


The initialization function will be called only once and before any other code when the first transaction arrives. Therefore, it is a great place to put the code preparing the backend application.

TBD