# Concepts

## Basic concepts

`@fluencelabs/fluence` package export the `FluencePeer` class. This class implements the Fluence protocol for javascript-based environments. It provides all the necessary features to communicate with Fluence network namely:

1. Connectivity with one or many Fluence Node which allows sending particles to and receiving from other Peers
2. The Peer Id identifying the node in the network
3. Aqua VM which allows the execution of air scripts inside particles
4. A set of builtin functions required by Fluence protocol
5. Support for the typescript code which is generated by Aqua compiler

Even though the js-based implementation closely resembles [node](https://github.com/fluencelabs/gitbook-docs/js-sdk/node.md) there are some considerable differences to the latter.

`FluencePeer` does not host services composed of wasm modules. Instead it allows to register service call handlers directly in javascript. The Aqua language compiler creates a typed helpers for that task. Using Aqua compiler is strontly advised when working with JS SDK.

Due to the limitations of browser-based environment `FluencePeer` cannot be discovered by it's Peer Id on it's own. To overcome this `FluencePeer` must use an existing node which will act as a `relay`. When a peer is connected through a relay it is considered to be `client`. The `FluencePeer` routes all it's particle through it's relay thus taking advantage of the peer discovery implemented on the node. A particle sent to the connected client must be routed through it's relay.

The js-based peer does not implement the full set of builtin functions due the limitations described previously. E.g there is no builtins implementation for _kad_ or _srv_ services. However _op_ service is fully implemented. For the full descriptions of implemented builtins refer to [Api reference](https://github.com/fluencelabs/gitbook-docs/js-sdk/js-sdk/6_reference/modules.md)

In contrast with the node implementation `FluencePeer` can initiate new particles execution. Aqua compiler generates executable functions from `func` definitions in aqua code.

## Creating applications with Aqua language

The official way to write applications for Fluence is using Aqua programming language. Aqua compiler emits TypeScript or JavaScript which in turn can be called from a js-based environemt. The compiler outputs code for the following entities:

1. Exported `func` declarations are turned into callable async functiokns
2. Exported `service` declarations are turned into functions which register callback handler in a typed manner

To learn more about Aqua see [aqua book](https://doc.fluence.dev/aqua-book/)

The building block of the application are:

* Aqua code for peer-to-peer communication
* Compiler cli package for aqua to \(java\)typescript compilation
* Initialization of the `FluencePeer`
* Application specific code \(java\)typescript in the framework of your choice

In the next section we see it in action