# Introduction to Clients & Transports

The Casper JS SDK uses a client-based architecture similar to viem. A **Client** groups related methods and is backed by a **Transport** that handles the actual network communication.

## Client Types

| Client | Purpose | Port |
|---|---|---|
| `RpcClient` | Query chain state, submit transactions | 7777 |
| `SseClient` | Subscribe to real-time events | 9999 |
| `SpeculativeClient` | Dry-run transactions without submitting | 7778 |

## Transport

Currently, `HttpHandler` is the only transport. It wraps axios for HTTP/HTTPS requests.

```ts
import { HttpHandler } from 'casper-js-sdk';

const handler = new HttpHandler('http://<Node Address>:7777/rpc', {
  timeout: 30_000,         // optional, ms
  headers: { 'X-Api-Key': 'secret' },  // optional
});
```

## Creating an RPC Client

```ts
import { HttpHandler, RpcClient } from 'casper-js-sdk';

const handler = new HttpHandler('http://<Node Address>:7777/rpc');
const rpcClient = new RpcClient(handler);

const status = await rpcClient.getStatus();
console.log(status.chainspecName); // 'casper' or 'casper-test'
```

## Creating an SSE Client

```ts
import { SseClient, EventName } from 'casper-js-sdk';

const sseClient = new SseClient('http://<Node Address>:9999/events');

sseClient.subscribe(EventName.BlockAddedEventType, (raw) => {
  const event = raw.parseAsBlockAddedEvent();
  if (!event.err) console.log('New block!');
});

sseClient.start();
```

## CasperNetwork (High-Level Client)

For most applications, `CasperNetwork` wraps everything and auto-detects the node version:

```ts
import { CasperNetwork } from 'casper-js-sdk';

const rpcClient = new RpcClient(new HttpHandler('http://<Node Address>:7777/rpc'));
const network = await CasperNetwork.create(rpcClient);
// auto-detects node version (1.x or 2.0) from getStatus()
```