# SSE Events

The Casper node exposes a Server-Sent Events (SSE) stream on port 9999 for real-time blockchain events.

## Setup

```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:', event.val.blockAdded);
  }
});

sseClient.start(); // opens the connection
```

## Event Types

| Event | When it fires |
|---|---|
| [`BlockAdded`](/sse/block-added) | A new block is finalized |
| [`TransactionProcessed`](/sse/transaction-processed) | A transaction completes execution |
| [`TransactionAccepted`](/sse/transaction-accepted) | A transaction is accepted to the mempool |
| [`TransactionExpired`](/sse/transaction-expired) | A transaction's TTL expires before execution |
| [`DeployProcessed`](/sse/deploy-processed) | A legacy deploy completes execution |
| [`FinalitySignature`](/sse/finality-signature) | A validator signs a block final |
| [`Fault`](/sse/fault) | A validator equivocates |
| [`Step`](/sse/step) | The end-of-era auction step runs |

## Parsing Pattern

Every event arrives as a `RawEvent`. Parse it to get the typed event:

```ts
sseClient.subscribe(EventName.TransactionProcessedEventType, (raw) => {
  // raw.data contains the JSON string
  const result = raw.parseAsTransactionProcessedEvent();

  if (result.err) {
    console.error('Parse error:', result.val);
    return;
  }

  const event = result.val; // TransactionProcessedEvent
  const payload = event.transactionProcessed;
});
```

All `parseAs*` methods return `Result<T, string>` from `ts-results`. Always check `.err` before accessing `.val`.

## Rules

- Only one handler per event type. Calling `subscribe` twice for the same event returns `Err`.
- Call `start()` after all `subscribe()` calls to avoid missing early events.
- EventSource reconnects automatically on connection loss.