> ## Documentation Index
> Fetch the complete documentation index at: https://www.helius.dev/docs/llms.txt
> Use this file to discover all available pages before exploring further.

# LaserStream Historical Replay: Backfill Missing Data

> Recover from disconnections and backfill missing Solana blockchain data with LaserStream's historical replay feature. Never miss a transaction again.

<Info>
  **Never miss a beat**: LaserStream's Historical Replay ensures you can recover from disconnections and backfill missing data from the last 24 hours of blockchain activity.
</Info>

## What is Historical Replay?

Historical Replay is LaserStream's feature that lets you replay recent blockchain data by setting a `fromSlot` starting point. This is useful for handling disconnections and ensuring data continuity in real-time apps.

## How Far Back You Can Replay

You can replay up to \~24 hours (\~216,000 slots) of history, regardless of which commitment level you subscribe with. Pass a `fromSlot` within that window and LaserStream streams forward from there to the present.

**Older replays return finalized data.** LaserStream keeps roughly the last \~20 minutes of slots in memory; anything older is served from a historical store that holds finalized blocks only. So when you replay a `fromSlot` older than \~20 minutes, the data reflects the finalized chain — you won't see forked or dropped slots or intra-slot account updates in that range, even on a `processed` or `confirmed` subscription. Slots within the last \~20 minutes carry true commitment semantics, including forks and intra-slot account updates.

<Warning>
  **Limited Time Window**: Historical replay only covers the last \~24 hours. You cannot replay data from arbitrary points in the past.
</Warning>

<CardGroup cols={2}>
  <Card title="Handle Disconnections" icon="clock-rotate-left">
    Recover data lost during brief disconnections (up to 24 hours)
  </Card>

  <Card title="Bootstrap Applications" icon="rocket">
    Start applications with recent context from the last 24 hours
  </Card>

  <Card title="Analyze Recent Events" icon="magnifying-glass">
    Review recent transactions and account changes
  </Card>

  <Card title="Test with Recent Data" icon="flask">
    Use real recent data for testing and development
  </Card>
</CardGroup>

## How It Works

<Steps>
  <Step title="Specify Starting Point">
    Use the `fromSlot` parameter to set your replay starting point (must be within last \~216,000 slots)
  </Step>

  <Step title="Stream Historical Data">
    LaserStream delivers all events from your specified slot forward
  </Step>

  <Step title="Catch Up to Real-Time">
    Historical data streams until you reach the current slot
  </Step>

  <Step title="Continue Live Streaming">
    Seamlessly transition to real-time data streaming
  </Step>
</Steps>

<Note>
  **Automatic Reconnection**: The [LaserStream SDK](https://github.com/helius-labs/laserstream-sdk) handles reconnections and replay automatically. No additional code required!
</Note>

## Quickstart

<Tip>
  Get started with LaserStream from your [Helius Dashboard](https://dashboard.helius.dev/laserstream). Mainnet requires a Business or Professional plan; Devnet is available on Developer and above. See [Plans & Pricing](/billing/plans) for details.
</Tip>

<Tabs>
  <Tab title="gRPC">
    ```typescript theme={"system"}
    import { subscribe, CommitmentLevel, LaserstreamConfig, SubscribeRequest } from 'helius-laserstream';

    // Pick a slot within the last ~216,000 slots (≈24 h). For a real start
    // value, call `getSlot` first and subtract however far back you want to replay.
    // Note: replays older than ~20 min return finalized data even at processed/confirmed;
    // slots within the last ~20 min carry true commitment semantics.
    const fromSlot = 419_800_000;

    const subscriptionRequest: SubscribeRequest = {
      transactions: {
        "token-filter": { // user-defined label for this filter
          accountInclude: ['TokenkegQfeZyiNwAJbNbGKPFXCWuBvf9Ss623VQ5DA'],
          vote: false,
          failed: false
        }
      },
      commitment: CommitmentLevel.CONFIRMED,
      accounts: {},
      slots: {},
      blocks: {},
      blocksMeta: {},
      entry: {},
      accountsDataSlice: [],
      fromSlot, // u64 slot number; must fall inside the replay window
    };

    const config: LaserstreamConfig = {
      apiKey: 'YOUR_API_KEY',
      endpoint: 'https://laserstream-mainnet-ewr.helius-rpc.com', // Choose your closest region
    };

    await subscribe(config, subscriptionRequest, 
      async (data) => {
        console.log('Received data:', data);
      }, 
      async (error) => {
        console.error('Error:', error);
      }
    );
    ```
  </Tab>
</Tabs>

## Configuration Options

<ParamField path="fromSlot" type="number" required>
  The slot number to start replaying from, as a `u64`. Must be within the replay window (last \~216,000 slots / \~24 hours from the current slot), at any commitment level.

  **Example**: `currentSlot - 1000`

  **Important**: If you pass a slot older than the window, LaserStream rejects the request with `Operation was attempted past the valid range`.
</ParamField>

## Use Cases

<AccordionGroup>
  <Accordion title="Reconnection After Brief Disconnection">
    When your application reconnects after a short disconnection (under 24 hours), you can use Historical Replay to ensure no data is missed. `getCurrentSlot` calls the Helius RPC; `lastProcessedSlot` is kept in memory below — persist it however suits your app (Redis, Postgres, a file, etc.).

    ```typescript theme={"system"}
    async function getCurrentSlot(): Promise<number> {
      const r = await fetch(`https://mainnet.helius-rpc.com/?api-key=${process.env.HELIUS_API_KEY}`, {
        method: 'POST',
        headers: { 'Content-Type': 'application/json' },
        body: JSON.stringify({ jsonrpc: '2.0', id: 1, method: 'getSlot', params: [{ commitment: 'confirmed' }] }),
      });
      const { result } = await r.json();
      return result as number;
    }

    // Load the last slot you processed from wherever you store it.
    let lastProcessedSlot = Number(process.env.LAST_PROCESSED_SLOT ?? 0);

    // Check if it's still within the replay window
    const currentSlot = await getCurrentSlot();
    const maxReplaySlot = currentSlot - 216_000;

    if (lastProcessedSlot < maxReplaySlot) {
      console.warn('Disconnection too long, some data may be lost');
      lastProcessedSlot = maxReplaySlot;
    }

    const subscriptionRequest: SubscribeRequest = {
      // ... your subscription config
      fromSlot: lastProcessedSlot, // pass as number, not string
    };

    await subscribe(config, subscriptionRequest,
      async (data) => {
        // your handler here
        if (data.transaction?.slot) {
          lastProcessedSlot = Number(data.transaction.slot);
          // persist `lastProcessedSlot` here so the next reconnect picks up
        }
      }
    );
    ```
  </Accordion>

  <Accordion title="Bootstrap with Recent Context">
    Start your application with recent context from the last few minutes:

    ```typescript theme={"system"}
    // Get a slot from 10 minutes ago (within the 24-hour window)
    const currentSlot = await getCurrentSlot();
    const startSlot = currentSlot - 1500; // ~10 minutes ago

    const subscriptionRequest: SubscribeRequest = {
      // ... your subscription config
      fromSlot: startSlot, // u64 number
    };
    ```
  </Accordion>

  <Accordion title="Testing with Recent Data">
    Use recent historical data for testing (limited to last 24 hours):

    ```typescript theme={"system"}
    // Test with data from the last 5 minutes
    const currentSlot = await getCurrentSlot();
    const testStartSlot = currentSlot - 750; // ~5 minutes ago
    const testEndSlot = currentSlot - 150;   // ~1 minute ago

    const subscriptionRequest: SubscribeRequest = {
      // ... your subscription config
      fromSlot: testStartSlot, // u64 number
    };

    // Stop processing when reaching the test end slot
    const stream = await subscribe(config, subscriptionRequest,
      async (data) => {
        const slot = Number(data.transaction?.slot ?? data.account?.slot ?? 0);
        if (slot >= testEndSlot) {
          stream.cancel();
          return;
        }
        // your test handler here
      }
    );
    ```
  </Accordion>
</AccordionGroup>

## Next Steps

<CardGroup cols={2}>
  <Card title="LaserStream gRPC" icon="bolt" href="/laserstream/grpc">
    Learn more about gRPC streaming capabilities and features
  </Card>

  <Card title="Get Started" icon="rocket" href="https://dashboard.helius.dev/laserstream">
    Enable LaserStream from your Helius Dashboard and start streaming.
  </Card>

  <Card title="SDK Documentation" icon="github" href="https://github.com/helius-labs/laserstream-sdk">
    View the complete SDK docs
  </Card>

  <Card title="Contact Support" icon="headset" href="/support">
    Get help with your implementation
  </Card>
</CardGroup>