Historical Replay

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Overview

One of LaserStream’s most powerful features is Historical Replay, which allows you to replay data from a specific point in time. This capability ensures you never miss critical on-chain events, even if your application disconnects temporarily.

With Historical Replay, you can:

• Backfill missing data after a disconnection
• Bootstrap new applications with recent historical data
• Analyze past events for specific accounts or transactions
• Test application behavior with real historical data

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How Historical Replay Works

When you establish a LaserStream connection, you can optionally specify a starting slot using the fromSlot parameter. LaserStream will then:

1. Begin streaming data from the specified slot
2. Deliver all relevant events from that slot forward
3. Eventually catch up to real-time data
4. Continue streaming real-time events seamlessly

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Using the fromSlot Parameter

The fromSlot parameter can be used with both gRPC and WebSocket connections.

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With gRPC

Add the fromSlot parameter to your SubscribeRequest object:

const subscriptionRequest: SubscribeRequest = {
  transactions: {
    client: {
      accountInclude: ['TokenkegQfeZyiNwAJbNbGKPFXCWuBvf9Ss623VQ5DA'],
      accountExclude: [],
      accountRequired: [],
      vote: false,
      failed: false
    }
  },
  commitment: CommitmentLevel.CONFIRMED,
  accounts: {},
  slots: {},
  blocks: {},
  blocksMeta: {},
  entry: {},
  accountsDataSlice: [],
  fromSlot: '224339000' // Specify the starting slot for historical replay
};

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Limitations

• Replay Window: Currently, you can replay data from up to 3000 slots in the past (approximately 20 minutes of blockchain activity).

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Best Practices

1. Use Recent Slots: When specifying a fromSlot, ensure it falls within the replay window (currently 3000 slots or about 20 minutes).
2. Handle High Volumes: During catch-up, you may receive data at a faster rate than during real-time streaming.
3. Set Appropriate Filters: To minimize the amount of data during replay, use specific filters.
4. Monitor Reconnections: The SDK handles reconnections automatically, but it’s good practice to log when they occur.

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Simplified Example

import { subscribe, CommitmentLevel, LaserstreamConfig, SubscribeRequest } from 'helius-laserstream';

async function main() {
  const subscriptionRequest: SubscribeRequest = {
    transactions: {
      client: {
        accountInclude: ['TokenkegQfeZyiNwAJbNbGKPFXCWuBvf9Ss623VQ5DA'],
        vote: false,
        failed: false
      }
    },
    commitment: CommitmentLevel.CONFIRMED,
    accounts: {},
    slots: {},
    blocks: {},
    blocksMeta: {},
    entry: {},
    accountsDataSlice: [],
    // Optional: specify a starting slot for replay
    fromSlot: '224339000'
  };

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

  // The subscribe function handles connection, reconnection, and replay automatically
  await subscribe(config, subscriptionRequest, async (data) => {
    // Process your data
    console.log(data);
  }, async (error) => {
    console.error(error);
  });
}

main().catch(console.error);

The LaserStream SDK will transparently handle:

• Initial connection and data streaming
• Automatic reconnection if the connection drops
• Resuming from where it left off after a reconnection
• Buffering and delivering data in the correct order