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Lasso supports Ethereum JSON-RPC subscriptions over WebSocket for real-time blockchain events. Use eth_subscribe to create subscriptions and eth_unsubscribe to cancel them.

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Subscription Types

Lasso supports the following subscription types: | Type | Description | |------|-------------|| | newHeads | New block headers as they arrive | | logs | Log events matching a filter |

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Creating Subscriptions

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Subscribe to New Block Headers

Request: 
{"jsonrpc":"2.0","method":"eth_subscribe","params":["newHeads"],"id":1}
Response: 
{"jsonrpc":"2.0","id":1,"result":"0xabc123..."}
The result field contains the subscription ID. Save this ID to unsubscribe later. Subscription Events: As new blocks arrive, the server pushes events: 
{
  "jsonrpc":"2.0",
  "method":"eth_subscription",
  "params":{
    "subscription":"0xabc123...",
    "result":{
      "parentHash":"0x...",
      "sha3Uncles":"0x...",
      "miner":"0x...",
      "stateRoot":"0x...",
      "transactionsRoot":"0x...",
      "receiptsRoot":"0x...",
      "logsBloom":"0x...",
      "difficulty":"0x0",
      "number":"0x8471c9a",
      "gasLimit":"0x1c9c380",
      "gasUsed":"0x5208",
      "timestamp":"0x65f3a2b0",
      "extraData":"0x...",
      "mixHash":"0x...",
      "nonce":"0x0000000000000000",
      "baseFeePerGas":"0x7",
      "hash":"0x..."
    }
  }
}

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Subscribe to Logs

Request with filter: 
{
  "jsonrpc":"2.0",
  "method":"eth_subscribe",
  "params":[
    "logs",
    {
      "address":"0xa0b86991c6218b36c1d19d4a2e9eb0ce3606eb48",
      "topics":["0xddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef"]
    }
  ],
  "id":2
}
This example subscribes to Transfer events from USDC on Ethereum. Response: 
{"jsonrpc":"2.0","id":2,"result":"0xdef456..."}
Subscription Events: As matching logs are emitted, the server pushes events: 
{
  "jsonrpc":"2.0",
  "method":"eth_subscription",
  "params":{
    "subscription":"0xdef456...",
    "result":{
      "address":"0xa0b86991c6218b36c1d19d4a2e9eb0ce3606eb48",
      "topics":[
        "0xddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef",
        "0x000000000000000000000000742d35cc6634c0532925a3b844bc454e4438f44e",
        "0x0000000000000000000000001e0049783f008a0085193e00003d00cd54003c71"
      ],
      "data":"0x0000000000000000000000000000000000000000000000000000000002faf080",
      "blockNumber":"0x8471c9a",
      "transactionHash":"0x...",
      "transactionIndex":"0x0",
      "blockHash":"0x...",
      "logIndex":"0x0",
      "removed":false
    }
  }
}

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Log Filter Options

The logs subscription accepts a filter object with: | Field | Type | Description | |-------|------|-------------|| | address | string or array | Contract address(es) to filter. Omit for all contracts. | | topics | array | Event signature and indexed parameters. Use null for wildcards. | Examples: All events from a contract: 
{
  "address":"0xa0b86991c6218b36c1d19d4a2e9eb0ce3606eb48"
}
Specific event signature across all contracts: 
{
  "topics":["0xddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef"]
}
Event from multiple contracts: 
{
  "address":[
    "0xa0b86991c6218b36c1d19d4a2e9eb0ce3606eb48",
    "0xdac17f958d2ee523a2206206994597c13d831ec7"
  ],
  "topics":["0xddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef"]
}
Event with indexed parameter filter: 
{
  "address":"0xa0b86991c6218b36c1d19d4a2e9eb0ce3606eb48",
  "topics":[
    "0xddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef",
    null,
    "0x0000000000000000000000001e0049783f008a0085193e00003d00cd54003c71"
  ]
}
This filters for Transfer events where the second indexed parameter (recipient) matches the specified address.

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Unsubscribing

Cancel a subscription using eth_unsubscribe: Request: 
{"jsonrpc":"2.0","method":"eth_unsubscribe","params":["0xabc123..."],"id":3}
Response: 
{"jsonrpc":"2.0","id":3,"result":true}
A result of true indicates successful unsubscription. After unsubscribing, no more events will be pushed for that subscription ID.

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Full Example

Here’s a complete subscription flow using wscat: 
# Connect to WebSocket endpoint
wscat -c 'ws://localhost:4000/ws/rpc/ethereum?key=lasso_abc123'

# Subscribe to new blocks
> {"jsonrpc":"2.0","method":"eth_subscribe","params":["newHeads"],"id":1}
< {"jsonrpc":"2.0","id":1,"result":"0xabc123..."}

# Receive block events
< {"jsonrpc":"2.0","method":"eth_subscription","params":{"subscription":"0xabc123...","result":{...}}}
< {"jsonrpc":"2.0","method":"eth_subscription","params":{"subscription":"0xabc123...","result":{...}}}

# Unsubscribe
> {"jsonrpc":"2.0","method":"eth_unsubscribe","params":["0xabc123..."],"id":2}
< {"jsonrpc":"2.0","id":2,"result":true}

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Subscription Lifecycle

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Automatic Cleanup

Subscriptions are automatically cleaned up when:
  • The WebSocket connection closes
  • The connection times out (see Connection Lifecycle)
  • An eth_unsubscribe request is received

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Multiple Subscriptions

You can create multiple subscriptions on a single WebSocket connection: 
// Subscribe to blocks
{"jsonrpc":"2.0","method":"eth_subscribe","params":["newHeads"],"id":1}

// Subscribe to logs
{"jsonrpc":"2.0","method":"eth_subscribe","params":["logs",{"address":"0x..."}],"id":2}
Each subscription receives a unique subscription ID and events are tagged with the corresponding ID.

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Error Handling

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Invalid Subscription Type

Request: 
{"jsonrpc":"2.0","method":"eth_subscribe","params":["invalidType"],"id":1}
Response: 
{
  "jsonrpc":"2.0",
  "id":1,
  "error":{
    "code":-32602,
    "message":"Invalid params: unsupported subscription type"
  }
}

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Invalid Filter Format

Request: 
{"jsonrpc":"2.0","method":"eth_subscribe","params":["logs",{"invalid":"field"}],"id":2}
Response: 
{
  "jsonrpc":"2.0",
  "id":2,
  "error":{
    "code":-32602,
    "message":"Invalid params: malformed filter"
  }
}

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Unsubscribe Non-Existent Subscription

Request: 
{"jsonrpc":"2.0","method":"eth_unsubscribe","params":["0xnonexistent"],"id":3}
Response: 
{"jsonrpc":"2.0","id":3,"result":false}
A result of false indicates the subscription ID was not found.

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Best Practices

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Track Subscription IDs

Always store subscription IDs returned by eth_subscribe to unsubscribe later: 
const subscriptionIds = new Map();

// Subscribe
const request = {jsonrpc: "2.0", method: "eth_subscribe", params: ["newHeads"], id: 1};
ws.send(JSON.stringify(request));

ws.on('message', (data) => {
  const response = JSON.parse(data);
  if (response.id === 1) {
    subscriptionIds.set('newHeads', response.result);
  }
});

// Unsubscribe
function cleanup() {
  const subId = subscriptionIds.get('newHeads');
  if (subId) {
    const request = {jsonrpc: "2.0", method: "eth_unsubscribe", params: [subId], id: 2};
    ws.send(JSON.stringify(request));
  }
}

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Handle Reconnection

Subscriptions don’t persist across reconnections. Re-subscribe after reconnecting: 
ws.on('open', () => {
  // Re-subscribe to all active subscriptions
  subscribeToNewHeads();
  subscribeToLogs();
});

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Filter Logs Efficiently

Use specific filters to reduce network traffic: 
// Good: Specific contract and event
{
  "address":"0xa0b86991c6218b36c1d19d4a2e9eb0ce3606eb48",
  "topics":["0xddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef"]
}

// Avoid: No filter (receives all logs)
{}

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Rate Limiting

Subscriptions count toward your API rate limit. Monitor your usage and adjust filters accordingly.