Request Metadata

Overview

Lasso can optionally include routing metadata in RPC responses, allowing clients to inspect provider selection, retry behavior, and request timing. This is useful for debugging, monitoring, and understanding routing decisions. Metadata is opt-in only — responses do not include metadata by default.

Opt-In Mechanisms

Clients control metadata visibility via:

Query Parameter

curl "http://localhost:4000/rpc/ethereum?include_meta=headers" \
  -H 'Content-Type: application/json' \
  -d '{"jsonrpc":"2.0","method":"eth_blockNumber","params":[],"id":1}'

Request Header

curl "http://localhost:4000/rpc/ethereum" \
  -H 'Content-Type: application/json' \
  -H 'X-Lasso-Include-Meta: body' \
  -d '{"jsonrpc":"2.0","method":"eth_blockNumber","params":[],"id":1}'

Values:

headers - Metadata in response headers
body - Metadata in response body
Omit parameter - No metadata (default)

Headers Mode

Request:

curl -i "http://localhost:4000/rpc/ethereum?include_meta=headers" \
  -H 'Content-Type: application/json' \
  -d '{"jsonrpc":"2.0","method":"eth_blockNumber","params":[],"id":1}'

Response:

HTTP/1.1 200 OK
Content-Type: application/json
X-Lasso-Request-ID: d12fd341cc14fc97ce9f09876fffa7a3
X-Lasso-Meta: eyJ2ZXJzaW9uIjoiMS4wIiwicmVxdWVzdF9pZCI6ImQxMmZkMzQxY2MxNGZjOTdjZTlmMDk4NzZmZmZhN2EzIiwic3RyYXRlZ3kiOiJsb2FkX2JhbGFuY2VkIiwiY2hhaW4iOiJldGhlcmV1bSIsInRyYW5zcG9ydCI6Imh0dHAiLCJzZWxlY3RlZF9wcm92aWRlciI6eyJpZCI6ImV0aGVyZXVtX2xsYW1hcnBjIiwicHJvdG9jb2wiOiJodHRwIn0sImNhbmRpZGF0ZV9wcm92aWRlcnMiOlsiZXRoZXJldW1fY2xvdWRmbGFyZTpodHRwIiwiZXRoZXJldW1fbGxhbWFycGM6aHR0cCJdLCJ1cHN0cmVhbV9sYXRlbmN5X21zIjo1MjUsInJldHJpZXMiOjEsImNpcmN1aXRfYnJlYWtlcl9zdGF0ZSI6ImNsb3NlZCIsImVuZF90b19lbmRfbGF0ZW5jeV9tcyI6NTI4fQ==

{"jsonrpc":"2.0","id":1,"result":"0x8471c9a"}

Decoding X-Lasso-Meta

The X-Lasso-Meta header contains base64url-encoded JSON:

const base64url = response.headers.get('x-lasso-meta');
const base64 = base64url.replace(/-/g, '+').replace(/_/g, '/');
const json = atob(base64);
const meta = JSON.parse(json);

console.log(meta);

Decoded metadata:

{
  "version": "1.0",
  "request_id": "d12fd341cc14fc97ce9f09876fffa7a3",
  "strategy": "load_balanced",
  "chain": "ethereum",
  "transport": "http",
  "selected_provider": {
    "id": "ethereum_llamarpc",
    "protocol": "http"
  },
  "candidate_providers": [
    "ethereum_cloudflare:http",
    "ethereum_llamarpc:http"
  ],
  "upstream_latency_ms": 525,
  "retries": 1,
  "circuit_breaker_state": "closed",
  "end_to_end_latency_ms": 528
}

Size Limit

If encoded metadata exceeds max_meta_header_bytes (default 4KB), only X-Lasso-Request-ID is included. Configuration:

config :lasso, :observability,
  max_meta_header_bytes: 4096  # Default: 4KB

Fallback to body mode:

# If headers mode exceeds limit, switch to body mode
curl "http://localhost:4000/rpc/ethereum?include_meta=body" \
  -H 'Content-Type: application/json' \
  -d '{"jsonrpc":"2.0","method":"eth_getLogs","params":[{"fromBlock":"0x0","toBlock":"latest"}],"id":1}'

Body Mode

Request:

curl "http://localhost:4000/rpc/ethereum?include_meta=body" \
  -H 'Content-Type: application/json' \
  -d '{"jsonrpc":"2.0","method":"eth_blockNumber","params":[],"id":1}'

Response:

{
  "jsonrpc": "2.0",
  "id": 1,
  "result": "0x8471c9a",
  "lasso_meta": {
    "version": "1.0",
    "request_id": "d12fd341cc14fc97ce9f09876fffa7a3",
    "strategy": "load_balanced",
    "chain": "ethereum",
    "transport": "http",
    "selected_provider": {
      "id": "ethereum_llamarpc",
      "protocol": "http"
    },
    "candidate_providers": [
      "ethereum_cloudflare:http",
      "ethereum_llamarpc:http"
    ],
    "upstream_latency_ms": 525,
    "retries": 1,
    "circuit_breaker_state": "closed",
    "end_to_end_latency_ms": 525
  }
}

No size limit — body mode always includes full metadata.

Metadata Fields

Complete Schema

interface LassoMeta {
  version: string;                    // Metadata schema version ("1.0")
  request_id: string;                 // UUID v4 request identifier
  strategy: string;                   // Routing strategy used
  chain: string;                      // Chain name
  transport: string;                  // "http" or "ws"
  selected_provider: {                // Chosen provider
    id: string;                       // Provider ID
    protocol: string;                 // "http" or "ws"
  };
  candidate_providers: string[];      // Providers considered ("id:protocol")
  upstream_latency_ms: number;        // Provider response time
  retries: number;                    // Retry count (0 = first try)
  circuit_breaker_state: string;      // "closed", "open", "half_open"
  end_to_end_latency_ms: number;      // Total request duration
}

Field Descriptions

Field	Type	Description
`version`	string	Metadata schema version (always “1.0”)
`request_id`	string	UUID v4 for request tracking
`strategy`	string	Routing strategy: `fastest`, `load_balanced`, `latency_weighted`
`chain`	string	Chain name (e.g., “ethereum”, “base”)
`transport`	string	Transport protocol: `http` or `ws`
`selected_provider`	object	Provider that fulfilled the request
`selected_provider.id`	string	Provider identifier
`selected_provider.protocol`	string	Transport used: `http` or `ws`
`candidate_providers`	array	Providers considered (format: `"provider_id:protocol"`)
`upstream_latency_ms`	number	Time waiting for provider response
`retries`	number	Number of retry attempts (0 = first try succeeded)
`circuit_breaker_state`	string	Circuit state: `closed`, `open`, `half_open`, `unknown`
`end_to_end_latency_ms`	number	Total request duration (selection + upstream + overhead)

Use Cases

1. Debugging Provider Selection

Scenario: Understand why a specific provider was chosen.

curl "http://localhost:4000/rpc/ethereum?include_meta=body" \
  -H 'Content-Type: application/json' \
  -d '{"jsonrpc":"2.0","method":"eth_blockNumber","params":[],"id":1}' | jq '.lasso_meta'

Output:

{
  "strategy": "fastest",
  "candidate_providers": [
    "ethereum_alchemy:http",
    "ethereum_infura:http"
  ],
  "selected_provider": {
    "id": "ethereum_alchemy",
    "protocol": "http"
  },
  "upstream_latency_ms": 95
}

Insight: Alchemy selected because it has lowest latency for eth_blockNumber.

2. Monitoring Retry Behavior

Scenario: Track failover attempts.

const response = await fetch('http://localhost:4000/rpc/ethereum?include_meta=body', {
  method: 'POST',
  headers: {'Content-Type': 'application/json'},
  body: JSON.stringify({jsonrpc: '2.0', method: 'eth_call', params: [...], id: 1})
});

const data = await response.json();

if (data.lasso_meta.retries > 0) {
  console.warn(`Request required ${data.lasso_meta.retries} retries`);
  console.log('Providers tried:', data.lasso_meta.candidate_providers);
}

Example output:

Request required 2 retries
Providers tried: ['ethereum_cloudflare:http', 'ethereum_llamarpc:http', 'ethereum_alchemy:http']

3. Performance Analysis

Scenario: Identify slow requests and upstream latency.

const meta = response.lasso_meta;

const overhead = meta.end_to_end_latency_ms - meta.upstream_latency_ms;

console.log({
  total: meta.end_to_end_latency_ms,
  upstream: meta.upstream_latency_ms,
  overhead: overhead,
  provider: meta.selected_provider.id
});

Output:

{
  "total": 595,
  "upstream": 592,
  "overhead": 3,
  "provider": "ethereum_llamarpc"
}

Insight: 592ms spent waiting for provider, 3ms Lasso overhead.

4. Circuit Breaker Monitoring

Scenario: Detect when providers are degraded.

if (data.lasso_meta.circuit_breaker_state === 'open') {
  console.error('Circuit breaker OPEN for', data.lasso_meta.selected_provider.id);
}

if (data.lasso_meta.circuit_breaker_state === 'half_open') {
  console.warn('Circuit breaker HALF-OPEN (recovery attempt)');
}

5. Client-Side Request Tracing

Scenario: Correlate client requests with server logs.

const response = await fetch('http://localhost:4000/rpc/ethereum?include_meta=headers', {
  method: 'POST',
  body: JSON.stringify({jsonrpc: '2.0', method: 'eth_getLogs', params: [...], id: 1})
});

const requestId = response.headers.get('x-lasso-request-id');

// Send to your monitoring service
analytics.track('rpc_request', {
  request_id: requestId,
  latency: parseInt(response.headers.get('x-lasso-latency')),
  provider: JSON.parse(atob(response.headers.get('x-lasso-meta'))).selected_provider.id
});

Server-side log lookup:

cat logs/app.log | grep 'd12fd341cc14fc97ce9f09876fffa7a3'

Client Library Examples

JavaScript/TypeScript

import { ethers } from 'ethers';

const provider = new ethers.JsonRpcProvider(
  'http://localhost:4000/rpc/ethereum?include_meta=body'
);

const blockNumber = await provider.getBlockNumber();
console.log('Block number:', blockNumber);

// Access metadata (custom ethers extension)
const meta = (await provider.send('eth_blockNumber', [])).lasso_meta;
console.log('Provider:', meta.selected_provider.id);
console.log('Latency:', meta.upstream_latency_ms, 'ms');

Python (web3.py)

from web3 import Web3
import base64
import json

w3 = Web3(Web3.HTTPProvider('http://localhost:4000/rpc/ethereum?include_meta=body'))

block_number = w3.eth.block_number
print(f'Block number: {block_number}')

# Access metadata from raw request
response = w3.provider.make_request('eth_blockNumber', [])
meta = response.get('lasso_meta', {})
print(f'Provider: {meta.get("selected_provider", {}).get("id")}')
print(f'Latency: {meta.get("upstream_latency_ms")}ms')

Go (go-ethereum)

package main

import (
	"context"
	"encoding/json"
	"fmt"
	"github.com/ethereum/go-ethereum/ethclient"
	"github.com/ethereum/go-ethereum/rpc"
)

type LassoMeta struct {
	RequestID          string `json:"request_id"`
	Strategy           string `json:"strategy"`
	SelectedProvider   struct {
		ID       string `json:"id"`
		Protocol string `json:"protocol"`
	} `json:"selected_provider"`
	UpstreamLatencyMs int `json:"upstream_latency_ms"`
}

func main() {
	rpcClient, _ := rpc.Dial("http://localhost:4000/rpc/ethereum?include_meta=body")
	client := ethclient.NewClient(rpcClient)

	var result struct {
		Result    string     `json:"result"`
		LassoMeta LassoMeta  `json:"lasso_meta"`
	}

	err := rpcClient.CallContext(context.Background(), &result, "eth_blockNumber")
	if err != nil {
		panic(err)
	}

	fmt.Printf("Block number: %s\n", result.Result)
	fmt.Printf("Provider: %s\n", result.LassoMeta.SelectedProvider.ID)
	fmt.Printf("Latency: %dms\n", result.LassoMeta.UpstreamLatencyMs)
}

Performance Impact

Overhead Breakdown

Operation	Overhead	Notes
Header encoding	<2ms	JSON encode + base64url
Body enrichment	<1ms	Map.put operation
Total (headers)	~2ms	Added to end-to-end latency
Total (body)	~1ms	Added to end-to-end latency
Total (none)	0ms	No metadata overhead

Metadata only computed when requested — no overhead when include_meta is omitted.

Configuration

Observability Settings

# config/config.exs
config :lasso, :observability,
  # Maximum size for X-Lasso-Meta header
  # If exceeded, only X-Lasso-Request-ID is sent
  max_meta_header_bytes: 4096

Increase limit for complex metadata:

config :lasso, :observability,
  max_meta_header_bytes: 8192  # 8KB

Troubleshooting

Metadata Not in Response

Symptom: Response does not include metadata despite include_meta parameter. Verify parameter:

# Correct
curl "http://localhost:4000/rpc/ethereum?include_meta=headers" ...

# Incorrect (missing parameter)
curl "http://localhost:4000/rpc/ethereum" ...

Check ObservabilityPlug:

# lib/lasso_web/router.ex
pipeline :api do
  plug LassoWeb.Plugs.ObservabilityPlug  # Must be present
end

Large Metadata Missing from Headers

Symptom: X-Lasso-Request-ID present but X-Lasso-Meta absent. Cause: Metadata exceeds max_meta_header_bytes. Solution 1: Use body mode instead:

curl "http://localhost:4000/rpc/ethereum?include_meta=body" ...

Solution 2: Increase header size limit:

config :lasso, :observability,
  max_meta_header_bytes: 8192

Invalid Base64 Decoding

Symptom: JavaScript atob() fails with “Invalid character” error. Cause: Header uses base64url encoding (not standard base64). Fix: Replace - and _ before decoding:

const base64url = response.headers.get('x-lasso-meta');
const base64 = base64url.replace(/-/g, '+').replace(/_/g, '/');
const meta = JSON.parse(atob(base64));

Summary

Lasso’s request metadata provides:

Opt-in visibility into routing decisions and performance
Two delivery modes: headers (compact) or body (no size limit)
Comprehensive data: provider selection, retries, circuit state, timing
Client library support for JavaScript, Python, Go
Low overhead (<2ms for headers, <1ms for body)
Request tracing via UUID request IDs
Production-safe with configurable size limits

Get Started

Core Concepts

Configuration

Deployment

Observability

Advanced

Overview

Opt-In Mechanisms

Query Parameter

Request Header

Headers Mode

Decoding X-Lasso-Meta

Size Limit

Body Mode

Metadata Fields

Complete Schema

Field Descriptions

Use Cases

1. Debugging Provider Selection

2. Monitoring Retry Behavior

3. Performance Analysis

4. Circuit Breaker Monitoring

5. Client-Side Request Tracing

Client Library Examples

JavaScript/TypeScript

Python (web3.py)

Go (go-ethereum)

Performance Impact

Overhead Breakdown

Configuration

Observability Settings

Troubleshooting

Metadata Not in Response

Large Metadata Missing from Headers

Invalid Base64 Decoding

Summary

Get Started

Core Concepts

Configuration

Deployment

Observability

Advanced

​Overview

​Opt-In Mechanisms

​Query Parameter

​Request Header

​Headers Mode

​Decoding X-Lasso-Meta

​Size Limit

​Body Mode

​Metadata Fields

​Complete Schema

​Field Descriptions

​Use Cases

​1. Debugging Provider Selection

​2. Monitoring Retry Behavior

​3. Performance Analysis

​4. Circuit Breaker Monitoring

​5. Client-Side Request Tracing

​Client Library Examples

​JavaScript/TypeScript

​Python (web3.py)

​Go (go-ethereum)

​Performance Impact

​Overhead Breakdown

​Configuration

​Observability Settings

​Troubleshooting

​Metadata Not in Response

​Large Metadata Missing from Headers

​Invalid Base64 Decoding

​Summary

Overview

Opt-In Mechanisms

Query Parameter

Request Header

Headers Mode

Decoding X-Lasso-Meta

Size Limit

Body Mode

Metadata Fields

Complete Schema

Field Descriptions

Use Cases

1. Debugging Provider Selection

2. Monitoring Retry Behavior

3. Performance Analysis

4. Circuit Breaker Monitoring

5. Client-Side Request Tracing

Client Library Examples

JavaScript/TypeScript

Python (web3.py)

Go (go-ethereum)

Performance Impact

Overhead Breakdown

Configuration

Observability Settings

Troubleshooting

Metadata Not in Response

Large Metadata Missing from Headers

Invalid Base64 Decoding

Summary