HAR Replay

Replay a HAR file as a mock Lightdash backend so the frontend renders with exact production data. No database, warehouse, or authentication needed.

The user must provide a path to a .har file. If not provided as $ARGUMENTS, ask for it.

Step 1: Analyze the HAR file

Run a Python script to extract key information from the HAR:

import json, sys, re
from collections import Counter
from urllib.parse import urlparse

har_path = "$HAR_FILE_PATH"
with open(har_path) as f:
    har = json.load(f)

entries = har['log']['entries']

# Extract the page URL from the HAR pages section
page_path = None
pages = har['log'].get('pages', [])
for p in pages:
    title = p.get('title', '')
    parsed = urlparse(title)
    if parsed.path and parsed.path != '/':
        page_path = parsed.path
        break

# Fallback: extract from referer headers
if not page_path:
    for e in entries:
        for h in e['request']['headers']:
            if h['name'].lower() == 'referer':
                parsed = urlparse(h['value'])
                if parsed.path and parsed.path != '/':
                    page_path = parsed.path
                    break
        if page_path:
            break

# Find the origin (first API call)
origin = None
for e in entries:
    url = urlparse(e['request']['url'])
    if url.path.startswith('/api/'):
        origin = f"{url.scheme}://{url.netloc}"
        break

# Filter to origin entries only
api_entries = [e for e in entries if origin and origin in e['request']['url']]

# Find dashboard URL if present
dashboard_uuid = None
for e in api_entries:
    path = urlparse(e['request']['url']).path
    m = re.search(r'/dashboards/([0-9a-f-]{36})', path)
    if m:
        dashboard_uuid = m.group(1)
        break

# Find project UUID
project_uuid = None
for e in api_entries:
    path = urlparse(e['request']['url']).path
    m = re.search(r'/projects/([0-9a-f-]{36})', path)
    if m:
        project_uuid = m.group(1)
        break

# Count request types
methods = Counter()
api_paths = Counter()
has_base64 = False
post_endpoints_needing_body_match = []

for e in api_entries:
    method = e['request']['method']
    path = urlparse(e['request']['url']).path
    methods[method] += 1
    normalized = re.sub(r'[0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12}', '{uuid}', path)
    api_paths[f'{method} {normalized}'] += 1
    if e['response']['content'].get('encoding') == 'base64':
        has_base64 = True

# Find POST endpoints with multiple entries (need body-based matching)
for key, count in api_paths.items():
    if key.startswith('POST') and count > 1:
        post_endpoints_needing_body_match.append((key, count))

print(f"Page path: {page_path}")
print(f"Origin: {origin}")
print(f"Total API entries: {len(api_entries)}")
print(f"Project UUID: {project_uuid}")
print(f"Dashboard UUID: {dashboard_uuid}")
print(f"Has base64 content: {has_base64}")
print(f"Methods: {dict(methods)}")
print(f"POST endpoints needing body match: {post_endpoints_needing_body_match}")
print()
print("API paths:")
for p, c in sorted(api_paths.items()):
    print(f"  {p}: {c}")

Report the findings to the user:

• Page path extracted from the HAR (this is the URL the user was viewing when the HAR was captured)
• Origin domain
• Number of API entries
• Notable POST endpoints that need body-based matching (e.g., dashboard-chart with multiple chart tiles)

Step 2: Write a tailored replay server

Based on the analysis, write a replay server to scripts/har-replay-server.ts. The server must handle these concerns:

Core structure

import http from 'node:http';
import fs from 'node:fs';
import path from 'node:path';

Use only Node.js built-in modules. Run with npx tsx scripts/har-replay-server.ts <har-path>.

HAR parsing rules

1. Filter by origin: Only index entries matching the detected origin domain
2. Base64 decoding: Check entry.response.content.encoding === 'base64' and decode with Buffer.from(text, 'base64').toString('utf-8')
3. Skip pending poll responses: For GET /api/v2/projects/{uuid}/query/{uuid} endpoints, parse the response body and skip entries where results.status === 'pending' — only keep the final ready response
4. Normalize 304s to 200: Serve 304 responses as 200 (they have full body in HAR)
5. Strip transport headers: Remove :pseudo-headers, transfer-encoding, content-encoding, content-length from response headers

Request matching strategy

• GET requests: Match by exact "METHOD /path?query". Fall back to path without query string.
• POST dashboard-chart: These all hit the same URL but carry different chartUuid in the request body. Index by chartUuid from the HAR request body, and match incoming requests by parsing their body.
• POST dashboard-sql-chart: Same pattern but with savedSqlUuid. If there’s only one, exact path match works.
• Other POST requests (e.g., availableFilters): Usually unique paths, exact match works.
• Any POST endpoint the analysis identified as having multiple entries to the same path: Must use body-based matching on a distinguishing field.

Server behavior

• Listen on port 3001
• Log every request with method, path, and whether it matched
• Return {"status":"error","error":{"message":"HAR replay: no matching entry"}} for 404s
• On startup, print the number of indexed responses and the dashboard URL to navigate to

Step 3: Start the replay server and frontend

1. Kill any existing processes on port 3001
2. Start the replay server in the background:

   npx tsx scripts/har-replay-server.ts <har-path>
   

3. Start the Vite frontend dev server pointing at the replay server:

   PORT=3001 pnpm -F frontend dev
   

4. Wait for both to be ready, then verify with:

   curl -s http://localhost:3001/api/v1/health
   

Step 4: Provide the URL

Once both servers are confirmed running, tell the user the URL to open in their browser. Construct it from the page path extracted in Step 1:

http://localhost:<vite-port><page-path>

For example, if the HAR was captured on /projects/abc-123/dashboards/def-456/view, the URL would be http://localhost:3002/projects/abc-123/dashboards/def-456/view.

Note: Vite may pick a port other than 3000 since 3001 is in use. Check the Vite startup output for the actual port.

Step 5: Debug any rendering errors

If the user reports errors:

1. Check the replay server logs for 404s (missing HAR entries)
2. Check for response format issues (base64 encoding, unexpected MIME types)
3. Fix the replay server script and restart

Notes

• The replay server is disposable — it’s tailored to the specific HAR file and can be deleted after use
• HAR files may contain session cookies and sensitive data — do not commit them
• The frontend will 404 on any navigation away from the captured pages
• To re-capture: Chrome DevTools > Network tab > right-click > “Save all as HAR with content”