Gateway API Compatibility: A Guide to Container Image Migration

Signed-off-by: Harshad Reddy Nalla <hnalla@redhat.com>

Author: harshad16

docs/gateway-api-migration-guide.md

@@ -0,0 +1,389 @@
+# Migrating Workbench Images to Kubernetes Gateway API
+
+## Overview
+
+When migrating from OpenShift Route + oauth-proxy to Kubernetes Gateway API + kube-rbac-proxy, workbench images require nginx configuration updates to properly handle path-based routing.
+
+## The Core Requirement
+
+**Your workbench image must serve all content from the base path `${NB_PREFIX}`.**
+
+Any call from a browser to a different path, for example `/index.html`, `/api/my-endpoint`, or simply `/`, won't be routed to the workbench container. This is because the routing, handled by the Gateway API, is path-based, using the same value as the environment variable `NB_PREFIX` that is injected into the workbench at runtime.
+
+Example `NB_PREFIX`: `/notebook/<namespace>/<workbench-name>`
+
+## Key Architectural Difference
+
+### OpenShift Route (Old)
+```
+External: /notebook/user/workbench/app/
+           ↓
+Route strips prefix
+           ↓
+Container receives: /app/
+```
+
+**Important**: The prefix stripping isn't automatic - it requires implementation:
+- **nginx** strips the prefix via rewrite rules
+- **Catch-all redirects** like `location / { return 302 /app; }`
+
+Both approaches work because the Route **forwards all traffic** to the pod regardless of path.
+
+### Gateway API (New)
+```
+External: /notebook/user/workbench/app/
+           ↓
+Gateway preserves full path (path-based routing)
+           ↓
+Container receives: /notebook/user/workbench/app/
+```
+
+**Critical Difference**: Gateway API uses **path-based routing**. Only requests matching the configured path prefix are forwarded to the pod.
+
+### Why Old Approaches Fail with Gateway API
+
+```
+App redirects: /notebook/user/workbench/app → /app
+                                               ↓
+Browser follows redirect to: /app
+                                ↓
+Gateway routing rule: /notebook/user/workbench/** (doesn't match /app!)
+                                ↓
+Pod receives NO traffic → 404 or routing failure
+```
+
+**The Problem**: If your application redirects to paths outside `${NB_PREFIX}`, the Gateway cannot route those requests back to your pod. The path-based matching at the Gateway level requires all traffic to stay within the configured prefix.
+
+**Critical Change**: Your application (or reverse proxy) must handle the **full path** including the prefix and never redirect outside of it.
+
+---
+
+## Part 1: For All Workbenches - General Requirements
+
+These requirements apply **regardless of whether you use nginx or application-level path handling**.
+
+### 1. Health Check Endpoints
+
+Your workbench **must** respond to health checks at:
+
+```
+GET /{NB_PREFIX}/api
+```
+
+This endpoint must return an HTTP 200 status for probes to succeed.
+
+**Example for Python Flask**:
+```python
+from flask import Flask
+import os
+
+app = Flask(__name__)
+nb_prefix = os.getenv('NB_PREFIX', '')
+
+@app.route(f'{nb_prefix}/api')
+def health_check():
+    return {'status': 'healthy'}, 200
+
+@app.route(f'{nb_prefix}/api/kernels')
+def kernels():
+    # Handle culler endpoint
+    return {'kernels': []}, 200
+
+@app.route(f'{nb_prefix}/api/terminals')  
+def terminals():
+    # Handle culler endpoint
+    return {'terminals': []}, 200
+```
+
+**Example for Node.js Express**:
+```javascript
+const express = require('express');
+const app = express();
+const nbPrefix = process.env.NB_PREFIX || '';
+
+app.get(`${nbPrefix}/api`, (req, res) => {
+    res.json({ status: 'healthy' });
+});
+
+app.get(`${nbPrefix}/api/kernels`, (req, res) => {
+    res.json({ kernels: [] });
+});
+
+app.get(`${nbPrefix}/api/terminals`, (req, res) => {
+    res.json({ terminals: [] });
+});
+```
+
+### 2. Culler Endpoints
+
+If your workbench supports culling idle workbenches, you must handle:
+
+```
+GET /{NB_PREFIX}/api/kernels
+GET /{NB_PREFIX}/api/terminals
+```
+
+These should return information about active kernels/terminals, or empty arrays if none exist.
+
+### 3. Use Relative URLs in Your Application
+
+**Critical**: Your application must generate relative URLs, not absolute ones.
+
+```html
+<!-- ❌ BAD - Hardcoded absolute path -->
+<a href="/menu1">Menu 1</a>
+<script src="/static/app.js"></script>
+<img src="/images/logo.png" />
+
+<!-- ✅ GOOD - Relative URLs -->
+<a href="menu1">Menu 1</a>
+<script src="static/app.js"></script>
+<img src="images/logo.png" />
+
+<!-- ✅ ALSO GOOD - Framework-generated URLs with base path -->
+<a href="{{ url_for('menu1') }}">Menu 1</a>
+```
+
+**Why**: Hardcoded absolute paths like `/menu1` will not include the `{NB_PREFIX}`, causing 404 errors. Relative URLs or framework-generated URLs will correctly resolve to `/{NB_PREFIX}/menu1`.
+
+### 4. Configure Your Application's Base Path
+
+If your framework supports it, configure the base path using the `NB_PREFIX` environment variable:
+
+**FastAPI**:
+```python
+from fastapi import FastAPI
+import os
+
+app = FastAPI(root_path=os.getenv('NB_PREFIX', ''))
+```
+
+**Flask**:
+```python
+from flask import Flask
+import os
+
+app = Flask(__name__)
+app.config['APPLICATION_ROOT'] = os.getenv('NB_PREFIX', '')
+```
+
+**Express.js**:
+```javascript
+const express = require('express');
+const app = express();
+const nbPrefix = process.env.NB_PREFIX || '';
+
+// Mount all routes under the prefix
+const router = express.Router();
+// ... define routes on router ...
+app.use(nbPrefix, router);
+```
+
+**Streamlit**:
+```toml
+# .streamlit/config.toml
+[server]
+baseUrlPath = "/notebook/namespace/workbench"  # Set via NB_PREFIX
+```
+
+### 5. Limitations: Applications with Hardcoded Absolute Paths
+
+**If your application has hardcoded absolute paths that cannot be changed**, migration becomes very difficult:
+
+```javascript
+// ❌ This cannot work with Gateway API unless rewritten
+const menuUrl = "/menu1";  // Hardcoded absolute path
+fetch(menuUrl).then(...);
+```
+
+**Solutions**:
+1. **Modify the application** - Change to relative URLs or configurable base path (preferred)
+2. **Use nginx with URL rewriting** - nginx can intercept and rewrite some URLs, but this is limited
+3. **HTML/JS post-processing** - Intercept responses and rewrite URLs (complex, not recommended)
+
+**Warning**: nginx can rewrite URLs in redirects and some headers, but it **cannot** rewrite URLs embedded in HTML/JavaScript content without complex content manipulation, which is error-prone and slow.
+
+---
+
+## Part 2: For nginx-based Workbenches - Reverse Proxy Configuration
+
+**Use this section if** your application does not support base path configuration and you need nginx to handle the path translation.
+
+### Required nginx Changes
+
+### 1. Remove Problematic Location Blocks
+
+**REMOVE** any overly broad location blocks that cause infinite redirects:
+
+```nginx
+# ❌ REMOVE THIS - Too broad, causes infinite loops
+location ${NB_PREFIX}/ {
+    return 302 $custom_scheme://$http_host/app/;
+}
+```
+
+**Why**: This matches ALL paths under the prefix, including your application endpoint itself (e.g., `/notebook/user/workbench/app/`), creating redirect loops.
+
+### 2. Update Redirects to Preserve NB_PREFIX
+
+**All redirects must include `${NB_PREFIX}`** to keep requests within the Gateway route:
+
+```nginx
+# ❌ BAD - Strips prefix
+location = ${NB_PREFIX} {
+    return 302 $custom_scheme://$http_host/myapp/;
+}
+
+# ✅ GOOD - Preserves prefix
+location ${NB_PREFIX} {
+    return 302 $custom_scheme://$http_host${NB_PREFIX}/myapp/;
+}
+```
+
+**Note**: Use `location ${NB_PREFIX}` (without `=`) to handle both with and without trailing slash.
+
+### 3. Add Prefix-Aware Proxy Location
+
+**Add a location block** that matches the full prefixed path and strips the prefix before proxying:
+
+```nginx
+location ${NB_PREFIX}/myapp/ {
+    # Strip the prefix before proxying to backend
+    rewrite ^${NB_PREFIX}/myapp/(.*)$ /$1 break;
+    
+    # Proxy to your application
+    proxy_pass http://localhost:8080/;
+    proxy_http_version 1.1;
+    
+    # Essential for WebSocket support
+    proxy_set_header Upgrade $http_upgrade;
+    proxy_set_header Connection $connection_upgrade;
+    
+    # Long timeout for interactive sessions
+    proxy_read_timeout 20d;
+    
+    # Pass through important headers
+    proxy_set_header Host $http_host;
+    proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
+    proxy_set_header X-Forwarded-Proto $custom_scheme;
+}
+```
+
+### 4. Update Health Check Endpoints
+
+Health checks must also preserve the prefix:
+
+```nginx
+# Health check endpoint
+location = ${NB_PREFIX}/api {
+    return 302 ${NB_PREFIX}/myapp/healthz/;
+    access_log off;
+}
+```
+
+### 5. Add Wildcard server_name Fallback
+
+Gateway API uses different hostnames than OpenShift Routes. Add fallback logic:
+
+```bash
+# In run-nginx.sh or startup script
+export BASE_URL=$(extract_base_url_from_notebook_args)
+
+# If BASE_URL is empty or invalid, use wildcard server_name
+if [ -z "$BASE_URL" ] || [ "$BASE_URL" = "$(echo $NB_PREFIX | awk -F/ '{ print $4"-"$3 }')" ]; then
+    export BASE_URL="_"
+fi
+```
+
+This sets `server_name _;` which accepts requests from any hostname.
+
+### 6. Update kube-rbac-proxy Configuration
+
+Remove trailing slashes from upstream URLs in pod/statefulset specs:
+
+```yaml
+# ❌ BAD
+args:
+  - '--upstream=http://127.0.0.1:8888/'
+
+# ✅ GOOD
+args:
+  - '--upstream=http://127.0.0.1:8888'
+```
+
+## HTTPRoute Configuration
+
+Ensure your HTTPRoute matches the full prefix path:
+
+```yaml
+apiVersion: gateway.networking.k8s.io/v1
+kind: HTTPRoute
+metadata:
+  name: my-workbench
+  namespace: <user-namespace>
+spec:
+  parentRefs:
+    - group: gateway.networking.k8s.io
+      kind: Gateway
+      name: data-science-gateway
+      namespace: openshift-ingress
+  rules:
+    - backendRefs:
+        - kind: Service
+          name: my-workbench-rbac
+          port: 8443
+          weight: 1
+      matches:
+        - path:
+            type: PathPrefix
+            value: /notebook/<namespace>/<workbench-name>
+```
+
+**Important**: The `value` must match the `NB_PREFIX` environment variable set in the pod.
+
+## Reference Implementation
+
+See these files for complete examples:
+
+### Code-Server
+- **nginx config**: `codeserver/ubi9-python-3.12/nginx/serverconf/proxy.conf.template_nbprefix`
+- **startup script**: `codeserver/ubi9-python-3.12/run-nginx.sh`
+
+### RStudio
+- **nginx config**: `rstudio/c9s-python-3.11/nginx/serverconf/proxy.conf.template_nbprefix`
+- **startup script**: `rstudio/c9s-python-3.11/run-nginx.sh`
+
+## Understanding nginx Location Matching
+
+nginx location blocks have different matching priorities:
+
+```nginx
+# 1. Exact match (highest priority)
+location = /exact/path {
+    # Only matches /exact/path (no trailing slash)
+}
+
+# 2. Prefix match (evaluated in order of length)
+location /prefix {
+    # Matches /prefix, /prefix/, /prefix/anything
+}
+
+# 3. Regex match (not covered here)
+```
+
+For Gateway API, you need:
+
+```nginx
+# Redirect root to app
+location ${NB_PREFIX} {
+    return 302 $custom_scheme://$http_host${NB_PREFIX}/myapp/;
+}
+
+# Proxy app traffic (longer prefix wins)
+location ${NB_PREFIX}/myapp/ {
+    proxy_pass http://localhost:8080/;
+}
+```
+
+Request `/notebook/ns/wb` → matches first location → redirects  
+Request `/notebook/ns/wb/myapp/` → matches second location (longer) → proxies