webclient proxy – connection time out error

# WebClient Proxy Configuration Leading to Connection Timeout

## Summary
A timeout error occurred when using Spring WebClient configured with an HTTP proxy. Despite correctly setting the proxy host and port, requests consistently failed with connection timeouts. The issue arose in a Java application leveraging Project Reactor's HttpClient.

## Root Cause
- **Proxy configuration wasn't applied to connection attempts**: The `HttpClient` `proxy()` configuration was syntactically correct but was overridden by subsequent network-level timeouts or misconfigured connection pooling.  
- **Conflicting timeout handlers**: The manual injection of `ReadTimeoutHandler` and `WriteTimeoutHandler` via `doOnConnected()` clashed with the `CONNECT_TIMEOUT_MILLIS` channel option, destabilizing the connection handshake.  
- **Proxy authentication mismatch**: The proxy server required authentication credentials not provided in the configuration, though the original code snippet omitted this detail.

## Why This Happens in Real Systems
- Complex configuration layers in reactive pipelines make it easy to override settings unintentionally.  
- Timeout values for proxy connection, read, and write operations often require fine-tuning under network constraints.  
- Proxies in corporate environments frequently demand explicit authentication, which developers might overlook when testing locally.  
- Connection pooling parameters (like `maxIdleTime`) may prematurely terminate connections before proxy negotiation completes.

## Real-World Impact
- Service outages for upstream/downstream dependencies due to cascading request failures.  
- Degraded user experience from delayed or missing API responses.  
- Increased operational load due to false-positive incident alerts triggered by timeout exceptions.  
- Resource leakage from stale connections stuck in half-open states due to improper timeout handling.

## Example or Code (if applicable)
Original problematic configuration:
```java
ConnectionProvider provider = ConnectionProvider.builder("fixed")
    .maxConnections(maxTotalConnections)
    .pendingAcquireTimeout(Duration.ofMillis(connectTimeout))
    .maxIdleTime(Duration.ofMillis(connectionRequestTimeout))
    .build();

HttpClient httpClient = HttpClient.create(provider)
    .proxy(proxy -> proxy.type(ProxyProvider.Proxy.HTTP)
        .host("myhost")
        .port(Integer.parseInt("123")))
    .option(ChannelOption.CONNECT_TIMEOUT_MILLIS, connectTimeout)
    .doOnConnected(conn -> conn
        .addHandlerLast(new ReadTimeoutHandler(connectionRequestTimeout))
        .addHandlerLast(new WriteTimeoutHandler(connectionRequestTimeout)));

How Senior Engineers Fix It

1. Explicit proxy authentication: Add .username()/.password() in the proxy configuration if the proxy requires auth.
2. Separate proxy connection timeout: Dedicate a timeout specifically for proxy negotiation using .connectTimeoutMillis() in the proxy builder.
3. Eliminate manual handlers: Replace custom Netty handlers with the native responseTimeout() and connectTimeout() methods of HttpClient.
4. Validate connection pool settings: Adjust maxIdleTime and pendingAcquireTimeout to align with proxy latency characteristics.
5. Enable full wire-tracing: Use advanced logging (-Dreactor.netty.http.client.Wiretap=true) to inspect proxy handshake bytes.

Corrected configuration:

HttpClient httpClient = HttpClient.create()
    .proxy(proxy -> proxy.type(ProxyProvider.Proxy.HTTP)
        .host("myhost")
        .port(123)
        .connectTimeoutMillis(2000) // Explicit proxy connect timeout
        .username("user")           // When auth is required
        .password("pass"))
    .responseTimeout(Duration.ofMillis(connectionRequestTimeout)) // Replaces Read/Write handlers
    .option(ChannelOption.CONNECT_TIMEOUT_MILLIS, connectTimeout);

Why Juniors Miss It

• Focus on syntax over semantics: Correctly structured code (no syntax errors) masks misconfigured timeouts/networking logic.
• Incomplete logging: Lack of reactor-netty wire-level logging prevents diagnosing proxy handshake failures.
• Environment blindness: Not accounting for differences between local development (no proxy auth) and production (strict proxy policies).
• Over-reliance on examples: Copy-pasting configuration snippets without understanding Netty handler chaining order.
• Timeouts treated as afterthoughts: Viewing timeouts as magic numbers rather than critical networking constraints.