When working with IIS web farms behind an F5 Big-IP load balancer, session management becomes critical. The current setup using ASP.NET State Service for OutProc session state works but introduces latency. Sticky sessions (session persistence) offer an alternative by routing subsequent requests from the same client to the same server.

Enabling sticky sessions on F5 Big-IP typically involves these configuration methods:

# F5 iRule example for cookie-based persistence
when HTTP_REQUEST {
    if { [HTTP::cookie exists "JSESSIONID"] } {
        persist uie [HTTP::cookie "JSESSIONID"]
    } else {
        persist add uie [HTTP::cookie insert "JSESSIONID" [format %x [expr {int(rand()*999999999)}]]]
    }
}

Sticky sessions reduce the overhead of serializing/deserializing session data to/from the state server, but create several challenges:

• Uneven Load Distribution: Long-lived sessions can create "hot spots" where some servers handle more traffic
• Failover Complexity: When a server fails, all its sticky sessions are lost unless you implement backup persistence
• Scaling Limitations: Adding/removing servers requires careful session draining

For high-traffic sites, consider combining approaches:

// C# example: Dual-mode session handling
public class HybridSessionModule : IHttpModule 
{
    public void Init(HttpApplication context)
    {
        context.BeginRequest += (sender, e) => {
            var app = (HttpApplication)sender;
            if (app.Request.Headers["X-Sticky-Session"] != null)
            {
                HttpContext.Current.Items["UseInProc"] = true;
            }
        };
    }
}

When using sticky sessions, implement these monitoring metrics:

• Per-server session counts
• Session duration distribution
• Load balancer persistence table size
• Failover event tracking

Optimize your Big-IP setup with these parameters:

# TCL snippet for optimized persistence profile
ltm persistence cookie {
    app-service none
    cookie-name SRV_ID
    default-from-profile cookie
    expiration "30 minutes"
    hash-length 8
    method insert
    secure enabled
}

When implementing sticky sessions (also called persistent sessions) with F5 Big-IP load balancers, the system uses cookies or IP-based affinity to route subsequent requests from the same client to the same backend server. While this enables InProc session state usage, it introduces architectural considerations:

// Example F5 iRule for cookie-based persistence
when HTTP_REQUEST {
    if { [HTTP::cookie exists "BIGipServerPool"] } {
        persist uie [HTTP::cookie "BIGipServerPool"]
    }
}
when HTTP_RESPONSE {
    if { [HTTP::cookie exists "BIGipServerPool"] } {
        persist add uie [HTTP::cookie "BIGipServerPool"]
    }
}

The primary drawbacks manifest in several critical scenarios:

• Uneven Load Distribution: Long-lived sessions create server imbalance despite round-robin algorithms
• Failure Resilience Impact: When a server fails, all its sticky sessions become orphaned
• Scalability Constraints: Adding new servers doesn't immediately rebalance existing sessions
• Maintenance Complexity: Draining servers requires careful session migration planning

In our stress tests with 50 IIS nodes, we observed:

For applications requiring some session persistence, consider this ASP.NET middleware approach:

public class SmartSessionMiddleware
{
    private readonly RequestDelegate _next;
    private readonly IMemoryCache _cache;

    public SmartSessionMiddleware(RequestDelegate next, IMemoryCache cache)
    {
        _next = next;
        _cache = cache;
    }

    public async Task Invoke(HttpContext context)
    {
        var sessionKey = context.Request.Cookies["SESSION_AFFINITY"];
        
        if(!string.IsNullOrEmpty(sessionKey) && 
           _cache.TryGetValue(sessionKey, out string preferredServer))
        {
            context.Items["TargetServer"] = preferredServer;
        }
        
        await _next(context);
        
        if(context.Items.TryGetValue("NewSession", out var newSession))
        {
            var options = new MemoryCacheEntryOptions()
                .SetSlidingExpiration(TimeSpan.FromMinutes(30));
                
            _cache.Set(sessionKey, context.Connection.LocalIpAddress, options);
        }
    }
}

When sticky sessions are unavoidable, optimize your Big-IP setup:

1. Set conservative timeout values (15-30 minutes max)
2. Enable connection mirroring for TCP sessions
3. Implement health monitoring at <5 second intervals
4. Configure fallback to round-robin when persistence fails

The ideal solution often involves combining sticky sessions for legacy components with stateless microservices for new development, gradually migrating to distributed caching solutions like Redis for session management.