The load balancer distributes incoming service requests among multiple servers in such a way that the load distribution is transparent to users. Load balancing helps achieve optimal resource use, maximizing throughput, minimizing response time, and avoiding overload.

The NSX load balancer supports two load balancing engines. The layer 4 load balancer is packet-based and provides fast-path processing. The layer 7 load balancer is socket-based and supports advanced traffic management strategies and DDOS mitigation for back end services.

Load balancing for an NSX Data Center for vSphere edge gateway is configured on the external interface because the edge gateway load balances incoming traffic from the external network. When configuring virtual servers for load balancing, specify one of the available IP addresses you have in your organization VDC.

Load Balancing Strategies and Concepts

A packet-based load balancing strategy is implemented on the TCP and UDP layer. Packet-based load balancing does not stop the connection or buffer the whole request. Instead, after manipulating the packet, it sends it directly to the selected server. TCP and UDP sessions are maintained in the load balancer so that packets for a single session are directed to the same server. You can select Acceleration Enable in both the global configuration and relevant virtual server configuration to enable packet-based load balancing.

A socket-based load balancing strategy is implemented on top of the socket interface. Two connections are established for a single request, a client-facing connection and a server-facing connection. The server-facing connection is established after server selection. For the HTTP socket-based implementation, the whole request is received before sending to the selected server with optional L7 manipulation. For HTTPS socket-based implementation, authentication information is exchanged either on the client-facing connection or server-facing connection. Socket-based load balancing is the default mode for TCP, HTTP, and HTTPS virtual servers.

The key concepts of the NSX load balancer are, virtual server, server pool, server pool member, and service monitor.

Virtual Server
Abstract of an application service, represented by a unique combination of IP, port, protocol and application profile such as TCP or UDP.
Server Pool
Group of back end servers.
Server Pool Member
Represents the back end server as member in a pool.
Service Monitor
Defines how to probe the health status of a back end server.
Application Profile
Represents the TCP, UDP, persistence, and certificate configuration for a given application.

Setup Overview

You begin by setting global options for the load balancer. You now create a server pool consisting of back end server members and associate a service monitor with the pool to manage and share the back end servers efficiently.

You then create an application profile to define the common application behavior in a load balancer such as client SSL, server SSL, x-forwarded-for, or persistence. Persistence sends subsequent requests with similar characteristic such as, source IP or cookie are required to be dispatched to the same pool member, without running the load balancing algorithm. The application profile can be reused across virtual servers.

You then create an optional application rule to configure application-specific settings for traffic manipulation such as, matching a certain URL or hostname so that different requests can be handled by different pools. Next, you create a service monitor that is specific to your application or you can use an existing service monitor if it meets your needs.

Optionally you can create an application rule to support advanced functionality of L7 virtual servers. Some use cases for application rules include content switching, header manipulation, security rules, and DOS protection.

Finally, you create a virtual server that connects your server pool, application profile, and any potential application rules together.

When the virtual server receives a request, the load balancing algorithm considers pool member configuration and runtime status. The algorithm then calculates the appropriate pool to distribute the traffic comprising one or more members. The pool member configuration includes settings such as, weight, maximum connection, and condition status. The runtime status includes current connections, response time, and health check status information. The calculation methods can be round-robin, weighted round-robin, least connection, source IP hash, weighted least connections, URL, URI, or HTTP header.

Each pool is monitored by the associated service monitor. When the load balancer detects a problem with a pool member, it is marked as DOWN. Only UP server is selected when choosing a pool member from the server pool. If the server pool is not configured with a service monitor, all the pool members are considered as UP.