XML Gateway - Load balancing Techniques

As an XML Gateway, Forum Sentry sits in front of your SOAP/XML/REST Web services protecting back-end services. For externally facing services (traffic comes in from outside your network), Sentry is responsible for handling all incoming XML traffic sent from your trading partner's client applications and destined for your services. Sentry processes these incoming requests and then sends them along to the back-end service (the remote server).

Often times the Forum Sentry gateway resides behind network load balancers which distribute the incoming requests among multiple Forum Sentry appliances. The load balancers ahead of Sentry allow for redundancy and increased throughput.  Forum Sentry,  also includes support for load balancing requests to multiple remote servers. The strategies are broken into two categories: Passive Load Balancing Strategies and Adaptive Load Balancing Strategies. Below is a quick summary of each.

Passive Load Balancing Strategies

Passive strategies choose a Remote policy without reference to the traffic passing through the system.

• Failover - Uses the order of the configured Remote policies in the group to signify priority. Always chooses the first Remote policy from the list of eligible Remote policies unless it is disabled or inaccessible, in which case it moves to the second, etc.
  
• Round Robin - Initially chooses an eligible Remote policy at random and then rotates through the list of eligible Remote policies in order, choosing the next eligible Remote policy for each new client request.
  
• Random - Chooses an eligible Remote policy at random for each new client request.
  
• Weighted Random - Chooses an eligible Remote policy at random for each new client
  request, using the relative weights configured for each Remote policy. The configured weights set the relative odds that each Remote policy will be selected if eligible.
  

Adaptive Load Balancing Strategies

Adaptive strategies gather statistics about current and past traffic passing through the system and choose a remote server based on the traffic patterns.

• Transfer Throughput - Chooses the highest performing eligible Remote policy. Performance is measured by the average transfer throughput of the last 100 requests, in bits per second.
  
• Active Requests - Chooses the eligible Remote policy which is the least busy, based on the number of concurrent requests the Remote policy is servicing.
  
• Response Time - Chooses the highest performing eligible Remote policy, measuring
  performance by the average response time of the last 100 requests. The Response Time strategy chooses the Remote policy with the lowest average response time.
  

XML Gateways, such as Forum Sentry are required to have sophisticated, flexible and dynamic load balancing capabilities within a corporate network.  Without such capabilities, scaling an XML/Web Services deployment becomes problematic.  A large number of non-XML, packet-based Load balancers are then used to work around the short comings of XML Gateways that lack sophisticated, content-based load balancing schemes.