Homogeneous Active/Active Systems for Master/Subordinate Peers Cell Phone Application


A major U.S. telecommunication service provider needed to offer the HPE OpenCall INS cell phone application with considerable extensions to its customers. One of its customers had nine total nodes in its architecture; and the system had to survive the loss of one or more of these nodes. Also, if the customer’s nodes were geographically distributed, disaster tolerance needed to be provided.


Shadowbase Solution: Homogeneous Active/Active Systems

  • This application is configured as a multimode active/active system, which the company calls the “N+1 Geographic Redundancy Option.” It allows the system to survive the loss of one or more nodes, depending upon the total number of nodes in the system.
  • Each node uses an HPE NonStop Server, which actively runs the application and processes any request.
  • The databases of the various nodes are kept in synchronization via asynchronous data replication.
  • One node is selected as the “master” node and the other nodes are “subordinate peer” nodes. Changes to the database of each subordinate peer node are directly replicated to the master node, which then replicates these changes to all subordinates, including the original subordinate.
  • If the master node detects a data collision, it resolves it by accepting the first of the conflicting updates which it has received.  It replicates this resultant change to all of the subordinate peer nodes, including the original subordinate. Rejected updates are logged for later review and resolution; often the change with the most recent time-stamp wins.
  • Collisions are resolved by the master, which is the final adjudicator. The databases of all subordinate peers are converged to the contents of the master database.
  • There are as many subordinate peer nodes as are needed for the anticipated transaction volume. Additional subordinate peers are provided to maintain full capacity in case of one or more node failures. If the master node fails, one of the subordinate nodes is designated as the new master.


  • Zero Downtime Migration
    • The telecommunication service provider uses the master/subordinate peer architecture to perform node upgrades without denying service to any of its users.
    • When a subordinate peer node requires upgrading, it is removed from service. Further user requests are simply distributed among the remaining subordinates

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The above was adapted from the book: Breaking the Availability Barrier, Volume III: Active/Active Systems in Practice by Paul J. Holenstein, Dr. Bruce Holenstein, and Dr. Bill Highleyman.