When to Build Three Tiers

Use a three-tier architecture if any of the following conditions apply:

• Systems with many application programs (more than 20)
  
• Mix of application programs of different languages or origins
  
• Two or more heterogeneous data sources (e.g., two DBMSs or a DBMS and a file system)
  
• Expected life of application beyond three years
  
• Many modifications and additions anticipated
  
• High-volume workload (more than 10,000 transactions per day)
  
• Significant inter-application communication, including inter-enterprise communication
  
• Upsizing; i.e., the application may grow over time so one or more previous conditions apply
  

Source: Gartner Group