Devoxx 2009 – Google Application Engine - A Real Live Voyage to The Cloud

17/11/2009, University Sessions, Sam Brodkin and Scott Stevenson

Google Application Engine for Java (GAE/J) is a cloud computing solution that provides

• a JVM
  
• OS support
  
• access to the Servlet API
  
• a management console
  
• access to Google utilities:
  
  • Logon / Sign up → com.google...User
    
  • image service: resize images
    
  • mails
    
• building blocks:
  
  • DataStore
    
  • MemCache
    

Cost model is based on actual usage, with a free quota for easy starting:

• bandwidth usage (in/out)
  
• cpu usage
  
• datastore size
  
• emails sent
  

Management console:

• access to logs
  
• shows bandwidth / cpu usage → cost
  
• users with admin role
  
• statistics
  
• manage multiple versions: one default, and many test versions (beware of data-incompatibilities)
  

Limits of GAE/J:

• requests timeout: 30sec
  
• no background processes allowed
  
• no server push
  
• no threads
  
• read-only file-system
  
• no transaction
  
• only access to white-listed classes (e.g. ImageIO class of java is not accessible → use Google Image service)
  
• a maximum of 3000 static resources per application
  
• Migrating your application off the GAE/J cloud: difficult, but not impossible → google for “GEA Bar”
  

DataStore for data persisency

• not a relational database
  
• BigTable → like a big HashMap
  
• no transaction
  
• JDO or JPA access
  

Development Environment:

• eclipse plugin
  
• Jetty based emulator → fair emulation, but not 100% relevant
  
• MemCache
  
• disk-based datastore
  
• compile-time checks
  
• no maven support yet
• DIY ant scripts are possible and recommended.
  

Demo: “swagswap.org”

• 3 tiers:
  
  • DAO
    
  • service layer
    
  • 4 frontends that use the same
    service layer:
    
    • jsf 2.0 (+IPhone version)
      
    • gwt 1.7
      
    • SmartGwt
      
    • Spring MVC
      
• wired through Spring 3.0
  
• get detailed cost of a request by setting a special HTTP header
  

Quirks:

• BLOB type
  
• Text type for long Strings
  
• white-listed classes
  
  • → find workarounds for blocked classes (e.g. Image services)
    
  • → some thirdparty libraries don't work
    
• JSF2.0:
  
  • disable threading in web.xml
    
  • no JNDI: hack WebConfiguration class
    
  • server side state doesn't work → use client side state
    
  • RichFaces, IceFaces and Commons FileUpload use non-whitelisted classes → incompatible with GAE
    
• GWT:
  
  • BLOB and Text are non-serializable
    
  • GWT <> Spring integration requires a special service class
    
• SmartGWT:
  
  • requires 1000 static resources → closer to the max of 3000 of GAE → workaround with zip-file and cache, but increases cpu-cost