The document provides 8 rules for building a web application effectively on Google App Engine: 1) Design a simple data model, 2) Handle data model updates via a non-default version, 3) Use techniques like Appstats and caching to reduce costs, 4) Improve cold startup time by minimizing dependencies, 5) Prefer Google Guice as a dependency injection framework, 6) Use GWT for a desktop-like interface, 7) Employ the GWT MVP pattern for large applications, and 8) Consider web frameworks like Apache Wicket that are optimized for App Engine.

1. 8 rules to build web app on
top of Google AppEngine
effectively
Lessons learned
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2. Google App Engine
• Keep in mind Google AppEngine supports
subset of Java EE specifications. Check at
http://code.google.com/p/googleappengine/wiki/W
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3. 1. Design Data model as simple as
possible
• The App Engine Datastore is not traditional relational SQL database and
there are several important differences.
• AppEngine datastore is schemaless. Entities of the same kind can have
different properties. The application itself is responsible for ensuring that
entities conform to it.
• If you want to migrate your existing data model to App Engine you will
probably need it to redesign.
• Data-model consistency will be split to the data-model and application.
• Check app engine datastore limitations to understand it. It gives you better
overview regarding your future data-model design.
http://code.google.com/appengine/articles/datastore/overview.html
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4. How to
• Use recommended High Replication Datastore HRD.
• Use Objectify with JPA annotations over built-in JPA or JDO.
• Prefer to use unowned relationships between entities.
• Forget about count entities over filtered query result. Design your UI so
that count is not being required.
• If you need statistic calculations, do it on entity update.
• Note: Building data model on top of built-in either JPA or JDO doesn't
guarantee that application will be vendor independent because of different
behavior of AppEngine datastore in some cases. Even it doesn't mean that's
not possible to do so just requires understanding of AppEngine datastore
behavior.
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5. 2. Handle your data model via non
default version of your GAE app
• The AppEngine allows you to deploy more than one non default version of
your application. Deploy specifically designed application to perform update
of your data model only as a non default version to be independent from
your main application release cycle.
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6. 3. Price shields tips
• Use Appstats out of the box tool to detect datastore bottleneck. Just plug
in. It helps you to find the hidden unnecessary datastore operations that are
dragging you down.
• Use memcache. Datastore operations costs money and caching of "static"
data could save you from using unnecessary datastore operations.
• Use as many static files as possible. Static files are handled with high
performance static front-end servers (CDN) which are highly optimized for
this type of content.
• See more
http://www.gregtracy.com/revisiting-google-app-engines-pricing-changes
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7. 4. Improve cold startup time
• AppEngine instances starts quite often thus cold startup time (known as
well as warmup request, loading request) is more important than in
traditional Java EE application. Good startup time improves user experience.
• Minimize number of your application java library dependencies as much as
possible. Make sure if all your libs are really needed. Otherwise it will slow
your cold startup time down.
• Don't use JSP if you are already using another Frontend framework
• Using JSPs will add additional libraries into your lib directory that are used
for processing of JSP.
http://www.listry.com/blog/2010/03/google-app-engine-cold-start-tip-dont
• Use Objectify over built-in JPA or JDO.
http://www.listry.com/blog/2010/03/google-app-engine-cold-start-guide-for
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8. 5. Prefer Google Guice as DI framework
• It's simple, small in size and powerful library and you probably won't need
more.
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9. 6. Use GWT for your desktop like web
app
• GWT together with GAE offers lot of built-in technics to improve your
web app performance. Built-in features coming as advice from the book
High Performance Web Sites.
• It's better to have only one framework for the UI even it means to build
some components yourself. Among other things it means less dependencies
and simplified upgrade process.
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10. How to GWT
• Use built-in optimization technics with help of GWT's ClientBundles.
• ClientBundles and ImageResource makes using images more efficient.
• CssResource for CSS minification, CSS image strips and much more, see
docs for ClientBundles (DataResource, TextResource,
ExternalTextResource).
• Switch on minification and obfuscation of the Java-JavaScript compiler.
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11. How to GAE
• Content Delivery Network CDN
• Gzip compression
• Memcache
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12. 7. Use GWT MVP pattern
• It's not easy to work with but worth of time. GWT has built-in support for
MVP pattern.
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13. Benefits
• Well suited for large scale application.
• Separation of functionality into components. Helps simplify UI unit testing.
• Built-in browser history management. MVP is not concerned with browser
history management, but Activities and Places may be used very well with
MVP development.
• Built-in event bus as a central point for app events.
• Caching all of your code books via GWT event bus as a central pipe to go
to the backend.
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14. 8. Use Apache Wicket for website
• The component based web framework. Version 1.5 or higher optimized for
Google AppEngine
• In Java ecosystem there is a lot of front-end frameworks thus to choose the
right one depends on many varying factors. We like Apache Wicket because
it's Java, feature rich component-oriented framework with good API. As
version 1.5 it very well operates on App Engine. So it's up to you and your
requirements but ones Apache Wicket is good fit for you we recommend it.
• Add wicketstuff-gae-initializer dependency along side with wicket core
dependencies
• Alternative Play framework as it has specific module for App Engine.
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15. • Visit http://www.comvai.com/google-app-engine-development and register
yourself to get development of your Google AppEngine project's Proof of
Concept for free.
• Validate suitability of your project to be built on top of Google App Engine.
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