3 Top things you were unaware about Google App engine

I found few specific useful tips from GAE blog and thought to repost. Here are they:

1. App Versions are strings, not numbers: So, by version things comes to mind some dot separted numerics. Here in GAE you can give versions like testinstance, dev, live as well which is much more meaningful than http://1.appspot.com.

2. You can batch put, get and delete operations for efficiency So, there may be cases when you have to update 100 entities at once. In that case no need to write the code in a loop to update n entities. There are batch update/delete queries to do that. Please follow here.

3. Datastore performance doesn't depend on how many entities you have: So the jist is no matter you have 100 or 1 million entities in the datastore the retrieval time is same. That is the power of GAE which stores everything in a bigTable. This is really different from mySql, or Oracle databases where number of rows in the table affectes query performance.

Rest you can follow on google app engine blog.

Enabling session in Google App engine

Well too short to qualify for a blog but still here is how to do that.

Just add and entry true in appengine-web.xml file in WEB-INF folder of your web app.

Implementing Distinct clause in Google Apple Engine Datastore

People coming from RDBMS background used to write typical sql queries involving distinct clause, doing table joins may be highly disappointed the way JDO handles all this. There are fundamental architectural differences between JDO and conventional RDBMS base model layer. Where RDBMS is a collection of logically related physical tables which can be queries or manipulated using sql queries on the other hand JDO persists the entities in the data store. Relations between entities are defined within entity definition to other entities. Based on relation groups these entities are persisted to data store. Google App Engine heavily believes in the philosophy of scalability. That is why, a lot of typical sql like operations are not supported.

So, RDBMS based community with zero or very less experience with JDO may initially take it as a flaw or painful stuff. In practice once you come to the GAE architecture mindset you start appreciating the way these things can be handled in GAE. Ok so let's take the example of implementing a distinct clause in JDO.

JDO Query Language (JDOQL) does not support the use of distinct clause. So, if you have a list of addresses where you have to find all the unique contury names then in RDBMS you will simply write:

"select distinct countryName from Address"

Now, to implement the same in JDO what you should do is you need to create a separate entity class say CountryList. So, whenever you create a new Address entity just check in the CountryList if the countryName already exists? If no then insert it. So, in a way you have already a list of distinct country names. You can simply query CountryList to get the names of distinct countries.

Now, the question comes why it is not supported in JDOQL. The answer is it is not scalable and will perform poorer along with number of Addresses added to the table. So, even in SQL ultimately when you write select distinct ... it behind the scene fetch all the addresses from the table and build a list of distinct countryName. Imagine if you have 1 million recs in your table then how bad the performance of distinct could be. But going the JDO ways that is maintaining a distinct list at the time of Address creation would be a good performance save and scalable as well.