What is AppEngine database "cold start time"?

One of the more common complaints I have read about the AppEngine d开发者_JS百科atabase (for Java) is that it is extremely slow when it come to "cold start time". What does this mean? And is it something I should be worried about?

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It is something you should be worried about.

Google App Engine spins up a new JVM to service requests when your app has not had any requests for a certain time period. Getting a handle on the datastore from "cold" - i.e. for the first time in a JVM - can take a considerable amount of time, as much as 5+ seconds.

After you have a handle on the datastore (normally an instance of PersistenceManager), everything is fine (for the life of the JVM!).

EDIT:

Spinning up a fresh JVM in GAE-Java is also slow. Read http://code.google.com/appengine/docs/java/datastore/overview.html and you will see that they use a Singleton class for the availability of a PersistenceManagerFactory, as they describe the operation of instantiating one as "expensive".

You could test it out for yourself. Create a brand new application on GAE-Java that merely returns "Hello World!" and you will find that the first request to the application takes a number of seconds.

Add a request for the PersistenceManagerFactory and you will find that the first request takes a few seconds more.

EDIT EDIT:

I have now created this test for your viewing pleasure:

http://stackoverflowanswers.appspot.com/helloworld

You will either instantly see "Hello, world 0" or "Hello, world xxxx" where xxxx is a count in MS of how long it took to get a handle on the datastore. I think that the complexity and number of indexes in the datastore may have an impact on how long it takes to get a handle on the datastore, as it is quicker in this app than in some of my other apps.

PMF is an exact copy of the one provided in the app engine docs.

@SuppressWarnings("serial")
public class HelloWorldServlet extends HttpServlet {
    public void doGet(HttpServletRequest req, HttpServletResponse resp)
            throws IOException {
        long a = System.currentTimeMillis();
        PersistenceManager p = PMF.get().getPersistenceManager();
        long b = System.currentTimeMillis() - a;
        resp.setContentType("text/plain");
        resp.getWriter().println("Hello, world "+b);
    }
}

EDIT EDIT EDIT:

I changed my code so that it instantiates a PersistenceManagerFactory with each request and now it throws 500 server errors, and in the logs:

javax.jdo.JDOFatalUserException: Application code attempted to create a PersistenceManagerFactory named transactions-optional, but one with this name already exists! Instances of PersistenceManagerFactory are extremely slow to create and it is usually not necessary to create one with a given name more than once. Instead, create a singleton and share it throughout your code. If you really do need to create a duplicate PersistenceManagerFactory (such as for a unittest suite), set the appengine.orm.disable.duplicate.pmf.exception system property to avoid this error.

I don't think I need to provide any more proof that getting a handle on the datastore in app engine is SLOW.