About a week ago, I saw this post https://groups.google.com/forum/?fromgroups=#!topic/google-appengine/GdBqSxqviYk about instances being unable to load and failing initialization. I took one look at the picture provided, saw the line This request started at [time] and was still executing at [time, 1 minute later], and immediately assumed that the application’s init function was taking too long to run.
In most cases, that’s a fair assumption to make. One of the bigger pitfalls of App Engine is that instances have only 60 seconds to start up (load in all files and run the init method of servlets). It’s very common for developers to write in a huge amount of code within the init method, and then have instances fail startup because initialization took too long. In this case I believed init was the problem for one reason alone: the picture of the logging stack trace included references to ZIP I/O streams. Uncompressing and processing large ZIP files within the init function could easily take more than a minute.
However, it turned out that the developer wasn’t uncompressing ZIP files in the init – the answer was that App Engine was having a slow day, and was exceeding the 60 second startup limit just trying to uncompress the JAR. Which is pretty amazing and notable enough to comment on – the application didn’t even get fully extracted before App Engine shut down the instance as a failure.