为什么在AppEngine / Java中默认禁用并发请求？

Question

Much to my surprise I found out that AppEngine applications server requests serially. Recently Java applications have the possiblity to server requests concurrently , which is disabled by default ( reference ).

What is the reason for concurrent request being disabled by default? Web applications are by their nature meant to serve multiple requests concurrently, whether they are written in Java, Python or PHP. It seems to me like this is an artificial limitation.

Answer 1

This may have something to do with AppEngine at the beginning not supporting concurrent requests. The feature was only added for Java in version 1.4.3 (March 30, 2011). So some developers may have coded and tested their applications to behave properly only against a single thread at a time.

Once Google built the infrastructure and code to support concurrent requests they may have been wary about enabling concurrency by default, as it could break existing applications. Hence the reason for the opt in approach.

There are some GAE folk who check StackOverflow, so we might need to hear from them to be sure.

Answer 2

I think that is the better option by default as many novice programmers are coding on appengine and expect minimalistic approach than scale/volume.

Concurrent PHP or python (process based) wont messup object data but Java can (thread based). That is the main difference.

Answer 3

可能是为了避免已经部署的应用程序的回归而不是线程安全...

Answer 4

In GAE doc, the "Note: If you wish to use concurrent requests, your application code needs to use proper thread synchronization before you enable ." is quite explicit about the danger of concurrent requesting.
As other people said above, this is less dangerous not to allow this feature by default and to provide it to advanced developers who go deeper in the documentation.
Yet, it is written quite small and at the end of the doc so it's not easy to see it :)