mugle-dev team mailing list archive

[Matt Giuca <matt.giuca@xxxxxxxxx>]

I don't know how much you guys have looked into App Engine datastore
indices, but a tricky aspect of it just bit me, so I have changed our
policy.

https://code.google.com/appengine/docs/java/datastore/queries.html

Basically, when you do certain queries such as searching in alphabetical
order, the datastore needs a pre-built index. The developer needs to
manually specify exactly which tables to index, which fields and the sort
orders, etc. If you don't specify it, you get a DatastoreNeedIndexException
at runtime. You put these in war/WEB-INF/datastore-indexes.xml.

*However*, by default the app engine dev mode automatically generates the
indices for you whenever one is needed, and saves them in
war/WEB-INF/appengine-generated/datastore-indexes-auto.xml. So in practice,
on the dev mode, you never get this exception. When you upload to the
production environment, it copies datastore-indexes-auto.xml as well, so you
*still* don't usually see these exceptions in practice. However, this is
quite brittle. If you recently cleaned out your appengine-generated (as I
did), or if you are uploading from a PC that hasn't fully tested the program
in dev mode, then you can upload a version of the program that doesn't work.

Therefore, the policy change is that I have *removed* the automatic
generation of indices. This means *you can now get
DatastoreNeedIndexExceptions* on the development mode, and you need to know
how to deal with them. If you do get such an exception, this is a good
thing: it means the dev mode is showing exactly what will happen on
production. It's quite easy to fix. There are instructions in
war/WEB-INF/datastore-indexes.xml.

   1. You don't seem to need to restart the dev server.
   2. Open up war/WEB-INF/datastore-indexes.xml in your text editor
   3. In datastore-indexes.xml, set autoGenerated="true".
   4. Test the program again; this time you won't get the
   DatastoreNeedIndexException.
   5. Open up war/WEB-INF/appengine-generated/datastore-indexes-auto.xml.
   There you will see a new index.
   6. Copy-paste the new index (anything that is not there already) into
   datastore-indexes.xml.
   7. *Be sure to* set autoGenerated="false" again or nobody will see any
   more DatastoreNeedIndexExceptions.
   8. Test the program again; you should not see any more
   DatastoreNeedIndexException.
   9. Commit the changes.