Question

This is a common case for a developer. You want to add a feature for every content types of your website that need to store data. It is a kind of metadata, not configuration data.

I see two solutions:

• zope.annotation
• setattr: add attribute to the persistent object

I don’t really know why but from Plone2.5 it was nice to use zope.annotation and now it seems to not be the prefered way to store additionnal data. For example plone.uuid use setattr to store the unique id.

Answer 1

Either one, depending on how often your data changes and how big it is. It’s all about how the ZODB stores this information.

Storing data directly on the object using setattr means that data is stored in the persistent record for that object. If that’s a big object that means there is a big transaction going to take place for that write.

Storing data in a zope.annotations annotation means you get a separate persistent record for each annotation entry, so any changes to your data will result in a smaller transaction. But if you want to access this data often, that extra persistent record will need to be loaded, on top of all the other persistent records. It’ll take a slot in your ZODB cache, your ZEO server or RelStorage server will need to serve it, etc.

plone.uuid uses setattr because it is generally generated only once for a given object, usually at a time it is being created already. It is also a piece of data that is accessed often and quite small. So, by putting it directly on the object itself it’ll be loaded as soon as you load that object, no extra trips to the ZODB required, and it’ll only be changed once in it’s lifetime.

Note: the above presumes that the annotations are stored with the AttributeAnnotations adapter, which is the most common method and the default for Plone content.