Good Way To Handle XML Change

Question

Our system stores XML strings in a database. I've recently had to change the Properties on a Class, and now when an XML string gets deserialized it will throw an exception. What is the best way to handle this change? Look for the Node in the application code using XPATH or LINQ, or change the xml string in sql database (ie do a mass update)?.

Answer 1

if you serialize the objects to the database you could try the approach I outlined here to load the old versions into a new version then when you save the new version will be saved. Not sure if having different versions of your class will be appropriate though...

Basically you create a factory to produce your objects from the xml. everytime you change your object you create a new factory and a new object class, which is given a version of the old class in its constructor and it creates itself from the old class. The new factory tries to create a new object from the xml, if it can, happy days, if it can't then it creates a new object and tells the next oldest factory to create a next oldest object from the xml. These factories can then be chained together so that you can always load a newest version of the objects from whatever data is in the db.

This assumes that its possible to always create a valid v2 object from a v1 object.

Answer 2

You might want to look at writing a custom XML deserializer (i.e. implementing IXmlSerializable, see here) to handle changes in your XML. If you've invested a lot of time into crafting your XML serialization attributes, you may want to look at another approach.

Consider batch-upgrading your XML, or deprecating (instead of removing) properties inside of your classes and mapping older behavior to newer behavior.

Longer term, you will want to come up with a strategy for dealing with this in the future, since you will most likely be continue to make changes to your schema/object definitions as you add/change the functionality of your system.

Answer 3

It's a good practice to store a version along your XML strings. Either at the database level or at the class level so that your code knows which version of the class it has to deserialize.

You might also look at XSLT. It allows you to transform one version of XML into another. In that case the logic to go from one version to another is not handle by code but by the XSLT. You can even store the XSLT into the database which makes it reusable by other programs.