4.7.4. Creating an update message
The update can be created in many ways but the most common approach is to build the update while modifications take place or to create one as the result of comparing old and new versions. It appears that comparing XML for differences is difficult. However, we can take advantage of the structure of calendaring entities to simplify the process. There are implementations available which take the diff approach to producing an update stream.
There are some special cases to consider when comparing. Some properties are multi-valued and may themselves appear more than once. There is no semantic information implied by any grouping though parameters may need to be taken into account. These properties need to be normalized before comparison and when updating them we produce a change which treats each value as a single property.
These properties are:
categoriesexdatefreebusyrdate
This normalization can take place before comparison.
Some properties are multi-valued and may only appear once. At the moment the only standard property is resource which may take a comma separated list. This should be treated as a single multi-valued property when comparing. The order is unimportant. Sorting the values may help.
Some properties may appear multiple times, for example comment. Comparison should take account of parameters. Ordering all properties appropriately allows for relatively simple comparison.