Problem: Today, lots of Solr code will create a NamedList and forget to consider creating a SimpleOrderedMap (subclassing NamedList) instead. The vast majority of NamedLists I've seen *should* be a SimpleOrderedMap but were not created as such. The distinction is highly subtle, affecting how Solr serializes a NamedList to JSON -- whether it should render it as a Map or another strategy dependent on the json.nl parameter. The subtle-ness means lack of testing and ease of breaking compatibility. And not using SimpleOrderedMap when we should is annoying to a JSON consumer who then has to parse it weirdly, maybe flip-flopping with json.nl.
Proposal: Strongly differentiate creation of NamedList instances between SimpleOrderedMap (exists), and a new subclass to convey that keys may repeat. NamedList will become abstract and gain some factory methods to instantiate one of these concisely that basically everyone will use. The exact naming is TBD for JIRA. Adding the factory methods and the type can come to 9.9 if a user wants to start using them, but making it abstract and making a sweeping change across the codebase is 10 only. The sweeping changes will *not* change any declared parameters/fields/variables to be different from NamedList to a specific type, thus the change won't be too huge. Fortunately, most/all of the javabin consuming code won't have a compatibility problem since NamedLists that become a SimpleOrderedMap are still nonetheless a NamedList. But users requesting JSON will in many cases find the JSON structure of many of Solr's APIs to have changed, and I'm not sure we can enumerate them. This can only be done in a major release -- Solr 10, especially so sweeping of a change. Are we okay with this? I know many of us have expressed a distaste for NamedList generally. There are several old JIRA issues about switching away from NamedList in many places. I imagine a distant Solr 11 with V2 complete (100% new v2 not old v2) and V1 gone, there will be far fewer NamedLists running around thanks to embracing annotated classes with JSON serialization instead. Still, NamedList should not be frozen, waiting for that future to unfold. For reference: all unresolved JIRA issues with "NamedList" in the summary: https://issues.apache.org/jira/issues/?jql=project%20%3D%20SOLR%20AND%20summary%20~%20NamedList%20AND%20resolution%20is%20EMPTY%20ORDER%20BY%20created%20DESC ~ David Smiley Apache Lucene/Solr Search Developer http://www.linkedin.com/in/davidwsmiley
