It would be beneficial to add an expression-based interface to Streaming API described in
SOLR-7082. Right now that API requires streaming requests to come in from clients as serialized bytecode of the streaming classes. The suggestion here is to support string expressions which describe the streaming operations the client wishes to perform.
With this syntax in mind, one can now express arbitrarily complex stream queries with a single string.
The syntax would support
1. Configurable expression names (eg. via solrconfig.xml one can map "unique" to a class implementing a Unique stream class) This allows users to build their own streams and use as they wish.
2. Named parameters (of both simple and expression types)
3. Unnamed, type-matched parameters (to support requiring N streams as arguments to another stream)
4. Positional parameters
The main goal here is to make streaming as accessible as possible and define a syntax for running complex queries across large distributed systems.