Low-level SolrNet

I recently got a question about how to handle multi-faceting in SolrNet, a nice feature of Solr that can be very useful to the end-user. eBay uses a kind of multi-faceting interface.
If you know nothing about Solr or SolrNet, read on, this article isn't so much about Solr as API design.

The Solr wiki has an example query with multi-faceting:

q=mainquery&fq=status:public&fq={!tag=dt}doctype:pdf&facet=on&facet.field={!ex=dt}doctype

For those of you that are not into Solr, this is just a regular URL query string that is passed to the Solr endpoint. The final URL looks like this (modulo encoding):

http://localhost:9983/solr/select/?q=mainquery&fq=status:public&fq={!tag=dt}doctype:pdf&facet=on&facet.field={!ex=dt}doctype 

And this is how you represent this query in the SolrNet object model:

var solr = ServiceLocator.Current.GetInstance<ISolrOperations<Document>>(); 
ISolrQueryResults<Document> results = solr.Query("mainquery", new QueryOptions { 
    FilterQueries = new[] { 
        Query.Field("status").Is("public"), 
        new LocalParams {{"tag", "dt"}} + Query.Field("doctype").Is("pdf") 
    }, 
    Facet = new FacetParameters { 
        Queries = new[] { 
            new SolrFacetFieldQuery(new LocalParams {{"ex", "dt"}} + "doctype") 
        } 
    } 
});

We build object models like this one because they're programmable, objects and methods can be programmatically combined to build (or compose) our intention. Like Hibernate's Criteria API.

Opposed to this is the string, and most of the time we hate it because it's opaque, it doesn't have any syntactical meaning within our object-oriented code. It has no programmability, no composability. We use very generic classes to build strings, like StringBuilders or StringWriters, which don't convey any syntactical information about what we're actually doing. If we need to extract information from a string, we have to write a parser, which is not a trivial task. But the string also has its advantages: it's naturally serializable (or should I say already serialized), it can be more readable and more concise. And those are some of the reasons why Hibernate also provides the HQL API. You might be thinking that this dichotomy of objects and strings is really a matter of serialization and deserialization, but I'm talking about human-readable strings here, whereas a serialized format is frequently for machine consumption only.

So if we already know what the query string is, how can we simplify the chunk of code above? Thanks to IoC, we can easily tap into some of SolrNet's "internal" components without worrying about what dependencies they need:

Func<string, string, KeyValuePair<string, string>> kv = (k, v) => new KeyValuePair<string, string>(k, v); 
var connection = ServiceLocator.Current.GetInstance<ISolrConnection>(); 
var xml = connection.Get("/select", new[] { 
    kv("q", "mainquery"), 
    kv("fq", "status:public"), 
    kv("fq", "{!tag=dt}doctype:pdf"), 
    kv("facet", "on"), 
    kv("facet.field", "{!ex=dt}doctype"), 
}); 
var parser = ServiceLocator.Current.GetInstance<ISolrQueryResultParser<Document>>(); 
ISolrQueryResults<Document> results = parser.Parse(xml); 

ISolrConnection is just a wrapper over the HTTP request, we give it the querystring parameters and get Solr's XML response, then we feed the response to the parser component and voilà, we have our results.

And since it's just a regular HTTP request, we can go even lower:

using (var web = new WebClient()) { 
    var xml = web.DownloadString("http://localhost:9983/solr/select/?q=mainquery&fq=status%3Apublic&fq=%7B!tag%3Ddt%7Ddoctype%3Apdf&facet=on&facet.field=%7B!ex%3Ddt%7Ddoctype");
    var parser = ServiceLocator.Current.GetInstance<ISolrQueryResultParser<Document>>(); 
    ISolrQueryResults<Document> results = parser.Parse(xml); 
} 

I'll leave it to you to decide which one to use. Like the choice between HQL and Criteria, sometimes you might prefer one over the other depending on the context. Just keep in mind that these components' interfaces are not as stable as the "really public" documented interfaces, they might have breaking changes more often.