Thursday, January 21, 2021

Normalizing FHIR Queries using FluentQuery

One the advantages of declarative specifications is that they tell a system what needs to be done without specifying how to accomplish it.  When I created FluentQuery for the SANER IG, it had a lot more to do with making FHIR Queries clearer, rather than providing a declarative form for query specifications, but it effectively does create a declarative form (because FHIRPath is a declarative language).

My initial implementation of this form is rather stupid, it just builds up the appropriate FHIR Query string to accomplish what has been asked for.  After going through Connectathon last week, we learned more about the differences in end-points, e.g., a FHIR Server vs. an EHR system implementing 21st Century Cures APIs, and I wound up having to "rewrite queries" to support the simpler syntaxes supported by the EHR.  This was completely expected, but what I hadn't realized beforehand, was that I could actually automate this process in the FluentQuery implementation itself.  

What I'd accidentally done by creating FluentQuery was enabled interoperability across varying FHIR Implementations so that a FHIR Path interpreter implementing FluentQuery could allow a user defined query to be implemented partially or fully by the target FHIR Server, with the rest of the filtering being handled by the receiver.  Doing so in such an implementation would allow a MeasureComputer actor to adapt to servers which are being queried based on their CapabilityStatement resources or other known factors within an implementation.

Let's look at some examples, starting with including().  Here's a query using including:

findAll('Encounter',including('subject','diagnosis','reasonReference'), …).onServers(%Base)

My simplistic implementation would write this as, and execute it using the resolve() function of FHIRPath.  My implementation of resolve already does some additional work here, addressing collection of all result pages when there is more than one page of results.


If the server supports includes, but does not support specific kinds of includes, the query can be written with _include=Encounter:*, and the resulting included references in the resulting Bundle can be filtered out of the results after the fact by removing those that aren't necessary after collecting all pages.

If the server does not support includes, then the resulting Bundle can be post-processed to call resolve() in the elements that should have been included, as if it was written:

   .select(Encounter | Encounter.subject.resolve() | encounter.diagnosis.condition.resolve() |

I've got several queries that work with ValueSet resources such as this one below:

findAll('Observation', for('patient', $, with('code').in(%Covid19Labs) )

If the server supports value set queries, the query would be written as:


But, if it does not support value set queries, but does support multiple parameters, and the value set is small (e.g., less than 10 values), it can be rewritten as:


And if it does not support multiple values in code, then it can be rewritten as multiple queries, and the resulting Bundles can be merged:


And finally, if it does not support the query parameter at all, it can be post-filtered as if it was written:


FluentQuery turns out to be a lot more powerful than I had expected.  A dumb implementation is VERY easy to implement, taking about 500 lines of Java.  The findAll and whereExists method write the query expression by writing out a ?, the resource name, and the remaining arguments to the function separated by & characters.  Each individual function argument writes the appropriate query expression where with() operates by simply writing out the field name, and the various comparison expressions concatenate that with the appropriate comparison operations.  And onServers simply prepends the server names to the query and calls resolve().

To change that to support an adaptive implementation, I would instead write out each comparison expression as a collection of Parameters (using the Parameters resource), and then parse the list of parameters in onServers based on the CapabilityStatement resource for the servers given in arguments to it to execute the appropriate FHIRPath expression.

In fact, I expect I might even compile some small FHIRPath expressions in onServers() that are associated with the Parameters resource used to memo-ize the query operation.  In HAPI, I can probably store the the compiled expressions in the Parameters resource as User data.  If I wanted to get fancier, I could even rewrite the compiled expression.

The rest is, as a former colleague of mine liked to say, a simple matter of programming.

1 comment:

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