Getting next page of search results without "Continue"

[TimothyMDean]

I am creating an internal-use API that simplifies access to FHIR resources for workflows being automated in Azure Logic Apps. My internal-use API is currently using the Firely SDK to make the back-end calls to the FHIR service. I am trying to implement the portion of my internal-use API that handles pagination of search results.

Because each API call to my API are their own session with their own FhirClient instance, I cannot just use the FhirClient.Continue method. I need to create a new call through the SDK to call and get the next page. I have managed to get the URIs for the next page from the response to my first page of results. The URI is embedded in the response to my API's client, and passed back in to my API when that client wants to retrieve the next page. So I have the URL that should allow me to go get the next page from the FHIR service.

Unfortunately, I have been unable to figure out how to call this URL and get the results, other than skipping the Firely FhirClient altogether and manually calling the FHIR service and parsing out results.

It seems like this should be more straightforward. Is there some way to emulate what the FhirClient.Continue method does but allows me to do it across different call contexts?

[alexzautke]

You can pass in a stub Bundle resource to the Continue method like:

    var stub = new Bundle
    {
        Type = Bundle.BundleType.Searchset,
        Link = new List<Bundle.LinkComponent>
        {
            new Bundle.LinkComponent
            {
                Relation = "next",  // or "previous"/"first"/"last"
                Url = nextLinkUrl
            }
        }
    };

[TimothyMDean]

Thanks @alexzautke - This seems to get me closer but still having some problems.

When I follow your suggested approach, I am able to perform an initial search and then to follow the link for the "next" relation. However, if I try to follow the link for the "self" relation then the Continue method returns a null bundle as its result.

Similarly, when getting the second page via the first's "next" relation, I tried to go back by following the second page's "previous" relation. That too seems to result in a null bundle being returned by the Continue method.

Is there something about the way that Continue works that would need the Link list to be populated with all of the related links even if only one of them is to be followed?

[TimothyMDean]

@alexzautke After looking through the source code for the ContinueAsync method I was able to get the previous/next working. The FHIR server I am hitting isn't currently returning first/last links so I can't test that out just yet.

The additional thing I was missing for getting the previous links to work was the optional second parameter for the ContinueAsync method. It is a PageDirection value that defaults to PageDirection.Next. By setting it to PageDirection.Previous AND setting the "previous" link I was able to get it working.

The one part I am still not able to get working is the "self" links. It appears that the ContinueAsync method cannot be used to follow the Self link because there is no PageDirection.Self enumerated value. But it isn't clear to me how I am supposed to perform a subsequent re-search by following the "self" link. Is there a different FhirClient method that I should be using to perform the search captured in the "self" link?

[alexzautke]

That's expected as only the next and prev links are provided for paging, the self link is just a "virtual" link for checking if the search was executed correctly. See https://build.fhir.org/search.html#paginglinks

[TimothyMDean]

OK, thanks for clarifying that. This is now working as I need it to work.