Add Previous Link

src/Microsoft.Health.Fhir.Api/Features/Routing/UrlResolver.cs

@@ -145,7 +145,7 @@
                 Request.Host.Value);
         }
 
-        public Uri ResolveRouteUrl(IReadOnlyCollection<Tuple<string, string>> unsupportedSearchParams = null, IReadOnlyList<(SearchParameterInfo searchParameterInfo, SortOrder sortOrder)> resultSortOrder = null, string continuationToken = null, bool removeTotalParameter = false)
+        public Uri ResolveRouteUrl(IReadOnlyCollection<Tuple<string, string>> unsupportedSearchParams = null, IReadOnlyList<(SearchParameterInfo searchParameterInfo, SortOrder sortOrder)> resultSortOrder = null, string continuationToken = null, bool removeTotalParameter = false, bool addPreviousLink = true)
         {
             string routeName = _fhirRequestContextAccessor.RequestContext.RouteName;
 
@@ -212,7 +212,15 @@
             // Update continuationToken if new value is provided.
             if (continuationToken != null)
             {
-                routeValues[KnownQueryParameterNames.ContinuationToken] = continuationToken;
+                if (Request.Headers.TryGetValue(KnownHeaders.AddPreviousLink, out var addPreviousLinkHeader) && addPreviousLinkHeader == "true" && addPreviousLink)
+                {
+                    var continuationTokenWithPreviousLink = continuationToken + "||" + routeValues[KnownQueryParameterNames.ContinuationToken];
+                    routeValues[KnownQueryParameterNames.ContinuationToken] = continuationTokenWithPreviousLink;
+                }
+                else
+                {
+                    routeValues[KnownQueryParameterNames.ContinuationToken] = continuationToken;
+                }
             }
 
             return GetRouteUri(
@@ -223,6 +231,21 @@
                 Request.Host.Value);
         }
 
+        public Uri ResolvePreviousRouteUrl()
+        {
+            if (Request.Headers.TryGetValue(KnownHeaders.AddPreviousLink, out var addPreviousLinkHeader) && addPreviousLinkHeader == "true")
+            {
+                if (Request.Query != null && Request.Query.TryGetValue(KnownQueryParameterNames.ContinuationToken, out var continuationToken))
+                {
+                    // Removes the most recent continuation token segment and returns the previous continuation token.
+                    var previousContinuationToken = continuationToken[0].Split("||", 2)[1];
+                    return ResolveRouteUrl(null, null, previousContinuationToken, false, false);
+                }
+            }
+
+            return null;
+        }
+
         public Uri ResolveRouteNameUrl(string routeName, IDictionary<string, object> routeValues)
         {
             var routeValueDictionary = new RouteValueDictionary(routeValues);

src/Microsoft.Health.Fhir.Core/Features/KnownHeaders.cs

@@ -31,6 +31,7 @@ public static class KnownHeaders
         public const string CustomAuditHeaderPrefix = "X-MS-AZUREFHIR-AUDIT-";
         public const string FhirUserHeader = "x-ms-fhiruser";
         public const string QueryLatencyOverEfficiency = "x-ms-query-latency-over-efficiency";
+        public const string AddPreviousLink = "x-ms-add-previous-link";
 
         // #conditionalQueryParallelism - Header used to activate parallel conditional-query processing.
         public const string ConditionalQueryProcessingLogic = "x-conditionalquery-processing-logic";

src/Microsoft.Health.Fhir.Core/Features/Routing/IUrlResolver.cs

@@ -46,8 +46,11 @@ public interface IUrlResolver
         /// <param name="resultSortOrder">The order the results are sorted in</param>
         /// <param name="continuationToken">The continuation token.</param>
         /// <param name="removeTotalParameter">True if the _total parameter should be removed from the url, false otherwise.</param>
+        /// <param name="addPreviousLink">Whether to add the previous link</param>
         /// <returns>The URL.</returns>
-        Uri ResolveRouteUrl(IReadOnlyCollection<Tuple<string, string>> unsupportedSearchParams = null, IReadOnlyList<(SearchParameterInfo searchParameterInfo, SortOrder sortOrder)> resultSortOrder = null, string continuationToken = null, bool removeTotalParameter = false);
+        Uri ResolveRouteUrl(IReadOnlyCollection<Tuple<string, string>> unsupportedSearchParams = null, IReadOnlyList<(SearchParameterInfo searchParameterInfo, SortOrder sortOrder)> resultSortOrder = null, string continuationToken = null, bool removeTotalParameter = false, bool addPreviousLink = true);
+
+        Uri ResolvePreviousRouteUrl();
 
         /// <summary>
         /// Resolves the URL for the specified routeName.

src/Microsoft.Health.Fhir.Shared.Core/Features/Operations/Everything/PatientEverythingService.cs

@@ -100,7 +100,7 @@ public async Task<SearchResult> SearchAsync(
         {
             EverythingOperationContinuationToken token = string.IsNullOrEmpty(continuationToken)
                 ? new EverythingOperationContinuationToken()
-                : EverythingOperationContinuationToken.FromJson(ContinuationTokenConverter.Decode(continuationToken));
+                : EverythingOperationContinuationToken.FromJson(ContinuationTokenConverter.Decode(continuationToken.Split("||", 2)[0]));
 
             if (token == null || token.Phase < 0 || token.Phase > 3)
             {

src/Microsoft.Health.Fhir.Shared.Core/Features/Search/BundleFactory.cs

@@ -156,6 +156,7 @@ private void CreateLinks(SearchResult result, Bundle bundle)
                         result.UnsupportedSearchParameters,
                         result.SortOrder,
                         ContinuationTokenConverter.Encode(result.ContinuationToken),
+                        true,
                         true);
                 }
                 catch (UriFormatException)
@@ -168,6 +169,13 @@ private void CreateLinks(SearchResult result, Bundle bundle)
             {
                 // Add the self link to indicate which search parameters were used.
                 bundle.SelfLink = _urlResolver.ResolveRouteUrl(result.UnsupportedSearchParameters, result.SortOrder);
+
+                // Add flag check
+                var previous = _urlResolver.ResolvePreviousRouteUrl();
+                if (previous != null)
+                {
+                    bundle.PreviousLink = previous;
+                }
             }
             catch (UriFormatException)
             {

src/Microsoft.Health.Fhir.Shared.Core/Features/Search/SearchOptionsFactory.cs

@@ -133,7 +133,8 @@ public SearchOptions Create(
                             string.Format(Core.Resources.MultipleQueryParametersNotAllowed, KnownQueryParameterNames.ContinuationToken));
                     }
 
-                    continuationToken = ContinuationTokenConverter.Decode(query.Item2);
+                    var fullContinuationToken = query.Item2;
+                    continuationToken = ContinuationTokenConverter.Decode(fullContinuationToken.Split("||", 2)[0]);
                     setDefaultBundleTotal = false;
                 }
                 else if (string.Equals(query.Item1, KnownQueryParameterNames.FeedRange, StringComparison.OrdinalIgnoreCase))