Introduced experimental X-Index-Revision header

Change-Id: Ida1bf82a59217ff8953cef0e1e5e2c5276f04a39
diff --git a/core/src/main/java/de/ids_mannheim/korap/service/SearchService.java b/core/src/main/java/de/ids_mannheim/korap/service/SearchService.java
index a40cbda..2d414f9 100644
--- a/core/src/main/java/de/ids_mannheim/korap/service/SearchService.java
+++ b/core/src/main/java/de/ids_mannheim/korap/service/SearchService.java
@@ -437,5 +437,11 @@
                     "Admin token is incorrect");
         }
     }
-    
+
+    /**
+     * Return the fingerprint of the latest index revision.
+     */
+    public String getIndexFingerprint () {
+        return searchKrill.getIndexFingerprint();
+    }
 }
diff --git a/core/src/main/java/de/ids_mannheim/korap/service/StatisticService.java b/core/src/main/java/de/ids_mannheim/korap/service/StatisticService.java
index 694e57b..ef016a1 100644
--- a/core/src/main/java/de/ids_mannheim/korap/service/StatisticService.java
+++ b/core/src/main/java/de/ids_mannheim/korap/service/StatisticService.java
@@ -95,4 +95,11 @@
         }
         return stats;
     }
+
+    /**
+     * Return the fingerprint of the latest index revision.
+     */
+    public String getIndexFingerprint () {
+        return searchKrill.getIndexFingerprint();
+    }
 }
diff --git a/core/src/main/java/de/ids_mannheim/korap/web/SearchKrill.java b/core/src/main/java/de/ids_mannheim/korap/web/SearchKrill.java
index bac5912..8096c77 100644
--- a/core/src/main/java/de/ids_mannheim/korap/web/SearchKrill.java
+++ b/core/src/main/java/de/ids_mannheim/korap/web/SearchKrill.java
@@ -359,4 +359,15 @@
                 .append(textID);
         return sb.toString();
     };
+
+    /**
+     * Return the fingerprint of the latest index revision.
+     */
+    public String getIndexFingerprint() {
+        if (index != null) {
+            return index.getFingerprint();
+        };
+        return "null";
+    }
+
 };
diff --git a/core/src/main/java/de/ids_mannheim/korap/web/controller/SearchController.java b/core/src/main/java/de/ids_mannheim/korap/web/controller/SearchController.java
index 3329a4f..128a0de 100644
--- a/core/src/main/java/de/ids_mannheim/korap/web/controller/SearchController.java
+++ b/core/src/main/java/de/ids_mannheim/korap/web/controller/SearchController.java
@@ -71,7 +71,10 @@
     @GET
     @Path("{version}")
     public Response index (){
-        return Response.ok(config.getApiWelcomeMessage()).build();
+        return Response
+            .ok(config.getApiWelcomeMessage())
+            .header("X-Index-Revision", searchService.getIndexFingerprint())
+            .build();
     }
     
     @POST
diff --git a/core/src/main/java/de/ids_mannheim/korap/web/controller/StatisticController.java b/core/src/main/java/de/ids_mannheim/korap/web/controller/StatisticController.java
index 9105af3..229fb08 100644
--- a/core/src/main/java/de/ids_mannheim/korap/web/controller/StatisticController.java
+++ b/core/src/main/java/de/ids_mannheim/korap/web/controller/StatisticController.java
@@ -82,7 +82,11 @@
             if (DEBUG) {
                 jlog.debug("Stats: " + stats);
             }
-            return Response.ok(stats).build();
+
+            return Response
+                .ok(stats)
+                .header("X-Index-Revision", service.getIndexFingerprint())
+                .build();
         }
         catch (KustvaktException e) {
             throw kustvaktResponseHandler.throwit(e);
@@ -96,7 +100,10 @@
             String koralQuery) {
         try {
             String stats = service.retrieveStatisticsForKoralQuery(koralQuery);
-            return Response.ok(stats).build();
+            return Response
+                .ok(stats)
+                .header("X-Index-Revision", service.getIndexFingerprint())
+                .build();
         }
         catch (KustvaktException e) {
             throw kustvaktResponseHandler.throwit(e);