If all scroll ids should be removed then the `_all` value should be used instead of not specifying any scroll ids.

docs/reference/search/request/search-type.asciidoc

@@ -142,5 +142,10 @@ resources to keep the view open will cleaned open. Example usage:
 curl -XDELETE 'localhost:9200/_search/scroll/c2NhbjsxOjBLMzdpWEtqU2IyZHlmVURPeFJOZnc7MzowSzM3aVhLalNiMmR5ZlVET3hSTmZ3OzU6MEszN2lYS2pTYjJkeWZVRE94Uk5mdzsyOjBLMzdpWEtqU2IyZHlmVURPeFJOZnc7NDowSzM3aVhLalNiMmR5ZlVET3hSTmZ3Ow=='
 --------------------------------------------------
 
-Multiple scroll ids can be specified in a comma separated manner, if no id is
-specified then all scroll ids will be cleared up.
+Multiple scroll ids can be specified in a comma separated manner.
+If all scroll ids need to be cleared the reserved `_all` value can used instead of an actual `scroll_id`:
+
+[source,js]
+--------------------------------------------------
+curl -XDELETE 'localhost:9200/_search/scroll/_all'
+--------------------------------------------------

src/main/java/org/elasticsearch/action/search/ClearScrollRequest.java

@@ -29,6 +29,7 @@ import java.util.Arrays;
 import java.util.List;
 
 import static com.google.common.collect.Lists.newArrayList;
+import static org.elasticsearch.action.ValidateActions.addValidationError;
 
 /**
  */
@@ -53,7 +54,11 @@ public class ClearScrollRequest extends ActionRequest {
 
     @Override
     public ActionRequestValidationException validate() {
-        return null;
+        ActionRequestValidationException validationException = null;
+        if (scrollIds == null || scrollIds.isEmpty()) {
+            validationException = addValidationError("no scroll ids specified", validationException);
+        }
+        return validationException;
     }
 
     @Override

src/main/java/org/elasticsearch/action/search/TransportClearScrollAction.java

@@ -69,7 +69,7 @@ public class TransportClearScrollAction extends TransportAction<ClearScrollReque
         private Async(ClearScrollRequest request, ActionListener<ClearScrollResponse> listener, ClusterState clusterState) {
             int expectedOps = 0;
             this.nodes = clusterState.nodes();
-            if (request.getScrollIds() == null || request.getScrollIds().isEmpty()) {
+            if (request.getScrollIds().size() == 1 && "_all".equals(request.getScrollIds().get(0))) {
                 expectedOps = nodes.size();
             } else {
                 for (String parsedScrollId : request.getScrollIds()) {

src/test/java/org/elasticsearch/search/scroll/SearchScrollTests.java

@@ -19,6 +19,7 @@
 
 package org.elasticsearch.search.scroll;
 
+import org.elasticsearch.AbstractSharedClusterTest;
 import org.elasticsearch.action.search.ClearScrollResponse;
 import org.elasticsearch.action.search.SearchResponse;
 import org.elasticsearch.action.search.SearchType;
@@ -27,7 +28,6 @@ import org.elasticsearch.common.settings.ImmutableSettings;
 import org.elasticsearch.common.unit.TimeValue;
 import org.elasticsearch.search.SearchHit;
 import org.elasticsearch.search.sort.SortOrder;
-import org.elasticsearch.AbstractSharedClusterTest;
 import org.junit.Test;
 
 import java.util.Map;
@@ -367,7 +367,7 @@ public class SearchScrollTests extends AbstractSharedClusterTest {
             assertThat(((Number) hit.sortValues()[0]).longValue(), equalTo(counter2++));
         }
 
-        ClearScrollResponse clearResponse = client().prepareClearScroll()
+        ClearScrollResponse clearResponse = client().prepareClearScroll().addScrollId("_all")
                 .execute().actionGet();
         assertThat(clearResponse.isSucceeded(), equalTo(true));