EHN: ETag property for the Query

CHANGELOG.md

@@ -9,6 +9,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Added
 
+* `ETag` property for the query. The `ETag` tags a server state and is being
+  updated each time the server state is being updated (i.e. the stored entities
+  change). This can be used to debug the query cache and also allows a client
+  to determine whether the server's state has changed between queries.
 * Basic caching for queries. The caching is enabled by default and can be
   controlled by the usual "cache" flag.
 

src/main/java/org/caosdb/server/query/Query.java

@@ -39,6 +39,7 @@ import java.util.LinkedList;
 import java.util.List;
 import java.util.Map;
 import java.util.Map.Entry;
+import java.util.UUID;
 import org.antlr.v4.runtime.CharStreams;
 import org.antlr.v4.runtime.CommonTokenStream;
 import org.apache.commons.jcs.access.behavior.ICacheAccess;
@@ -238,6 +239,14 @@ public class Query implements QueryInterface, ToElementable, TransactionInterfac
    */
   private boolean cachable = true;
 
+  /**
+   * Tags the query cache and is renewed each time the cache is being cleared, i.e. each time the
+   * database is being updated.
+   *
+   * <p>As the name suggests, the idea is similar to the ETag header of the HTTP protocol.
+   */
+  private static String cacheETag = UUID.randomUUID().toString();
+
   public Type getType() {
     return this.type;
   }
@@ -668,6 +677,7 @@ public class Query implements QueryInterface, ToElementable, TransactionInterfac
 
   /** Remove all cached queries from the cache. */
   public static void clearCache() {
+    cacheETag = UUID.randomUUID().toString();
     cache.clear();
   }
 
@@ -678,10 +688,12 @@ public class Query implements QueryInterface, ToElementable, TransactionInterfac
    * @param resultSet
    */
   private void setCache(String key, List<IdVersionPair> resultSet) {
-    if (resultSet instanceof Serializable) {
-      cache.put(key, (Serializable) resultSet);
-    } else {
-      cache.put(key, new ArrayList<>(resultSet));
+    synchronized (cache) {
+      if (resultSet instanceof Serializable) {
+        cache.put(key, (Serializable) resultSet);
+      } else {
+        cache.put(key, new ArrayList<>(resultSet));
+      }
     }
   }
 
@@ -859,6 +871,7 @@ public class Query implements QueryInterface, ToElementable, TransactionInterfac
       ret.setAttribute("results", "0");
     }
     ret.setAttribute("cached", Boolean.toString(this.cached));
+    ret.setAttribute("etag", cacheETag);
 
     final Element parseTreeElem = new Element("ParseTree");
     if (this.el.hasErrors()) {