DATAMONGO-1061 - ReadPreference now gets applied on the cursor.

DBCollection instances are cached in the MongoDB driver, which makes them unsuitable for per-operation customization. We previously set the read preference configured on MongoTemplate but that might cause issues if multiple templates with different read preferences are now trying to manipulated the same collection instance.

We now rather set the read preference configured on MongoTemplate on the DBCursor as we get a fresh instance for every query execution.

Related tickets: https://jira.mongodb.org/browse/JAVA-2293.

Author: odrotbohm

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/MongoTemplate.java

@@ -805,11 +805,7 @@ protected void ensureNotIterable(Object o) {
 	 * 
 	 * @param collection
 	 */
-	protected void prepareCollection(DBCollection collection) {
-		if (this.readPreference != null) {
-			collection.setReadPreference(readPreference);
-		}
-	}
+	protected void prepareCollection(DBCollection collection) {}
 
 	/**
 	 * Prepare the WriteConcern before any processing is done using it. This allows a convenient way to apply custom
@@ -2340,32 +2336,41 @@ public DBCursor prepare(DBCursor cursor) {
 			}
 
 			if (query.getSkip() <= 0 && query.getLimit() <= 0 && query.getSortObject() == null
-					&& !StringUtils.hasText(query.getHint()) && !query.getMeta().hasValues()) {
+					&& !StringUtils.hasText(query.getHint()) && !query.getMeta().hasValues() && readPreference == null) {
 				return cursor;
 			}
 
 			DBCursor cursorToUse = cursor.copy();
 
 			try {
+
 				if (query.getSkip() > 0) {
 					cursorToUse = cursorToUse.skip(query.getSkip());
 				}
+
 				if (query.getLimit() > 0) {
 					cursorToUse = cursorToUse.limit(query.getLimit());
 				}
+
 				if (query.getSortObject() != null) {
 					DBObject sortDbo = type != null ? getMappedSortObject(query, type) : query.getSortObject();
 					cursorToUse = cursorToUse.sort(sortDbo);
 				}
+
 				if (StringUtils.hasText(query.getHint())) {
 					cursorToUse = cursorToUse.hint(query.getHint());
 				}
+
 				if (query.getMeta().hasValues()) {
 					for (Entry<String, Object> entry : query.getMeta().values()) {
 						cursorToUse = cursorToUse.addSpecial(entry.getKey(), entry.getValue());
 					}
 				}
 
+				if (readPreference != null) {
+					cursorToUse.setReadPreference(readPreference);
+				}
+
 			} catch (RuntimeException e) {
 				throw potentiallyConvertRuntimeException(e, exceptionTranslator);
 			}

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/MongoTemplateTests.java

@@ -92,7 +92,6 @@
 import com.mongodb.DBRef;
 import com.mongodb.Mongo;
 import com.mongodb.MongoException;
-import com.mongodb.ReadPreference;
 import com.mongodb.WriteConcern;
 import com.mongodb.WriteResult;
 
@@ -1115,26 +1114,6 @@ public void testFindOneWithSort() {
 		assertThat(p5.getFirstName(), is("Mark"));
 	}
 
-	@Test
-	public void testUsingReadPreference() throws Exception {
-		this.template.execute("readPref", new CollectionCallback<Object>() {
-			public Object doInCollection(DBCollection collection) throws MongoException, DataAccessException {
-				assertThat(collection.getOptions(), is(0));
-				assertThat(collection.getDB().getOptions(), is(0));
-				return null;
-			}
-		});
-		MongoTemplate slaveTemplate = new MongoTemplate(factory);
-		slaveTemplate.setReadPreference(ReadPreference.secondary());
-		slaveTemplate.execute("readPref", new CollectionCallback<Object>() {
-			public Object doInCollection(DBCollection collection) throws MongoException, DataAccessException {
-				assertThat(collection.getReadPreference(), is(ReadPreference.secondary()));
-				assertThat(collection.getDB().getOptions(), is(0));
-				return null;
-			}
-		});
-	}
-
 	/**
 	 * @see DATADOC-166
 	 */

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/QueryCursorPreparerUnitTests.java

@@ -33,6 +33,7 @@
 import org.springframework.data.mongodb.core.query.Query;
 
 import com.mongodb.DBCursor;
+import com.mongodb.ReadPreference;
 
 /**
  * Unit tests for {@link QueryCursorPreparer}.
@@ -133,6 +134,19 @@ public void appliesSnapshotCorrectly() {
 		verify(cursorToUse).addSpecial(eq("$snapshot"), eq(true));
 	}
 
+	/**
+	 * @see DATAMONGO-1061
+	 */
+	@Test
+	public void appliesTemplateReadPreferenceToCursor() {
+
+		MongoTemplate template = new MongoTemplate(factory);
+		template.setReadPreference(ReadPreference.secondary());
+		template.new QueryCursorPreparer(query(where("foo").is("bar")), getClass()).prepare(cursor);
+
+		verify(cursorToUse).setReadPreference(ReadPreference.secondary());
+	}
+
 	private DBCursor pepare(Query query) {
 
 		CursorPreparer preparer = new MongoTemplate(factory).new QueryCursorPreparer(query, null);