Github user rmetzger commented on a diff in the pull request:
https://github.com/apache/flink/pull/659#discussion_r29853065
--- Diff:
flink-staging/flink-streaming/flink-streaming-core/src/main/java/org/apache/flink/streaming/api/functions/source/Source.java
---
@@ -0,0 +1,87 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements. See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership. The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.flink.streaming.api.functions.source;
+
+import org.apache.flink.api.common.functions.Function;
+import org.joda.time.Instant;
+
+import java.io.Serializable;
+
+/**
+ * Base interface for all stream data sources in Flink. The contract of a
stream source
+ * is similar to an iterator - it is consumed as in the following pseudo
code:
+ *
+ * <pre>{@code
+ * StreamSource<T> source = ...;
+ * Collector<T> out = ...;
+ * while (!source.reachedEnd()) {
+ * out.collect(source.next());
+ * }
+ * }
+ * </pre>
+ *
+ * <b>Note about blocking behavior</b>
+ * <p>This implementations of the methods in the stream sources must have
certain guarantees about
+ * blocking behavior. One of the two characteristics must be fulfilled.</p>
+ * <ul>
+ * <li>The methods must react to thread interrupt calls and break out
of blocking calls with
+ * an {@link InterruptedException}.</li>
+ * <li>The method may ignore interrupt calls and/or swallow
InterruptedExceptions, if it is guaranteed
+ * that the method returns quasi immediately irrespectively of the
input. This is true for example
+ * for file streams, where the call is guaranteed to return after
a very short I/O delay in
+ * the order of milliseconds.</li>
+ * </ul>
+ *
+ * @param <T> The type of the records produced by this source.
+ */
+public interface Source<T> extends Function, Serializable {
+
+ /**
+ * Checks whether the stream has reached its end.
+ *
+ * <p>This method must obey the contract about blocking behavior
declared in the
+ * description of this class.</p>
+ *
+ * @return True, if the end of the stream has been reached, false if
more data is available.
+ *
+ * @throws InterruptedException The calling thread may be interrupted
to pull the function out of this
+ * method during checkpoints.
+ * @throws Exception Any other exception that is thrown causes the
source to fail and results in failure of
+ * the streaming program, or triggers recovery,
depending on the program setup.
+ */
+ boolean reachedEnd() throws Exception;
+
+
+ /**
+ * Produces the next record.
+ *
+ * <p>This method must obey the contract about blocking behavior
declared in the
+ * description of this class.</p>
+ *
+ * @return The next record produced by this stream source.
+ *
+ * @throws InterruptedException The calling thread may be interrupted
to pull the function out of this
+ * method during checkpoints.
+ * @throws Exception Any other exception that is thrown causes the
source to fail and results in failure of
+ * the streaming program, or triggers recovery,
depending on the program setup.
+ */
+ T next() throws Exception;
+
+ Instant getLowWatermark();
--- End diff --
How about turning the interface into an abstract class which always returns
`Instant.now()` ?
A lot of sources are probably doing it like that.
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