Github user joshelser commented on a diff in the pull request:
https://github.com/apache/incubator-ratis/pull/4#discussion_r213797086
--- Diff:
ratis-logservice/src/main/java/org/apache/ratis/logservice/api/LogReader.java
---
@@ -0,0 +1,53 @@
+/**
+ * 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.ratis.logservice.api;
+
+import java.io.IOException;
+import java.nio.ByteBuffer;
+import java.util.List;
+
+/**
+ * Synchronous client interface to read from a LogStream.
+ */
+public interface LogReader extends AutoCloseable {
+
+ /**
+ * Seeks to the position before the record at the provided {@code
offset} in the LogStream.
+ *
+ * @param offset A non-negative, offset in the LogStream
+ * @return A future for when the operation is completed.
+ */
+ void seek(long recordId) throws IOException;
--- End diff --
Moving to a specific point in a LogStream is absolutely a necessary API
call. We must be able to resume reading from a known point in the Log. HBase
Replication needs that functionality.
A naive implementation can read all records from the LogStream to know
where to start reading. A slightly more advanced implementation can use the
underlying segment files to find the right segment file to read from. An
advance implementation could consult an index which we explicitly maintain over
all segment files.
We haven't touched on implementation of how the LogReader class would
negotiate this call with the back-end StateMachine (that depends on what the
StateMachine and/or Ratis APIs look like, I think).
---
