ASF GitHub Bot commented on FLINK-8547:

Github user pnowojski commented on a diff in the pull request:

    --- Diff: 
    @@ -0,0 +1,61 @@
    + * 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.runtime.io;
    +import org.apache.flink.annotation.Internal;
    +import java.io.IOException;
    + * The buffer blocker takes the buffers and events from a data stream and 
adds them in a sequence.
    + * After a number of elements have been added, the blocker can "roll 
over": It presents the added
    + * elements as a readable sequence, and creates a new sequence.
    + */
    +public interface BufferBlocker {
    +   /**
    +    * Adds a buffer or event to the blocker.
    +    *
    +    * @param boe The buffer or event to be added into the blocker.
    +    */
    +   void add(BufferOrEvent boe) throws IOException;
    +   /**
    +    * Starts a new sequence of buffers and event and returns the current 
sequence of buffers for reading.
    +    * This method returns {@code null}, if nothing was added since the 
creation, or the last call to this method.
    +    *
    +    * @param newBuffer only works for {@link BufferSpiller} implements 
    --- End diff --
    Java doc in this interface shouldn't mention implementation specific 
details. On the other hand, this java doc doesn't explain what `newBuffer` is 
doing and for this information one must check the `BufferSpiller`'s java doc 
    Can you add appropriate java doc here, or better add java doc to proposed 
in the comment below two methods:  `rollOverWithoutReusingResources()` and 
`rollOverReusingResources()`. Comment in 
`CachedBufferBlocker.java#rollOverReusingResources` should state that it is 
never reusing resources and is defaulting to 

> Implement CheckpointBarrierHandler not to spill data for exactly-once based 
> on credit-based flow control
> --------------------------------------------------------------------------------------------------------
>                 Key: FLINK-8547
>                 URL: https://issues.apache.org/jira/browse/FLINK-8547
>             Project: Flink
>          Issue Type: Sub-task
>          Components: Network
>    Affects Versions: 1.5.0
>            Reporter: zhijiang
>            Assignee: zhijiang
>            Priority: Major
> Currently in exactly-once mode, the {{BarrierBuffer}} would block inputs with 
> barriers until all inputs have received the barrier for a given checkpoint. 
> To avoid back-pressuring the input streams which may cause distributed 
> deadlocks, the {{BarrierBuffer}} has to spill the data in disk files to 
> recycle the buffers for blocked channels.
> Based on credit-based flow control, every channel has exclusive buffers, so 
> it is no need to spill data for avoiding deadlock. Then we implement a new 
> {{CheckpointBarrierHandler}} for only buffering the data for blocked channels 
> for better performance.
> And this new {{CheckpointBarrierHandler}} can also be configured to use or 
> not in order to rollback the original mode for unexpected risks.

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