[jira] [Commented] (KAFKA-7326) Let KStream.print() to flush on each printed line

["ASF GitHub Bot (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/KAFKA-7326?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16599673#comment-16599673 ] 

ASF GitHub Bot commented on KAFKA-7326:
---------------------------------------

guozhangwang closed pull request #5579: KAFKA-7326: KStream.print() should flush on each line for PrintStream
URL: https://github.com/apache/kafka/pull/5579
 
 
   

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diff --git a/docs/streams/developer-guide/dsl-api.html b/docs/streams/developer-guide/dsl-api.html
index beb83a3970a..da1b78dafcb 100644
--- a/docs/streams/developer-guide/dsl-api.html
+++ b/docs/streams/developer-guide/dsl-api.html
@@ -631,6 +631,7 @@ <h2><a class="toc-backref" href="#id7">Overview</a><a class="headerlink" href="#
                             caveats.
                             (<a class="reference external" href="../../../javadoc/org/apache/kafka/streams/kstream/KStream.html#print--">details</a>)</p>
                             <p>Calling <code class="docutils literal"><span class="pre">print()</span></code> is the same as calling <code class="docutils literal"><span class="pre">foreach((key,</span> <span class="pre">value)</span> <span class="pre">-&gt;</span> <span class="pre">System.out.println(key</span> <span class="pre">+</span> <span class="pre">&quot;,</span> <span class="pre">&quot;</span> <span class="pre">+</span> <span class="pre">value))</span></code></p>
+                            <p><code class="docutils literal"><span class="pre">print</span></code> is mainly for debugging/testing purposes, and it will try to flush on each record print. Hence it <strong>should not</strong> be used for production usage if performance requirements are concerned.</p>
                             <div class="last highlight-java"><div class="highlight"><pre><span></span><span class="n">KStream</span><span class="o">&lt;</span><span class="kt">byte</span><span class="o">[],</span> <span class="n">String</span><span class="o">&gt;</span> <span class="n">stream</span> <span class="o">=</span> <span class="o">...;</span>
 <span class="c1">// print to sysout</span>
 <span class="n">stream</span><span class="o">.</span><span class="na">print</span><span class="o">();</span>
diff --git a/streams/src/main/java/org/apache/kafka/streams/kstream/KStream.java b/streams/src/main/java/org/apache/kafka/streams/kstream/KStream.java
index ae3b28a35ce..cf2ce75450e 100644
--- a/streams/src/main/java/org/apache/kafka/streams/kstream/KStream.java
+++ b/streams/src/main/java/org/apache/kafka/streams/kstream/KStream.java
@@ -359,6 +359,9 @@
 
     /**
      * Print the records of this KStream using the options provided by {@link Printed}
+     * Note that this is mainly for debugging/testing purposes, and it will try to flush on each record print.
+     * It <em>SHOULD NOT</em> be used for production usage if performance requirements are concerned.
+     *
      * @param printed options for printing
      */
     void print(final Printed<K, V> printed);
diff --git a/streams/src/main/java/org/apache/kafka/streams/kstream/internals/PrintForeachAction.java b/streams/src/main/java/org/apache/kafka/streams/kstream/internals/PrintForeachAction.java
index 174319fec4b..861dfd3b84e 100644
--- a/streams/src/main/java/org/apache/kafka/streams/kstream/internals/PrintForeachAction.java
+++ b/streams/src/main/java/org/apache/kafka/streams/kstream/internals/PrintForeachAction.java
@@ -51,6 +51,9 @@
     public void apply(final K key, final V value) {
         final String data = String.format("[%s]: %s", label, mapper.apply(key, value));
         printWriter.println(data);
+        if (!closable) {
+            printWriter.flush();
+        }
     }
 
     public void close() {


 

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> Let KStream.print() to flush on each printed line
> -------------------------------------------------
>
>                 Key: KAFKA-7326
>                 URL: https://issues.apache.org/jira/browse/KAFKA-7326
>             Project: Kafka
>          Issue Type: Improvement
>          Components: streams
>    Affects Versions: 2.0.0
>            Reporter: Guozhang Wang
>            Assignee: huxihx
>            Priority: Major
>              Labels: newbie++
>
> Today, {{KStream.print()}} is implemented as a special "foreach" function as below:
> {code}
> @Override
>     public void apply(final K key, final V value) {
>         final String data = String.format("[%s]: %s", label, mapper.apply(key, value));
>         printWriter.println(data);
>     }
> {code}
> Note that since {{this.printWriter = new PrintWriter(new OutputStreamWriter(outputStream, StandardCharsets.UTF_8));}}, without flushing the writer we do not guarantee that printed lines are written to the underlying `outputStream` in time.
> Since {{KStream.print()}} is mainly for debugging / testing / demoing purposes, not for performance, I think it is okay to enforce auto flushing.
> This would include:
> 1. set {{autoFlush}} in the constructor of printWriter.
> 2. document in java-docs of {{KStream.print}} that this is for debug / testing purposes only, and it will try to flush on each record print, and hence should not be used for production usage if performance requirement is key.



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