I find the answer on the [QUICKSTART][1]:

&gt; Option `zookeeper` is deprecated, use `--bootstrap-server` instead.

Now it works:

    bin/kafka-console-consumer.sh --bootstrap-server localhost:9092 --topic test --from-beginning


  [1]: https://kafka.apache.org/quickstart

For windows 

**To start zookeeper**

    C:\kafka-2.12&gt;.\bin\windows\zookeeper-server-start.bat .\config\server.properties

**To start Kafka Broker**

    C:\kafka-2.12&gt;.\bin\windows\kafka-server-start.bat .\config\server.properties

**To Create topic** 

    C:\kafka-2.12\bin\windows&gt;kafka-topics.bat --create --zookeeper localhost:2181 --replication-factor 1 --partitions 1 --topic test

**Start Producer**

    C:\kafka-2.12\bin\windows&gt;kafka-console-producer.bat --broker-list localhost:9092 --topic test

**To create consumer**

    C:\kafka-2.12\bin\windows&gt;kafka-console-consumer.bat --bootstrap-server localhost:9092 --topic test --from-beginning

**Topic creation With multi partition with replication factor 1**

    C:\kafka-2.12\bin\windows&gt;kafka-topics --zookeeper localhost:2181 --topic first_topic --create --partitions 3 --replication-factor 1

**To get the list of topic created in system**

    C:\kafka-2.12\bin\windows&gt;kafka-topics --zookeeper localhost:2181 --list

**To get the description of topic created in system**

    C:\kafka-2.12\bin\windows&gt;kafka-topics.bat --describe --zookeeper localhost:2181 --topic test

**To delete topic test created in system**

    C:\kafka-2.12\bin\windows&gt;kafka-run-class.bat kafka.admin.TopicCommand --delete --topic test --zookeeper localhost:2181

**To read message from beginning of topic test created in system(version&gt;2.0)**

    C:\kafka-2.12\bin\windows&gt;kafka-console-consumer.bat --bootstrap-server localhost:9092 --topic test --from-beginn

For windows users use

    bin/kafka-console-consumer.bat --bootstrap-server localhost:9092 --topic test --from-beginning



 **UPDATE**

 - From version 2.1.* the previous answers may not work, always check the official [documentation][1] , this answer provided with version `3.1.0` .

For Linux kernel base systems

    ./kafka-topics.sh --create --topic yourTopicname --bootstrap-server localhost:9092

For Windows systems

    ./kafka-topics.bat --create --topic quickstart-events --bootstrap-server localhost:9092

No need to pass `--from-beginning` when there is no initial offset and overload the creation command with desired [configs][2]


  [1]: https://kafka.apache.org/quickstart
  [2]: https://kafka.apache.org/documentation/#topicconfigs

I know that general answer to unfocusing is to use this piece of code: ```FocusScope.of(context).requestFocus(new FocusNode());``` 

But when TextField has custom focusNode, this code doesn&#39;t seem to work.

```SystemChannels.textInput.invokeMethod(&#39;TextInput.hide&#39;);``` still works, but it only removes the keyboard - field itself is still selected.

The code (irrelevant parts removed):

    class RegisterScreen extends StatelessWidget {
      final phoneNumberTEC = TextEditingController();
      final passwordTEC = TextEditingController();
      final passwordFocusNode = FocusNode();
    
      @override
      Widget build(BuildContext context) {
        return this.keyboardDismisser(
          context: context,
          child: Scaffold(
            appBar: new AppBar(
              title: new Text(&quot;Register&quot;),
            ),
            body: this.page(context),
            resizeToAvoidBottomPadding: false,
          ),
        );
      }
    
      Widget keyboardDismisser({BuildContext context, Widget child}) {
        final gesture = GestureDetector(
          onTap: () {
            this.passwordFocusNode.unfocus();
            FocusScope.of(context).requestFocus(new FocusNode());
            SystemChannels.textInput.invokeMethod(&#39;TextInput.hide&#39;);
          },
          child: child,
        );
        return gesture;
      }
    
      Widget page(BuildContext context) {
        return Column(
            mainAxisAlignment: MainAxisAlignment.spaceBetween,
            children: &lt;Widget&gt;[
              Container(
                padding: new EdgeInsets.all(16.0),
                child: Column(
                    mainAxisAlignment: MainAxisAlignment.center,
                    crossAxisAlignment: CrossAxisAlignment.center,
                    children: &lt;Widget&gt;[
                      this.phoneNumberTextField(context),
                      this.passwordTextField(context),
                    ]
                ),
              ),
              // cutting irrelevant widgets out
              )
            ]
        );
      }
    
      Widget phoneNumberTextField(BuildContext context) {
        return TextField(
          controller: this.phoneNumberTEC,
          decoration: InputDecoration(hintText: &quot;Phone number&quot;),
          onSubmitted: (string) {
            FocusScope.of(context).requestFocus(this.passwordFocusNode);
          },
        );
      }
    
      Widget passwordTextField(BuildContext context) {
        return TextField(
          controller: this.passwordTEC,
          decoration: InputDecoration(hintText: &quot;Password&quot;),
          obscureText: true,
          focusNode: this.passwordFocusNode,
          onSubmitted: (string) {
            this.performRegister(context);
          },
        );
      }
    
    }

How to unfocus TextField that has custom FocusNode?

I am having trouble figuring out how to type `useState` function since it returns a tuple. In essence, I have to provide `null` as initial value for `email` i.e. lets assume I can&#39;t use empty string here.

I then have `setEmail` function to update this state value, which takes in email as string.

ideally I would like to type my `useState` so it expects email to be either string or null if possible. At the moment it inherits it as only `null`

    import * as React from &quot;react&quot;;
    
    const { useState } = React;
    
    function Example() {
      const [state, setState] = useState({ email: null, password: null });
    
      function setEmail(email: string) {
        setState(prevState =&gt; ({ ...prevState, email }))
      }
    
      return &lt;p&gt;{state.email}&lt;/p&gt;
    }

Following error is returned for `setEmail` function since `string` in function argument is not valid type for `null` specified in `useState()`

    [ts]
    Argument of type &#39;(prevState: { email: null; password: null; }) =&gt; { email: string; password: null; }&#39; is not assignable to parameter of type &#39;SetStateAction&lt;{ email: null; password: null; }&gt;&#39;.
      Type &#39;(prevState: { email: null; password: null; }) =&gt; { email: string; password: null; }&#39; is not assignable to type &#39;(prevState: { email: null; password: null; }) =&gt; { email: null; password: null; }&#39;.
        Type &#39;{ email: string; password: null; }&#39; is not assignable to type &#39;{ email: null; password: null; }&#39;.
          Types of property &#39;email&#39; are incompatible.
            Type &#39;string&#39; is not assignable to type &#39;null&#39;. [2345]
    (parameter) prevState: {
        email: null;
        password: null;
    }

Correct way to type nullable state when using React&#39;s useState hook

I&#39;m learning how to use Kafka on this website [link][1](except I&#39;m using port 2182 in zookeeper), but it shows:

&gt; zookeeper is not a recognized option

after executing:

&gt; sudo ./bin/kafka-console-consumer.sh --topic test --zookeeper
&gt; localhost:2182

How to fix it?

Env:

    kafka_2.11-2.1.0
    zookeeper-3.4.10


  [1]: https://data-artisans.com/blog/kafka-flink-a-practical-how-to
  [2]: https://stackoverflow.com/questions/53341838/kafka-consumer-zookeeper-is-not-a-recognized-option

zookeeper is not a recognized option when executing kafka-console-consumer.sh

I'm learning how to use Kafka on this website <a href="https://data-artisans.com/blog/kafka-flink-a-practical-how-to" target="_blank" rel="noopener noreferrer">link</a>(except I'm using port 2182 in zookeeper), but it shows:
> zookeeper is not a recognized option
after executing:
> sudo ./bin/kafka-console-consumer.sh --topic test --zookeeper
> localhost:2182
How to fix it?
Env:
<pre><code class="hljs">kafka_2.11-2.1.0
zookeeper-3.4.10
</code></pre>

For my application, the memory used by the Java process is much more than the heap size.

The system where the containers are running starts to have memory problem because the container is taking much more memory than the heap size.

The heap size is set to 128 MB (`-Xmx128m -Xms128m`) while the container takes up to 1GB of memory. Under normal condition, it needs 500MB. If the docker container has a limit below (e.g. `mem_limit=mem_limit=400MB`) the process gets killed by the out of memory killer of the OS.

**Could you explain why the Java process is using much more memory than the heap? How to size correctly the Docker memory limit? Is there a way to reduce the off-heap memory footprint of the Java process?**


----------


I gather some details about the issue using command from [Native memory tracking in JVM][1].

From the host system, I get the memory used by the container.

    $ docker stats --no-stream 9afcb62a26c8
    CONTAINER ID        NAME                                                                                        CPU %               MEM USAGE / LIMIT   MEM %               NET I/O             BLOCK I/O           PIDS
    9afcb62a26c8        xx-xxxxxxxxxxxxx-xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx.0acbb46bb6fe3ae1b1c99aff3a6073bb7b7ecf85   0.93%               461MiB / 9.744GiB   4.62%               286MB / 7.92MB      157MB / 2.66GB      57

From inside the container, I get the memory used by the process.

    $ ps -p 71 -o pcpu,rss,size,vsize
    %CPU   RSS  SIZE    VSZ
    11.2 486040 580860 3814600

****

    $ jcmd 71 VM.native_memory
    71:
    
    Native Memory Tracking:
    
    Total: reserved=1631932KB, committed=367400KB
    -                 Java Heap (reserved=131072KB, committed=131072KB)
                                (mmap: reserved=131072KB, committed=131072KB) 
     
    -                     Class (reserved=1120142KB, committed=79830KB)
                                (classes #15267)
                                (  instance classes #14230, array classes #1037)
                                (malloc=1934KB #32977) 
                                (mmap: reserved=1118208KB, committed=77896KB) 
                                (  Metadata:   )
                                (    reserved=69632KB, committed=68272KB)
                                (    used=66725KB)
                                (    free=1547KB)
                                (    waste=0KB =0.00%)
                                (  Class space:)
                                (    reserved=1048576KB, committed=9624KB)
                                (    used=8939KB)
                                (    free=685KB)
                                (    waste=0KB =0.00%)
     
    -                    Thread (reserved=24786KB, committed=5294KB)
                                (thread #56)
                                (stack: reserved=24500KB, committed=5008KB)
                                (malloc=198KB #293) 
                                (arena=88KB #110)
     
    -                      Code (reserved=250635KB, committed=45907KB)
                                (malloc=2947KB #13459) 
                                (mmap: reserved=247688KB, committed=42960KB) 
     
    -                        GC (reserved=48091KB, committed=48091KB)
                                (malloc=10439KB #18634) 
                                (mmap: reserved=37652KB, committed=37652KB) 
     
    -                  Compiler (reserved=358KB, committed=358KB)
                                (malloc=249KB #1450) 
                                (arena=109KB #5)
     
    -                  Internal (reserved=1165KB, committed=1165KB)
                                (malloc=1125KB #3363) 
                                (mmap: reserved=40KB, committed=40KB) 
     
    -                     Other (reserved=16696KB, committed=16696KB)
                                (malloc=16696KB #35) 
     
    -                    Symbol (reserved=15277KB, committed=15277KB)
                                (malloc=13543KB #180850) 
                                (arena=1734KB #1)
     
    -    Native Memory Tracking (reserved=4436KB, committed=4436KB)
                                (malloc=378KB #5359) 
                                (tracking overhead=4058KB)
     
    -        Shared class space (reserved=17144KB, committed=17144KB)
                                (mmap: reserved=17144KB, committed=17144KB) 
     
    -               Arena Chunk (reserved=1850KB, committed=1850KB)
                                (malloc=1850KB) 
     
    -                   Logging (reserved=4KB, committed=4KB)
                                (malloc=4KB #179) 
     
    -                 Arguments (reserved=19KB, committed=19KB)
                                (malloc=19KB #512) 
     
    -                    Module (reserved=258KB, committed=258KB)
                                (malloc=258KB #2356) 

    $ cat /proc/71/smaps | grep Rss | cut -d: -f2 | tr -d &quot; &quot; | cut -f1 -dk | sort -n | awk &#39;{ sum += $1 } END { print sum }&#39;
    491080

The application is a web server using Jetty/Jersey/CDI bundled inside a fat far of 36 MB.

The following version of OS and Java are used (inside the container). The Docker image is based on `openjdk:11-jre-slim`.

    $ java -version
    openjdk version &quot;11&quot; 2018-09-25
    OpenJDK Runtime Environment (build 11+28-Debian-1)
    OpenJDK 64-Bit Server VM (build 11+28-Debian-1, mixed mode, sharing)
    $ uname -a
    Linux service1 4.9.125-linuxkit #1 SMP Fri Sep 7 08:20:28 UTC 2018 x86_64 GNU/Linux

https://gist.github.com/prasanthj/48e7063cac88eb396bc9961fb3149b58


  [1]: https://gist.github.com/prasanthj/48e7063cac88eb396bc9961fb3149b58

Java using much more memory than heap size (or size correctly Docker memory limit)

The following Java program takes on average between 0.50 secs and 0.55 secs to run:

    public static void main(String[] args) {
        long startTime = System.nanoTime();
        int n = 0;
        for (int i = 0; i &lt; 1000000000; i++) {
            n += 2 * (i * i);
        }
        System.out.println((double) (System.nanoTime() - startTime) / 1000000000 + &quot; s&quot;);
        System.out.println(&quot;n = &quot; + n);
    }

If I replace `2 * (i * i)` with `2 * i * i`, it takes between 0.60 and 0.65 secs to run. How come?

I ran each version of the program 15 times, alternating between the two. Here are the results:

     2*(i*i)  |  2*i*i
    ----------+----------
    0.5183738 | 0.6246434
    0.5298337 | 0.6049722
    0.5308647 | 0.6603363
    0.5133458 | 0.6243328
    0.5003011 | 0.6541802
    0.5366181 | 0.6312638
    0.515149  | 0.6241105
    0.5237389 | 0.627815
    0.5249942 | 0.6114252
    0.5641624 | 0.6781033
    0.538412  | 0.6393969
    0.5466744 | 0.6608845
    0.531159  | 0.6201077
    0.5048032 | 0.6511559
    0.5232789 | 0.6544526

The fastest run of `2 * i * i` took longer than the slowest run of `2 * (i * i)`. If they had the same efficiency, the probability of this happening would be less than `1/2^15 * 100% = 0.00305%`.


Why is 2 * (i * i) faster than 2 * i * i in Java?

[I ran cross this puzzler from an advanced programming course at a UK university exam][1].

Consider the following loop, in which i is, so far, undeclared:

    while (i == i + 1) {}

Find the definition of `i`, that precedes this loop, *such that the while loop
continues for ever.*

The next question, which asked the same question for this code snippet:

    while (i != i) {}

was obvious to me. Of course in this other situation it is `NaN` but I am really stuck on the prior one. Does this have to do with overflow? What would cause such a loop to loop for ever in Java?

  [1]: http://homepages.inf.ed.ac.uk/perdita/inf1opadvanced2015.pdf

For what value of i does while (i == i + 1) {} loop forever?

I&#39;m trying to install Spark on my Mac. I&#39;ve used home-brew to install spark 2.4.0 and Scala. I&#39;ve installed PySpark in my anaconda environment and am using PyCharm for development. I&#39;ve exported to my bash profile:

    export SPARK_VERSION=`ls /usr/local/Cellar/apache-spark/ | sort | tail -1`
    export SPARK_HOME=&quot;/usr/local/Cellar/apache-spark/$SPARK_VERSION/libexec&quot;
    export PYTHONPATH=$SPARK_HOME/python/:$PYTHONPATH
    export PYTHONPATH=$SPARK_HOME/python/lib/py4j-0.9-src.zip:$PYTHONPATH

However I&#39;m unable to get it to work. 

I suspect this is due to java version from reading the traceback. I would really appreciate some help fixed the issue. Please comment if there is any information I could provide that is helpful beyond the traceback.

I am getting the following error: 

    Traceback (most recent call last):
      File &quot;&lt;input&gt;&quot;, line 4, in &lt;module&gt;
      File &quot;/anaconda3/envs/coda/lib/python3.6/site-packages/pyspark/rdd.py&quot;, line 816, in collect
        sock_info = self.ctx._jvm.PythonRDD.collectAndServe(self._jrdd.rdd())
      File &quot;/anaconda3/envs/coda/lib/python3.6/site-packages/py4j/java_gateway.py&quot;, line 1257, in __call__
        answer, self.gateway_client, self.target_id, self.name)
      File &quot;/anaconda3/envs/coda/lib/python3.6/site-packages/py4j/protocol.py&quot;, line 328, in get_return_value
        format(target_id, &quot;.&quot;, name), value)
    py4j.protocol.Py4JJavaError: An error occurred while calling z:org.apache.spark.api.python.PythonRDD.collectAndServe.
    : java.lang.IllegalArgumentException: Unsupported class file major version 55


  [1]: https://stackoverflow.com/questions/24342886/how-to-install-java-8-on-mac/55774255#55774255

Spark Error - Unsupported class file major version

I&#39;m new to project [Reactor][1] and reactive programming in general.

I&#39;m currently working on a piece of code similar to this:

	Mono.just(userId)
		.map(repo::findById)
		.map(user-&gt; {
			if(user == null){
				throw new UserNotFoundException();
			}
			return user;
		})
		// ... other mappings

This example is probably silly and there are surely better ways of implementing this case, but the point is: 

Is it wrong to use a `throw new` exception in a `map` block or should I replace this with a `return Mono.error(new UserNotFoundException())`?

Is there any actual difference in these two ways of doing?



  [1]: https://projectreactor.io/

Correct way of throwing exceptions with Reactor

Nowadays in microservice world, i’m seeing alot of design in my workplace that uses kafka messaging when you can achieve similar results using rest api calls between microservices. Technically you can stop using rest api calls altogether and instead use kafka messaging. I really want to know the best practice, pros and cons, when to use api calls between microsevices, when to use kafka messaging. 


Lets put a real life example:

I have an inventory service and a vendor service. Everyday vendor service calls the vendor API to get new items and these need to be moved into inventory service. The number of items can be up to 10,000 objects.

For this use case, is it better to :

1. After getting new data from vendor API, call REST API of inventory service to store the new items.

2. After getting new data from vendor API, send them as message to a kafka topic, to be consumed by inventory service 

Which way would you choose and what is the consideration

Content Type	Original Author	Original Content on Stackoverflow
Question	xingbin	View Question on Stackoverflow
Solution 1 - Java	xingbin	View Answer on Stackoverflow
Solution 2 - Java	black_pottery_beauty	View Answer on Stackoverflow
Solution 3 - Java	Gurpreet Singh	View Answer on Stackoverflow
Solution 4 - Java	Lunatic	View Answer on Stackoverflow

zookeeper is not a recognized option when executing kafka-console-consumer.sh

Java Problem Overview

Java Solutions

Solution 1 - Java

Solution 2 - Java

Solution 3 - Java

Solution 4 - Java

Correct way to type nullable state when using React's useState hook

How to unfocus TextField that has custom FocusNode?

Attributions