Analyze Linux disk usage

Fixing a disk space problem on a Linux server without a graphical interface is tedious without the right command line tools handy.

Disk usage

A quick df -h  lists all disks and their current usage.

Find large files

To find large folders with files to clean up, you can use ncdu. Install it ( sudo apt-get install ncdu ) and start it pointing to a certain folder.

Why I love Kotlin

At avisec we decided to use Kotlin when we started to rewrite our backend service. This has proven to be a very good decision. Here are my top 10 reasons why I do love Kotlin:

  1. Data classes eliminate a lot of boilerplate code.
  2. The nullability support of the compiler forces you to think about nulls everywhere. This reduces the number of NullpointerException related bugs to a minimum.
  3. The lambda syntax is very, very lean.
  4. No more guessing which functional Java interface to use.
  5. The standard library provides is a great help.
  6. The IntelliJ support is seamless.
  7. Java and Kotlin integrate both ways – you can start writing Kotlin code in Java projects today, no big deal.
  8. Return values are supported for most (?) structures: if, try/catch, when
  9. Assign default values to parameters and used named parameters when calling functions.
  10. Generics in Kotlin are more powerful than in Java.

Let’s hope the current adoption rate of Kotlin continues and it will become a widely used alternative for Java on the JVM.

Mosquitto TLS instability

We’ve been facing recurring TLS issues with Mosquitto, our MQTT broker. The clients tried to send a message and lost the connection in a random and non-reproducible manner. In the Mosquitto error log we always found the following problem:

This issue is reported on GitHub and there seems to be no solution to it yet. However wiebeytec recommended to use an NGINX stream proxy to terminate the TLS connection and forward the decrypted traffic locally to Mosquitto. This is what we did and it solved our issue too!


Launch Jenkins using Docker Compose

To launch a Jenkins server in a Docker container using Docker Compose, the following docker-compose.yml defines a Jenkins server with a persistent job configuration.

And start then start it:

You should now have a Jenkins running on localhost:8080.

If you need a bash to change any configuration, start it like this:


Java startup parameters

The following Java startup parameters should be used (and the values adjusted) when running a Java process on a server.

When a Java process starts, it overwrites an existing GC log file. I would recommend to either use a timestamp in the filename or use the %pid placeholder.

If you want to use JMX, also add the following parameters:

Functional Interfaces in Java 8

With Java 8, the concept of Lambdas was finally added to the language specification. Often you’re using Lambdas on the fly without assigning them to a variable.

However if you want to save a Lambda into a variable and pass it as a method parameter or use it later, you need to pick the applicable type.

Here is the list of the Interfaces you might use from the Java API:

  • Runnable // nothing
  • Supplier<T> // a return value
  • Consumer<T> // a parameter
  • BiConsumer<T, U> // two parameters
  • Function<T, R> // a parameter and a return value
  • BiFunction<T, U, R> // two parameters and a return value
  • UnaryOperator<T> // the parameter and the return value have the same type
  • BinaryOperator<T> // two parameters and a return value of the same type
  • Predicate<T> // a parameter and a boolean return value
  • BiPredicate<T, U> // two parameters and a boolean return value

InformIT presents all those Interfaces using a nice table in the article “Java SE 8 for the Really Impatient: Programming with Lambdas“.


Linux kills a process when when starving memory

I recently observed a rather strange problem with randomly dying Java processes under heavy load. The vital clue to this problem was hidden in /var/log/messages as shown below:

It turns out that Linux may decide to kill a process when the system runs out of memory. It may make sense to cap the memory limit (-Xmx) of any Java processes running on the system to a value that sums up below the server’s available memory.

Jaan Angerpikk explained this problem in great detail, follow the link below to read more on this matter.

Out of memory: Kill process or sacrifice child