When people ask for my mail address, they usually get a personalized address from me. That is particularly true for all commercial email. So you don’t get to send mail to my main account, but to kris-yourbusiness@koehntopp.de , and that will end up going into INBOX.special.yourbusiness. At least until it leaks, receives spam or is otherwise burned. In which case I will short it out and route all incoming mail on that address to /dev/null. Here is how it is done.

Deutschland ist im Lockdown, die Schulen sind endlich geschlossen und es wird remote unterrichtet. Weil es Deutschland ist, passiert das in jedem Bundesland anders und uneinheitlich. In Baden-Württemberg verwendet man Moodle . Wer sich da drunter nichts vorstellen kann, kann es sich hier ansehen.

In Bawü wird eine getrennte Moodle-Instanz pro Schule installiert, aber halt viele Instanzen pro Server, weil Server recht groß sind. In Summe muß man bummelig 940.000 Schüler abfrühstücken. Die Strukturen in Moodle sind kleinräumig (Klassen, Jahrgänge, Schulen) und nicht stark quer verbunden, sodaß sich das im Grunde relativ leicht skalieren lassen sollte. Dennoch kam es im Frühjahr zum Engpässen, weil das Moodle-Projekt auf andere Projektziele und -größen geplant war (“Pilotschulen”) als es gebraucht wurde (“Lockdown”).

My son wants labyrinths. Ok, let’s make them like it’s the first semester.

We need a class Labyrinth that holds the dimensions of our maze, and the actual maze, in grid - a List of Lists of integers. That’s not exactly a two-dimensional array (it can have a ragged right edge), but it will do for us.

A basic container for labyrinths

class Labyrinth:
    """Store a labyrinth as a List of Lists of Integers.

    Passages exist in the 4 cardinal directions, N, E, S, and W. We store them
    as bit flags (N=1, E=2, S=4, W=8). When set, a passage exists from the current
    cell into the direction indicated by the bitflag.
    """

    # Grid size
    width: int
    height: int
    grid: List[List[int]]

The integer Zero will indicate an unused cell. We then use bits to store passages to the four cardinal directions: 1 for a passage to the North, 2 for a passage to the East, 4 for a passage to the South, and 8 for a passage to the West.

Heise writes an introduction to bash programming (in german) :

Bash ist eine vollwertige Programmiersprache, mit der Sie alltägliche Aufgaben leicht automatisieren.

Bash is a fully featured programming language that you can use to automate everyday tasks.

Bash is not a fully featured programming language at all, and nothing in bash is ever easy. You are advised to use a proper programming language early on in development, and if possible never put bash commands into a file.

This is the english version of a 2007 article .

In de.comp.os.unix.linux.misc somebody asked:

• Are commands in a script executed strictly sequentially, that is, will the next command only be executed when the previous command has completed, or will the shell automatically start the next command if the system has spare capacity?
• Can I change the default behavior - whatever it may be - in any way?

If you are looking into the fine manual, it may explain at some point that the shell starts each command in a separate process. Then you may continue your thought process and ask what that actually means. As soon as you get to this stage, you may want to have a look at the Unix process lifecycle.

Olya Kudriavtseva has an ugly Christmas sweater :

He’s making a table. He’s sorting it twice. SELECT * FROM contacts WHERE behavior = “nice”; SQL Clause is coming town! (buy here )

Katie Bauer observes :

I mean, except for the fact that sorting something twice is TERRIBLY optimized

So how bad is this? Let’s find out.

Some test data

We are defining a table santa, where we store peoples names (GDPR, EU Regulation 2016/679 applies!), their behavior (naughty or nice), their age, their location, and their wishlist items.

Kalenderwoche 51/2020: Lockdown mit Schulschließungen. Nachdem es im März schon einmal Schulschließungen wegen Corona gab, und dort die Defizite technischer und organisatorischer Natur offenbar wurden, hat man in Deutschland die Zeit genutzt und sich auf die vorhergesagte 2. Welle vorbereitet, die jetzt genau eingetroffen ist. Deutschland ist schließlich nicht nur das Land der Dichter und Denker, sondern auch ein Land der Ingenieure und Tüftler, und bekannt für seine funktionierende und effiziente Bürokratie.

So this happened: CentOS Project shifts focus to CentOS Stream

The future of the CentOS Project is CentOS Stream, and over the next year we’ll be shifting focus from CentOS Linux, the rebuild of Red Hat Enterprise Linux (RHEL), to CentOS Stream, which tracks just ahead of a current RHEL release. CentOS Linux 8, as a rebuild of RHEL 8, will end at the end of 2021. CentOS Stream continues after that date, serving as the upstream (development) branch of Red Hat Enterprise Linux.

The tables in PERFORMANCE_SCHEMA (P_S) are not actually tables. You should not think of them as tables, even if your SQL works on them. You should not JOIN them, and you should not GROUP or ORDER BY them.

Unlocked memory buffers without indexes

The stuff in P_S has been created with “keep the impact on production small” in mind. That is, from a users point of view, you can think of them as unlocked memory buffers - the values in there change as you look at them, and there are precisely zero stability guarantees.

There was a question at work about MySQL backups and restore. I needed to explain more.

We use databases to make state persistent. That is: As a developer you can think of your database as a single giant, structured global variable with a weird access method, and to make things worse, concurrent access.

A database is just a global variable to your code

We can log statements that change the state of our database in a log. In MySQL, we call this The Binlog.