Wednesday, January 31, 2018

Diagnosing problems and running maintenance tasks in a network with services deployed by Disnix

I have been maintaining a production system with Disnix for quite some time. Although deployment works quite conveniently for me (I may probably be a bit biased, since I created Disnix :-) ), you cannot get around unforeseen incidents and problems, such as:

  • Crashing processes due to bugs or excessive load.
  • Database problems, such as inconsistencies in the data.

Errors in distributed systems are typically much more difficult to debug than single machine system failures. For example, tracing the origins of an error in distributed systems is generally hard -- one service's fault may be caused by a message propagated by another service residing on a different machine in the network.

But even if you know the origins of an error (e.g. you can clearly observe that a web application is crashing or a database connection), you may face other kinds of challenges:

  • You have to figure out to which machine in the network a service has been deployed.
  • You have to connect to the machine, e.g. through an SSH connection, to run debugging tasks.
  • You have to know the configuration properties of a service to diagnose it -- in Disnix, as explained in earlier blog posts, services can take any form -- they can be web services, but also web applications, databases and processes.

Because of these challenges, diagnosing errors and running maintenance tasks in a system deployed by Disnix is always unnecessarily time-consuming and inconvenient.

To alleviate this burden, I have developed a small tool and extension that establishes remote shell connections with environments providing all relevant configuration properties. Furthermore, the tool gives suggestions to the end-user explaining what kinds of maintenance tasks he could carry out.

The shell activity of Dysnomia

As explained in previous Disnix-related blog posts, Disnix carries out all activities to deploy a service oriented system to a network machines (i.e. to bring it in a running state), such as building services from source code, distributing their intra-dependency closures to the target machines, and activating or deactivating every service.

For the build and distribution activities, Disnix uses, as its name implies, the Nix package manager because it offers a number of powerful properties, such as strong reproducibility guarantees and atomic upgrades and rollbacks.

For the remaining activities that Nix does not support, e.g. activating or deactivating services, Disnix uses a companion tool called Dysnomia. Because services in a Disnix context could take any form, there is no generic means to activate or deactivate them -- for this reason, Dysnomia provides a plugin system with modules that carry out specific activities for a specific service type.

One of the plugins that Dysnomia provides is the deployment of MySQL databases to a MySQL DBMS server. Dysnomia deployment activities are driven by two kinds of configuration specifications. A component configuration defines the properties of a deployable unit, such as a MySQL database:

create table author
  FirstName  VARCHAR(255)  NOT NULL,
  LastName   VARCHAR(255)  NOT NULL,

create table books
( ISBN       VARCHAR(255)  NOT NULL,
  Title      VARCHAR(255)  NOT NULL,
  FOREIGN KEY(AUTHOR_ID) references author(AUTHOR_ID) on update cascade on delete cascade

The above configuration is a MySQL script (~/testdb) that creates the database schema consisting of two tables.

The container configuration captures properties of the environment in which the component should be hosted, which is in this particular case, a MySQL DBMS server:


The above component configuration (~/mysql-production) defines the type stating that mysql-database plugin must be used, and provides the authentication credentials required to connect to the DBMS server.

The Dysnomia plugin for MySQL implements various kinds of deployment activities for MySQL databases. For example, the activation activity is implemented as follows:


case "$1" in
        # Initalize the given schema if the database does not exists
        if [ "$(echo "show databases" | @mysql@ --user=$mysqlUsername --password=$mysqlPassword -N | grep -x $componentName)" = "" ]
            ( echo "create database $componentName;"
              echo "use $componentName;"
              if [ -d $2/mysql-databases ]
                  cat $2/mysql-databases/*.sql
            ) | @mysql@ $socketArg --user=$mysqlUsername --password=$mysqlPassword -N


The above code fragment checks whether a database with the given schema exists and if it does not, it will create it by running the database initialization script provided by the component configuration. As may also be observed, the above activity uses the container properties (such as the authentication credentials) as environment variables.

Dysnomia activities can be executed by invoking the dysnomia command-line tool. For example, the following command will activate the MySQL database in the MySQL database server:

$ dysnomia --operation activate \
  --component ~/testdb --container ~/mysql-production

To make the execution of arbitrary tasks more convenient, I have created a new Dysnomia option called: shell. The shell operation is basically an activity that does not execute anything, but instead spawns a shell session that provides the container configuration properties as environment variables.

Moreover, the shell activity of a Dysnomia plugin typically displays suggestions for shell commands that the user may want to carry out.

For example, when we run the following command:

$ dysnomia --shell \
  --component ~/testdb --container ~/mysql-production

Dysnomia spawns a shell session that shows the following:

This is a shell session that can be used to control the 'staff' MySQL database.

Module specific environment variables:
mysqlUsername  Username of the account that has the privileges to administer
               the database
mysqlPassword  Password of the above account
mysqlSocket    Path to the UNIX domain socket that is used to connect to the
               server (optional)

Some useful commands:
/nix/store/h0kcf5g2ssyancr9m2i8sr09b3wq2zy0-mariadb-10.1.28/bin/mysql  --user=$mysqlUsername --password=$mysqlPassword staff Start a MySQL interactive terminal

General environment variables:
this_dysnomia_module     Path to the Dysnomia module
this_component           Path to the mutable component
this_container           Path to the container configuration file


By executing the command-line suggestion shown above in the above shell session, we get a MySQL interactive terminal allowing us to execute arbitrary SQL commands. It saves us the burden looking up all the MySQL configuration properties, such as the authentication credentials and the database name.

The Dysnomia shell feature is heavily inspired by nix-shell that works in quite a similar way -- it will take the build dependencies of a package build as inputs (which typically manifest themselves as environment variables) and fetches the sources, but it will not execute the package build procedure. Instead, it spawns an interactive shell session allowing the user to execute arbitrary build tasks. This Nix feature is particularly useful for development projects.

Diagnosing services with Disnix

In addition to extending Dysnomia with the shell feature, I have also extended Disnix to make this feature available in a distributed context.

The following command can be executed to spawn a shell for a particular service of the ridiculous staff tracker example (that happens to be a MySQL database):

$ disnix-diagnose -S staff
[test2]: Connecting to service: /nix/store/yazjd3hcb9ds160cq03z66y5crbxiwq0-staff deployed to container: mysql-database
This is a shell session that can be used to control the 'staff' MySQL database.

Module specific environment variables:
mysqlUsername  Username of the account that has the privileges to administer
               the database
mysqlPassword  Password of the above account
mysqlSocket    Path to the UNIX domain socket that is used to connect to the
               server (optional)

Some useful commands:
/nix/store/h0kcf5g2ssyancr9m2i8sr09b3wq2zy0-mariadb-10.1.28/bin/mysql  --user=$mysqlUsername --password=$mysqlPassword staff Start a MySQL interactive terminal

General environment variables:
this_dysnomia_module     Path to the Dysnomia module
this_component           Path to the mutable component
this_container           Path to the container configuration file


The above command-line instruction will lookup the location of the staff database in the configuration of the system that is currently deployed, connects to it (typically through SSH) and spawns a Dysnomia shell for the given service type.

In addition to an interactive shell, you can also directly run shell commands. For example, the following command will query all the staff records:

$ disnix-diagnose -S staff \
  --command 'echo "select * from staff" | mysql -u $mysqlUsername -p $mysqlPassword staff'

In most cases, only one instance of a service exists, but Disnix can also deploy redundant instances of the same service. For example, we may want to deploy two redundant instances of the web application front end in the distribution.nix configuration file:

stafftracker = [ infrastructure.test1 infrastructure.test2 ];

When trying to spawn a Dysnomia shell, the tool returns an error because it does not know to which instance to connect to:

$ disnix-diagnose -S stafftracker
Multiple mappings found! Please specify a --target and, optionally, a
--container parameter! Alternatively, you can execute commands for all possible
service mappings by providing a --command parameter.

This service has been mapped to:

container: apache-webapplication, target: test1
container: apache-webapplication, target: test2

In this case, we must refine our query with a --target parameter. For example, the following command connects to the web front-end on the test1 machine:

$ disnix-diagnose -S stafftracker --target test1

It is still possible to execute remote shell commands for redundantly deployed services. For example, the following command gets executed twice, because we have two instances deployed:

$ disnix-diagnose -S stafftracker \
  --command 'echo I will see this message two times!'

In some cases, you may want to execute other kinds of maintenance tasks or you simply want to know where a particular service resides. This can be done by running the following command:

$ disnix-diagnose -S stafftracker --show-mappings
This service has been mapped to:

container: apache-webapplication, target: test1
container: apache-webapplication, target: test2


In this blog post, I have described a new feature of Dysnomia and Disnix that spawns interactive shell sessions making problem solving and maintenance tasks more convenient.

disnix-diagnose and the shell extension are part of the development versions of Disnix and Dysnomia and will become available in the next release.

Monday, January 8, 2018

Syntax highlighting Nix expressions in mcedit

The year 2017 has passed and 2018 has now started. For quite a few people, this is a good moment for reflection (as I have done in my previous blog post) and to think about new year's resolutions. New year's resolutions are typically about adopting good new habits and rejecting old bad ones.

Orthodox file managers

One of my unconventional habits is that I like orthodox file managers and that I extensively use them. Orthodox file managers have a number of interesting properties:

  • They typically display textual lists of files, as opposed to icons or thumbnails.
  • They typically have two panels for displaying files: one source and one destination panel.
  • They may also have third panel (typically placed underneath the source and destination panels) that serves as a command-line prompt.

The first orthodox file manager I ever used was DirectoryOpus on the Commodore Amiga. For nearly all operating systems and desktop environments that I touched ever since, I have been using some kind of a orthodox file manager, such as:

Over the years, I have received many questions from various kinds of people -- they typically ask me what is so appealing about using such a "weird program" and why I have never considered switching to a more "traditional way" of working, because "that would be more efficient".

Aside from the fact that it may probably be mostly inertia, my motivating factors are the following:

  • Lists of files allow me to see more relevant and interesting details. In many traditional file managers, much of the screen space is wasted by icons and the spacing between them. Furthermore, traditional file managers may typically hide properties of files that I also typically want to know about, such as a file's size or modification timestamp.
  • Some file operations involve a source and destination, such as copying or moving files. In an orthodox file manager, these operations can be executed much more intuitively IMO because there is always a source and destination panel present. When I am using a traditional file manager, I typically have to interrupt my workflow to open a second destination window, and use it to browse to my target location.
  • All the orthodox file managers I have mentioned, implement virtual file system support allowing me to browse compressed archives and remote network locations as if they were directories.

    Nowadays, VFS support is not exclusive to orthodox file managers anymore, but they existed in orthodox file managers much longer.

    Moreover, I consider the VFS properties of orthodox file managers to be much more powerful. For example, the Windows file explorer can browse Zip archives, but Total Commander also has first class support for many more kinds of archives, such as RAR, ACE, LhA, 7-zip and tarballs, and can be easily extended to support many other kinds of file systems by an add-on system.
  • They have very powerful search properties. For example, searching for a collection of files having certain kinds of text patterns can be done quite conveniently.

    As with VFS support, this feature is not exclusive to orthodox file managers, but I have noticed that their search functions are still considerably more powerful than most traditional file managers.

From all the orthodox file managers listed above, Midnight Commander is the one I have been using the longest -- it was one of the first programs I used when I started using Linux (in 1999) and I have been using it ever since.

Midnight Commander also includes a text editor named: mcedit that integrates nicely with the search function. Although I have experience with half a dozen editors (such as vim and various IDEs, such as Eclipse and Netbeans), I have been using mcedit, mostly for editing configuration files, shell scripts and simple programs.

Syntax highlighting in mcedit

Earlier in the introduction I mentioned: "new year's resolutions", which may probably suggest that I intend to quit using orthodox file managers and an unconventional editor, such as mcedit. Actually, this is not something I am planning to :-).

In addition to Midnight Commander and mcedit, I have also been using another unconventional program for quite some time, namely: the Nix package manager since late 2007.

What I noticed is that, despite being primitive, mcedit has reasonable syntax highlighting support for a variety of programming languages. Unfortunately, what I still miss is support for the Nix expression language -- the DSL that is used to specify package builds and system configurations.

For quite some time, editing Nix expressions was a primitive process for me. To improve my unconventional way of working a bit, I have decided to address this shortcoming in my Christmas break by creating a Nix syntax configuration file for mcedit.

Implementing a syntax configuration for the Nix expression language

mcedit provides syntax highlighting (the format is described in the manual page) for a number of programming languages. The syntax highlighting configurations seem to follow similar conventions, probably because of the fact that programming languages influence each other a lot.

As with many programming languages, the Nix expression language has its own influences as well, such as Haskell, C, bash, JavaScript (more specifically: the JSON subset) and Perl.

I have decided to adopt similar syntax highlighting conventions in the Nix expression syntax configuration. I started by examining Nix's lexer module (src/libexpr/lexer.l):

  • First, I took the keywords and operators, and configured the syntax highlighter to color them yellow. Yellow keywords is a convention that other syntax highlighting configurations also seem to follow.
  • Then I implemented support for single line and multi-line comments. The context directive turned out to be very helpful -- it makes it possible to color all characters between a start and stop token. Comments in mcedit are typically brown.
  • The next step were the numbers. Unfortunately, the syntax highlighter does not have full support for regular expressions. For example, you cannot specify character ranges, such as [0-9]+. Instead you must enumerate all characters one by one:

    keyword whole \[0123456789\]

    Floating point numbers were a bit trickier to support, but fortunately I could steal them from the JavaScript syntax highlighter, since the formatting Nix uses is exactly the same.
  • Strings were also relatively simple to implement (with the exception of anti-quotations) by using the context directive. I have configured the syntax highlighter to color them green, similar to other programming languages.
  • The Nix expression language also supports objects of the URL or path type. Since there is no other language that I am aware of that has a similar property, I have decided to color them white, with the exception of system paths -- system paths look very similar to the C preprocessor's #include path arguments, so I have decided to color them red, similar to the C syntax highlighter.

    To properly support paths, I implemented an approximation of the regular expression used in Nix's lexer. Without full regular expression support, it is extremely difficult to make a direct translation, but for all my use cases it seems to work fine.

After configuring the above properties, I noticed that there were still some bits missing. The next step was opening the parser configuration (src/libexpr/parser.y) and look for any missing characters.

I discovered that there were still separators that I needed to add (e.g. parenthesis, brackets, semi-colons etc.). I have configured the syntax highlighter to color them bright cyan, with the exception of semi-colons -- I colored them purple, similar to the C and JavaScript syntax highlighter.

I also added syntax highlighting for the builtin functions (e.g. derivation, map and toString) so that they appear in cyan. This convention is similar to bash' syntax highlighting.

The implementation process of the Nix syntax configuration was generally straight forward, except for one thing -- anti-quotations. Because we only have a primitive lexer and no parser, it is impossible to have a configuration that covers all possibilities. For example, anti-quotations in strings that embed strings cannot be properly supported. I ended up with an implementation that only works for simple cases (e.g. a reference to an identifier or a file).


The syntax highlighter works quite well for the majority of expressions in the Nix packages collection. For example, the expression for the Disnix package looks as follows:

The top-level expression that contains the package compositions looks as follows:

Also, most Hydra release.nix configurations seem to work well, such as the one used for node2nix:


The Nix syntax configuration can be obtained from my GitHub page. It can be used by installing it in a user's personal configuration directory, or by deploying a patched version of Midnight Commander. More details can be found in the README.