Dynamic Disnix

Dynamic Disnix is a (very experimental!) prototype extension framework for Disnix supporting dynamic (re)deployment of service-oriented systems.

The basic Disnix toolset implements models and mechanisms to execute the deployment of a service oriented system into a network of machines. The deployment models of the basic toolset are static -- one must explicitly write down the available services and their characteristics, the available target machines in the network and their properties, and a distribution mapping services to machines.

In practice, a network of machines may be quite dynamic -- a machine may crash and disappear from the network, we may add machines to provide extra system resources, or we may change their characteristics, e.g. a RAM upgrade.

These events may require reconfiguration of the services defined in the services model, the target machines in the infrastructure model and the mapping of services to machines in the distribution model, which is quite costly to do by hand, in particular in large networks of machines.

The extension framework provides a number of components to cope with the dynamism of the infrastructure:

• A discovery service is responsible for discovering the machines in the network and their characteristics and generating an infrastructure model
• The infrastucture augmenter adds privacy sensitive information to the discovered infrastructure model that cannot be auto-discovered
• The distribution generator computes a mapping of services to machines based on their technical requirements and non-functional requirements
• The ID assigner automatically assigns unique IDs (such as TCP ports) to services that require them

Prerequisites

In order to build Dynamic Disnix from source code, the following packages are required:

• Disnix
• libxml2
• libxslt
• glib

Installation

Dynamic Disnix is a typical autotools based package which can be compiled and installed by running the following commands in a shell session:

$ ./configure
$ make
$ make install

When building from the Git repository, you should run the bootstrap script first:

$ ./bootstrap

For more information about using the autotools setup or for customizing the configuration, take a look at the ./INSTALL file.

Usage

This package contains a number of command-line utilities. The most important use cases are the following:

Augmenting a discovered infrastructure model

The following command takes an infrastructure model and augmentation model and produces a new infrastructure model with the properties augmented:

$ dydisnix-augment-infra -i infrastructure-discovered -a augment.nix

The first parameter refers to a Disnix infrastructure model that can be written by hand or generated through disnix-capture-infra or the Dynamic Disnix Avahi client.

An augmentation model could look as follows:

{infrastructure, lib}:

lib.mapAttrs (targetName: target:
  target // (if target ? containers && target.containers ? mysql-database then {
    containers = target.containers // {
      mysql-database = target.containers.mysql-database //
        { mysqlUsername = "root";
          mysqlPassword = "secret";
        };
    };
  } else {})
) infrastructure

An augmentation model is a function in which infrastructure refers to the original infrastructure model and lib to the set of library functions in Nixpkgs. In the body, a policy should be written that augments the infrastructure model with additional data.

In the above example, we search for all machines that provide a MySQL DBMS as a container service, and we manually configure a password.

Generating a distribution model

We can generate a distribution model from a services model, infrastructure model and quality of service (QoS) model in which the last model describes how to map services to machines based on their technical and non-functional properties:

$ dydisnix-gendist -s services.nix -i infrastructure.nix -q qos.nix

A QoS model is a Nix expression declaring a function that has the following header:

{services, infrastructure, initialDistribution, previousDistribution, filters, lib}:

The parameters have the following properties:

• The services parameter refers to a Disnix services model
• The infrastructure parameter refers to a Disnix infrastructure model
• The initialDistribution refers to a cartesian product mapping each service in the services model to each machine in the infrastructure model
• The previousDistribution refers to the distribution model of the previous deployment, or null in case of an initial deployment
• The filters parameter exposes a set of utility functions to dynamically compose mappings
• The lib parameter exposes the utility functions from Nixpkgs

A simple QoS model would be the following:

{services, infrastructure, initialDistribution, previousDistribution, filters, lib}:

filters.divide {
  strategy = "greedy";

  inherit services infrastructure;
  distribution = initialDistribution;
  
  serviceProperty = "requireMem";
  targetProperty = "mem";
}

The above QoS model uses a greedy division method that takes the mem propery of each machine, representing the total amount of RAM that it provides, and divides services over them each having their own memory requirements (indicated by requireMem).

We can also combine filter functions:

{services, infrastructure, initialDistribution, previousDistribution, filters, lib}:

filters.divide {
  strategy = "greedy";
  inherit services infrastructure;
  
  distribution = filters.mapAttrOnList {
    inherit services infrastructure;
    distribution = initialDistribution;
    serviceProperty = "type";
    targetPropertyList = "supportedTypes";
  };
  
  serviceProperty = "requireMem";
  targetProperty = "mem";
}

In the above example, we first filter each service (that has a specific type) in such a way that that they are only mapped to machines that support them (through a list property named supportedTypes indicating which types of service the machine can run). Then we use the same division method as in the previous example to divide the services over the candidates.

The notation used in the above example is a bit inconvenient as for each transformation step on the candidate distribution, we nest one indentation level deeper. A better practice would be to write a QoS model as follows:

{services, infrastructure, initialDistribution, previousDistribution, filters, lib}:

let
  # Filter the candidate distribution by types.
  # This prevents services of a type to get distributed to a machine that is
  # incapable of activating it.

  distribution1 = filters.mapAttrOnList {
    inherit services infrastructure;
    distribution = initialDistribution;
    serviceProperty = "type";
    targetPropertyList = "supportedTypes";
  };

  # Divide the services over the candidates based on their memory constraints
  # using the greedy division method.

  distribution2 = filters.divide {
    strategy = "greedy";
    inherit services infrastructure;
    distribution = distribution1;
    serviceProperty = "requireMem";
    targetProperty = "mem";
  };
in
distribution2

In the revised example shown above, we compose a let-block in which each attribute composes a new candidate distribution, using the previous candidate distribution as an input. Each transformation step can be done on the same indentation level.

When implementing QoS policies, it is a good practice to divide them in to two phases -- the candidate selection phase determines which services a specific target can host, the division phase divides the services over the candidates according to some strategy,

The Dynamic Disnix toolset provides a collection of algorithms described in the academic literature. For more information on filter functions, consult the API documentation of the $PREFIX/share/dydisnix/filters.nix module. Currently, the following algorithms are provided:

• A collection of service to target machine mapping functions that filters candidate mappings on relationships, e.g. one to many, many to one, one to one.
• A round robin division method.
• A function that orders candidate target machines on priority
• A one dimensional division method, using a greedy, lowest-bidder or highest-bidder strategy
• An approximation alogrithm for the minimum set cover problem (optimize costs when machines have a fixed price, no matter the amount of services they run).
• An approximation algorithm for the multiway cut problem (try to reduce the amount of network links between machines, to make deployments more reliable)
• An approximation algorithm for the graph coloring problem (try to manifest all dependencies as network links between machines, with the minimum amount of machines). This can be useful to test a system for reliability and recoverability.

Implementing custom filter functions

In addition to using the deployment planning algorithms provided by the Dynamic Disnix framework, it is also possible to make a catalog of custom filter functions, such as:

{pkgs, referenceFilters}:

{
  clear = {distribution}:
    pkgs.lib.mapAttrs (serviceName: target: []);
}

A custom filter function catalog has the following structure:

• It defines a function that takes two arguments: pkgs refers to the Nixpkgs collection and referenceFilters refers to the set of reference filters included with Dynamic Disnix. The body defines an attribute set of custom filter functions.
• The clear function is a (dummy) example function that takes an existing distribution model and erases the targets for every service.

The clear filter function is implemented in the Nix expression language, that is a domain-specific language less suitable for arbitrary programming tasks. For more complicated tasks, it is also possible to invoke external executables that generate Disnix distributions:

{pkgs, referenceFilters}:

{
  multiwaycut = {distribution}:
    import "${(pkgs.stdenv.mkDerivation {
      name = "distribution.nix";
      buildInputs = [ dydisnix ];
      buildCommand =
      ''
        dydisnix-multiwaycut \
          --distribution ${referenceFilters.generateDistributionXML distribution} \
          --xml \
          > $out
      '';
    })}";
}

The above example invokes the dydisnix-multiwaycut executable to calculate a distribution by using an approximation algorithm for the multiway cut problem. It uses the referenceFilters.generateDistributionXML function to convert the distribution model to a standardized XML file so that it can be consumed by the dydisnix-multiwaycut executable.

Invoking a custom filter function from a QoS model can be done by referring to the filters parameter:

{services, infrastructure, initialDistribution, previousDistribution, filters, lib}:

filters.clear {
  distribution = initialDistribution;
}

The filters catalog expression can be specified by providing the -F parameter to the appropriate command-line tools, such as dydisnix-gendist that dynamically generates a distribution model:

$ dydisnix-gendist -s services.nix -i infrastructure.nix -q qos.nix -F customfilters.nix

Using the external distribution filters on the command-line

Apart from QoS models, it is also possible to use some filter algorithms (that are implemented as external executables) directly from the command-line for experimentation purposes.

External filter programs accept both Nix expression as well as XML representations of input models, and can write Nix or XML representations of filtered distributions to the standard output.

For example, the following command will apply the one-dimensional division strategy on a set of input models:

$ dydisnix-divide --strategy greedy -s services.nix -i infrastructure.nix -d distribution.nix --service-property requireMem --target-property mem

The above command-line invocation reads services from the given service model (services.nix), uses the target machines in the infrastructure model (infrastructure.nix) considering all targets for each service in the distribution model (distribution.nix) candidate targets.

It does a one-to-many mapping from the requireMem property of each service onto to the mem property of each target.

It is also possible to use XML representations of all input models. Internally, all external filter programs will work with XML representations of the input models, converting them from Nix when necessary.

The Nix representations of the inputs can also be explicitly converted to XML by running the dydisnix-xml command, such as:

$ dydisnix-xml -s services.nix

The above command-line instruction generates an XML representation of a service model implemented in the Nix expression language.

Most tools can be instructed to accept XML versions of input models by passing the --xml parameter. For example, the following command does exactly the same as the previously shown dydisnix-divide invocation:

$ dydisnix-divide --xml --strategy greedy -s services.xml -i infrastructure.xml -d distribution.xml --service-property requireMem --target-property mem

By default, the filter programs will output a Nix expression representation of a new distribution. It is also possible to use XML by providing the --output-xml parameter:

$ dydisnix-divide --output-xml --xml --strategy greedy -s services.xml -i infrastructure.xml -d distribution.xml --service-property requireMem --target-property mem

ID assigner

Some service require unique IDs, for example to allow them to bind to an unallocated TCP port or to use stable UIDs and GIDs when a service needs to run as an unprivileged user.

The process of assigning unique IDs to services can be automated. To do this you need to specify an ID resources model that has the following structure:

{
  ports = {
    min = 3000;
    max = 4000;
    scope = "global";
    step = 1;
  };

  uids = {
    min = 2000;
    max = 3000;
    scope = "global";
  };

  gids = {
    min = 2000;
    max = 3000;
    scope = "global";
  };
}

The above idresources.nix configuration file defines three kinds of numeric ID resources:

• ports refers to a pool of unique TCP port numbers
• uids refers to a pool of unique user IDs (UIDs)
• gids refers to a pool of unique group IDs (GIDs)

Each resource defines the following configuration properties:

• min specifies the minimum allowed ID
• max specifies the maximum allowed ID
• scope defines the scope of the ID. global (the default value) means that the IDs should be globally unique for the entire system. machine says that an ID should be unique to the machine where a service is deployed to. In case the scope is machine, a service becomes target-specific and can only be deployed to one machine in the network only.
• step specifies the step size of the search algorithm. The default the value is 1, but can be increased to any number. A higher number is useful to auto assign port numbers to services that requires multiple port assignments in which port numbers are derived from a base port number.

To make it possible to automatically assign IDs to services, we can annotate a service model as follows:

{distribution, system, pkgs}:

let
  ids = if builtins.pathExists ./ids.nix then (import ./ids.nix).ids else {};
in
rec {
  roomservice = rec {
    name = "roomservice";
    pkg = customPkgs.roomservicewrapper {
      port = ids.ports.roomservice;
      uid = ids.uids.roomservice;
      gid = ids.gids.roomservice;
    };
    dependsOn = {
      inherit rooms;
    };
    type = "process";
    requiresUniqueIdsFor = [ "ports" "uids" "groups" ];
  };

  ...

  stafftracker = rec {
    name = "stafftracker";
    pkg = customPkgs.stafftrackerwrapper {
      port = ids.ports.stafftracker;
      uid = ids.uids.stafftracker;
      gid = ids.gids.stafftracker;
    };
    dependsOn = {
      inherit roomservice staffservice zipcodeservice;
    };
    type = "process";
    baseURL = "/";
    requiresUniqueIdsFor = [ "ports" "uids" "groups" ];
  };
}

In the above partial service model, every service defines a requireUniqueIdsFor property to tell the ID generator for which resources it requires a unique ID. (The requireUniqueIdsFor property can be changed by passing the --service-property parameter to the generator tool).

The generated IDs expression (ids.nix) is imported in the beginning of the expression. To allow the services to use the unique IDs in the ids attribute set, they are passed as parameters to each pkg constructor function.

With the following command, you can automatically generate an ids.nix expression assigning unique IDs to every services that is deployed in the distribution model that requires a unique ID:

$ dydisnix-id-assign -s services.nix -i infrastructure.nix -d distribution.nix --id-resources idresources.nix --output-file ids.nix

The output of the ids.nix expression may look as follows:

{
  ids = {
    ports = {
      roomservice = 3000;
      stafftracker = 3001;
    };

    uids = {
      roomservice = 2000;
      stafftracker = 2001;
    };

    gids = {
      roomservice = 2000;
      stafftracker = 2001;
    };
  };
  lastAssignments = {
    ports = 3001;
    uids = 2001;
    gids = 2001;
  };
}

The above Nix expression specifies for each resource type, a mapping from a service to a unique ID (that might be globally unique or unique to the target machine where the service is deployed to).

It also memorizes the last assigned IDs per resource (and optionally per target) so that it will not reuse any previously assigned IDs, unless the limit has been reached.

In addition to creating ID assignments from scratch, it is also possible to update an existing ids.nix expression:

$ dydisnix-id-assign -s services.nix -i infrastructure.nix -d distribution.nix --id-resources idresources.nix --ids ids.nix --output-file ids.nix

The above command retains all previous ID assignments that are still valid, and will only assign IDs to service that have none assigned yet. Retaining valid assignments prevents unnecessary redeployments.

The above commands will only assign IDs to services that are deployed to target machines. It is also possible to assign IDs to all services in a services model regardless whether they are actually used or not:

$ dydisnix-id-assign -s services.nix --id-resources idresources.nix --output-file ids.nix

When assigning IDs to all services, you can only use resource types that work on a global scope.

Dynamically deploying a system

The following command deploys a service-oriented system in which the infrastructure is dynamically discovered, and the distribution dynamically generated:

$ dydisnix-env -s services.nix -a augment.nix -q qos.nix

When adding the --ids parameter, it will also automatically assign unique IDs as part of the deployment process:

$ dydisnix-env -s services.nix -a augment.nix -q qos.nix --id-resources idresources.nix --ids ids.nix

Self adaptive deployment of a system

We can also run a basic feedback loop regularly checking for changes and redeploying the machine if any change has been detected:

$ dydisnix-self-adapt -s services.nix -a augment.nix -q qos.nix

When adding the --ids parameter, it will also automatically assign unique IDs as part of the deployment process:

$ dydisnix-self-adapt -s services.nix -a augment.nix -q qos.nix --id-resources idresources.nix --ids ids.nix

We can also preemtively take snapshots of all stateful services by providing the --snapshot parameter, so that if any of the machines disappears, a service's state can be restored when it is redeployed elsewhere:

$ dydisnix-self-adapt -s services.nix -a augment.nix -q qos.nix --snapshot

Finally, it may also happen that a machine with an existing deployment configuration gets added to a network. In order to be able to manage them, their deployment configurations must be reconstructed. This can be done by adding the --reconstruct parameter:

$ dydisnix-self-adapt -s services.nix -a augment.nix -q qos.nix --reconstruct

Generating functional architecture documentation

Another use case of the toolset is to use the model parsing infrastructure to generate functional architecture documentation.

Running the following command will generate a HTML documentation catalog for the provided services expression, using SVG images for the diagrams, storing the output artefacts in the out/ sub folder:

$ dydisnix-generate-services-docs -s services.nix -f svg --output-dir out

The catalog can be inspected by opening the root page in the output folder:

$ firefox out/index.html

For complex architectures consisting of many services, it is also possible to group services (for example, by clustering services that implement a feature group). Grouping can be done by annotating a service in a service Nix expression (e.g. services.nix) with a group property:

{distribution, invDistribution, pkgs, system}:

let
  customPkgs = import ../top-level/all-packages.nix {
    inherit pkgs system;
  };

  groups = {
    homework = "Homework";
    literature = "Literature";
  };
in
{
  homeworkdb = {
    name = "homeworkdb";
    pkg = customPkgs.homeworkdb;
    type = "mysql-database";
    group = groups.homework;
    description = "Database backend of the Homework subsystem";
  };

  homework = {
    name = "homework";
    pkg = customPkgs.homework;
    dependsOn = {
      inherit usersdb homeworkdb;
    };
    type = "apache-webapplication";
    appName = "Homework";
    group = groups.homework;
    description = "Front-end of the Homework subsystem";
  };

  ...
}

In the above services model, the homeworkdb and homework service are grouped together in a feature group called Homework. In the above example, this is a means to group a database and a web application front-end into one "logical" group" that makes more sense from a functional point of view.

When services are grouped, the documentation catalog generator will partition the diagrams and their descriptions over multiple pages using a layered organisation allowing you to zoom in from the highest abstraction layer to deeper layers. There are multiple layers of sub groups possible, by using the `/' symbol as a delimiter in a group identifier.

It may also be desired adjust certain kinds of properties for the generated documentation catalog. These adjustments can be specified in a docs configuration expression (e.g. docs.nix):

{
  groups = {
    Homework = "Homework subsystem";
    Literature = "Literature subsystem";
  };

  fields = [ "description" "type" ];

  descriptions = {
    type = "Type";
    description = "Description";
  };
}

The above configuration provides descriptions to the group identifiers and specifies that the type and description attributes of every service should be displayed in the overview.

By adding the docs.nix configuration as a command-line parameter, we can adjust the documentation generation process with additional descriptions and fields:

$ dydisnix-generate-services-docs -s services.nix --docs docs.nix -f svg --output-dir out

In addition to generating a full catalog, it is also possible to generate the diagrams (using dydisnix-visualize-services) and descriptions separately (using dydisnix-document-services). These tools can be used for showing the details of a single layer, or in batch mode to generate artefacts for all abstraction layers.

License

Disnix is free software; you can redistribute it and/or modify it under the terms of the GNU Lesser General Public License as published by the Free Software Foundation either version 2.1 of the License, or (at your option) any later version. Disnix is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public License for more details.