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<h1 id="managing-complex-configurations"><a class="header" href="#managing-complex-configurations">Managing Complex Configurations</a></h1>
<p>It is sometimes useful to generate shadow configuration files dynamically.
Since Shadow accepts configuration files in <a href="https://yaml.org/spec/1.2.2/">YAML
1.2</a> format, there are many options
available; even more so since <a href="https://en.wikipedia.org/wiki/JSON">JSON</a> is
also valid YAML 1.2.</p>
<h2 id="yaml-templating"><a class="header" href="#yaml-templating">YAML templating</a></h2>
<p>YAML itself has some features to help avoid repetition. When using these
features, it can be helpful to use shadow's <code>--show-config</code> flag to examine the
"flat" generated config.</p>
<p>An individual node can be made into an <em>anchor</em> (<code>&AnchorName x</code>), and
referenced via an <em>alias</em> (<code>*AnchorName</code>). For example, here we create
and use the anchors <code>Node</code>, <code>Fast</code>, <code>Slow</code>, <code>ClientPath</code>, and <code>ServerPath</code>:</p>
<pre><code class="language-yaml">general:
stop_time: 10s
network:
graph:
type: 1_gbit_switch
hosts:
fast_client:
network_node_id: &Node 0
bandwidth_up: &Fast "100 Mbit"
bandwidth_down: *Fast
processes:
- path: &ClientPath "/path/to/client"
# ...
slow_client:
network_node_id: *Node
bandwidth_up: &Slow "1 Mbit"
bandwidth_down: *Slow
processes:
- path: *ClientPath
# ...
fast_server:
network_node_id: *Node
bandwidth_up: *Fast
bandwidth_down: *Fast
processes:
- path: &ServerPath "/path/to/server"
# ...
slow_server:
network_node_id: *Node
bandwidth_up: *Slow
bandwidth_down: *Slow
processes:
- path: *ServerPath
</code></pre>
<p>We can use <a href="https://docs.docker.com/compose/compose-file/#extension">extension
fields</a> to move our
constants into one place:</p>
<pre><code class="language-yaml">x-constants:
- &Node 0
- &Fast "100 Mbit"
- &Slow "1 Mbit"
- &ClientPath "/path/to/client"
- &ServerPath "/path/to/server"
general:
stop_time: 10s
network:
graph:
type: 1_gbit_switch
hosts:
fast_client:
network_node_id: *Node
bandwidth_up: *Fast
bandwidth_down: *Fast
processes:
- path: *ClientPath
slow_client:
network_node_id: *Node
bandwidth_up: *Slow
bandwidth_down: *Slow
processes:
- path: *ClientPath
fast_server:
network_node_id: *Node
bandwidth_up: *Fast
bandwidth_down: *Fast
processes:
- path: *ServerPath
slow_server:
network_node_id: *Node
bandwidth_up: *Slow
bandwidth_down: *Slow
processes:
- path: *ServerPath
</code></pre>
<p>We can also use <a href="https://yaml.org/type/merge.html">merge keys</a> to make
extendable templates for fast and slow hosts:</p>
<pre><code class="language-yaml">x-constants:
- &Node 0
- &Fast "100 Mbit"
- &Slow "1 Mbit"
- &ClientPath "/path/to/client"
- &ServerPath "/path/to/server"
- &FastHost
network_node_id: *Node
bandwidth_up: *Fast
bandwidth_down: *Fast
- &SlowHost
network_node_id: *Node
bandwidth_up: *Slow
bandwidth_down: *Slow
general:
stop_time: 10s
network:
graph:
type: 1_gbit_switch
hosts:
fast_client:
<<: *FastHost
processes:
- path: *ClientPath
slow_client:
<<: *SlowHost
processes:
- path: *ClientPath
fast_server:
<<: *FastHost
processes:
- path: *ServerPath
slow_server:
<<: *SlowHost
processes:
- path: *ServerPath
</code></pre>
<h2 id="dynamic-generation"><a class="header" href="#dynamic-generation">Dynamic Generation</a></h2>
<p>There are many tools and libraries for generating YAML and JSON. These can be helpful for
representing more complex relationships between parameter values.</p>
<p>Suppose we want to add a cleanup process to each host that runs one second
before the simulation ends. Since YAML doesn't support arithmetic, the
following <em>doesn't</em> work:</p>
<pre><code class="language-yaml">x-constants:
- &StopTimeSec 10
- &CleanupProcess
# This will evaluate to the invalid time string "10 - 1"; not "9"
start_time: *StopTimeSec - 1
...
# ...
</code></pre>
<p>In such cases it may be helpful to write your configuration in a language that does support
more advanced features that can generate YAML or JSON.</p>
<h3 id="python-example"><a class="header" href="#python-example">Python example</a></h3>
<p>We can achieve the desired effect in Python like so:</p>
<pre><code class="language-python">#!/usr/bin/env python3
Node = 0
StopTimeSec = 10
Fast = "100 Mbit"
Slow = "1 Mbit"
ClientPath = "/path/to/client"
ServerPath = "/path/to/server"
FastHost = {
'network_node_id': Node,
'bandwidth_up': Fast,
'bandwidth_down': Fast,
}
SlowHost = {
'network_node_id': Node,
'bandwidth_up': Slow,
'bandwidth_down': Slow,
}
CleanupProcess = {
'start_time': f'{StopTimeSec - 1}s',
'path': '/path/to/cleanup',
}
config = {
'general': {
'stop_time': '10s',
},
'network': {
'graph': {
'type': '1_gbit_switch'
},
},
'hosts': {
'fast_client': {
**FastHost,
'processes': [
{'path': ClientPath},
CleanupProcess,
],
},
'slow_client': {
**SlowHost,
'processes': [
{'path': ClientPath},
CleanupProcess,
],
},
'fast_server': {
**FastHost,
'processes': [
{'path': ServerPath},
CleanupProcess,
],
},
'slow_server': {
**SlowHost,
'processes': [
{'path': ServerPath},
CleanupProcess,
],
},
},
}
import yaml
print(yaml.safe_dump(config))
</code></pre>
<h3 id="nix-example"><a class="header" href="#nix-example">Nix example</a></h3>
<p>There are also languages that specialize in doing this kind of advanced configuration generation.
For example, using <a href="https://nixos.org/manual/nix/stable/language/index.html">NixOs's config language</a>:</p>
<pre><code class="language-nix">let
Node = 0;
StopTimeSec = 10;
Fast = "100 Mbit";
Slow = "1 Mbit";
ClientPath = "/path/to/client";
ServerPath = "/path/to/server";
FastHost = {
network_node_id = Node;
bandwidth_up = Fast;
bandwidth_down = Fast;
};
SlowHost = {
network_node_id = Node;
bandwidth_up = Slow;
bandwidth_down = Slow;
};
CleanupProcess = {
start_time = (toString (StopTimeSec - 1)) + "s";
path = "/path/to/cleanup";
};
in
{
general = {
stop_time = (toString StopTimeSec) + "s";
};
network = {
graph = {
type = "1_gbit_switch";
};
};
hosts = {
fast_client = FastHost // {
processes = [
{path = ClientPath;}
CleanupProcess
];
};
slow_client = SlowHost // {
processes = [
{path = ClientPath;}
CleanupProcess
];
};
fast_server = FastHost // {
processes = [
{path = ServerPath;}
CleanupProcess
];
};
slow_server = SlowHost // {
processes = [
{path = ServerPath;}
CleanupProcess
];
};
};
}
</code></pre>
<p>This can be converted to JSON, which is also valid YAML, with:</p>
<pre><code>nix eval -f example.nix --json
</code></pre>
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