Bind charm architecture¶

The following diagram shows the different modules of this charm and how they interact with each other.

        C4Component
title Bind Operator Components

UpdateLayoutConfig($c4ShapeInRow="3", $c4BoundaryInRow="2")

Container_Boundary(bind-operator, "Bind Operator") {
    Component(events, "events.py", "Module", "Defines custom events")
    Component(exceptions, "exceptions.py", "Module", "Defines custom exceptions")
    Component(empty1, "")
    Component(charm, "charm.py", "Module", "Handles charm events and interactions with Juju")
    Component(empty2, "")
    Component(dns_data, "dns_data.py", "Module", "Manages DNS record data and conversions")
    Component(empty3, "")
    Component(empty4, "")
    Component(empty5, "")
    Component(models, "models.py", "Module", "Defines data models using Pydantic")
    Component(empty7, "")
    Component(bind, "bind.py", "Module", "Controls Bind workload execution")
    Component(empty6, "")
    Component(constants, "constants.py", "Module", "Defines constant values")
    Component(templates, "templates.py", "Module", "Provides templates for Bind configuration files")

    UpdateElementStyle(empty1,  $bgColor="#0000", $borderColor="#0000")
    UpdateElementStyle(empty2,  $bgColor="#0000", $borderColor="#0000")
    UpdateElementStyle(empty3,  $bgColor="#0000", $borderColor="#0000")
    UpdateElementStyle(empty4,  $bgColor="#0000", $borderColor="#0000")
    UpdateElementStyle(empty5,  $bgColor="#0000", $borderColor="#0000")
    UpdateElementStyle(empty6,  $bgColor="#0000", $borderColor="#0000")
    UpdateElementStyle(empty7,  $bgColor="#0000", $borderColor="#0000")

    Rel(charm, bind, "")
    Rel(bind, constants, "")
    Rel(bind, dns_data, "")
    Rel(bind, exceptions, "")
    Rel(bind, models, "")
    Rel(bind, templates, "")
    Rel(charm, constants, "")
    Rel(charm, dns_data, "")
    Rel(charm, exceptions, "")
    Rel(charm, models, "")
    Rel(dns_data, models, "")
    Rel(models, constants, "")
    Rel(charm, events, "")
}

UpdateElementStyle(bind, $fontColor="black", $bgColor="lightblue", $borderColor="gray")
UpdateElementStyle(charm, $fontColor="black", $bgColor="lightblue", $borderColor="gray")
UpdateElementStyle(dns_data, $fontColor="black", $bgColor="lightblue", $borderColor="gray")
UpdateElementStyle(models, $fontColor="black", $bgColor="lightblue", $borderColor="gray")
UpdateElementStyle(templates, $fontColor="black", $bgColor="lightblue", $borderColor="gray")
UpdateElementStyle(constants, $fontColor="black", $bgColor="lightblue", $borderColor="gray")
UpdateElementStyle(events, $fontColor="black", $bgColor="lightblue", $borderColor="gray")
UpdateElementStyle(exceptions, $fontColor="black", $bgColor="lightblue", $borderColor="gray")

Scaling behavior¶

When Bind is scaled, only one unit gets the active status, all the others are in standby. A new active unit is elected whenever the previous one goes down.
The active unit updates the zone configuration of its workload and has Bind working as a primary authoritative server over them.
All the standby units have Bind working as a secondary server, replicating the records via the usual DNS zone transfer.

Charm code overview¶

The src/charm.py is the default entry point for a charm and has the BindCharm Python class which inherits from CharmBase. CharmBase is the base class from which all charms are formed, defined by Ops Python framework for developing charms.

Workload¶

The workload of this charm is a snap whose source code is in the charmed-bind directory of the dns-operators repository. The workload is operated through the src/bind.py module which is able to set up, start, stop, restart, and change the workload configuration files.
The workload doesn’t communicate with the charm. It merely executes the charm’s orders.
Outside of the snap, the charm also sets up a systemd timer to awaken every minute through the custom event ReloadBindEvent. The hook responding to this event makes sure that the configuration of Bind is up to date with the charm’s relations.