The lifecycle of ephemeral Kubernetes worker nodes is driven by events
emitted by Amazon EventBridge and delivered via Amazon Simple
Notification Service. These events trigger the *dynk8s* provisioner to
take the appropriate action based on the state of an EC2 instance.
In order to add a node to the cluster using `kubeadm`, a "bootstrap
token" needs to be created. When manually adding a node, this would be
done e.g. using `kubeadm token create`. Since bootstrap tokens are just
a special type of Secret, they can be easily created programmatically as
well. When a new EC2 instance enters the "running" state, the
provisioner creates a new bootstrap token and associates it with the
instance by storing the instance ID in a label in the Secret resource's
metadata.
The initial implementation of the event handler is rather naïve. It
generates a token for every instance, though some instances may not be
intended to be used as Kubernetes workers. Ideally, the provisioner
would only allocate tokens for instances matching some configurable
criteria, such as AWS tags. Further, a token is allocated every time
the instance enters the running state, even if a token already exists or
is not needed.
Upon receipt of a notification or unsubscribe confirmation message from
SNS, after the message signature has been verified, the receiver will
now write the re-serialized contents of the message out to the
filesystem. This will allow the messages to be inspected later in order
to develop additional functionality for this service.
The messages are saved in a `messages` director within the current
working directory. This directory contains a subdirectory for each SNS
topic. Within the topic subdirectories, the each message is saved in a
file named with the message timestamp and ID.
The `model::sns::Message` enumeration provides a mechanism for
deserializing a JSON document into the correct type. It will be used by
the HTTP operation that receives messages from SNS in order to determine
the correct action to take in response to the message.
In order to prevent arbitrary clients from using the provisioner to
retrieve WireGuard keys and Kubernetes bootstrap tokens, access to those
resources *must* be restricted to the EC2 machines created by the
Kubernetes Cloud Autoscaler. The key to the authentication process will
be SNS notifications from AWS to indicate when new EC2 instances are
created; everything that the provisioner does will be associated with an
instance it discovered through an SNS notification.
SNS messages are signed using PKCS#1 v1.5 RSA-SHA1, with a public key
distributed in an X.509 certificate. To ensure that messages received
are indeed from AWS, the provisioner will need to verify those
signatures. Messages with missing or invalid signatures will be
considered unsafe and ignored.
The `model::sns` module includes the data structures that represent SNS
messages. The `sns::sig` module includes the primitive operations for
implementing signature verification.
Dustin