Cluster and Services Configurations and Persistence¶
MetalK8s comes with a set of tools and services that may need to be configured on site. At the same time, we don’t want the administrator of the cluster to master each and every service of the cluster. We also don’t want to allow all kind of configurations since it would make the system even more complex to test and maintain over time.
In addition to those services, MetalK8s deployment may have to be adapted depending on the architecture of the platform or depending on the different use cases and applications running on top of it.
It can be:
The various roles and taints we set on the node objects of the cluster
The configurations associated to solutions, such as the list of available solutions, the environments and namespaces created for a solution
Be it services or MetalK8s configurations, we need to ensure it is persisted and resilient to various type of events such as node reboot, upgrade, downgrade, backup, restore.
As a cluster administrator, I have access to a finite list of settings I can customize on-site in order to match with my environment specificities:
List of static users and credentials configured in Dex
Integration with an external IDP configuration in Dex
Existing Prometheus rules edition and new rules addition
Alert notifications configuration in Alert Manager
New Grafana dashboards or new Grafana datasources
Number of replicas for the Prometheus, Alert Manager, Grafana or Dex deployments
Changing the path on which the MetalK8s UI is deployed
Modifying OIDC provider, client ID or scopes
Adding custom menu entries
Other items will appear as we add new configurable features in MetalK8s
As a cluster administrator, I have access to a documented list of settings I can configure in the Operational Guide.
Persistence of Configurations¶
As a cluster administrator, I can upgrade or downgrade my cluster without losing any of the customised settings described above.
Backup and Restoration¶
As a cluster administrator, when I am doing a backup of my cluster, I backup all the customised settings described above and I can restore it when restoring the MetalK8s cluster or I can re apply part or all of it on a fresh new cluster.
As a MetalK8s expert, I can use
kubectl command(s) in order to edit all
settings that are exposed. The intent is to have a method / API that an expert
could use, if the right CLI tool or GUI is not available or not functioning as
ConfigMap is chosen as a unified data access and storage media for cluster and service configurations in a MetalK8s cluster based on the above requirements for the following reasons:
Ability to support Update operations on ConfigMaps with CLI and UI easily using our already existing python kubernetes module.
Guarantee of adaptability and ease of changing the design and implementation in cases where customer needs evolve rapidly.
ConfigMaps are stored in the etcd database which is generally being backed up. This ensures that user settings cannot be lost easily.
How it works¶
During Bootstrap, Upgrade or Downgrade stages, when we are assertive that the K8s cluster is fully ready and available we could perform the following actions:
Firstly, create and deploy ConfigMaps that will hold customizable cluster and service configurations. These ConfigMaps should define an empty config.yaml in the data section of the ConfigMap for later use.
A standard layout for each customizable field could be added in the documentation to assist MetalK8s administrator in adding and modifying customizations.
To simplify the customizing efforts required from MetalK8s administrators, each customizable ConfigMap will include an example section with inline documented directives that highlight how users should add, edit and remove customizations.
In an Addon config file for example; salt/metalk8s/addons/prometheus-operator/config/alertmanager.yaml, define the keys and values for default service configurations in a YAML structured format.
The layout of service configurations within this file could follow the format:# Configuration of the Alertmanager service apiVersion: addons.metalk8s.scality.com/v1alpha1 kind: AlertmanagerConfig spec: # Configure the Alertmanager Deployment deployment: replicas: 1
During Addon manifest rendering, call a Salt module that will merge the configurations defined within the customizable ConfigMap to those defined in alertmanager.yaml using a Salt merge strategy.
Amongst other merge technique such as aggregate, overwrite, list, the recurse merge technique is chosen to merge the two data structures because it allows deep merging of python dict objects while being able to support the aggregation of list structures within the python object.
Aggregating list structures is particularly useful when merging the pre-provisioned Dex static users found in the default configurations to those newly defined by Administrators especially during upgrade. Without support for list merge, pre-provisioned Dex static users will be overwritten during merge time.
Recurse merge strategy example:
Merging the following structures using salt.utils.dictupdate.merge:
Object (a) (MetalK8s defaults):apiVersion: addons.metalk8s.scality.com/v1alpha1 kind: AlertmanagerConfig spec: deployment: replicas: 1
Object (b) (User-defined configurations from ConfigMap):apiVersion: addons.metalk8s.scality.com/v1alpha1 kind: AlertmanagerConfig spec: deployment: replicas: 2 notification: config: global: resolve_timeout: 5m
Result of Salt recurse merge:apiVersion: addons.metalk8s.scality.com/v1alpha1 kind: AlertmanagerConfig spec: deployment: replicas: 2 notification: config: global: resolve_timeout: 5m
The resulting configuration (a python object) will be used to populate the desired configuration fields within each Addon chart at render time.
The above approach is flexible and fault tolerant because in a MetalK8s cluster, once the user-defined ConfigMaps are absent or empty during Addon deployment, merging will yield no changes and we can effectively use default values packaged alongside each MetalK8s Addon to run the deployment.
Using Salt states
Once a ConfigMap is updated by the user (say a user changes the number of replicas for Prometheus deployments to a new value), then perform the following actions:
Apply a Salt state that reads the ConfigMap object, validates the schema and checks the new values passed and re-applies this configuration value to the deployment in question.
Restart the Kubernetes deployment to pickup newly applied service configurations.
A YAML (K8s-like) format was chosen to represent the data field instead of a flat key-value structure for the following reasons:
YAML formatted configurations are easy to write and understand hence it will be simpler for users to edit configurations.
The YAML format benefits from bearing a schema version, which can be checked and validated against a version we deploy.
YAML is a format for describing hierarchical data structures, while using a flat key-value format would require a form of encoding (and then, decoding) of this hierarchical structure.
A sample ConfigMap can be defined with the following fields.
Use case 1:
Configure and store the number of replicas for service specific Deployments found in the metalk8s-monitoring namespace using the ConfigMap format.
This section contains requirements stated above which the current design choice does not cater for and will be addressed later:
Persisting newly added Grafana dashboards or new Grafana datasources especially for modifications added via the Grafana UI cannot be stored in ConfigMaps and hence will be catered for later.
As stated in the requirements, adding and editing Prometheus alert rules is also not covered by the chosen design choice and will need to be addressed differently. Even if we could use ConfigMaps for Prometheus rules, we prefer relying on the Prometheus Operator and it’s CRD (PrometheusRule).
Rejected design choices¶
Consul KV vs ConfigMap¶
This approach offers a full fledge KV store with a /kv endpoint which allows CRUD operations on all KV data stored in it. Consul KV also allows access to past versions of objects and has an optimistic concurrency when manipulating multiple objects.
Note that, Consul KV store was rejected because managing operations such as performing full backups, system restores for a full fledged KV system requires time and much more efforts than the ConfigMap approach.
Operator (Custom Controller) vs Salt¶
Operators are useful in that, they provide self-healing functionalities on a reactive basis. When a user changes a given configuration, it is easy to reconcile and apply these changes to the in-cluster objects.
The Operator approach was rejected because it is much more complex, requires much more effort to realize and there is no real need for applying changes using this method because configuration changes are not frequent (for a typical MetalK8s admin, changing the number of replicas for a given deployment could happen once in 3 months or less) as such, having an operator watch for object changes is not significant and not very useful at this point in time.
In the Salt approach, Salt Formulas are designed to be idempotent ensuring that service configuration changes can be applied each time a new configuration is introduced.
Define and deploy new ConfigMap stores that will hold cluster and service configurations as listed in the requirements. For each ConfigMap, define its schema, its default values, and how it impacts the configured services
Template and render Deployment and Pod manifests that will make use of this persisted cluster and service configurations
Document how to change cluster and service configurations using kubectl
Document the entire list of configurations which can be changed by the user
Provide a CLI tool for changing any of the cluster and service configurations:
Count of replicas for chosen Deployments (Prometheus)
Updating a Dex authentication connector (OpenLDAP, AD and staticUser store)
Updating the Alertmanager notification configuration
Provide a UI interface for adding, updating and deleting service specific configurations for example Dex-LDAP connector integration.
Provide a UI interface for listing MetalK8s available/supported Dex authentication Connectors
Provide a UI interface for enabling or disabling Dex authentication connectors (LDAP, Active Directory, StaticUser store)
Add a UI interface for listing Alertmanager notification systems MetalK8s will support (Slack, email)
Provide a UI interface for adding, modifying and deleting Alertmanager configurations from the listing above
In the Operational Guide:
Document how to customize or change any given service settings using the CLI tool
Document how to customize or change any given service settings using the UI interface
Document the list of service settings which can be configured by the user
Document the default service configurations files which are deployed along side MetalK8s addons
Add test that ensures that update operations on user configurations are propagated down to the various services
Add test that ensures that after a MetalK8s upgrade, we do not lose previous customizations.
Other corner cases that require testing to reduce error prone setups include:
Checking for invalid values in a user defined configuration (e.g setting the number of replicas to a string (“two”))
Checking for invalid formats in a user configuration
Add tests to ensure we could merge a service configuration at render time while keeping user-defined modifications intact