Helm – Configuration Management for Kubernetes Resources

Mon, 26 Jul 2021 19:28:22 -0400

Developer ships application in Docker container, so it can eventually hosted in Kubernetes cluster. However, there are still some installation steps, before the application can operate online in production. In this post, we use the container image of Orthanc application as a starting point. We first build services in Kubernetes to go through these steps. Then, to automate the steps, we build a helm chart. The code is kept in Korthweb project, in which the manual directory has the files requirement for manual deployment, and the helm directory is the helm chart.

Manual Deployment

The manual deployment steps include different kinds of activities, such as creating X.509 certificates, apply config map, create Kubernetes deployment using the YAML declarations, and use helm to install dependency. The steps need to take place in a particular sequence. Some step requires pulling information from secrets created in the previous step. This is why the deployment is not portable.

In order to automate the steps, one might think of wrapper script, which is very limited. A configuration management tool is needed in this scenario. Two common options are Kustomize, and Helm. Kustomize is a native tool which can be run by kubectl. It is also driven by declarative statement in YAML, which is simple to grasp. However, in lack of a templating mechanism, Kustomize may require wordy statements. Helm, on the other hand, comes with a templating mechanism which greatly increase reusability, making it more suitable for complex steps required in installation.

Helm Repo and Chart

Helm is known as package manager for applications running on Kubernetes. Helm defines an application as a collection of related Kubernetes resources, and it manages application deployment through a templated approach. An installation workbook is called a chart. Charts are kept in repositories. There are some well-known repositories, such as Bitnami, Helm stable. You need to add a repostory before using the Helm Charts in it. To add a repo, run:

helm repo add bitnami https://charts.bitnami.com/bitnami

You can host your own repo (public or private) as well. To search for charts across repositories, the best place is artifact hub, which indexes charts from a lot of public repositories. To search for charts from the repositories added, run:

helm search repo postgres

Template is the soul of Helm chart. A Helm chart consists of a directory of files following specific pattern so Helm can understand how to deploy the application. For example, the chart name is the name of the working directory. Under the directory, the values.yaml and chart.yaml defines variables and constants, both serving as template inputs. The template directory is the most important part of the directory where the installation logics are defined. Helm runs the entire directory hierarchy (except for paths specified in .helmignore file) through a Go template rendering engine. The template result spec out the detailed steps.

A great example of using Helm chart to simplify installation is the wordpress chart by Bitnami. You can install all the required components in a single command:

helm install my-release bitnami/wordpress

The helm chart in Korthweb project is also an evolving helm chart I created for installing Orthanc application.

Helm V3 (released in late 2019) includes an important architectural change – the removal of tiller. This means Helm can operate on the client-side – a significant simplification. Helm graduated from CNCF project in 2020. There are also a few changes in V3, as outlined here, including the consolidation of requirements.yaml into Charts.yaml.

Template and Function

As discussed, templating is the key towards reusability and flexibility in configuration management. We’ve worked with Jinja2 template engine in Ansible and Python. Here in Helm, we use Go templates with some enhancement. The syntax is mostly based on Go template, which is somewhat similar to Jinja2. Helm also added all functions from the Sprig library, making it more powerful and flexible than Jinja2. Helm chart developer should be very familiar with these functions, as well as the best practices. For example, the cryptographic and security functions in Sprig library gives us the ability to create self-signed X509 certificates during installation.

Since template introduces another layer of abstraction, to help troubleshooting we should be able to preview rendered template with the template command:

helm template orthanc | less

The command above simply renders template without attempting to execute the chart. To go one step further, you can dry-run the installation with:

helm install orthweb ./orthanc --debug --dry-run | less

Although Jinja2 (using {% … %} to express template control) and Go (using {{ … }} to express template control) have different syntaxes, one aspect that is similar between them, is chomping whitespace with minus sign (-). This is pretty common in templating language. The documentation of both Jinja2 and Helm have a section on whitespace control. Not paying attention to this nuance may cause pesky errors.

Dependency

The Orthanc application relies on Postgres database, which itself is deployed by a separate helm chart. This can be specified in Chart.yaml (Helm V3), like this:

dependencies:
 - condition: postgresql-ha.enabled
 name: postgresql-ha
 repository: https://charts.bitnami.com/bitnami
 version: 7.8.x

The values of variables of the dependency chart can be specified in values.yaml of the root chart. They can also be imperatively specified as a parameter of helm install command.

The section above also requires the dependency chart to be downloaded into the charts sub-directory. This can be done with:

helm dependency update

Then you will notice a file with tgz extension in the charts sub-directory. Note that when you change the version of the dependency package in Chart.yaml, then you will need to run the command again. Alternatively, this command can be automatically executed before helm install if you specify the switch –dependency-update with helm install.

The main chart (e.g. wordpress) is referred to as parent chart, and the charts it depends on are referred to as sub-chart (e.g. mariadb, memcached). When it comes to managing property values, values from parent chart can override those from sub-chart, as explained here. On the other hand, values from sub-chart can override those from parent chart in two formats: export format (keyword exports) and child-parent format (keyword import-values). This is something to be careful and we can use the aforementioned template command to display the rendered values.

Hooks

Helm does a great job in figuring out the dependency relationship between kubernetes objects defined in the chart, and create them in order. So typically you do not need hooks for objects in the chart. However, in certain circumstances, such as cleaning up after uninstallation, we may need hooks. Here is a list of available hooks.

It is worth-noting that hook is not tied to an action. Instead it is tied to a kubernetes resource. The resource could be a job, a config map, etc. The resource is tied to a hook simply by resource annotation.

Moving to GUI

Helm is a command-line tool. For a team with varying levels of familiarity with command-line, GUI-based tool is a better option. For that, some enterprises adopt Rancher, a comprehensive Kubernetes cluster management platform. Rancher manages many aspects of Kubernetes cluster through web portal. One aspect is the support of helm chart. Rancher can be install on a cluster of its own. For demo, it can also be installed on docker desktop, a single-node Kubernetes cluster by Docker. In both cases, Nginx ingress controller needs to be configured.

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