ktempl - render Go templates with Nodes data from a Kubernetes

⚠️ Early stage of development! Use with caution! No warranties provided. 🚧

ktempl renders template files formatted using Go template with data returned from a Kubernetes cluster.

Primary data for ktempl is a list of nodes returned by a query to a Kubernetes. By default, ktempl fetches all nodes from the Kubernetes cluster. You can limit/filter nodes list using -l and available kubernetes key=value labels. Also you get list of nodes that host specific pods only by providing -p and -l to select specific pods.

Secondary data is a key=value data provided by user using --set arguments.

Optionally, after successful render, ktemp might execute command provided by user.

Initial motivation was to write piece of software which can be used as a glue between not flexible services, without built-in service discovery capabilities, and Kubernetes. One such example is the Varnish HTTP accelerator, where you need to pass backend servers configuration, and reload Varnish daemon if configuration changes.

Inspired by many configuration management frameworks which operate templates to generate configs, e.g., Helm, consul-template, Ansible, Puppet, etc.

Prerequisites

You should have a kubeconfig file with proper authentication and context details supplied.

User listed in the kubeconfig should be allowed to list nodes and pods.

As a minimum user should be allowed to list either nodes or pods.

Default kubeconfig location ~/.kube/config

More details on how to obtain that file can be found here.

Install

Download from the releases page. Example setup on linux machine:

curl -OL https://github.com/den-is/ktempl/releases/download/0.0.1/ktempl_0.0.1_`uname -s`_x86_64.tar.gz &&\
tar xzvf ktempl_0.0.1_`uname -s`_x86_64.tar.gz &&\
cp ktempl_0.0.1_`uname -s`_x86_64/ktempl /usr/local/bin

Compile it yourself. Minimum recommended Go version is 1.17+.

go get -u github.com/den-is/ktempl

Template language and scope variables

Templates are formated using Go template language and support Sprig template library.

Two main variables are passed to template for rendering:

• .Nodes - list of nodes returned by Kubernetes
• .Values - dictionary of key=values passed to ktempl using --set in the terminal or values: in the config file.

Each node in .Nodes has next fields

• .mynode.Name
• .mynode.InternalIP
• .mynode.Annotations
• .mynode.Labels

Example template file

# Welcome to {{ .Values.title  | lower | repeat 5 }} <<<<
{{- range $i, $n := .Nodes }}
Node {{$n.Name }} has {{ $n.InternalIP }} IP and port {{ $.Values.port }}
{{- end }}

Example usage

More examples here

# query nodes with a specific label
ktempl -l disk=ssd -t myexamle.tpl

# query pods with a specific label
ktempl -p -l app=myapp1 -t myexamle.tpl

# extended example
ktempl -l app=stagingapps -t example.tpl -o output.conf --set port=32456 --exec="touch success_exec.txt"

Config file

Optionally you can supply ktempl with a config file. The default is config.yaml in the same directory as binary. Or /etc/ktempl/config.yaml. Or whatever you supply with -c command-line option.

List of available configuration options

Config file settings	CLI flags	Description
kubeconfig	-k, --kubeconfig	Path to kubeconfig
pods	-p, --pods	Query pods and get nodes they are running on
namespace	-n, --namespace	Kubernetes namespace where to look Pods for. Used with -p
selector	-l, --selector	Kubernetes label selectors string
template	-t, --template	Path to template
output	-o, --output	Path where to put rendered results. default stdout
permissions	N/A	Output file permissions. default 0644. should be in 4 digit format!
set	--set	Additional key=values passed to template rendering engine
exec	-e, --exec	Command to execute after successful template render
log.file	--log-file	Path to log file. Allowed values disabled, stdout, stderr, path_to_log_file_dst. Default disabled.
log.level	--log-level	Minimum log message level to log. Default info. Available levels by hierarchy: trace, debug, info, warn, error, fatal, panic
N/A	-c, --config	Path to ktempl config file
daemon	-d, --daemon	Run ktempl in service mode, rather than singleshot
interval	-i, --interval	Interval between polls. default 15s. Valid time units are "s", "m", "h".
retries	N/A	NOT YET IMPLEMENTED Number of retries to fetch data from Kubernetes
timeout	N/A	NOT YET IMPLEMENTED ktempl operations timeout

Contact

Denis Iskandarov denis@cloudboom.io