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KubeTemplate

KubeTemplate (package kubetemplate) is a library to facilitate the construction of reactive application configurations when running inside Kubernetes.

It provides a common Go-based templating engine to construct application configuration files using a number of kubernetes resources as sources.

Usage

A templating engine can be used on any number of templates, but before calling Render() on a template, you should run that template through a Learn() cycle.

Thus, the usual process is to run each template through a Learn() whenever it is added or changes, and then call Render() any number of times. Note that Learn() takes out a lock on the entire engine, so you should not attempt to run it concurrently.

The network portion of this tool relies on a netdiscover instance. See NetDiscover for more information. At a minimum, just create one with discover.NewDiscoverer().

Templates

Values for the templates may come from a number of sources:

ConfigMap and Secret

To obtain ConfigMap or Secret entries, KubeTemplate will use the Kubernetes API to attempt to pull in the ConfigMap/Secret and key requested.

Format: {{.ConfigMap "<name>" "<namespace>" "<key>"}}

Format: {{.Secret "<name>" "<namespace>" "<key>"}}

The provided namespace may be "" if both the ConfigMap/Secret is in the same namespace as the Pod and the POD_NAMESPACE environment variable is properly set.

The ConfigMap/Secret will be monitored by the engine, and if it is updated, the configuration files will be regenerated, and a reload will be performed.

Note that this will likely require an RBAC entry to allow the ServiceAccount under which the engine is running to access the referenced ConfigMap.

NOTE: it is generally easier to use the standard kubernetes Pod methods to translate ConfigMap and Secret values to environment variables. However, doing so does not currently result in changes to those referent ConfigMaps and Secrets being propagated to running applications. Therefore, the choice between using these dynamic references and the native kubernetes environment variable bindings is left to the user.

Environment Variables

Format: {{.Env "<name>"}}

It is useful to note that IP addresses of services within the same namespace will automatically be populated as environment variables by kubernetes. These will be of the form <SERVICE_NAME>_SERVICE_HOST. For instance, the IP of a service named "kamailio" will be stored in the environment variable KAMAILIO_SERVICE_HOST. This is a normal, default feature of all kubernetes containers. See the documentation for more information.

Service

Data from a kubernetes Service may be obtained using the Kubernetes API.

Format: {{.Service "<name>" "<namespace>]"}}

The provided namespace may be "" if both the Service is in the same namespace as the Pod and the POD_NAMESPACE environment variable is properly set.

The value returned here is the Kubernetes Service. Keep in mind that Go uses PascalCase for the fields, so "clusterIP" becomes "ClusterIP".

For example, to get the ClusterIP of a service named "kamailio" in the "voip" namespace:

{{ with .Service "kamailio" "voip"}}{{.Spec.ClusterIP}}{{end}}

Note that the IP address of a service within the same namespace can be obtained more simply by environment variable, as described above.

ServiceIP

Since the most common reason to probe a service is to retrieve its ClusterIP, we have also included a macro which does just that.

Format: {{.ServiceIP "<name>" "<namespace>]"}}

This works as Service, but instead of returning a structure, it just returns the ClusterIP of the Service, as a string.

Endpoints

Data from the kubernetes Endpoints of a Service may be obtained using the Kubernetes API.

Format: {{.Service "<name>" "<namespace>"}}

The provided namespace may be "" if both the Service is in the same namespace as the Pod and the POD_NAMESPACE environment variable is properly set.

The value returned is the Kubernetes Endpoints.

The Endpoints will be monitored by the engine, and if it is updated, the reload channel will be signaled.

This is usually used to obtain the dynamic set of proxy servers, but since the most common reason to do this is to obtain the set of IPs for endpoints of a service, we provide a second helper function just for that.

Endpoint IPs

One of the most common pieces of dynamic data to retrieve is the set of IPs for the endpoints of a service. Therefore, to simplify the relatively tedious iteration of these directly from the Endpoints spec, we provide the EndpointIPs macro, which returns the list of IPs of all Endpoints of the given service name.

Format: {{.EndpointIPs "<name>" "<namespace>"}}

The provided namespace may be "" if both the Service is in the same namespace as the Pod and the POD_NAMESPACE environment variable is properly set.

Using this is then easy. For example, to create a PJSIP endpoint from the set of proxy servers running as the "kamailio" service:

pjsip.d/proxies.conf:

[proxies]
type=endpoint
transport=k8s-internal-ipv4-external-media
context=from-proxies
disallow=all
allow=ulaw
aors=proxies
rtp_symmetric=yes

[proxies]
type=aor
{{range .EndpointIPs "kamailio"}}
contact=sip:{{.}}
{{end}}

[proxies]
type=identify
endpoint=proxies
{{range .EndpointIPs "kamailio"}}
match={{.}}
{{end}}

[proxies]
type=acl
deny=0.0.0.0/0.0.0.0
{{range .EndpointIPs "kamailio"}}
permit={{.}}
{{end}}

The Endpoints IPs will be monitored by the engine, and if they are updated, the configuration files will be regenerated, and a reload will be performed.

Network data

The IP addresses for the running Pod are made available, as well.

Format: {{.Network "<kind>"}}

The available data kinds correspond to the data available from NetDiscover:

  • "hostname"
  • "privatev4"
  • "publicv4"
  • "publicv6"