coredns/middleware/kubernetes/nametemplate/nametemplate.go

package nametemplate

import (
	"errors"
	"fmt"
	"strings"

	"github.com/miekg/coredns/middleware/kubernetes/util"
)

// Likely symbols that require support:
// {id}
// {ip}
// {portname}
// {protocolname}
// {servicename}
// {namespace}
// {type}              "svc" or "pod"
// {zone}

// SkyDNS normal services have an A-record of the form "{servicename}.{namespace}.{type}.{zone}"
// This resolves to the cluster IP of the service.

// SkyDNS headless services have an A-record of the form "{servicename}.{namespace}.{type}.{zone}"
// This resolves to the set of IPs of the pods selected by the Service. Clients are expected to
// consume the set or else use round-robin selection from the set.

var symbols = map[string]string{
	"service":   "{service}",
	"namespace": "{namespace}",
	"type":      "{type}",
	"zone":      "{zone}",
}

var types = []string{
	"svc",
	"pod",
}

// TODO: Validate that provided NameTemplate string only contains:
//			* valid, known symbols, or
//			* static strings

// TODO: Support collapsing multiple segments into a symbol. Either:
//			* all left-over segments are used as the "service" name, or
//			* some scheme like "{namespace}.{namespace}" means use
//			  segments concatenated with a "." for the namespace, or
//			* {namespace2:4} means use segements 2->4 for the namespace.

// TODO: possibly need to store length of segmented format to handle cases
//       where query string segments to a shorter or longer list than the template.
//		 When query string segments to shorter than template:
//			* either wildcards are being used, or
//			* we are not looking up an A, AAAA, or SRV record (eg NS), or
//			* we can just short-circuit failure before hitting the k8s API.
//		 Where the query string is longer than the template, need to define which
//		 symbol consumes the other segments. Most likely this would be the servicename.
//		 Also consider how to handle static strings in the format template.
type NameTemplate struct {
	formatString string
	splitFormat  []string
	// Element is a map of element name :: index in the segmented record name for the named element
	Element map[string]int
}

func (t *NameTemplate) SetTemplate(s string) error {
	var err error
	fmt.Println()

	t.Element = map[string]int{}

	t.formatString = s
	t.splitFormat = strings.Split(t.formatString, ".")
	for templateIndex, v := range t.splitFormat {
		elementPositionSet := false
		for name, symbol := range symbols {
			if v == symbol {
				t.Element[name] = templateIndex
				elementPositionSet = true
				break
			}
		}
		if !elementPositionSet {
			if strings.Contains(v, "{") {
				err = errors.New("Record name template contains the unknown symbol '" + v + "'")
				fmt.Printf("[debug] %v\n", err)
				return err
			} else {
				fmt.Printf("[debug] Template string has static element '%v'\n", v)
			}
		}
	}

	return err
}

// TODO: Find a better way to pull the data segments out of the
//       query string based on the template. Perhaps it is better
//		 to treat the query string segments as a reverse stack and
//       step down the stack to find the right element.

func (t *NameTemplate) GetZoneFromSegmentArray(segments []string) string {
	if index, ok := t.Element["zone"]; !ok {
		return ""
	} else {
		return strings.Join(segments[index:len(segments)], ".")
	}
}

func (t *NameTemplate) GetNamespaceFromSegmentArray(segments []string) string {
	return t.GetSymbolFromSegmentArray("namespace", segments)
}

func (t *NameTemplate) GetServiceFromSegmentArray(segments []string) string {
	return t.GetSymbolFromSegmentArray("service", segments)
}

func (t *NameTemplate) GetTypeFromSegmentArray(segments []string) string {
	typeSegment := t.GetSymbolFromSegmentArray("type", segments)

	// Limit type to known types symbols
	if util.StringInSlice(typeSegment, types) {
		return ""
	}

	return typeSegment
}

func (t *NameTemplate) GetSymbolFromSegmentArray(symbol string, segments []string) string {
	if index, ok := t.Element[symbol]; !ok {
		return ""
	} else {
		return segments[index]
	}
}

// GetRecordNameFromNameValues returns the string produced by applying the
// values to the NameTemplate format string.
func (t *NameTemplate) GetRecordNameFromNameValues(values NameValues) string {
	recordName := make([]string, len(t.splitFormat))
	copy(recordName[:], t.splitFormat)

	for name, index := range t.Element {
		if index == -1 {
			continue
		}
		switch name {
		case "type":
			recordName[index] = values.TypeName
		case "service":
			recordName[index] = values.ServiceName
		case "namespace":
			recordName[index] = values.Namespace
		case "zone":
			recordName[index] = values.Zone
		}
	}
	return strings.Join(recordName, ".")
}

type NameValues struct {
	ServiceName string
	Namespace   string
	TypeName    string
	Zone        string
}