1 files changed, 53 insertions, 0 deletions
diff --git a/plugin/metadata/metadataer.go b/plugin/metadata/metadataer.go
new file mode 100644
index 000000000..bff12e92d
--- /dev/null
+++ b/plugin/metadata/metadataer.go
@@ -0,0 +1,53 @@
+package metadata
+
+import (
+	"context"
+
+	"github.com/miekg/dns"
+)
+
+// Provider interface needs to be implemented by each plugin willing to provide
+// metadata information for other plugins.
+// Note: this method should work quickly, because it is called for every request
+// from the metadata plugin.
+type Provider interface {
+	// List of variables which are provided by current Provider. Must remain constant.
+	MetadataVarNames() []string
+	// Metadata is expected to return a value with metadata information by the key
+	// from 4th argument. Value can be later retrieved from context by any other plugin.
+	// If value is not available by some reason returned boolean value should be false.
+	Metadata(context.Context, dns.ResponseWriter, *dns.Msg, string) (interface{}, bool)
+}
+
+// MD is metadata information storage
+type MD map[string]interface{}
+
+// metadataKey defines the type of key that is used to save metadata into the context
+type metadataKey struct{}
+
+// newMD initializes MD and attaches it to context
+func newMD(ctx context.Context) (MD, context.Context) {
+	m := MD{}
+	return m, context.WithValue(ctx, metadataKey{}, m)
+}
+
+// FromContext retrieves MD struct from context.
+func FromContext(ctx context.Context) (md MD, ok bool) {
+	if metadata := ctx.Value(metadataKey{}); metadata != nil {
+		if md, ok := metadata.(MD); ok {
+			return md, true
+		}
+	}
+	return MD{}, false
+}
+
+// Value returns metadata value by key.
+func (m MD) Value(key string) (value interface{}, ok bool) {
+	value, ok = m[key]
+	return value, ok
+}
+
+// setValue adds metadata value.
+func (m MD) setValue(key string, val interface{}) {
+	m[key] = val
+}