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// Copyright 2018 Google LLC
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// See the License for the specific language governing permissions and
// limitations under the License.
package metadata
import (
// Metadata holds Google Cloud Functions metadata.
type Metadata struct {
// EventID is a unique ID for the event. For example: "70172329041928".
EventID string `json:"eventId"`
// Timestamp is the date/time this event was created.
Timestamp time.Time `json:"timestamp"`
// EventType is the type of the event. For example: "google.pubsub.topic.publish".
EventType string `json:"eventType"`
// Resource is the resource that triggered the event.
Resource *Resource `json:"resource"`
// Resource holds Google Cloud Functions resource metadata.
// Resource values are dependent on the event type they're from.
type Resource struct {
// Service is the service that triggered the event.
Service string `json:"service"`
// Name is the name associated with the event.
Name string `json:"name"`
// Type is the type of event.
Type string `json:"type"`
type contextKey string
// GCFContextKey satisfies an interface to be able to use contextKey to read
// metadata from a Cloud Functions context.Context.
// Be careful making changes to this function. See FromContext.
func (k contextKey) GCFContextKey() string {
return string(k)
const metadataContextKey = contextKey("metadata")
// FromContext extracts the Metadata from the Context, if present.
func FromContext(ctx context.Context) (*Metadata, error) {
if ctx == nil {
return nil, errors.New("nil ctx")
// The original JSON is inserted by the Cloud Functions worker. So, the
// format must not change, or the message may fail to unmarshal. We use
// JSON as a common format between the worker and this package to ensure
// this package can be updated independently from the worker. The contextKey
// type and the metadataContextKey value use an interface to avoid using
// a built-in type as a context key (which is easy to have collisions with).
// If we need another value to be stored in the context, we can use a new
// key or interface and avoid needing to change this one. Similarly, if we
// need to change the format of the message, we should add an additional key
// to keep backward compatibility.
b, ok := ctx.Value(metadataContextKey).(json.RawMessage)
if !ok {
return nil, errors.New("unable to find metadata")
meta := &Metadata{}
if err := json.Unmarshal(b, meta); err != nil {
return nil, fmt.Errorf("json.Unmarshal: %v", err)
return meta, nil
// NewContext returns a new Context carrying m. If m is nil, NewContext returns
// ctx. NewContext is only used for writing tests which rely on Metadata.
func NewContext(ctx context.Context, m *Metadata) context.Context {
if m == nil {
return ctx
b, err := json.Marshal(m)
if err != nil {
return ctx
return context.WithValue(ctx, metadataContextKey, json.RawMessage(b))