google-app-engine

Google App Engine

原文：https://echo.labstack.com/docs/cookbook/google-app-engine

Google App Engine (GAE) provides a range of hosting options from pure PaaS (App Engine Classic) through Managed VMs to fully self-managed or container-driven Compute Engine instances. Echo works great with all of these but requires a few changes to the usual examples to run on the AppEngine Classic and Managed VM options. With a small amount of effort though it’s possible to produce a codebase that will run on these and also non-managed platforms automatically.

We’ll walk through the changes needed to support each option.

Standalone

Wait? What? I thought this was about AppEngine! Bear with me - the easiest way to show the changes required is to start with a setup for standalone and work from there plus there’s no reason we wouldn’t want to retain the ability to run our app anywhere, right?

We take advantage of the go build constraints or tags to change how we create and run the Echo server for each platform while keeping the rest of the application (e.g. handler wireup) the same across all of them.

First, we have the normal setup based on the examples but we split it into two files - app.go will be common to all variations and holds the Echo instance variable. We initialise it from a function and because it is a var this will happen before any init() functions run - a feature that we’ll use to connect our handlers later.

cookbook/google-app-engine/app.go

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package main

// reference our echo instance and create it early
var e = createMux()

A separate source file contains the function to create the Echo instance and add the static file handlers and middleware. Note the build tag on the first line which says to use this when not bulding with appengine or appenginevm tags (which thoese platforms automatically add for us). We also have the main() function to start serving our app as normal. This should all be very familiar.

cookbook/google-app-engine/app-standalone.go

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// +build !appengine,!appenginevm

package main

import (
	"github.com/labstack/echo/v4"
	"github.com/labstack/echo/v4/middleware"
)

func createMux() *echo.Echo {
	e := echo.New()

	e.Use(middleware.Recover())
	e.Use(middleware.Logger())
	e.Use(middleware.Gzip())

	e.Static("/", "public")

	return e
}

func main() {
	e.Logger.Fatal(e.Start(":8080"))
}

The handler-wireup that would normally also be a part of this Echo setup moves to separate files which take advantage of the ability to have multiple init() functions which run after the e Echo var is initialized but before the main() function is executed. These allow additional handlers to attach themselves to the instance - I’ve found the Group feature naturally fits into this pattern with a file per REST endpoint, often with a higher-level api group created that they attach to instead of the root Echo instance directly (so things like CORS middleware can be added at this higher common-level).

cookbook/google-app-engine/users.go

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package main

import (
	"net/http"

	"github.com/labstack/echo/v4"
	"github.com/labstack/echo/v4/middleware"
)

type (
	user struct {
		ID   string `json:"id"`
		Name string `json:"name"`
	}
)

var (
	users map[string]user
)

func init() {
	users = map[string]user{
		"1": user{
			ID:   "1",
			Name: "Wreck-It Ralph",
		},
	}

	// hook into the echo instance to create an endpoint group
	// and add specific middleware to it plus handlers
	g := e.Group("/users")
	g.Use(middleware.CORS())

	g.POST("", createUser)
	g.GET("", getUsers)
	g.GET("/:id", getUser)
}

func createUser(c echo.Context) error {
	u := new(user)
	if err := c.Bind(u); err != nil {
		return err
	}
	users[u.ID] = *u
	return c.JSON(http.StatusCreated, u)
}

func getUsers(c echo.Context) error {
	return c.JSON(http.StatusOK, users)
}

func getUser(c echo.Context) error {
	return c.JSON(http.StatusOK, users[c.Param("id")])
}

If we run our app it should execute as it did before when everything was in one file although we have at least gained the ability to organize our handlers a little more cleanly.

AppEngine Classic and Managed VM(s)

So far we’ve seen how to split apart the Echo creation and setup but still have the same app that still only runs standalone. Now we’ll see how those changes allow us to add support for AppEngine hosting.

Refer to the AppEngine site for full configuration and deployment information.

Configuration file

Both of these are Platform as as Service options running on either sandboxed micro-containers or managed Compute Engine instances. Both require an app.yaml file to describe the app to the service. While the app could still serve all it’s static files itself, one of the benefits of the platform is having Google’s infrastructure handle that for us so it can be offloaded and the app only has to deal with dynamic requests. The platform also handles logging and http gzip compression so these can be removed from the codebase as well.

The yaml file also contains other options to control instance size and auto-scaling so for true deployment freedom you would likely have separate app-classic.yaml and app-vm.yaml files and this can help when making the transition from AppEngine Classic to Managed VMs.

cookbook/google-app-engine/app-engine.yaml

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application: my-application-id  # defined when you create your app using google dev console
module: default                 # see https://cloud.google.com/appengine/docs/go/
version: alpha                  # you can run multiple versions of an app and A/B test
runtime: go                     # see https://cloud.google.com/appengine/docs/go/
api_version: go1                # used when appengine supports different go versions

default_expiration: "1d"        # for CDN serving of static files (use url versioning if long!)

handlers:
# all the static files that we normally serve ourselves are defined here and Google will handle
# serving them for us from it's own CDN / edge locations. For all the configuration options see:
# https://cloud.google.com/appengine/docs/go/config/appconfig#Go_app_yaml_Static_file_handlers
- url: /
  mime_type: text/html
  static_files: public/index.html
  upload: public/index.html

- url: /favicon.ico
  mime_type: image/x-icon
  static_files: public/favicon.ico
  upload: public/favicon.ico

- url: /scripts
  mime_type: text/javascript
  static_dir: public/scripts

# static files normally don't touch the server that the app runs on but server-side template files
# needs to be readable by the app. The application_readable option makes sure they are available as
# part of the app deployment onto the instance.
- url: /templates
  static_dir: /templates
  application_readable: true

# finally, we route all other requests to our application. The script name just means "the go app"
- url: /.*
  script: _go_app

Router configuration

We’ll now use the build constraints again like we did when creating our app-standalone.go instance but this time with the opposite tags to use this file if the build has the appengine or appenginevm tags (added automatically when deploying to these platforms).

This allows us to replace the createMux() function to create our Echo server without any of the static file handling and logging + gzip middleware which is no longer required. Also worth nothing is that GAE classic provides a wrapper to handle serving the app so instead of a main() function where we run the server, we instead wire up the router to the default http.Handler instead.

cookbook/google-app-engine/app-engine.go

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// +build appengine

package main

import (
	"net/http"

	"github.com/labstack/echo/v4"
)

func createMux() *echo.Echo {
	e := echo.New()
	// note: we don't need to provide the middleware or static handlers, that's taken care of by the platform
	// app engine has it's own "main" wrapper - we just need to hook echo into the default handler
	http.Handle("/", e)
	return e
}

Managed VMs are slightly different. They are expected to respond to requests on port 8080 as well as special health-check requests used by the service to detect if an instance is still running in order to provide automated failover and instance replacement. The google.golang.org/appengine package provides this for us so we have a slightly different version for Managed VMs:

cookbook/google-app-engine/app-managed.go

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// +build appenginevm

package main

import (
	"net/http"

	"github.com/labstack/echo/v4"
	"google.golang.org/appengine"
)

func createMux() *echo.Echo {
	e := echo.New()
	// note: we don't need to provide the middleware or static handlers
	// for the appengine vm version - that's taken care of by the platform
	return e
}

func main() {
	// the appengine package provides a convenient method to handle the health-check requests
	// and also run the app on the correct port. We just need to add Echo to the default handler
	e := echo.New(":8080")
	http.Handle("/", e)
	appengine.Main()
}

So now we have three different configurations. We can build and run our app as normal so it can be executed locally, on a full Compute Engine instance or any other traditional hosting provider (including EC2, Docker etc…). This build will ignore the code in appengine and appenginevm tagged files and the app.yaml file is meaningless to anything other than the AppEngine platform.

We can also run locally using the Google AppEngine SDK for Go either emulating AppEngine Classic:

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goapp serve

Or Managed VM(s):

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gcloud config set project [your project id]
gcloud preview app run .

And of course we can deploy our app to both of these platforms for easy and inexpensive auto-scaling joy.

Depending on what your app actually does it’s possible you may need to make other changes to allow switching between AppEngine provided service such as Datastore and alternative storage implementations such as MongoDB. A combination of go interfaces and build constraints can make this fairly straightforward but is outside the scope of this example.