适用于Appengine的跨平台Go代码

Question

What is the GO appropriate way to create a FetchUrl/GetURL function that works from the command line and works from google app engine with its custom way to fetch a url.

I have basic code that fetches and processes some data on a URL. I want to be able to call it from code I use on my desktop, and code deployed to app engine.

Hopefully thats clear, if not please let me know and Ill clarify.

Answer 1

If you have some code which works both on local machine and on AppEngine environment, you have nothing to do.

If you need to do something which should or must be done differently on AppEngine, then you need to detect the environment and write different code for the different environments.

This detection and code selection is easiest done using build constraints. You can put a special comment line in the beginning of your .go file, and it may or may not be compiled and run depending on the environment.

Quoting from The Go Blog: The App Engine SDK and workspaces (GOPATH):

The App Engine SDK introduces a new build constraint term: "appengine". Files that specify

// +build appengine

will be built by the App Engine SDK and ignored by the go tool. Conversely, files that specify

// +build !appengine

are ignored by the App Engine SDK, while the go tool will happily build them.

So for example you can have 2 separate .go files, one for AppEngine and one for local (non-AppEngine) environment. Define the same function in both (with same parameter list), so no matter in which environment the code is built, the function will have one declaration. We will use this signature:

func GetURL(url string, r *http.Request) ([]byte, error)

Note that the 2nd parameter (*http.Request) is only required for the AppEngine (in order to be able to create a Context), so in the implementation for local env it is not used (can even be nil).

An elegant solution can take advantage of the http.Client type which is available in both the standard environment and in AppEngine, and which can be used to do an HTTP GET request. An http.Client value can be acquired differently on AppEngine, but once we have an http.Client value, we can proceed the same way. So we will have a common code that receives an http.Client and can do the rest.

Example implementation can look like this:

url_local.go:

// +build !appengine

package mypackage

import (
    "net/http"
)

func GetURL(url string, r *http.Request) ([]byte, error) {
    // Local GetURL implementation
    return GetClient(url, &http.Client{})
}

url_gae.go:

// +build appengine

package mypackage

import (
    "google.golang.org/appengine"
    "google.golang.org/appengine/urlfetch"
    "net/http"
)

func GetURL(url string, r *http.Request) ([]byte, error) {
    // Appengine GetURL implementation
    ctx := appengine.NewContext(r)
    c := urlfetch.Client(ctx)
    return GetClient(url, c)
}

url_common.go:

// No build constraint: this is common code

package mypackage

import (
    "net/http"
)

func GetClient(url string, c *http.Client) ([]byte, error) {
    // Implementation for both local and AppEngine
    resp, err := c.Get(url)
    if err != nil {
        return nil, err
    }
    defer resp.Body.Close()
    body, err := ioutil.ReadAll(resp.Body)
    if err != nil {
        return nil, err
    }
    return body, nil
}