google app engine - Cross platform go code for appengine -

what go appropriate way create fetchurl/geturl function works command line , works google app engine custom way fetch url.

i have basic code fetches , processes data on url. want able call code use on desktop, , code deployed app engine.

hopefully thats clear, if not please let me know , ill clarify.

if have code works both on local machine , on appengine environment, have nothing do.

if need should or must done differently on appengine, need detect environment , write different code different environments.

this detection , code selection easiest done using build constraints. can put special comment line in beginning of .go file, , may or may not compiled , run depending on environment.

quoting the go blog: app engine sdk , workspaces (gopath):

the app engine sdk introduces new build constraint term: "appengine". files specify

// +build appengine 

will built app engine sdk , ignored go tool. conversely, files specify

// +build !appengine 

are ignored app engine sdk, while go tool happily build them.

so example can have 2 separate .go files, 1 appengine , 1 local (non-appengine) environment. define same function in both (with same parameter list), no matter in environment code built, function have 1 declaration. use signature:

func geturl(url string, r *http.request) ([]byte, error) 

note 2nd parameter (*http.request) required appengine (in order able create context), in implementation local env not used (can nil).

an elegant solution can take advantage of http.client type available in both standard environment , in appengine, , can used http request. http.client value can acquired differently on appengine, once have http.client value, can proceed same way. have common code receives http.client , can rest.

example implementation can 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: common code  package mypackage  import (     "net/http" )  func getclient(url string, c *http.client) ([]byte, error) {     // implementation both local , 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 }