Future method is not invoked from PostInstall script

Question

I'm trying to create a future method that makes a callout, and for some reason it's not invoked when the script is done.

postInstallScript.cls:

global class postInstallScript implements system.InstallHandler {
   global void onInstall(InstallContext ctx) {
      // ...
      myFutureMethod('test');
   }
   @future(callout=true)
   global static void myFutureMethod(string s) {
      // sending email on invoke
   }
}

I'm trying to simply send an email as a test once the method is invoked, and it works only when I take off the @future annotation from the myFutureMethod method, but as the method contains a callout, it cannot be executed from the onInstall method, and trows an error:

System.CalloutException: You have uncommitted work pending. Please commit or rollback before calling out

Answer 1

Sending Apex Email doesn't instantaneously sends the email. It actually locks the database, and the email will be sent at the end of the transaction. As email has locked the database it actually an uncommitted work pending. No Callout after DML, in our case, no callout after Email.

Solution: Do callout first and then send email.