[SalesForce] Calling future method from Batch

Move your callouts before your DML statements.

In other words make sure that all update, insert and delete statements in your execute method occur after any callout statements. A quick search of "uncommitted work" will reveal that there are plenty of questions on this site that address this issue

Here's an example :

Assume you have a queue of "contacts" off accounts, each contact has a url (Url__c) and a state (State__c) . State would be set to EMAIL-PENDING just before you make your callout

This would be a problem and fire the uncommited error :

public void execute( ... )
{

    for(Sobject s : contacts)
    {
        contact.put('State__c', 'EMAIL-PENDING');
        update contact;

        // http callout (contact.Url__c)

        // send-email( contact.emailAddress )'
        contact.put('State__c', 'EMAIL-SENT');
        update contact;
    }
}

Possible solution : modify your logic so that you focus on state showing the emails actually sent ( EMAIL-SENT )

    public void execute( Database.BatchableContext BC, List<SObject> contacts )
    {

        for(Sobject s : contacts)
        {
             // http callout (contact.Url__c)

            // send-email( contact.emailAddress )'
            contact.put('State__c', 'EMAIL-SENT');
            update contact;
        }
    }

Alternative solution, move from a query locator to an iterable in your batch and use the start context to set all work as EMAIL-PENDING. This approach allows you to perform state changes / DML before you make the callout

   global Iterable<...> start(Database.BatchableContext info) {

      for(Sobject s : contacts)
        {
            contact.put('State__c', 'EMAIL-PENDING');
            update contact;
        }

        return....
    }


    public void execute( Database.BatchableContext BC, List<SObject> contacts )
    {
        for(Sobject s : contacts)
        {
            // http callout (contact.Url__c)
            // send-email( contact.emailAddress )'
        }

        for(Sobject s : contacts)
        {
            contact.put('State__c', 'EMAIL-SENT');
            update contact;
        }
    } 

Definitely you can use an HTTP callout as an escape mechanism. But this is pretty naughty. You'll need a Remote Site Setting and a Session ID and a global class etc etc etc (red flags, alarm bells).

May I humbly recommend the use of System.Queueable call instead of @Future. You can create a class that is constructed with any kind of argument that you want to hand in, like so:

public class DeferredHandler implements System.Queueable
{

    private Map<String,String> key2val;

    public DeferredHandler(Map<String,String> key2val) {
        this.key2val = key2val;
    }

    public void execute(System.QueueableContext objContext)
    {
        //do your @Future stuff here with this.key2val
    }

}

And invoke him from inside your batch like this:

//defer stuff
System.Queueable job = new DeferredHandler(key2val);
System.enqueueJob(job);

There is some literature about this in the docs and Josh Kaplan's awesome Queueable blog post.