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Wondering about @future

What a hectic day:

18:04:53.589 (1589740000)|FATAL_ERROR|System.CalloutException: Callout from triggers are currently not supported.

Actually I try to call a class (contain Http that retrieve data from external server) from trigger and retrieve the error above.

Based on googling, it said something like this : to prevent exception above we need to add @future annotation at the top of our method like below.


@future
public static void executeSearch(string criteria)  {  
        String returnStr='';
        String baseUrl =URL + '/2.0/search?format=xml&';
        String[] searchArgs = new List<string>();
        searchArgs.add(criteria);
        String relativeUrl = String.format('criteria={0}', searchArgs);
             
        HTTPRequest request = new HTTPRequest();              
        request.setEndpoint(baseUrl + relativeUrl);
        request.setHeader('Authorization', authorizationHeader);
        request.setHeader('Content-Type', 'application/xml');
        request.setMethod('GET');
        request.setTimeout(120000);
       
        HTTP http = new HTTP();
        HTTPResponse response =  http.send(request);

         if(response.getStatusCode()!=200){
           returnStr=response.getStatusCode() +' : '+ response.getStatus();
           errorCode=response.getStatusCode();
           errorMessage=returnStr;
         }
        
        else {
          returnStr=response.getBody();
         }
        system.debug( returnStr);
    }

Or if we want it to execute callout use @future(callout=true) ,if want to prevent callout use @future(callout=false)

Why @future?
To identify methods that are executed asynchronously. When we specify future, the method executes when Salesforce has available resources.

But @future will have some limitation.It only works for static void method, it won't work if our method got return value example if I want to retrieve response string .Other work around on this is to store the return value in somewhere else that we can refer it later.

Because it depends on resource availability, I have no idea when it run.If our process contains several method that doing call out and depend on each other , we might not able to control it.In other word,the method won't get executed in the same order it is called.


More explanation from Salesforce site.Below I copy some note from the Force IDE Help for my own note.
Methods with the future annotation have the following limits:
  • No more than 10 method calls per Apex invocation
  • Salesforce also imposes a limit on the number of future method invocations: 200 method calls per full Salesforce user license, Salesforce Platform user license, or Force.com - One App user license, per 24 hours. This is an organization-wide limit. Chatter Only, Chatter customer users, Customer Portal User, and partner portal User licenses aren’t included in this limit calculation. For example, suppose your organization has three full Salesforce licenses, two Salesforce Platform licenses, and 100 Customer Portal User licenses. Your entire organization is limited to only 1,000 method calls every 24 hours, calculated as 200 * (3+2), not 200 * (3+2+100).
  • The specified parameters must be primitive data types, arrays of primitive data types, or collections of primitive data types.
  • Methods with the future annotation cannot take sObjects or objects as arguments.
  • Methods with the future annotation cannot be used in Visualforce controllers in either getMethodName or setMethodName methods, nor in the constructor.


Other references link that I've read.
http://salesforce.stackexchange.com/questions/5602/http-callout-from-triggers
http://www.sundoginteractive.com/sunblog/posts/four-gotchas-when-using-future-calls-in-apex-in-salesforce
http://boards.developerforce.com/t5/Apex-Code-Development/Synchronous-callout-in-trigger/td-p/271947

Well,maybe instead of using trigger, I will  use Scheduler and Batch.

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