Callout loop not allowed error

Question

I have an APEX REST POST method that is being called from an external system 'A'.

Processing this POST I need to callout to another external system 'B' and based on its response continue with my application logic, eventually returning the result to system 'A'.

The problem is that I'm getting an Error: "System.CalloutException: Callout loop not allowed"

is there any way to get around this ?

Answer 1

No, there is not an immediate workaround to this :-(

The mechanism by which they measure that 'callout depth' is by transmitting an HTTP header on requests originating from Apex, Sfdc-Stack-Depth: 1 - if you try and trick the platform by explicitly setting this header to '' etc, you'll find the value is very sticky!

Unfortunately for us, there is a good business reason behind this "feature" on Salesforce' part. It is by design, to prevents callouts from blocking threads between multiple orgs / instances.

(hopefully this sequence diagram illustrates the intention; as an HTTP callout could take a while, the number of loopback connections is restricted to 1, otherwise imagine the DoS implications)

You might be able to overcome this issue if your B callout were read-only and its contents were updated infrequently.

You could schedule a batch process to regularly perform the callout and avail its response in a Document (or such). Then read that Document when responding to A, decoupling the two. But I admit that may be unlikely in your case...

Answer 2

@user31's answer is absolutely incorrect (or I am misunderstanding the question). There is nothing preventing you from making callouts from Apex REST methods to external systems. Here is some sample code:

@RestResource(urlMapping='/restexample/')
global with sharing class RestExample 
{
    @HttpGet
    global static string doGet() 
    {
        HttpRequest req = new HttpRequest();

        req.setMethod('GET');
        req.setEndpoint('https://www.google.com');
        system.debug('request body: '+req.getbody());

        HTTPResponse res = new http().send(req);

        return res.getBody();
    }
}

Note that you cannot make callouts to Salesforce as that will give you the callout loop error.

You can test this code through curl or something similar. Testing it through anonymous Apex will give the callout loop error as you are making a callout from within Salesforce to Salesforce.

Answer 3

You can solve this by placing the second API call in an implements Queueable class. It doesn't work for @future.

Answer 4

I've encountered this, but different scenario. I have a setup where we have two orgs connected via an integration tool. In addition to these two SF orgs, we are connected to an external service.

A - SF Org 1 B - SF Org 2 C - 3rd party service

B is connected to A & C directly. To save licensing costs if A wants to talk to C it must go through B.

This encounters the callout loop without a workaround.

What happens is on org B there is a service that runs and passes calls along in a new session.

Now the packages are managed, but here is what I know about how it works: https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/apex_interface_system_schedulablecontext.htm

https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/apex_scheduler.htm

https://help.salesforce.com/articleView?id=000340086&type=1&mode=1 https://developer.salesforce.com/forums/?id=906F0000000g27DIAQ

Answer 5

I have the similar situation but simpler - external system A sends REST API request to create a record in Salesforce, trigger is executed on the object, future method is executed to make a callout to another external system B. This future method fails with the System.CalloutException: Callout loop not allowed error. The solution for me was to make this callout from Scheduled class. So instead of executing future method, I scheduled class to be executed in 2 seconds from current time.