Continuation Callouts

Continuation Callouts

Use this skill when a Salesforce page (Visualforce or LWC) needs to call an external HTTP service that may take longer than the 10-second synchronous Apex callout limit allows, or when you need to fire up to three callouts in parallel without blocking the UI. The Apex Continuation class offloads callout execution to Salesforce infrastructure, freeing the request thread while the external service responds.

Before Starting

Gather this context before working on anything in this domain:

• Surface type: Is the entry point a Visualforce page action method, or an LWC @AuraEnabled method? The wiring differs. Visualforce uses a controller action that returns a Continuation object. LWC uses invokeAction from the Lightning Data Service continuation API.
• Endpoint registration: The external endpoint must be in the Remote Site Settings (or preferably a Named Credential). Continuation callouts do not bypass this requirement.
• Most common wrong assumption: Practitioners assume Continuation works from Apex triggers, Batch Apex, or standard @future methods — it does not. Continuation is exclusively for user-initiated requests from Visualforce pages or Lightning Experience components.
• Platform limits in play:
  • Maximum Continuation timeout: 120 seconds per callout (configurable via Continuation.timeout).
  • Maximum parallel callouts per Continuation: 3.
  • Continuation requests do not count toward the org-wide long-running synchronous request governor (since Winter '20, all callouts are excluded from that limit, so this is no longer a differentiating factor — but the 120s ceiling still applies).
  • The callback method (processResponse) runs in a new Apex transaction with its own governor limits.

Core Concepts

Concept 1: The Continuation Object and Two-Phase Execution

A Continuation request follows a two-phase pattern:

1. Start phase — The Visualforce action method (or LWC Apex method) creates a Continuation object, adds one or more HttpRequest objects to it, sets the callback method name, and returns it. Salesforce intercepts the return value, fires the HTTP request(s) off-platform, and releases the request thread.
2. Callback phase — When the external service responds (or the timeout elapses), Salesforce invokes the callback method in a fresh Apex execution context. The callback receives the response via Continuation.getResponse(label) and returns the final value to the page.

The Salesforce platform acts as a proxy. The request thread is freed between phases, so no page timeout occurs while waiting for the external service.

Concept 2: State Passing Between Phases

Because the callback runs in a new transaction, instance variables on a Visualforce controller are not automatically preserved. State must be passed explicitly:

• Set continuation.state to any serializable Apex object (String, Map, custom class with JSON.serialize-compatible structure) before returning from the start method.
• In the callback, retrieve it from the state parameter: public Object processResponse(List<String> labels, Object state).
• Non-serializable types (Blob, SObject with relationship fields, iterators) cannot be passed as state. This is a common source of runtime failures.

Concept 3: Parallel Callouts

A single Continuation can encapsulate up to three simultaneous HTTP requests. Each call to continuation.addHttpRequest(req) returns a unique String label. All requests in the batch fire in parallel. The callback receives a List<String> labels and uses Continuation.getResponse(label) to retrieve the individual responses. Order of response arrival is non-deterministic; always retrieve by label, not by index.

Concept 4: Visualforce vs. LWC Invocation

Common Patterns

Pattern 1: Single Continuation Callout from Visualforce Controller

When to use: A Visualforce page button triggers a callout to a slow external REST API (e.g., a quote-generation service that routinely takes 30–90 seconds).

How it works:

public class QuoteContinuationController {

    // State to pass between phases — must be serializable
    public String quoteRequestId { get; set; }
    public String quoteResult    { get; set; }

    // Phase 1 — Start the callout, return Continuation
    public Continuation startQuoteCallout() {
        // Max timeout in seconds (1–120). Default is 30.
        Continuation con = new Continuation(60);
        // Name of the callback method on THIS class
        con.continuationMethod = 'processQuoteResponse';

        HttpRequest req = new HttpRequest();
        req.setMethod('POST');
        req.setEndpoint('callout:QuoteService/api/quotes'); // Named Credential
        req.setHeader('Content-Type', 'application/json');
        req.setBody('{"requestId":"' + quoteRequestId + '"}');

        con.addHttpRequest(req);

        // Pass serializable state to the callback
        con.state = quoteRequestId;

        return con; // Salesforce intercepts and fires the HTTP request
    }

    // Phase 2 — Callback after callout completes
    public Object processQuoteResponse(List<String> labels, Object state) {
        // labels[0] is the label for the first (and only) request
        HttpResponse res = Continuation.getResponse(labels[0]);

        if (res.getStatusCode() == 200) {
            quoteResult = res.getBody();
        } else {
            quoteResult = 'Error: ' + res.getStatusCode();
        }

        // Return null or a PageReference to navigate after callback
        return null;
    }
}

Visualforce page binding:

<apex:page controller="QuoteContinuationController">
    <apex:form>
        <apex:commandButton value="Generate Quote"
                            action="{!startQuoteCallout}"
                            reRender="resultPanel"/>
        <apex:outputPanel id="resultPanel">
            <apex:outputText value="{!quoteResult}"/>
        </apex:outputPanel>
    </apex:form>
</apex:page>

Why not the alternative: A synchronous Http.send() in a Visualforce action method times out at 10 seconds and blocks the Salesforce request thread. For slow services, this causes System.CalloutException: Read timed out before the service responds.

Pattern 2: Parallel Callouts from a Single Continuation

When to use: A dashboard page needs to aggregate data from three different APIs simultaneously (e.g., ERP inventory, CRM pricing, logistics tracking). Firing them sequentially wastes time.

How it works:

public class DashboardContinuationController {

    public String inventoryData  { get; set; }
    public String pricingData    { get; set; }
    public String trackingData   { get; set; }

    private String inventoryLabel;
    private String pricingLabel;
    private String trackingLabel;

    public Continuation startDashboardLoad() {
        Continuation con = new Continuation(90);
        con.continuationMethod = 'processDashboardResponses';

        HttpRequest invReq = new HttpRequest();
        invReq.setMethod('GET');
        invReq.setEndpoint('callout:ERPService/inventory');
        inventoryLabel = con.addHttpRequest(invReq); // store label

        HttpRequest priceReq = new HttpRequest();
        priceReq.setMethod('GET');
        priceReq.setEndpoint('callout:PricingService/current');
        pricingLabel = con.addHttpRequest(priceReq);

        HttpRequest trackReq = new HttpRequest();
        trackReq.setMethod('GET');
        trackReq.setEndpoint('callout:LogisticsService/track');
        trackingLabel = con.addHttpRequest(trackReq);

        // Pass labels as state so callback can retrieve by name
        con.state = new Map<String, String>{
            'inv'   => inventoryLabel,
            'price' => pricingLabel,
            'track' => trackingLabel
        };

        return con;
    }

    public Object processDashboardResponses(List<String> labels, Object state) {
        Map<String, String> labelMap = (Map<String, String>) state;

        HttpResponse invRes   = Continuation.getResponse(labelMap.get('inv'));
        HttpResponse priceRes = Continuation.getResponse(labelMap.get('price'));
        HttpResponse trackRes = Continuation.getResponse(labelMap.get('track'));

        inventoryData = invRes.getStatusCode()  == 200 ? invRes.getBody()  : 'Error';
        pricingData   = priceRes.getStatusCode() == 200 ? priceRes.getBody() : 'Error';
        trackingData  = trackRes.getStatusCode() == 200 ? trackRes.getBody() : 'Error';

        return null;
    }
}

Why not sequential: Three sequential Continuation calls would require three separate user interactions or a chained pattern. Using a single Continuation with three requests fires them in parallel on Salesforce infrastructure, typically reducing wall-clock latency to the slowest single response rather than the sum of all three.

Pattern 3: LWC with @AuraEnabled Continuation

When to use: A Lightning Web Component (not Visualforce) needs to invoke a long-running callout. Available since Summer '19.

How it works:

// Apex controller
public with sharing class ProductSearchController {

    @AuraEnabled(continuation=true)
    public static Object startSearch(String searchTerm) {
        Continuation con = new Continuation(60);
        con.continuationMethod = 'processSearchResults';

        HttpRequest req = new HttpRequest();
        req.setMethod('GET');
        req.setEndpoint('callout:ProductCatalog/search?q=' +
                        EncodingUtil.urlEncode(searchTerm, 'UTF-8'));
        con.addHttpRequest(req);

        con.state = searchTerm;
        return con;
    }

    @AuraEnabled(continuation=true)
    public static Object processSearchResults(List<String> labels, Object state) {
        HttpResponse res = Continuation.getResponse(labels[0]);
        if (res.getStatusCode() == 200) {
            return res.getBody();
        }
        throw new AuraHandledException('Search failed: ' + res.getStatusCode());
    }
}

// LWC JavaScript — productSearch.js
import { LightningElement } from 'lwc';
import { invokeAction }      from 'lightning/actions'; // Aura-compatible wire
import startSearch            from '@salesforce/apex/ProductSearchController.startSearch';

export default class ProductSearch extends LightningElement {
    results;

    handleSearch(event) {
        const term = event.target.value;
        invokeAction(this, startSearch, { searchTerm: term })
            .then(result => { this.results = result; })
            .catch(error => console.error(error));
    }
}

Decision Guidance

Recommended Workflow

Step-by-step instructions for an AI agent or practitioner activating this skill:

1. Gather context — confirm the org edition, relevant objects, and current configuration state
2. Review official sources — check the references in this skill's well-architected.md before making changes
3. Implement or advise — apply the patterns from Core Concepts and Common Patterns sections above
4. Validate — run the skill's checker script and verify against the Review Checklist below
5. Document — record any deviations from standard patterns and update the template if needed

Review Checklist

Run through these before marking work in this area complete:

• Continuation object timeout set explicitly (1–120 seconds); not left at default 30 if service needs more.
• continuationMethod name matches the actual callback method signature exactly.
• Callback method signature is public Object methodName(List<String> labels, Object state).
• All state passed between phases is JSON-serializable (no Blob, no SObject with relationship queries).
• Each addHttpRequest return label is stored and used to retrieve the correct response in the callback.
• Endpoint is registered in Remote Site Settings or uses a Named Credential.
• Error status codes handled in the callback (service may return 4xx/5xx on timeout or failure).
• For LWC: Apex method annotated @AuraEnabled(continuation=true), not plain @AuraEnabled.
• Continuation is NOT used from a trigger, batch, scheduled, or @future context.
• Test class uses Test.setContinuationResponse() and Test.invokeContinuationMethod() to mock both phases.

Salesforce-Specific Gotchas

Non-obvious platform behaviors that cause real production problems:

1. Continuation is not supported in triggers or async Apex — Attempting to instantiate Continuation from a trigger, Batch Apex execute, Scheduled Apex, or @future method throws a runtime exception. The class is only valid in the context of a Visualforce action request or LWC @AuraEnabled(continuation=true) invocation.

2. Non-serializable state silently fails or throws at callback time — The state property is serialized by the platform between phases. Passing an SObject that includes relationship query results, a Type reference, or an Iterator causes a System.SerializationException at callback invocation. Always pass simple scalar types, Map<String, String>, or a plain class with @TestVisible and primitive fields.

3. Callback method must be public and on the same class — continuationMethod must be a public (not private) method on the same Apex class. It cannot be a static method. If the name is misspelled or the method is private, the callback silently never fires — no exception is thrown at start time.

Output Artifacts

Related Skills

• apex/callouts-and-http-integrations — use for synchronous HTTP callouts under 10 seconds, Named Credential setup, and HttpRequest/HttpResponse fundamentals.
• apex/async-apex — use when the core design choice is whether the callout belongs in @future, Queueable, Batch, or Scheduled Apex (none of which support Continuation).
• apex/exception-handling — use when callback error handling and retry logic need a structured pattern.
• apex/test-class-standards — use for mocking Continuation responses in test classes with Test.setContinuationResponse().