(C++) Execution Plan Failure Due to Circular Dependencies

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This example demonstrates what happens when an execution plan cannot be created due to a cycle in the dependency graph.

The target curl command requires {{report_id}}. A helper function is defined to produce report_id, but it depends on user_id. In turn, user_id depends on account_id, and account_id depends back on report_id.

This creates a circular dependency:

• report_id → user_id
• user_id → account_id
• account_id → report_id

Because each required value ultimately depends on itself, there is no starting point with a known or independently resolvable input. As a result, it is impossible to construct a valid execution plan.

When ExaminePlan is called, it detects this cycle and reports that the execution plan cannot be created.

This example highlights that dependency resolution requires an acyclic graph. Every chain of dependencies must eventually terminate in a known value. If a cycle exists, the system correctly identifies it and prevents execution.

Note: This example requires Chilkat v11.5.0 or greater.

For more information, see https://www.chilkatsoft.com/curl_dependency_engine.asp

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#include <CkHttpCurl.h>
#include <CkJsonObject.h>

void ChilkatSample(void)
    {
    bool success = false;

    CkHttpCurl httpCurl;

    // This example uses fake URLs and fake variables to demonstrate a dependency cycle.
    // No requests are sent because we only call ExaminePlan.

    // The target curl command requires {{report_id}}.
    const char *targetCurl = "curl -X GET https://api.example.com/reports/{{report_id}}";

    // getReportId can produce report_id, but it requires user_id.
    const char *fnName = "getReportId";
    httpCurl.AddFunction(fnName,"curl -X GET https://api.example.com/report-id?user={{user_id}}");
    httpCurl.AddOutput(fnName,"report.id","report_id");

    // getUserId can produce user_id, but it requires account_id.
    fnName = "getUserId";
    httpCurl.AddFunction(fnName,"curl -X GET https://api.example.com/user-id?account={{account_id}}");
    httpCurl.AddOutput(fnName,"user.id","user_id");

    // getAccountId can produce account_id, but it requires report_id.
    // This creates a cycle:
    // 
    //   report_id -> user_id -> account_id -> report_id
    // 
    // In other words:
    //   - targetCurl needs report_id
    //   - report_id requires user_id
    //   - user_id requires account_id
    //   - account_id requires report_id
    // 
    // There is no starting point where the dependency chain can be resolved.
    fnName = "getAccountId";
    httpCurl.AddFunction(fnName,"curl -X GET https://api.example.com/account-id?report={{report_id}}");
    httpCurl.AddOutput(fnName,"account.id","account_id");

    // Examine the execution plan without sending any requests.
    // The plan cannot be created because the dependencies contain a cycle.
    CkJsonObject planJson;
    planJson.put_EmitCompact(false);

    success = httpCurl.ExaminePlan(targetCurl,planJson);

    // Success is expected to be false.
    std::cout << "success = " << success << "\r\n";

    std::cout << planJson.emit() << "\r\n";

    // Expected result:
    // 
    // {
    //   "errors": [{
    //     "variable": "report_id",
    //     "msg": "Cycle detected"
    //   },{
    //     "variable": "account_id",
    //     "msg": "Unable to resolve"
    //   },{
    //     "variable": "user_id",
    //     "msg": "Unable to resolve"
    //   },{
    //     "variable": "report_id",
    //     "msg": "Unable to resolve"
    //   }]
    // 
    }