Handling concurrency exceptions with external API calls - asp.net-mvc

Question

I have the following POST edit action method, which mainly perform two Update actions:-
Edit the object on the external system suing API calls.
Edit the object on our system database.
[HttpPost]
public ActionResult Create(RackJoin rj, FormCollection formValues)
{string controllername = RouteData.Values["controller"].ToString();
if (ModelState.IsValid)
{ var message = "";
var status = "";
long assetid = new long();
XmlDocument doc = new XmlDocument();
using (var client = new WebClient())
{
var query = HttpUtility.ParseQueryString(string.Empty);
foreach (string key in formValues)
{
query[key] = this.Request.Form[key];
}
query["username"] = System.Web.Configuration.WebConfigurationManager.AppSettings["ApiUserName"];
query["password"] = System.Web.Configuration.WebConfigurationManager.AppSettings["ApiPassword"];
string apiurl = System.Web.Configuration.WebConfigurationManager.AppSettings["ApiURL"];
var url = new UriBuilder(apiurl);
url.Query = query.ToString();
try
{
string xml = client.DownloadString(url.ToString());
doc.LoadXml(xml);
status = doc.SelectSingleNode("/operation/operationstatus").InnerText;
message = doc.SelectSingleNode("/operation/message").InnerText;
}
catch (WebException ex)
{
ModelState.AddModelError(string.Empty, "Error occurred:" + ex.InnerException);
}
}
if (status.ToUpper() == "SUCCESS")
{
repository.InsertOrUpdateRack(rj.Rack, User.Identity.Name, rj.Resource.RESOURCEID);
repository.Save();
return RedirectToAction("Index");
}
else
{
ModelState.AddModelError(string.Empty, message.ToString());
}
}
}
catch (DbUpdateConcurrencyException ex)
{
As shown in the above code I will not do a repository.save() to update the object on our system, unless the API return a “success”.
But currently I am facing the following problem:-
If the API return a “success” but a concurrency exception occurred, then the API will update the object on the external system, but the object will not be updated on our system?
So is there a way to handle this situation?

Answer 1

There's no easy way to solve this situation. One way to handle it would be to ask the designers of the external API expose a method allowing to commit the transaction done in a previous call. Basically your first call will make modifications to the external system but with some boolean flag indicating that those changes are still pending. Then you update your system and in case of success you would call the external API to flag the data from pending to valid.
If you have no control over the external API and it makes the changes to the data from the first call irreversible, then I am afraid that you do not have much choices left. You might remember the state of the object you are modifying on the external system before calling the API and in case of an exception on your system, revert back to the previous state by calling the API with the previous values.