Let's imagine I have WCF service and a client that consumes some methods from a given service.
There are tons of posts of how to handle various exceptions during the client and service communication. Only thing which is still confusing me is a following case:
Service:
[ServiceContract]
public interface IService1
{
[OperationContract]
bool ExportData(object data);
}
public class Service1 : IService1
{
public bool ExportData(object data)
{
// Simulate long operation (i.e. inserting data to the DB)
Thread.Sleep(1000000);
return true;
}
}
Client:
class Program
{
static wsService1.Service1Client client1 = new wsService1.Service1Client();
static void Main(string[] args)
{
object data = GetRecordsFromLocalDB();
bool result = client1.ExportData(data);
if (result)
{
DeleteRecordsFromLocalDB();
}
}
}
Client gets some data from local db and sending it to the server. If result is successful, then client is going to remove exported rows from local DB. Now imagine, when data is already sent to the server, suddenly connection failed (i.e. WiFi was disconnected). In this case data is successfully processed on a server side, but client is never know about it. And yes, I can catch connection exception, but still I don't know what should I do with a records in my local DB. I can send this data again later, but I'll get some duplication on a server DB (i.e. duplication is allowed on remote DB), but I don't want to send same data multiple times.
So, my question is how to handle such cases? What is the best practices?
I checked some info about asynchronous operations. But still this is about when I have stable connection.
As a workaround I can store my export operation under some GUID remotelly and check status for this GUID later. Only thing I can't change remote DB. So, please, suggest what would be better in my case?
Here are some points to consider
On server side you can catch all kinds of error (custom class deriving IErrorHandler) and provide specific error to client letting him know about error's reason.
The concept of service is that it is kind of intermediary between client and database so why would client retrieve data and then send it to service?
One way out is to use transaction which assures that if error occurres then no changes are going to be retained.
By the way, If you expect service to throw an exception do not create global service object since it will end up being in faulted state. Create new instance for every single call instead (make use of using statement so as to dispose its instance). Bool return type does not provide extensive information about the error if any takes place. Let it have void return type and wrap in try/catch block which gives a change to learn more about the source and nature of error.
Related
I had a troubling issue dealing with SignalR (v 2.4.1) hubs last week, and despite doing enough by the documents, I could not broadcast messages without hacking my way through it. For some extra context, this is a self hosted (Owin) hub attached to a windows service.
MSDN documents, and runtime errors (Using a Hub instance not created by the HubPipeline is unsupported), suggest that we are supposed to get the hub context by calling GlobalHost.ConnectionManager.GetHubContext<ThisTypeOfHub>(), and at that point we can make calls to the client.
(https://learn.microsoft.com/en-us/aspnet/signalr/overview/guide-to-the-api/hubs-api-guide-server#callfromoutsidehub)
When I originally set up calls to the clients, I was doing it this way:
public void OnDisplayMessage(string message)
{
var hub = GlobalHost.ConnectionManager.GetHubContext<ThisTypeOfHub>();
hub.Clients.All.BroadcastToClient(message);
}
The browsers that contained the method and had connected to signalR never got called, however. The clients could call server methods, and even the server methods would send out a callback which the clients responded to, but the hub context could never call out to the client methods when called from outside the hub. In the end, I directly brought back the hub reference in the IoC container, and called out to clients using that, as shown below.
public class LogicWithUI : Logic
{
Hub hub;
public LogicWithUI(IDependencyInjectionContainer container)
{
this.hub = container.Resolve<ThisTypeOfHub>(); // ThisTypeOfHub inherits from Hub
}
public class OnDisplayMessage(string message)
{
try
{
this.hub.Clients.All.DisplayMessage(string);
}
catch (Exception)
{
//do nothing, no webpage has connected yet
}
}
}
This way it is finally working fine (you can see the catch placed there for when an error occurs -- only when no webpages have connected to this yet), but it doesn't make sense.
What would cause the HubContext called from outside the hub to not actually broadcast the method?
Why would the unsupported error get thrown only when no clients are connected?
Are there any obvious mistakes I'm overlooking here?
The primary goal here is to have a functioning product, but I also want to do it the right/documented way. It's a little confusing when that way isn't working.
I also understand I might have left out some important details regarding the SignalR configuration, I can answer any follow up questions, but wanted to start with the basic explanation.
I'm trying to develop a system to share information across 2 windows applications with different update loops.
I developed a solution that uses a WCF service to store and retrieve data. However this data is different across clients and therefore showing different values for each applications.
The service I tried to implement are similar to this
namespace TEST_Service_ServiceLibrary
{
[ServiceContract]
public interface TEST_ServiceInterface
{
[OperationContract]
string GetData();
[OperationContract]
void StoreData(string data);
}
}
namespace TEST_Service_ServiceLibrary
{
// Core service of the application, stores and provides data:
public class TEST_Service : TEST_ServiceInterface
{
string TEST_string;
// Used to pull stored data
public string GetData()
{
return TEST_string;
}
// Used to store data
public void StoreData(string data)
{
TEST_string = data;
}
}
}
Each of the applications creates a TEST_Service client.
I tested the GetData and StoreData functions and they work fine independently, however when I use StoreData on one application and test the GetData method from the other the data appears to be empty.
I have looked around but haven't found a solution to this problem, is there a work around for this? or should I change my approach? I thought of using a local data base but I'm not sure this is the best way to solve it
Thanks a lot
You have more than one instance of your service class. If you want to have your data in memory, you will need to run it in single instance mode:
[ServiceBehavior(InstanceContextMode = InstanceContextMode.Single)]
Now keeping your data in memory might not be the best option anyway. You should look for a data store of some kind and then make that store a persistent instance with a single interface. Then it does not matter how many of your service instances are used.
If your WCF service was storing information in a database, then information stored on one request would go to the database, and when another request retrieved it, the result would come from that database. So if one client stored something, another could retrieve it.
The reason why this isn't working is because in response to each request your application is creating a new instance of the TEST_Service class. That means TEST_string, where you are storing values between requests, is a new string. It doesn't contain the previous value.
For experimentation you could try changing the string to static:
static string TEST_string;
...and then the value would persist between instances of the service class. But that still wouldn't be effective because your WCF service could be deployed to multiple servers, and then each one would have a separate instance of the class. Updating one wouldn't update the others. Or, if the service application restarted then the value would be lost. (From the context I assume that you're just experimenting with this.)
So ultimately you'd want some way to persist data that wouldn't depend on any of those factors, but would "survive" even when the instance of the service class goes out of scope or the application shuts down.
I have a button, that when clicked will/should notify the server. The server will then save a value to the database. If all went well, it should return true, otherwise return false.
I instantiate a hub in my view
var signalRhub = $.connection.hubSignalR;
Start the connection:
$.connection.hub.start().done(function () {
$("#submitBut").click(function () {
signalRhub.server.cardAdded();
});
});
Define the function that will be used by the server to return the boolean value:
signalRhub.client.cardAddedRes = function (isSuccess) {
alert("From server: " + isSuccss);
}
My Hub class:
public class HubSignalR : Hub
{
public bool isSuccess = false; <-- Will be set from controller
public void CardAdded()
{
Clients.Caller.CardAddedRes(isSuccess); <-- Notice the isSuccess
}
}
My problem is that the isSuccess value is coming from my controller, that interacts with the model/database.
So I get the error:
Using a Hub instance not created by the HubPipeline is unsupported.
I tried using: GlobalHost.ConnectionManager.GetHubContext<HubSignalR>()
but I can't make it work.
Here is the relevant code in my controller:
private HubSignalR signalR = new HubSignalR(); <-- Field variable
[HttpPost]
public ActionResult AttachCard(Card model, int MemberID)
{
var hub = GlobalHost.ConnectionManager.GetHubContext<HubSignalR>();
...
//We saved to the database, so we call the client function with bool = true
hub.Clients.All.CardAdded(true); <-- Actually I want to send to one client, NOT ALL
//Something like hub.Clients.Caller.CardAdded();
}
I'm forced to make the isSuccess field in my: HubSignalR class, since I need to return that as the parameter from my controller. But when the button is clicked, this value has not yet been set (I think).
I can see from the debugger, that I do reach: signalRhub.server.cardAdded();
But the server never responds, so I don't reach this function:
signalRhub.client.cardAddedRes = function (isSuccess) {
alert("From server: " + isSuccss);
}
I don't really get to call the CardAdded() method from my controller, cus of the GlobalHost.ConnectionManager.GetHubContext. But you can see
If you got a nicer solution than what I'm trying to do, please tell. I'm total new with SignalR and fairly new with ASP.net MVC
Hope I got the problem right: you want to set isSuccess in controller and send it to specific client, but don't reach specific client from outside the hub?
As to "how":
You should find a way to identify your client, i.e. implement some kind of authentication. More on this here: Authentication in .net core When clients connect to SignalR, they get a connection ID. You can map the connection ID to real client identity. More info here: Mapping clients to connections
Than in you server method:
Get authenticated client identity
Get signalR hub context
Map client identity to existing signalR connection ID
Send message to that signalR connection
As I can see from you code you might be working on some personal-use/exploratory project and probably not interested in implementing authentication/don't care about security. You could get signalR connection ID in your client after connecting through $.connection.hub.id. Than you could send this ID to the server method as parameter or header. This is by no way should be used in production environment, as you would trust the client who he is, and pass parameters that are not strictly needed by your method.
As to "why":
In fact I don't think you need signalR for your use case. You call a server method, it saves to DB and returns you OK or not OK, client is happy. No need to pass it through signalR.
You do need signalR e.g. when:
- Same client is logged in on several devices and wants to get updates if changes were made on one of them
- Client works on something and another one changes same data. You want to inform the first client.
- Inform your client of an event that was not triggered by him (new notification)
In all this cases you have some kind of authentication and sending signalR message to the right client is not a problem.
Answer to comment below
I have little experience with ajax, i guess it might work. Another idea, if you want to avoid authentication, is a subscription model with SignalR.
You have to find out which specific resources you have, let's say "game" in your case, which have IDs. Than clients, interested in that particular resource, shall subscribe to changes.
All participants of a particular card game instance shall call a hub method defined like this:
public async Task SubscribeToGameChanges(long id)
{
await this.Groups.AddToGroupAsync(this.Context.ConnectionId, Helper.GetGameGroupName(id));
}
public static class Helper
{
public static string GetGameGroupName(long id)
{
return $"Game_{id}";
}
}
Than call it from client. Note the AddToGroupAsync. SignalR shall create a group with given name and add a client to it. If group exists, it will just add another client. So you have a group per game instance with a list of interested clients (players).
Now when a change happens to the game, you notify all your clients by calling from your controller on hub context:
await hubContext.Clients.Groups(Helper.GetGameGroupName(id)).SendAsync("myNotifyMethod", myParameters);
You could pack all your changes into parameters, or just inform the clients that the state of the game (or whatever other resource) has changed and clients shall requery the state through normal API call.
Also I noticed you use GlobalHost in your code. My code samples are for .net core SignalR version and might slightly differ in your case. See here on differences between .net core and full .net SignalR.
I am developing server side logic to process requests and respond with data to front-end server as well as to mobile app direct connections.
I have implemented SessionContext class that basically ensures, that there is matching session record in DB for every service that is called (with exception for forgot password cases, etc).
I am now trying to implement event logging. I want to have common logic so I may log all requests, exceptions, data, etc.
I have come up with this code, but somehow I don't feel good about it - too much code for each service method. Are there any clever tricks that I might implement to make it shorter and easier to read/code?
Current implementation would use EventLogic class to log event to event table. At some point some events might be related to session so I am passing eventLog as parameter to SessionContext (to create link between event and session). SessionContext saves entity data on successful dispose... i have a gut feeling that something is wrong with my design.
public Session CreateUser(string email, string password, System.Net.IPAddress ipAddress)
{
using (var eventLog = new EventLogic())
{
try
{
eventLog.LogCreateUser(email, password, ipAddress);
using (var context = SessionContext.CreateUser(eventLog, email, password, ipAddress))
{
return new Session()
{
Id = context.Session.UId,
HasExpired = context.Session.IsClosed,
IsEmailVerified = context.Session.User.IsEmailVerified,
TimeCreated = context.Session.TimeCreated,
PublicUserId = CryptoHelper.GuidFromId(context.Session, context.Session.UserId, CryptoHelper.TargetTypeEnum.PublicUser),
ServerTime = context.Time
};
}
}
catch (Exception e)
{
eventLog.Exception(e);
}
}
}
You should consider using something like SLF4J + LogBack (for example) for logging.
If your classes follow SRP, you should have not more than one call type like LogCreateUser per your application. And that means, there is no need to extract logging logic into a new class.
I am learning SignalR and I have some doubts about it.
First: Does the Hub works as static object? This question moves to the next question.
Second: Is right to start a transaction inside a method inside the hub?
I want to use the SignalR to send and save information in real time. For example, I want to create a chat, and, when the server receive the message, it saves in the database.
My question is about the method that receive the message will be in memory forever (while the webapp is running).
My concern is about the transaction/connection. Does the transaction will be always active?
For example:
public void Send(string name, string message)
{
Message m = new Message() { n = name, m = message};
using(Entities db = new Entities()
{
db.Messages.Add(m);
db.Save();
}
// Call the addNewMessageToPage method to update clients.
Clients.All.addNewMessageToPage(name, message);
}
I am using EntityFramework 6 and SignalR 2.
Hub instances are created for each request. You can read more about hub instance lifetime here.
EF is creating a transaction each time it needs to save changes. However the transactions created by EF are committed/rolled back and disposed once saving changes is completed and transactions do not leak outside the SaveChanges call. You are also disposing your context (which is good) so you do not leak transactions or connections. (I actually don't hub instance lifetime is relevant at all in your case since you don't try to store anything in class variables).