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Design pattern for implementing the Polling approach in c#

I want to implement polling approach in c# using design patterns with some generic code with the below scenario.
I have 3 console application names Employee, Orders, Salary and in this from the Employee application I am initiating the Orders and Salary console application , once I will initiate application Orders and Salary console application are long running application which will keep on running and we are updating the local text file with the completion status for each of the application.
From the Employee application I have created a Timers objected and using Elapsed Event for 2 sec to check the file text whether it has status of both the file has been completed or not.
So I want to make the below implementation in more generic way or using some other approach/design patterns so that we can achieve the similar polling concept as I have ore complex situation where I have to use the same repetitive code for each time . Any suggestion on this ?
var poolingTimer = new Timer(2 * 1000);
poolingTimer.AutoReset = false;
poolingTimer.Elapsed += async delegate {
if (logic to check if File text has console application completed status) {
poolingTimer.Stop();
} else {
poolingTimer.Start();
}
};
poolingTimer.Start();

C# run SQL server query and not wait for result

I'm using Serenity C# to develop a website.
When I click on a button it should run a SQL command, which starts a stored procedure.
My code
public ListResponse<MyRow> RunSQL(IDbConnection connection, ListRequest request)
{
string sql = "EXEC SP_A #Username='" + Authorization.UserDefinition.Username + "'";
SqlHelper.ExecuteNonQuery(connection, sql);
return new MyRepository().List(connection, request);
}
This code works fine, but it makes my web slow because my web needs to wait for the query to finish.
I want to kick off the SQL command and not wait for the result. Can I use the Task Parallel Library (TPL) for this?

Can I use TPL (Task Parallel Library)??
No, you can not. You execute ONE statement, I am not even sure where you get the idea that paralellism of one item will do anything. if that query takes a long time, analyze whether it is defective. if it is not defective...
...change the API to be async an return a come back later with a token. It is waiting for the return value because you degiend the API to be synchroneous . you this is not acceptable, then the API is a design error and the design at least of this method should change.
Nothing in async/await/paralellism will change the API design and it will not magically make the request finish faster.

Hangfire causing locks in SQL Server

We are using Hangfire 1.7.2 within our ASP.NET Web project with SQL Server 2016. We have around 150 sites on our server, with each site using Hangfire 1.7.2. We noticed that when we upgraded these sites to use Hangfire, the DB server collapsed. Checking the DB logs, we found out there were multiple locking queries. We have identified one RPC Event “sys.sp_getapplock;1” In the all blocking sessions. It seems like Hangfire is locking our DB rendering whole DB unusable. We noticed almost 670+ locking queries because of Hangfire.
This could possibly be due to these properties we setup:
SlidingInvisibilityTimeout = TimeSpan.FromMinutes(30),
QueuePollInterval = TimeSpan.FromHours(5)
Each site has around 20 background jobs, a few of them run every minute, whereas others every hour, every 6 hours and some once a day.
I have searched the documentation but could not find anything which could explain these two properties or how to set them to avoid DB locks.
Looking for some help on this.
EDIT: The following queries are executed at every second:
exec sp_executesql N'select count(*) from [HangFire].[Set] with (readcommittedlock, forceseek) where [Key] = #key',N'#key nvarchar(4000)',#key=N'retries'
select distinct(Queue) from [HangFire].JobQueue with (nolock)
exec sp_executesql N'select count(*) from [HangFire].[Set] with (readcommittedlock, forceseek) where [Key] = #key',N'#key nvarchar(4000)',#key=N'retries'
irrespective of various combinations of timespan values we set. Here is the code of GetHangfirServers we are using:
public static IEnumerable<IDisposable> GetHangfireServers()
{
// Reference for GlobalConfiguration.Configuration: http://docs.hangfire.io/en/latest/getting-started/index.html
// Reference for UseSqlServerStorage: http://docs.hangfire.io/en/latest/configuration/using-sql-server.html#configuring-the-polling-interval
GlobalConfiguration.Configuration
.SetDataCompatibilityLevel(CompatibilityLevel.Version_170)
.UseSimpleAssemblyNameTypeSerializer()
.UseRecommendedSerializerSettings()
.UseSqlServerStorage(ConfigurationManager.ConnectionStrings["abc"]
.ConnectionString, new SqlServerStorageOptions
{
CommandBatchMaxTimeout = TimeSpan.FromMinutes(5),
SlidingInvisibilityTimeout = TimeSpan.FromMinutes(30),
QueuePollInterval = TimeSpan.FromHours(5), // Hangfire will poll after 5 hrs to check failed jobs.
UseRecommendedIsolationLevel = true,
UsePageLocksOnDequeue = true,
DisableGlobalLocks = true
});
// Reference: https://docs.hangfire.io/en/latest/background-processing/configuring-degree-of-parallelism.html
var options = new BackgroundJobServerOptions
{
WorkerCount = 5
};
var server = new BackgroundJobServer(options);
yield return server;
}
The worker count is set just to 5.
There are just 4 jobs and even those are completed (SELECT * FROM [HangFire].[State]):
Do you have any idea why the Hangfire is hitting so many queries at each second?

We faced this issue in one of our projects. The hangfire dashboard is pretty read heavy and it polls the hangfire db very frequently to refresh job status.
Best solution that worked for us was to have a dedicated hangfire database.
That way you will isolate the application queries from hangfire queries and your application queries won't be affected by the hangfire server and dashboard queries.

There is a newer configuration option called SlidingInvisibilityTimeout when configuring SqlServerStorage that causes these database locks as part of newer fetching non-transactional message fetching algorithm. It is meant for long running jobs that may cause backups of transactional logs to error out (as there is a database transaction that is still active as part of the long running job).
.UseSqlServerStorage(
"connection_string",
new SqlServerStorageOptions { SlidingInvisibilityTimeout = TimeSpan.FromMinutes(5) });
Our DBA did not like the database locks, so I just removed this SlidingInvisibilityTimeout option to use the old transactional based message fetching algorithm since I didn't have any long running jobs in my queue.
Whether you enable this option or not is dependent on your situation. You may want to consider moving your queue database outside of your application database if it isn't already and enable the SlidingInvisibilityTimeout option. If your DBA can't live with the locks even if the queue is a separate database, then maybe you could refactor your tasks into many more smaller tasks that are shorter lived. Just some ideas.
https://www.hangfire.io/blog/2017/06/16/hangfire-1.6.14.html

SqlServerStorage runs Install.sql which takes an exclusive schema lock on the Hangfire-schema.
DECLARE #SchemaLockResult INT;
EXEC #SchemaLockResult = sp_getapplock #Resource = '$(HangFireSchema):SchemaLock',
#LockMode = 'Exclusive'
From the Hangfire documentation:
"SQL Server objects are installed automatically from the SqlServerStorage constructor by executing statements
described in the Install.sql file (which is located under the tools folder in the NuGet package). Which contains
the migration script, so new versions of Hangfire with schema changes can be installed seamlessly, without your
intervention."
If you don't want to run this script everytime you could set SqlServerStorageOptions.PrepareSchemaIfNecessary to false.
var options = new SqlServerStorageOptions
{
PrepareSchemaIfNecessary = false
};
var sqlServerStorage = new SqlServerStorage(connectionstring, options);
Instead run the Install.sql manually by using this line:
SqlServerObjectsInstaller.Install(connection);

hangfire recurring job on every server

I have a situation where I need a recurring job registered with hangfire to run on every server in the cluster.
(The job is to copy some files locally so needs to run on every server regularly)
So far I have tried registering the same job with an id of the server name resulting in n job for n servers:
RecurringJob.AddOrUpdate(Environment.MachineName, () => CopyFiles(Environment.MachineName), Cron.MinuteInterval(_delay));
and the job itself checks if it is the correct server and only does something if it is:
public static void CopyFiles(string taskId)
{
if (string.IsNullOrWhiteSpace(taskId) || !taskId.Equals(Environment.MachineName))
{
return;
}
// do stuff here if it matches our taskname
}
The problem with this is that all jobs executes on the first server to come along, is marked as complete and as a result is not executed by the other servers.
Is there any way to ensure that the job runs on all servers?
or is there a way to ensure that only one server can process a given job? i.e. target the job at the server that created it

Found an answer using this link.
Simply assign the job to a queue that is specific to the server you want it processing on.
So I changed my enqueue to:
RecurringJob.AddOrUpdate(Environment.MachineName,
() => CopyFiles(Environment.MachineName),
Cron.MinuteInterval(_delay),
queue: Environment.MachineName.ToLower(CultureInfo.CurrentCulture));
And when I start my server I do this:
_backgroundJobServer = new BackgroundJobServer(new BackgroundJobServerOptions
{
Queues = new[] { Environment.MachineName.ToLower() }
});

Send notification on specific date and time

In our App, We are storing questions with Question's startdate, enddate and resultdate. We need to send notification to app (iPhone and Andorid) once startdate of question is arrives.
Can anybody let me know how can we achieve this?
We don't want to use pull method. like in particular time interval it will check for question startdate and send notification.
I have a URL to send Notification for question. I need to call this URL when question's startdate is arrived.
Thanks.

Take a look at Quartz :
Quartz.NET is a full-featured, open source job scheduling system that can be used from smallest apps to large scale enterprise systems
Quartz Enterprise Scheduler .NET
You can create a new Quarts Job, lets call it QuestionSenderJob. Then your application can schedule a task in Quartz scheduler, jobs can have many instances of same Job with custom data - in your case QuestionId.
Additionally it supports storing Job scheduling in your SQL database (there are DDL Scripts included) so you can create some relations if you need for UI for example.
You can find table-creation SQL scripts in the "database/dbtables" directory of the Quartz.NET distribution
Lesson 9: JobStores
This way you leave firing in right moment to Quartz engine.
When you will go through Quartz .NET basics, see this code snippet I made a for your case to schedule job. Perhaps some modifications will be necessary thought.
IDictionary<string, object> jobData = new Dictionary<string, object> { { "QuestionId", questionId } };
var questionDate = new DateTime(2016, 09, 01);
var questionTriggerName = string.Format("Question{0}_Trigger", questionId);
var questionTrigger = TriggerBuilder.Create()
.WithIdentity(questionTriggerName, "QuestionSendGroup")
.StartAt(questionDate)
.UsingJobData(new Quartz.JobDataMap(jobData))
.Build();
scheduler
.ScheduleJob(questionSenderJob, questionTrigger);
Then in Job you will get your questionId through JobExecutionContext.
public class QuestionSenderJob: IJob
{
public void Execute(JobExecutionContext context)
{
JobDataMap dataMap = context.JobDetail.JobDataMap;
// Extract question Id and send message
}
}

What about using the Task Scheduler Managed Wrapper?
You do not want to use pooling, but if you write your own class that will encapsulate Timer (e.g. System.Thread.Timer) and check for the time each second, that will not take much resources. Depending on how exact you need it, you could check also less often, e.g. each minute. Maybe you should reconsider it.

If you use any third party service to manage your push notification such as Azure Notification Hub, Parse.com, ... they offer an integrated way to schedule push notifications. Either by passing in a send date or let them run a job periodically. I'm a user of the Azure service and it works very well.

The best implementation i can advice right now is for you to send the notification from a server.
All you just need is a good scheduler that can dispatch operation.
For me, my server is powered by Javascript (NodeJS) so i use "node-schedule". All i just do is
var schedule = require('node-schedule');
//Reporting rule at minute 1 every hour
var rule = new schedule.RecurrenceRule();
rule.minute = 1;
schedule.scheduleJob(rule, function () {
console.log(new Date().toTimeString() + ' Testing Scheduler! Executing Every other minute');
//sendPush()
});