Re: Help - Timing Logic

Thanks a lot everyone... I'l re-read all messages and work this out. I appreciate all the help and assistance. I'm sure with your suggestions everything with get resolved. Thanks a lot.



"jeff" <jhersey at allnorth dottt com> wrote in message news:umKysi8EHHA.4016@xxxxxxxxxxxxxxxxxxxxxxx
Jay,

Cant have a trigger execute a function in a remote vb.net app.

Install the application on you DATABASE SERVER- make it a local APP... If the messages are that important ... this will give you the quickest result! A function CAN call a local exe ... I do it all the time - for broadcasting status changes and sending critical alerts in an Patient Tracking system for a hospital emergancy department...works like a hot dam ... even sends messages to peoples pagers! If you DBA screams and yells and jumps up and down - this is a database server, we are not install you little program on it ... find out the persons name at the bottom of the DBA's paycheck, go to them, tell them what is going on ... tell them what you are doing ... tell them how critical this application / process is to organization - MISSION CRITICAL ... tell them how much this program will impact the overall performance of the database server ... sell it to them - the persons whos name is at the bottom of the paycheck!

Again, if this is so mission critical, you must capture it at the moment the data is made available...and you need fall-over, redundancy in you messaging application.

what happens ... message in table ... 3 more seconds til the messaging thread is fired ... 2 ... 1 ... oops, messaging machine is off-line! No message sent! Now what? If on the database server ... if the data is saved, the message is sent ... and if it is not sent ... alerts are sent... there is a fail proof plan in place when messages are not SENT simply because the system knows a message was suppose to be sent! However, if you are sending messages for a machine that is off line and not polling for new messages (because some network guy re-started the box and for got to restart your process) ... there is now way of detecting unsent messages.

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What I do not understand ... is your reason to use '4' threads??? I know speed ... mission critical . But why 4 threads??? Why not have a thread for EACH MESSAGE ... 16 messages .... 16 threads! Using your reasoning, would this not be even quicker!

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What I am saying is ... and have said before ...and is being repeated by Chris ... he has taken my suggestion one step further by giving you useable code... is you approach to multi threading the entire process is flawed ... I am not argueing the need for multithreading in the process ... I am just disagreeing with the need to multithread the entire process!

Example ... in plain text ... you figure out the code ...

1. Scenerio: Retrieve list of messages from the database... MessageBroker ... have MessageBroker create a seperate thread for each message it has to send!

So, proces is...

- Get list of message from database ...

- Loop through list ...

- for each record ... call a SendMessage method ... include all the necessary information (parameters) for selecting or sending the message

- Have the SendMessage start a new thread ... send the message information to this new thread ... so its know which message to send ... have this new thread send the message.

- while this thread is processing (sending the message) ... return to your list of messages in the messagebroker ...

- messagebroker gets next message ... messagebroker calls sendmessage ... sendmessage creates a new thread ... sendmessage send message in new thread, while program returns to the loop ...

Do you see what I am doing here ... the prgram will create a seperate thread for EACH message. The messagebroker is your control, it will ensure you do not send duplicate messages...

This will only work, if you have a single machine designated as your MessageBroker!


2. Scenerio - Use your 4 threads ...

x = 1 to 4

- Thread x retrieves a message from the database ...

- Thread x prepares message to send ... before it sends ... it checks the database ... to see if any other thread has sent the message ...
YES another thread has sent it ... Thread x , drops message, and gets another message ...
NO, nobody has sent, thread x locks the message row in the database (transactions and isololation levels ... have you looked at these yet), updates the sending flagging ... sends the message ... updates the sent flag...

- Thread x gets another message ...

This process ... will create a lot of wasted CHECKING and UPDATING and DATABASE LOCKING to ensure messages are not duplicated between recipients. This is not faster!

By having 4 threads working independant of each other, you are slowing down the process! and adding 'more threads', you could be compound the issue!

You need some mechanism for communicating between your threads ...
- an exe ... the messagebroker approach ... telling the thread which message to send.
- the database ... implement row level locking ... look at Isolation and Transactions ... to ensure each message row in only processed by 1 thread...
- hard code it ... implement a design structure - add the ThreadToProcess field - in to your solution. This will cause maintenance and scalability issues later in life! And is poor design...and a poor work-around!

So, Jay, I have tried to shine some light on the problem for you, it is up to you to decide your next steps ... if this is so mission critical, and you are having these difficults, maybe consultant out for a solution :-)

Jeff.











"Jay" <someone@xxxxxxxxxxxxx> wrote in message news:OGJfd9xEHHA.3188@xxxxxxxxxxxxxxxxxxxxxxx
Thanks Jeff, answers under your questions...


"jeff" <jhersey at allnorth dottt com> wrote in message news:uekJE1xEHHA.2356@xxxxxxxxxxxxxxxxxxxxxxx

I know it is a time critical system ... employees need the information ASAP... but what I do not understand is ...

- where is the application running ... on a server ... on a users work station?

Server (2003)

- who will be responsible for executing / sending messages ... one computer or many different computers?

One computer. One multithreaded vb.net app.

- will this message sender reside on a server / database server / or a user's workstation ... ???

Remote SQL Server 2005 (EE)

- what database are you using ???

SQL Server 2005 EE

- is this functionality wrapped up in a large application?

Not very large. The logic around this loop cycle is pretty much the entire application.

- why 4 threads ?

Allows sending 4 messages at a time.

- how does you program determine which message to send ... parameter list please.

Read from a table in the db server. A Queue table. (employeeid, messageaddress)




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If this information is so mission critical, and employees must get their message ASAP ... put a trigger on the database table.

Cant have a trigger execute a function in a remote vb.net app.


... wrap the necessary functionality is a small stand alone exe application

Exactly what I need to do.


...
... build your application so it receives commandline parameters / string

Considering it.

...
... build a trigger on the database .. fire on inserts ... have it call you message program with a command parameter (messageID) ...
... install you application on the database server .... this will speed up the connection / retrievals and so on ... no network latency.

Each time a message is inserted in the table, the trigger fires, calling your EXE with the appropriate parameter string, exe starts, fires off message ... done. Small exe can run as many times on the server...

------------------------------------

If you are using a timestamp for determining which messages to send ... incorporate another table ... LastTimeExecuted ...

table: MessengerExecution
field: LastDateTimeFired

begin transactions ... lock table.
select lastdatetimefired from this table...in a variable
update field with thread date/time ...
end transaction.
return the select value and the update value

select messages where date is between lastdatetimefired and the updateddatetime I just used...

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Again, there are many solutions ... just do not completely understand what you are doing...

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Bottom line ...

- you have determined that you need to run a mutli-threaded process for this.

- so, in order to avoid DUPLICATE MESSAGES .... you either have to employee...

Transaction and Database Locking - look at isolation levels / settings

or

A booker type of system...

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have the broker continually pool the table ... this will only work it 1 machine is designated for messaging! If more than one machine will be used for messaging ... you will have to roll up your sleeves and look at Transactions and Isolation Levels.

CheckMessage

Do Until Company.Revenues < 0
MessegeID = broker.getNextMessage()
broker.sendMessage(MessageID)
Loop

SendMessage(MessageID)

Create a new thread...
SendMessage(MessageID) in this new thread...
Return to the MessageBroker.CheckMessage...while the other thread is preparing and sending the message...

This will continuious poll the database server for new messages! ...

------------------------------------

However, if it is mission criticial users receive this information ASAP ... TRIGGER on database table! If the user can wait 10 seconds .. build a BROKER ... and have it spawn as many threads needed to send the messages in the QUEUE ... have it control the process...and sending!

------------------------------------

Again, many solutions, depends on your needs ... code sample, table structure ... some thing to trying and figure out how exactly your 'threads' are getting 1 message...

Jeff.


"Jay" <someone@xxxxxxxxxxxxx> wrote in message news:OLc3QDxEHHA.996@xxxxxxxxxxxxxxxxxxxxxxx
It is extremely important for the employees to receive the messages almost immediately after identifying (time val in table). So, threading ensures I can execute multiple processes at the same time... only problem is these threads read from the same queue table and have the potential to send the same message to the same guy x number of times. where x is the number of threads running.

"jeff" <jhersey at allnorth dottt com> wrote in message news:uQ7%23Y4wEHHA.4404@xxxxxxxxxxxxxxxxxxxxxxx

Still do not know why you need 4 threads ? Speed ? Does runnig the process in one thread not work fast enough for you since timing is import ? Why 4 threads why not 6,12 , 3, 36 ... ? How do you determine which row a 'thread retrieves' ??? does it simply select the next available row from the DATABASE ??? How does the thread determine 'the next row'???

If mutli-threading is duplicating 'send messages' it is not working properly...
If you are multi-threading to improve performance ... maybe look at the design...
If you need to implement a locking mechanism / or / logging mechanism / or / a checking mechanism to avoid duplicate messages caused by multi-threading ... these will all come at a cost ... performance cost!

What is bottle necking your process that you need 4 threads? Is it the READ from the database ... is it the SENDING the text message? Is it connecting to the database ??? do not know here ...

Look at what is causing problem ...

- you need better performance ... so, multiple thread it! However, mutli-threading the entire process is causing issues ... duplicate records ... now you either need to incorporate a locking procedure ... or a checking procedure to avoid duplicate messages... all this has a cost to the overall performance ... adding more threads may in fact negatively impact the overall performance....

Maybe implement a message broker ...

- message broker gets all the necessary messages or message id's from the database to be sent ... in-memory list...
- message broker loops through list of messageIDs ...
- message broker starts another thread for processing the sendtextmessage functionality for each messageID...
- message broker will include the messageID so the process knows which message to get and process...
- message broker will ensure each database row is only processed once...

It is very hard to help you without knowing exactly what you are doing...

Again, I will ask, what rational / reasoning did you use for using 4 threads ? Performance ? Speed ? ... Where is the bottle neck in the process that requires you to multi-thread it? Maybe just move the 'bottle neck section' to another thread ... ?

Grasping at straws ...

Code would be nice how to see what you are doing ...

Jeff















"Jay" <someone@xxxxxxxxxxxxx> wrote in message news:uBX7QewEHHA.1224@xxxxxxxxxxxxxxxxxxxxxxx
Thanks. I do need to select an individual row at a time and all 4 threads need to do this. What trasaction isolation level would you recommend? Perhaps a stored proc may be faster to execute and return the values as opposed to building the transaction in the code. What has to happen is every 5 seconds, and for each thread, a sub runs to get a single row then send the message to dtr("numbertosendto"). Because this app heavily relies on timing it is important that all threads run and only one distinct row can be returned at a time for each thread.



"jeff" <jhersey at allnorth dottt com> wrote in message news:%23qVFIYwEHHA.3576@xxxxxxxxxxxxxxxxxxxxxxx

are you selecting individual rows at a time from the database table ...

if so ... use a transaction ...

begin transaction ...

select a row from database ...

update the row in table set flag = 'Processed'

end transaction

this will lock the row until the end of transaction is issued ... as long as the isolation level is set accordingly...

Again, please let us know how you are getting the information from the database... then we can help!

If you are reading a bunch of rows in one statement, storing the rows in in-memory datasets on the workstation, looping through the rows one by one ... then you may need to either ...

implement as above locking only the records you retrieve / update - need to watch out here for table locking ... may impact performance,
implement using an update flag at sent and 'check before send method'...
...
lots of options here, just need to know how you are retrieving your data...and how you are processing the data.

Question, why do you need 4 threads running ... are they doing 'different processes', sending different 'types' of messages ... sending the same message ... just need 4 to make it faster ... what is the logic ...

Tieing a record to a thread my cause problems in the future ... what happens when a thread stops or hangs ... those messages will not be processed... When happens if somebody changes the ThreadID ... in the program ...

trying to help ..

Jeff

PS: you can lock individual rows ... look at how database transactions work and incorporate it in you program ...








"Jay" <someone@xxxxxxxxxxxxx> wrote in message news:%23f70LJwEHHA.2328@xxxxxxxxxxxxxxxxxxxxxxx
Thanks for the reply.

I do need all 4 threads running (maybe even more in the future). I can not delete the row from the table... it needs to be there for later update use. I am considering marking each row at insert (1 - 4) then having each of the 4 threads only select a row based on the mark. It's becoming a little tricky... it would be great if I could lock a single row when selecting it.


"jeff" <jhersey at allnorth dottt com> wrote in message news:OPCyvDwEHHA.4740@xxxxxxxxxxxxxxxxxxxxxxx

Question:

What is the reason for the 4 threads?

Do you need all these threads processing the same dataset?

Just asking, maybe you could change your approach to avoid this problem...

If not, investigate Record Locking / Transactions...

How are you retrieving your records in to memory for processing ...

ie,
- are you selecting on the entire contents of a 'message' table and looping through an in-memory dataset...

- do you retrieve 'chunks' or rows from the message table based on parameters...

You could use the internal locking mechanisms of the database to achieve this ... but procede with caution...

Start Transaction...
SELECT * FROM MESSAGE TABLE
...
stuff this into a data table on the desktop
...
DELETE * FROM MESSAGE TABLE
...
punt / clean the table.
...
End Transaction.

The transactions places locks on the table ... but selecting the entire tables contents ... you are essentially 'LOCKING' the entire table until the transaction is over...

So, each thread's 'retrieve' process will have to wait in line until the select and delete are processed. As well, anything triggering 'New Messages to be Added' will be delayed until the Transaction is completed...

You may impact the overall performance of you application ... need to investigate.

If locking does not work for you, you will need to implement some type of logging / checking approach...

ie... have a log table...once a process has sent the message, write to a log table ... message sent. Before each message is compiled and sent, check the log table to see if another process has sent the message...if not, send you message...


A snip of code would be great to figure out how you are retrieving a list of messages and how you are processing each record...

Jeff.


"Jay" <someone@xxxxxxxxxxxxx> wrote in message news:O2phzwvEHHA.4280@xxxxxxxxxxxxxxxxxxxxxxx
I have a multi threaded VB.NET application (4 threads) that I use to send text messages to many, many employees via system.timer at a 5 second interval. Basically, I look in a SQL table (queue) to determine who needs to receive the text message then send the message to the address. Only problem is, the employee may receive up to 4 of the same messages because each thread gets the recors then sends the message. I need somehow to prevent this... just can't think of how. Somehow I need the other threads to know that another thread is already using that record and move on to the next record. I thought of getting the record then marking it (column value) as in use and writing the code to look for records only where not in use... problem is all 4 run fast enough to all use/mark the row. Any thoughts?

Thanks a lot.

Jay
















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