Re: Help - Timing Logic


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?
- who will be responsible for executing / sending messages ... one computer
or many different computers?
- will this message sender reside on a server / database server / or a
user's workstation ... ???
- what database are you using ???
- is this functionality wrapped up in a large application?
- why 4 threads ?
- how does you program determine which message to send ... parameter list
please.

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

.... wrap the necessary functionality is a small stand alone exe application
....
.... build your application so it receives commandline parameters / string
....
.... 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...

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

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

------------------------------------
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...

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