Delay in Asynchronous Procedure Calls

Question

Delay in Asynchronous Procedure Calls

blueazul

Ten Centuries

Points: 1397
More actions
February 23, 2012 at 8:57 pm

#256417

I would like to call stored procs asynchronously, but there seems to be a delay in service broker so that not all of the procedures finish at around the same time. Actually, the delay is significant at several seconds. I have max readers higher than the amount of procedures I am calling. I have tried using the same conversation, multiple conversations, receiving more than top(1) --- although for high concurrency top(1) should be best --- and using waitfor with a timeout. Here's what it looks like:
create table AsyncExecResults (
token uniqueidentifier primary key,
start_time datetime null,
finish_time datetime null)
go
create queue AsyncExecQueue;
go
create service AsyncExecService on queue AsyncExecQueue (DEFAULT);
GO
if object_id('usp_AsyncExecActivated') is not null
drop procedure usp_AsyncExecActivated;
go
create procedure usp_AsyncExecActivated
as
set nocount on;
declare @h uniqueidentifier,
@messageTypeName sysname,
@messageBody varbinary(max),
@xmlBody xml,
@procedureName sysname,
@startTime datetime,
@finishTime datetime,
@token uniqueidentifier;
waitfor(
receive top(1)
@h = conversation_handle,
@messageTypeName = message_type_name,
@messageBody = message_body
from AsyncExecQueue
), TIMEOUT 5000
if (@h is not null)
begin
if (@messageTypeName = N'DEFAULT')
begin
select @xmlBody = CAST(@messageBody as xml);
select @procedureName = @xmlBody.value('(//procedure/name)[1]', 'sysname');
select @startTime = CURRENT_TIMESTAMP;
exec(@procedureName);
select @finishTime = CURRENT_TIMESTAMP;
select @token = conversation_id
from sys.conversation_endpoints
where conversation_handle = @h;
insert AsyncExecResults (start_time, finish_time, token)
values(
@startTime,
@finishTime,
@token);
end
else if (@messageTypeName = N'http://schemas.microsoft.com/SQL/ServiceBroker/EndDialog')
begin
end conversation @h;
end
end
go
alter queue AsyncExecQueue
with activation (
procedure_name = usp_AsyncExecActivated
, max_queue_readers = 25
, execute as owner
, status = on
), status = on, retention = off;
go
if object_id('usp_AsyncExecInvoke') is not null
drop procedure usp_AsyncExecInvoke;
go
create procedure usp_AsyncExecInvoke
@procedureName sysname,
@token uniqueidentifier output
as
declare @h uniqueidentifier,
@xmlBody xml
set nocount on;
begin dialog conversation @h
from service AsyncExecService
to service N'AsyncExecService', 'current database'
with encryption = off;
select @xmlBody = (
select @procedureName as name
for xml path('procedure'), type);
send on conversation @h (@xmlBody);
go
if object_id('usp_MyLongRunningProcedure') is not null
drop procedure usp_MyLongRunningProcedure;
go
create procedure usp_MyLongRunningProcedure
as
begin
waitfor delay '00:00:10';
end
go
truncate table AsyncExecResults;
declare @token uniqueidentifier;
exec usp_AsyncExecInvoke N'usp_MyLongRunningProcedure', @token output;
go
declare @token uniqueidentifier;
exec usp_AsyncExecInvoke N'usp_MyLongRunningProcedure', @token output;
go
declare @token uniqueidentifier;
exec usp_AsyncExecInvoke N'usp_MyLongRunningProcedure', @token output;
go
declare @token uniqueidentifier;
exec usp_AsyncExecInvoke N'usp_MyLongRunningProcedure', @token output;
go
declare @token uniqueidentifier;
exec usp_AsyncExecInvoke N'usp_MyLongRunningProcedure', @token output;
go
declare @token uniqueidentifier;
exec usp_AsyncExecInvoke N'usp_MyLongRunningProcedure', @token output;
go
declare @token uniqueidentifier;
exec usp_AsyncExecInvoke N'usp_MyLongRunningProcedure', @token output;
go
declare @token uniqueidentifier;
exec usp_AsyncExecInvoke N'usp_MyLongRunningProcedure', @token output;
go
declare @token uniqueidentifier;
exec usp_AsyncExecInvoke N'usp_MyLongRunningProcedure', @token output;
go
declare @token uniqueidentifier;
exec usp_AsyncExecInvoke N'usp_MyLongRunningProcedure', @token output;
go
select * from AsyncExecResults
order by finish_time;
Code borrowed heavily from here[/url] for reference.
I have really taken it down to just the bare-bones on purpose here so that I can test performance. What I would like is for there to be as little delay between the first start-time and last finish-time when results are sorted in ascending order. I have done some research and it seems the issue is that service broker is not constantly scanning the queue and then activating whatever is on it. It waits for a bit and then scans again. However, after I run all of the waitfor delay procedures, they should all enter the queue. So by the time SB checks a second time, they should surely all be there for SB to dispatch activation procedures.
Thanks for any help!

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R.P.Rozema SSChampion Points: 12307 More actions · Answer 1

Your async handler routine should not exit after each message: Service broker expects its handlers to continue processing messages for as long as there are more messages available. As you discovered, it only rechecks after 5 seconds to see if the queue has emptied. Only if it hasn't then it starts another handler, for as far as the max_queue_readers setting permits it.

So the problem you need to fix is first of all in your handler procedure. Try this for example (not tested):

if object_id('usp_AsyncExecActivated') is not null

drop procedure usp_AsyncExecActivated;

go

create procedure usp_AsyncExecActivated

as

set nocount on;

declare @h uniqueidentifier,

@messageTypeName sysname,

@messageBody varbinary(max),

@xmlBody xml,

@procedureName sysname,

@startTime datetime,

@finishTime datetime,

@token uniqueidentifier;

while 1 = 1

begin

begin tran;

waitfor(

receive top(1)

@h = conversation_handle,

@messageTypeName = message_type_name,

@messageBody = message_body

from AsyncExecQueue

), TIMEOUT 5000;

if not @@rowcount > 0

begin

commit tran;

break; -- exit while 1 = 1

end

if (@messageTypeName = N'DEFAULT')

begin

select @xmlBody = CAST(@messageBody as xml);

select @procedureName = @xmlBody.value('(//procedure/name)[1]', 'sysname');

select @startTime = CURRENT_TIMESTAMP;

exec(@procedureName);

select @finishTime = CURRENT_TIMESTAMP;

select @token = conversation_id

from sys.conversation_endpoints

where conversation_handle = @h;

insert AsyncExecResults (start_time, finish_time, token)

values(

@startTime,

@finishTime,

@token);

end

else if (@messageTypeName = N'http://schemas.microsoft.com/SQL/ServiceBroker/EndDialog')

begin

end conversation @h;

end

commit tran;

end

go

This will process your messages sequentially, until for more than 5 seconds the queue could not be emptied by a single instance. If the queue is not empty 5 seconds after the 1st instance got started a 2nd instance will be started, and 2 messages will be processed simultaneously. Then after 10 seconds and the queue is still not empty, a 3rd instance is started and 3 messages are processed at the same time (provided your server has at least 3 cpu's, otherwise you'll just be context switching) and so on, until either the maximum number of max_queue_readers is reached or the queue becomes empty for at least 5 seconds (= the timeout value on your waitfor( receive) statement).

In practice 25 queue readers is probably way to high.

Posting Data Etiquette - Jeff Moden[/url]
Posting Performance Based Questions - Gail Shaw[/url]
Hidden RBAR - Jeff Moden[/url]
Cross Tabs and Pivots - Jeff Moden[/url]
Catch-all queries - Gail Shaw[/url]

If you don't have time to do it right, when will you have time to do it over?

R.P.Rozema SSChampion Points: 12307 More actions · Answer 2

For those trying to duplicate this experiment, for example to learn some Service broker basics from it, here is the same experimental code with documented improvements in it:

create database t2

go

use master

go

alter database t2 set enable_broker

go

use t2

go

create table AsyncExecResults (

token uniqueidentifier primary key,

start_time datetime null,

finish_time datetime null)

go

create queue AsyncExecQueue;

go

create service AsyncExecService

authorization dbo

on queue AsyncExecQueue

( [DEFAULT] -- default is NOT a keyword here. i.e. it MUST be

-- enclosed by square brackets.

);

GO

if object_id('usp_AsyncExecActivated') is not null

drop procedure dbo.usp_AsyncExecActivated;

go

if object_id('usp_AsyncExecActivated') is not null

drop procedure dbo.usp_AsyncExecActivated;

go

alter procedure dbo.usp_AsyncExecActivated

as

set nocount on;

declare @h uniqueidentifier,

@messageTypeName sysname,

@messageBody varbinary(max),

@xmlBody xml,

@procedureName sysname,

@startTime datetime,

@finishTime datetime,

@token uniqueidentifier;

while 1 = 1

begin

begin tran;

waitfor(

receive top(1)

@h = conversation_handle,

@messageTypeName = message_type_name,

@messageBody = message_body

from AsyncExecQueue

), TIMEOUT 5000;

if not @@rowcount > 0

begin

commit tran;

break; -- exit while 1 = 1

end

if (@messageTypeName = N'DEFAULT')

begin

select @xmlBody = CAST(@messageBody as xml);

select @procedureName = @xmlBody.value('(//procedure/name)[1]', 'sysname');

select @startTime = CURRENT_TIMESTAMP;

exec(@procedureName);

select @finishTime = CURRENT_TIMESTAMP;

select @token = conversation_id

from sys.conversation_endpoints

where conversation_handle = @h;

insert AsyncExecResults (start_time, finish_time, token)

values(

@startTime,

@finishTime,

@token);

-- The chosen protocol says the conversation ends after the 1st

-- message is received. So let SSSB know we're done by ending the

-- conversation after each received message. Not ending the conversation

-- will leave all conversations open. When you get into the millions

-- of open conversations (see sys.conversation_endpoints), your server

-- will go slower and slower.

end conversation @h;

end

else if (@messageTypeName = N'http://schemas.microsoft.com/SQL/ServiceBroker/EndDialog')

begin

end conversation @h;

end

-- Don't forget to handle error messages too.

-- These also indicate the end of a conversation, just like EndDialog.

else if (@messageTypeName = N'http://schemas.microsoft.com/SQL/ServiceBroker/Error')

begin

-- Read the xml document that we were sent in the message body.

declare @nCode int;

declare @vchDescription nvarchar(2000);

declare @xmlError xml; -- (document

select @xmlError = convert(xml, @messageBody);

with xmlnamespaces( default 'http://schemas.microsoft.com/SQL/ServiceBroker/Error')

select

@nCode = Errors.error.value( 'Code[1]', 'int'),

@vchDescription = Errors.error.value( 'Description[1]', 'nvarchar(max)')

from @xmlError.nodes( '/Error[1]') as Errors(error);

-- Any text output by a SSSB queue reader will by default end up in the sql log,

-- so I just write out the output with nowait to get errors shown in the sql log.

-- Using "with log" would put the error in the sql log twice: once because of

-- the with log, plus once more because all output from an auto activated procedure

-- is written into the log. So don't specify with log here.

raiserror('Service broker error received. Error %d:%s. (%s)', 0, 0, @nCode, @vchDescription, '%PROC%') with nowait;

-- An Error message means the conversation has ended, just like with a EndDialog message.

-- So we tell SSSB we got the message and have it clean up our end of the conversation too.

end conversation @h;

end

commit tran;

end

go

alter queue dbo.AsyncExecQueue

with activation (

procedure_name = usp_AsyncExecActivated

, max_queue_readers = 25

, execute as owner

, status = on

), status = on, retention = off;

go

if object_id('usp_AsyncExecInvoke') is not null

drop procedure dbo.usp_AsyncExecInvoke;

go

alter procedure dbo.usp_AsyncExecInvoke

@procedureName sysname,

@token uniqueidentifier output

as

declare @h uniqueidentifier,

@xmlBody xml

set nocount on;

begin dialog conversation @h

from service AsyncExecService

to service N'AsyncExecService'--, 'current database'

with encryption = off;

select @xmlBody = (

select @procedureName as name

for xml path('procedure'), type);

send on conversation @h (@xmlBody);

go

if object_id('usp_MyLongRunningProcedure') is not null

drop procedure dbo.usp_MyLongRunningProcedure;

go

create procedure dbo.usp_MyLongRunningProcedure

as

begin

waitfor delay '00:00:10';

end

go

truncate table AsyncExecResults;

declare @token uniqueidentifier;