Queuing Sql Server: Utilise queues to increase performance in SQL Server

Messages & Queues in SQL Server
Enhance performance by using queues in
SQL Server

Bio - Niels Berglund
 Software Specialist - Derivco
 Software Specialist - lots of production dev. plus figuring out
ways to "use and abuse" existing and new technologies

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Author - "First Look at SQL Server 2005 for Developers"
Researcher / Instructor - DevelopMentor
Speaker - TechEd, DevWeek, SQL Pass, etc.
Longtime user of SQL Server

 www.derivco.com
 niels.it.berglund@gmail.com
 @nielsberglund

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10/28/201
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Derivco
 World's leading development house for online
gaming software; Casino, Poker, Bingo etc.
 Offices in Durban, Cape Town, Pretoria (soon)
 Estonia, Hong Kong, Sweden, UK

 Technology company
 One of the world's largest install base of SQL Server's
(~300 SQL Server servers, multiple instances & db's).
 SQL Server 2008 / 2012, research into 2014
 Hadoop, Windows Azure
 .NET 4.5, SignalR, WebSockets, mobile (Android, iOS)

WE ARE HIRING

Disclaimer

I really, really like Service Broker!!
Honest, I Do!

Background
 In certain scenarios beneficial to off-load work to other
processes/threads.
 In SQL Server we have no real way to control threading
 Historically difficult to create messaging applications in SQL
Server, due to queuing is highly concurrent.
 how do we ensure that multiple consumers does not block each
other
 how do we de-queue without causing deadlocks
 etc.

 SQL Server 2005 introduced new functionality which makes
it easier to remove a de-queued row and return it to the
caller in one atomic statement:
 DELETE TOP(x)
 OUTPUT statement

De-queuing old vs. new
--old way
DECLARE @tab TABLE(RowID bigint, MsgTypeID int, Msg varchar(500),
EnqueueDate datetime2)
INSERT INTO @tab
SELECT TOP (1) * FROM dbo.tb_QueueTabClIdx WITH(UPDLOCK, READPAST)
DELETE clix FROM dbo.tb_QueueTabClIdx clix
JOIN @tab t ON clix.RowID = t.RowID

--new way
DECLARE @tab TABLE(RowID bigint, MsgTypeID int, Msg varchar(500),
EnqueueDate datetime2)
DELETE TOP(1) dbo.tb_QueueTabClIdx WITH(ROWLOCK, READPAST)
OUTPUT deleted.RowID, deleted.MsgTypeID, deleted.Msg, deleted.EnqueueDate
INTO @tab(RowID, MsgTypeID, Msg, EnqueueDate)

Queue types
 Heap queue
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backed by heap table
heap table is a table with no clustered index
cannot guarantee order of en-queue, de-queue
useful for specific queuing scenarios
beware of side effects (data growth)

 Ordered queue (FIFO, LIFO)
 First In First Out, Last In First Out
 in LIFO queues beware of hot-spots (inserting, deleting same group of
rows)
 backed by table with clustered index (can be non-unique or unique)
 de-queuing has to be done with an ORDER BY clause

 SQL Server Service Broker Queue (SSB Queue)
 Bound queues (more later)

De-queuing ordered queues
 De-queue has to be done with an ORDER BY clause
 Unfortunately DELETE does not support ORDER BY 
 By using a CTE we can however do it in a very efficient way
 the query will execute as a DELETE,
 NOT SELECT followed by DELETE, nor as SELECT in context
of a DELETE
 SELECT automatically taking UPDATE lock
;WITH delCte
AS
(
SELECT TOP(1) RowID, Msg, EnqueueDate
FROM dbo.tb_QueueTabClIdx WITH(ROWLOCK, READPAST)
ORDER BY RowId
)
DELETE delCte
OUTPUT deleted.*

SSB queues
 SSB allows internal or external processes to send
and receive guaranteed, asynchronous messages
by using extensions to DML
 It has the concept of queues, meant for use by SSB
 When using it in a "normal" queuing application, it
has several short-comings:
 convoluted way to en-queue messages
 rigid structure
 performance

 One big plus though: Activation
 stored procedure that automatically is activated when a
message is en-queued on the SSB queue.
 can control concurrency

Performance I
 Multiple producers running concurrently
 One consumer running in parallel
 Each producer en-queuing one record
 running in a tight loop, 10,000 times

 Consumer started together with the
producers, de-queuing until no more data.

Queuing Sql Server: Utilise queues to increase performance in SQL Server

Bound queues
 High volume inserts/deletes causes problems in
a db:
 heap table unlimited growth
 statistics are never up to date
 auto updates of statistics constantly kicking in,
creating jagged CPU patterns and throughput
 new pages constantly allocated, de-allocated stresses internal allocation structures

 Quite a few high performing queuing
applications are using a bound queue in a ringbuffer scenario
 minimizes memory allocation, de-allocation

Table backing ring-buffer
CREATE TABLE dbo.tb_QueueTabRingBuffer
(
MsgSlot BIGINT NOT NULL
, MessageID BIGINT NULL
, MsgTypeID int NULL
, EnqueueDate datetime2 NOT NULL
, Msg char(500) NOT NULL
, ReferenceCount TINYINT NOT NULL
)

Using a ring-buffer like table
 After creation of table, prefill with data (keep track of how
many rows inserted)
 To enqueue:
 create a new message id
 find the next available slot
 update the message column and add one to the reference count

 To de-queue
 find the smallest message id that has not yet been de-queued.
 read and return the message column
 decrement reference count, marking the slot as available.

 We need to be able to extremely efficiently locate the next
available slot
 For this we use a sequence object

Sequence object
 A high scale data structure to generate "next number"
 quite a few "neat" features

 Introduced in SQL Server 2012
 When used in conjunction with the modulo operator (%)
we can efficiently get next slot etc.
CREATE SEQUENCE dbo.seq_Test AS BIGINT
START WITH 1 INCREMENT BY 1
CACHE 5000;
GO
SELECT NEXT VALUE FOR dbo.seq_Test;

En-queuing
 When en-queuing into a ring-buffer, the en-queue may
block if no available slots.
 Need to handle that in en-queue code (not shown below)
DECLARE
DECLARE
DECLARE
DECLARE

@enqueuSequence bigint;
@slot int;
@QueueSize int = 5000;
@msg char(500) = 'this is a new message';

--grab the next value, and slot
SET @enqueuSequence = NEXT VALUE FOR dbo.seq_Enqueue;
SET @Slot = @enqueuSequence % @QueueSize
--do the update
UPDATE dbo.tb_QueueTabRingBuffer WITH(ROWLOCK)
SET EnqueueDate = SYSDATETIME(), Msg = @Msg, MessageID =
@enqueuSequence, MsgTypeID = 2, ReferenceCount = 1
WHERE MsgSlot = @Slot AND ReferenceCount = 0

De-queuing
 De-queuing is similar to en-queue. Need however to
ensure that the de-queue sequence does not out of
synch with the en-queue.
DECLARE @dequeuSequence bigint;
DECLARE @slot int;
DECLARE @queueSize int = 5000;
--grab the next MessageID, and slot
SET @dequeuSequence = NEXT VALUE FOR dbo.seq_Dequeue;
SET @Slot = @dequeuSequence % @QueueSize
UPDATE dbo.tb_QueueTabRingBuffer WITH(ROWLOCK)
SET EnqueueDate = SYSDATETIME(), MessageID = NULL, MsgTypeID = NULL
, Msg = 'dummy', ReferenceCount = 0
OUTPUT deleted.*
WHERE MsgSlot = @Slot AND MessageID = @dequeuSequence
AND ReferenceCount = 1;

Summary
 Implementing efficient queuing in SQL Server is definitely
doable
 SQL Server 2008R2
 the best table structure for overall performance is a queue
backed by clustered non-unique index
 for certain specialized scenarios a heap-table can come into play
(beware of data growth)

 SQL Server 2012
 a ring-buffer like structure using sequence objects are the
absolutely best performing table structure
 beware of more convoluted code to keep the en-queue and dequeue in synch

 SQL Server Service Broker
 too convoluted, not enough performance 
 activation procedures its strong point

Queuing Sql Server: Utilise queues to increase performance in SQL Server

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