Replicate Stored Procedure Execution

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When most of build transactional publications we think of articles as tables,

but articles can actually be other objects including stored procedures, the

focus of this article. To get started I've created a database with a single

table called Employees and two stored procedures, one to add an employee and the

other to increase the pay of all employees. In the screenshot below you can set

the publication configuration.

You can see that I've elected to publish both stored procedures as articles.

If we highlight a stored proc and click on the article properties button, we can

see that we have three choices for how the stored procedure gets replicated. I'm

going to leave it set at the default 'Stored Procedure Definition Only' for now.

Once we add a subscriber and let the snapshot run, this is what we have on

the subscriber:

Now let's do a couple quick tests. The first is to run the Add Employee proc

and see how the change is transmitted to the subscriber:

exec [sp_MSins_dboEmployees] 21,'2007-08-19 11:18:16:077','John','Public',NULL

This shows that we're calling the replication generated procedure on the

subscriber. Our Add Employee proc will never get called by the publisher, but it

might be called by some process on the subscriber. Now let's see what happens if

we modify the proc on the publisher - I'll add a comment to it. The following

was executed on the subscriber automatically:

ALTER PROCEDURE [dbo].[usp_Employee_Add] @FirstName varchar(50), @LastName varchar(50)
as
set nocount on
--TEST COMMENT
insert into dbo.Employees (
	FirstName,
	LastName)
values (
	@FirstName,
	@LastName)

Changes to stored procedures are propagated to subscribers automatically if

you make them part of the publication. Remember, the stored procedure won't get

called as part of standard replication. It makes sense to replicate the

definition if you plan to use the subscriber as a standby server (having all

your proc changes kept in sync is pretty important) or if you've configured

replication for updating subscribers (would need the same stored procedures that

would normally get executed on the publisher).

Now let's revisit our settings and change it to replicate stored procedure

execution:

I'll execute the same IncreasePay proc on the publisher:

And then we see the same call executed on the subscriber:

This can be a huge performance advantage. I called a single proc that updated

all the rows on the publisher and then instead of transmitting a replication

proc call on the subscriber for each row changed, we just executed the same proc

on the subscriber(s). Given that the proc definition and data hasn't been manually

adjusted on the subscriber you wind up with exactly the same results for a lot

less work - one row logged and processed in the distribution database and one

row instead of x rows sent across the network We still have to do the actual

work on the subscriber of course; if the proc modifies a million rows there is a

certain amount of overhead that we pay regardless of how the update was issued.

And even though we've elected to replication stored procedure execution in this

case we still have the ability to directly modify the table and have those

changes replicated as usual.

The last option is 'Execution in a serialized transaction of the SP' and has

the same advantages of replicating procedure execution but it does so within a

serialized transaction, that is, one with the isolation level set to

SERIALIZABLE. Most of us don't use that high a setting often because it

typically has a negative impact on concurrency. If you need absolute perfection,

this is the way to go, because it eliminates the chance that the stored

procedure can affect a different number of rows because of other transactions

that are occurring at the same time. But in practice do you really need this?

The easy answer is to say yes and the possibly risky answer is to say no. As

with most things that have to do with SQL performance the answer is 'it depends'

and in this case, it depends on you looking at how and when data access occurs,

and your tolerance for having data wind up slightly different. It's probably not

a fair line to draw but I'm usually comfortable skipping serialization if the

changes are just being executed on a reporting server. No, it's not good to have

incorrect data, but if something happens I definitely do still have the

original/correct data. Even if you do select the serialize option you will

probably wind up a net gain compared to processing the individual row changes

that would normally be done.

As a follow up to that point, nothing says that you have to allow replication

of all stored procedures, or that they all have to be serialized or not. It's

entirely worthwhile to just create one stored procedure called something like 'usp_DBA_AdminProc'

and add it to the publication when you create it and set it to replicate stored

procedure execution. It's just for you, will never get called by an application.

But the next time there are a large number of rows to be modified you can alter

the proc to have the needed definition and then run it on the publisher,

probably at night or other lull. You get all the benefits of replicating

procedure execution and almost none of the possible downside because it's being

executed off peak. You can modify the proc again and again, changing it however

is needed, knowing that the right definition will be on each subscriber at the

right time.

The scenario where replicating execution makes the most sense would be where

you have a slow network connection, followed by when you are frequently updating

or deleting large numbers of rows, and the last would be when you just need to

reduce the latency between execution on the publisher and having the changes

fully applied on the subscriber. There can also be some reduction of load on the

distributor if the procs are updating large numbers of rows, but probably much

less savings if you are just replicating OLTP type procs that only affect a

small number of rows with each call.

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