Managing synonyms in a proc with nested procs using sp_getapplock - sql

Question

Building on the topics previously addressed by gbn in these questions
Q1, Q2, Q3, Q4, and regarding the use of synonyms and re-creating the synonyms to keep the synonyms pointing at live data, it is not clear to me how prevent a "race condition" by using
"sp_getapplock after BEGIN TRAN in Transaction mode and trap/handle the return status as required."
MSDN documentation for sp_getapplock is a bit cryptic for me. For instance, can resource_name be any made-up string? But more to the point: if I run a single proc containing nested procs, where the first step is to build tables, and if those succeed, the next major step is to DROP and CREATE the existing synonyms, how would I properly implement sp_getapplock?
CREATE PROCEDURE [dbo].[some_old_proc]
AS
SET XACT_ABORT, NOCOUNT ON
DECLARE #nested_build_success varchar(3) = 'No'
DECLARE #starttrancount int
BEGIN TRY
SET #starttrancount = ##TRANCOUNT
IF #starttrancount = 0
BEGIN TRANSACTION
-- fill the tables that the synonyms don't point to yet...
EXEC dbo.nested_proc_1
EXEC dbo.nested_proc_2
EXEC dbo.nested_proc_3
EXEC dbo.nested_proc_4
EXEC dbo.nested_proc_5
IF #starttrancount = 0
BEGIN
COMMIT TRANSACTION
SET #nested_build_success = 'Yes'
END
END TRY
BEGIN CATCH
IF XACT_STATE() <> 0 AND #starttrancount = 0
ROLLBACK TRANSACTION
-- RAISERROR... log error event
END CATCH
IF #nested_build_success = 'Yes'
BEGIN TRAN
-- simple talk article
-- http://www.simple-talk.com/sql/t-sql-programming/developing-modifications-that-survive-concurrency/
DECLARE #ret INT -- does it matter what the resource_name is?
EXEC #ret = sp_getapplock #Resource = 'DoesNameMatterHere', #LockMode = 'Exclusive';
IF #ret < 0
BEGIN
-- log error message?
END
ELSE
BEGIN
-- call the proc that a does a DROP and CREATE of the relevant synonyms
-- so the synonyms point at a different set of tables...
EXEC dbo.change_the_synonyms
END
COMMIT TRAN
Perhaps a different and better way exists to avoid a race condition than the use of sp_getapplock, or a good example of what I'm trying to do is available?

Answer 1

If I understand your question correctly the preparation steps and the synonym setup must be in a single transaction, not two separate transactions. Here is an example, based on the Exception handling and nested transactions template:
create procedure [usp_my_procedure_name]
as
begin
set nocount on;
declare #trancount int;
set #trancount = ##trancount;
begin try
if #trancount = 0
begin transaction
else
save transaction usp_my_procedure_name;
EXEC sp_getapplock 'usp_my_procedure_name',
'Exclusive',
'TRANSACTION';
EXEC dbo.nested_proc_1;
EXEC dbo.nested_proc_2;
EXEC dbo.nested_proc_3;
EXEC dbo.nested_proc_4;
EXEC dbo.nested_proc_5;
EXEC dbo.change_the_synonyms;
lbexit:
if #trancount = 0
commit;
end try
begin catch
declare #error int, #message varchar(4000), #xstate int;
select #error = ERROR_NUMBER(),
#message = ERROR_MESSAGE(),
#xstate = XACT_STATE();
if #xstate = -1
rollback;
if #xstate = 1 and #trancount = 0
rollback
if #xstate = 1 and #trancount > 0
rollback transaction usp_my_procedure_name;
raiserror ('usp_my_procedure_name: %d: %s',
16, 1, #error, #message) ;
end catch
end
This will do all the work atomically. It will use an applock to serialize access so that no two procedures execute this work concurrently. In case of error the work will either completely roll back or, in the case when the caller already has a transaction, it rolls back the work to a consistent state at the procedure entry w/o rolling back the caller (this is extremely useful in batch processing). XACT_ABORT has its use in deployment script, but mixing XACT_ABORT with TRY/CATCH is a big no-no in my book.