Running only one instance of Sql Server stored procedure

run-single-instance-stored-proc.sql

if not exists (select * from dbo.sysobjects 
              where id = object_id(N'dbo.p_AcquireLock') and               
              OBJECTPROPERTY(id, N'IsProcedure') = 1)
  exec('create procedure dbo.p_AcquireLock as select 1')
go

alter procedure dbo.p_AcquireLock
(
    @LockName nvarchar(255)
)
as
begin
    declare @i int
    declare @lckName nvarchar(100)
    set @lckName='ServiceBook->'+@LockName
    exec @i=sp_getapplock @lckName,'Exclusive','Session',0
    select @i as LockStatus
    return @i
end
go


-- Usage
exec @i=dbo.p_AcquireLock @MyProcName
if  @i < 0
begin
    print 'Another instance is active'
    return
end
go