sp_MSensure_single_instance (Transact-SQL MetaData) Definition

Please note: that the following source code is provided and copyrighted by Microsoft and is for educational purpose only.
The meta data is from an SQL 2012 Server.

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Definition:

sys.sp_MSensure_single_instance(nvarchar @application_name
, int @agent_type)

MetaData:

 CREATE PROCEDURE sys.sp_MSensure_single_instance  
(  
    @application_name     sysname,  
    @agent_type         int = NULL  
)  
AS  
BEGIN  
    SET NOCOUNT ON  
  
    DECLARE @retcode     int,  
            @type         sysname,  
            @dbprincipal sysname  
  
    if @agent_type = 4 and exists (select 1 from sys.database_principals where name=N'MSmerge_PAL_role' and type = 'R')  
    begin  
        --  for merge any pal user (PAL of any publication) can do the instance check  
        exec @retcode = sys.sp_MSrepl_PAL_rolecheck   
        if (@retcode <> 0) or (@@error <> 0)  
            return 1  
  
        --  use the merge pal role for merge    
        select @dbprincipal = N'MSmerge_PAL_role'  
    end  
    else  
    begin  
        IF IS_MEMBER('db_owner') != 1  
        BEGIN  
            --  "Only members of the sysadmin or db_owner roles can perform this operation."  
            RAISERROR(21050, 14, -1)  
            RETURN 1  
        END  
  
        --  use db_owner principal  
        select @dbprincipal = N'db_owner'          
    end  
  
    IF @agent_type = 1  
    BEGIN   
        SELECT @type = 'snapshot'  
    END  
    ELSE IF @agent_type = 2  
    BEGIN  
        SELECT @type = 'logreader'  
    END  
    ELSE IF @agent_type = 9  
    BEGIN  
        SELECT @type = 'queuereader'  
    END  
    ELSE IF @agent_type = 10  
    BEGIN  
        SELECT @type = 'distribution'  
    END  
    ELSE IF @agent_type = 4  
    BEGIN  
        SELECT @type = 'merge'  
    END  
    ELSE  
    BEGIN  
        --  "Invalid value given for parameter @type."  
        RAISERROR(15021, 16, -1, '@type')  
        RETURN 1  
    END  
      
    --  get an applock at the db_owner level to ensure we   
    --  only allow one agent to run at a time. only dbo  
    --  for the distribution db will be able to aquire this  
    EXEC @retcode = sys.sp_getapplock @Resource = @application_name,   
                                      @LockMode = 'Exclusive',   
                                      @LockOwner = 'Session',  
                                      @LockTimeout = 0,  
                                      @DbPrincipal = @dbprincipal  
    IF @@ERROR <> 0 OR @retcode NOT IN (0, 1)  
    BEGIN  
        --  "Another '@type' agent for the subscription(s) is running or the server is working on a previous request by the same agent."  
        RAISERROR(21036, 16, -1, @type)  
        RETURN 1  
    END  
  
    --  Select the name of the server this executes on if the physical server name is not the same as the virtual server name  
    if lower(@@servername) <> lower(publishingservername())  
        select @@servername  
    else  
        select NULL  
          
    RETURN 0  
END