Asynchronous Triggers in SQL Server 2005/2008

问题:

I have triggers that manipulate and insert a lot of data into a Change tracking table for audit purposes on every insert, update and delete.

This trigger does its job very well, by using it we are able to log the desired oldvalues/newvalues as per the business requirements for every transaction.

However in some cases where the source table has a lot columns, it can take up to 30 seconds for the transaction to complete which is unacceptable.

Is there a way to make the trigger run asynchronously? Any examples.

回答1:

You can't make the trigger run asynchronously, but you could have the trigger synchronously send a message to a SQL Service Broker queue. The queue can then be processed asynchronously by a stored procedure.

回答2:

these articles show how to use service broker for async auditing and should be useful:

Centralized Asynchronous Auditing with Service Broker

Service Broker goodies: Cross Server Many to One (One to Many) scenario and How to troubleshoot it

回答3:

SQL Server 2014 introduced a very interesting feature called Delayed Durability. If you can tolerate loosing a few rows in case of an catastrophic event, like a server crash, you could really boost your performance in schenarios like yours.

Delayed transaction durability is accomplished using asynchronous log writes to disk. Transaction log records are kept in a buffer and written to disk when the buffer fills or a buffer flushing event takes place. Delayed transaction durability reduces both latency and contention within the system

The database containing the table must first be altered to allow delayed durability.

ALTER DATABASE dbname SET DELAYED_DURABILITY = ALLOWED

Then you could control the durability on a per-transaction basis.

begin tran

insert into ChangeTrackingTable select * from inserted

commit with(DELAYED_DURABILITY=ON)

The transaction will be commited as durable if the transaction is cross-database, so this will only work if your audit table is located in the same database as the trigger.

There is also a possibility to alter the database as forced instead of allowed. This causes all transactions in the database to become delayed durable.

ALTER DATABASE dbname SET DELAYED_DURABILITY = FORCED

For delayed durability, there is no difference between an unexpected shutdown and an expected shutdown/restart of SQL Server. Like catastrophic events, you should plan for data loss. In a planned shutdown/restart some transactions that have not been written to disk may first be saved to disk, but you should not plan on it. Plan as though a shutdown/restart, whether planned or unplanned, loses the data the same as a catastrophic event.

This strange defect will hopefully be addressed in a future release, but until then it may be wise to make sure to automatically execute the 'sp_flush_log' procedure when SQL server is restarting or shutting down.

回答4:

I wonder if you could tag a record for the change tracking by inserting into a "too process" table including who did the change etc etc.

Then another process could come along and copy the rest of the data on a regular basis.

回答5:

There's a basic conflict between "does its job very well" and "unacceptable", obviously.

It sounds to me that you're trying to use triggers the same way you would use events in an OO procedural application, which IMHO doesn't map.

I would call any trigger logic that takes 30 seconds - no, more that 0.1 second - as disfunctional. I think you really need to redesign your functionality and do it some other way. I'd say "if you want to make it asynchronous", but I don't think this design makes sense in any form.

As far as "asynchronous triggers", the basic fundamental conflict is that you could never include such a thing between BEGIN TRAN and COMMIT TRAN statements because you've lost track of whether it succeeded or not.

回答6:

Create history table(s). While updating (/deleting/inserting) main table, insert old values of record (deleted pseudo-table in trigger) into history table; some additional info is needed too (timestamp, operation type, maybe user context). New values are kept in live table anyway.

This way triggers run fast(er) and you can shift slow operations to log viewer (procedure).

回答7:

To perform asynchronous processing you can use Service Broker, but it isn't the only option, you can also use CLR objects.

The following is an example of an stored procedure (AsyncProcedure) that asynchronous calls another procedure (SyncProcedure):

using System;
using System.Data;
using System.Data.SqlClient;
using System.Data.SqlTypes;
using Microsoft.SqlServer.Server;
using System.Runtime.Remoting.Messaging;
using System.Diagnostics;

public delegate void AsyncMethodCaller(string data, string server, string dbName);

public partial class StoredProcedures
{
    [Microsoft.SqlServer.Server.SqlProcedure]
    public static void AsyncProcedure(SqlXml data)
    {
        AsyncMethodCaller methodCaller = new AsyncMethodCaller(ExecuteAsync);
        string server = null;
        string dbName = null;
        using (SqlConnection cn = new SqlConnection("context connection=true"))
        using (SqlCommand cmd = new SqlCommand("SELECT @@SERVERNAME AS [Server], DB_NAME() AS DbName", cn))
        {
            cn.Open();
            using (SqlDataReader reader = cmd.ExecuteReader())
            {
                reader.Read();
                server = reader.GetString(0);
                dbName = reader.GetString(1);
            }
        }
        methodCaller.BeginInvoke(data.Value, server, dbName, new AsyncCallback(Callback), null);
        //methodCaller.BeginInvoke(data.Value, server, dbName, null, null);
    }

    private static void ExecuteAsync(string data, string server, string dbName)
    {
        string connectionString = string.Format("Data Source={0};Initial Catalog={1};Integrated Security=SSPI", server, dbName);
        using (SqlConnection cn = new SqlConnection(connectionString))
        using (SqlCommand cmd = new SqlCommand("SyncProcedure", cn))
        {
            cmd.CommandType = CommandType.StoredProcedure;
            cmd.Parameters.Add("@data", SqlDbType.Xml).Value = data;
            cn.Open();
            cmd.ExecuteNonQuery();
        }
    }

    private static void Callback(IAsyncResult ar)
    {
        AsyncResult result = (AsyncResult)ar;
        AsyncMethodCaller caller = (AsyncMethodCaller)result.AsyncDelegate;
        try
        {
            caller.EndInvoke(ar);
        }
        catch (Exception ex)
        {
            // handle the exception
            //Debug.WriteLine(ex.ToString());
        }
    }
}

It uses asynchronous delegates to call SyncProcedure:

CREATE PROCEDURE SyncProcedure(@data xml)
AS
  INSERT INTO T(Data) VALUES (@data)

Example of calling AsyncProcedure:

EXEC dbo.AsyncProcedure N'<doc><id>1</id></doc>'

Unfortunatelly, the assembly requires UNSAFE permission.

回答8:

Not that I know of, but are you inserting values into the Audit table that also exist in the base table? If so, you could consider tracking just the changes. Therefore an insert would track the change time, user, extra and a bunch of NULLs (in effect the before value). An update would have the change time, user etc and the before value of the changed column only. A delete has the change at, etc and all values.

Also, do you have an audit table per base table or one audit table for the DB? Of course the later can more easily result in waits as each transaction tries to write to the one table.

回答9:

I suspect that your trigger is of of these generic csv/text generating triggers designed to log all changes for all table in one place. Good in theory (perhaps...), but difficult to maintain and use in practice.

If you could run asynchronously (which would still require storing data somewhere for logging again later), then you are not auditing and neither do have history to use.

Perhaps you could look at the trigger execution plan and see what bit is taking the longest?

Can you change how you audit, say, to per table? You could split the current log data into the relevant tables.

回答10:

From sql server 2008 you can use CDC feature for automatically logging changes, which is purely asynchronous. Find more details in here