Background
To make the story short, our company is facing the task of making our application redudant and more resilent to heavy loads by load-balancing. The task is on my desk and I've been doing some research as I've never done it before.
Fact
Today we host our application on 1 server and the goal is to have another one to even out serverload with load-balacing.
Issue
I've been doing some research and got stuck on how to setup the MSSQL Replication. If one server goes down the other one must be in sync as the users will be redirected there insted by the load-balancer
The tenthousand view to the solution goes something like: Have a Publisher/Distributor on same server and then add subscriber databases and they will sync between eachother.
Question
What happens if the Publisher/Distrubtor server goes down? Suddenly the system isn't redudant at all. Do we have to setup a Publisher/Distrubtor on each subscriber server to take over the role? I've been search around and haven't found a good answer.
Just hint if the explanation is confusing and I'll fill in the blanks..
Thanks in advance!
Related
We have a client deployment of our software that is showing intermittent SQL server connection failures, and we are struggling to understand them.
Our system consists of a SQL Server DB (2012) and 14 identical engines, each installed on a Windows 2012 VM. Each of these was created from the same template so they should be identical. The engines consist of a Windows service that connects to the DB on startup by reading a single row from a table. If the connection fails they will wait a few seconds and try again, until they get a connection.
In this particular case, the VMs were all rebooted due to a Windows Update. (The SQL server had the update/reboot about 12 hours before). They came online within a few minutes of each other. 12 of the engines started up without any problem. Two of them, however, failed to connect to the DB with:
"The underlying provider failed on Open."
Those two engines then started to poll, and continued to get this error for many hours. The rest of the engines had started up and were fine. We have a broker service too that was accessing the DB throughout and showed no connection issues.
When the client noticed this issue, they restarted the engine services on the two problem VMs, and the two engines connected to the DB just fine.
We are trying to understand what could have happened here. I guess my main questions are:
What could be an explanation of why 12 connections succeed and two fail? There's absolutely no difference as far as we know between the engines. The query itself is very simple.
Why did the connection continue to fail for those two engines until the service was restarted? This suggests to me that there is some process-level failed state that is only cleared when restarting the services. I've looked at the code to see if it was reusing the connections. It uses Entity Framework to read the single table row, and we create a fresh DbContext each time. I don't understand how this could go wrong.
We noted that there was a CheckDb operation proceeding on the DB around the time the services were coming up, and we wondered if this could be related to the issue. However, the client says that this runs every night and hasn't caused problems in the past. And it wouldn't explain why the engines didn't come back up again.
Thanks in advance for any help.
We had an issue yesterday that we are trying to figure out. Out of nowhere everything on the database changed,
We know it was an update without a where clause, but we are just a few developers. So if any of us would have done it we would know it.
It was at a strange time of the day, very late at night and only a few ip addresses are allowed into the server.
Is there any way to get the full log with ips of all the transactions on azure?
Did anyone had a similar problem? can it be a break through?
Are there any software protections, scripts that we can add to limit this?
Is there any way to get the full log with ips of all the transactions on azure?
Few options i could think off,Even this is not possible in onpremises..if you don't have correct measures to detect this...else contact support for a request to read TLOG of the database(Azure support won't read the log,unless you have a business justification,as this involves involving many teams due to safety reasons)
1.) You could use activity log to know more details..
2.) There is an sys.event_log (Azure SQL Database) DMV ,which shows connections successfull or not .you can correlate to know the users based on your office set up..this won't show success or failures
To avoid this happening again,Audit data and Azure offers many features to know more on whats happening like
1..Get started with SQL database auditing
2. Enable rules to get alerted when some thing happens..
Enable Auditing and Threat Detection on the server if you hadn't
For more information, please read this page.
I've got a writing and reading database application holding a local cache. In case of an application server fault a backup server shall start working.
The primary and backup application can only run exclusively because of its local cache and some low isolation level on the database.
As far as my communication knowledge goes it is impossible to let both servers always figure out who is allowed to run exclusively.
Can I somehow solve this communication conflict through using the database as a third entity? I think this is a quite typical problem and there might not be a 100% safe method, but I would be happy to know how other people recommend to solve such issues? Or if there is some best practice to this.
It's okay if both application are not working for 30 minutes or so, but there is not enough time to get people out of bed and let them figure out what the problem is.
Can you set up a third server which is monitoring both application servers for health? This server could then decide appropriately in case one of the servers appears to be gone: Instruct the hot standby to start processing.
if i get the picture right, your backup server constantly polls the primary server for data updates, it wouldn't be hard to check if the poll fails, schedule it again for 30s later 3 times and in the third failure dynamically update the DNS entry to the database server to reflect the change in active server. Both Windows DNS and Bind accept dynamic updates signed and unsigned.
In our college, we are conducting a contest, which is in the form of Multiple choice questions.
For that we are using VB as a front-end , MS Access as a back-end.
Ref:
The Application VB 6 runs with no problem and the participant entry is added into the database along with their scores, when 1 or 2 clients access the database simultaneously.
Problem:
But when more than 2 clients access the database simultaneously, the Application crashes.
1.In some clients, we are getting runtimeerror printing some large negative value with a
message "Operations query cannot modify the database".
2.In some clients, the VB 6.0 crashes and closes suddenly.
These errors occurs when we are tried to access the database using
OK,SUBMIT buttons.
Could you tell me why this error occurs and how can I correct it.
My Questions are..
1.Is giving the burden of all clients to a single laptop is the problem here?If there is some other problem please explain it.
2.Why I am getting the error as "Operation query cannot modify database", If so then how it works when 1 or 2 clients access the database simultaneously.
Access databases (and other directly file-based DBs) are not really built for multi-access. There are some facilities in place to help it work, but in my experience, it is quite unreliable.
You need a database server running, which can allow multiple clients to the same database simultaneously. A free option is MySQL. There is also a free version of Microsoft SQL Server available.
"Operation query cannot modify database"
Badly designed Access databases can have issues with users trying to modify records if they don't have defined Primary keys. This is especially true when multiple people are accessing because it literally can't tell which record to modify if two people try to do the same thing. Sometimes it will let you insert but not update.
Further if you are looking for performance, Access is just the wrong tool. It has very little in the way of performance tuning options or abilities. SQl Server Express or mySQl would have more things available to diagnose and fix a performance issue.
I have a very strange and complicated situation. I have data being erased from one of my SQL Server tables, and I am not sure by what application. I would like to be able to track this.
As I am sure you are wondering how I could find myself in this situation, here is some background. We have 2 servers, Web and Database running IIS6 and SQL Server 2005 respectively. They were setup by the previous developer who left the company without giving me any sort of introduction to the system so I am left "hunting" for everything.. I have been able to figure out most of the system on my own except for this, which remains a mystery. All I know for sure is this:
Data is being erased at a set time every day (I have setup a TRIGGER to capture this)
It is not a SQL Server Agent Job
It is not a Windows Scheduled Task
It is not a Windows Service
All database logins are done with the sa user so login history cannot help me... (again, I didn't set this up)
How the heck do I debug something like this? If anything, I want to know if this is coming from something running on the database server, or from a request from an outside source. Please help :-)
As you know the time it happens you should set up a SQL Profiler trace at that time to catch the statements being sent.
This will show you the SQL being sent, the spid of the connection, user name, application name sent by the connection and other useful info to track down the culprit.
In case the time that it happens is not convenient for you to do this you can script SQL traces (which is more lightweight than running the full GUI anyway)
Edit: Be careful when using it not to record so much information that you bog down the server. You can filter for activity on the database of interest for example.