I'm using an SQL Server 2008 R2 as a queuing mechanism. I add items to the table, and an external service reads and processes these items. This works great, but is missing one thing - I need mechanism whereby I can attempt to select a single row from the table and, if there isn't one, block until there is (preferably for a specific period of time).
Can anyone advise on how I might achieve this?
The only way to achieve a non-pooling blocking dequeue is WAITFOR (RECEIVE). Which implies Service Broker queues, with all the added overhead.
If you're using ordinary tables as queues you will not be able to achieve non-polling blocking. You must poll the queue by asking for a dequeue operation, and if it returns nothing, sleep and try again later.
I'm afraid I'm going to disagree with Andomar here: while his answer works as a generic question 'are there any rows in the table?' when it comes to queueing, due to the busy nature of overlapping enqueue/dequeue, checking for rows like this is a (almost) guaranteed deadlock under load. When it comes to using tables as queue, one must always stick to the basic enqueue/dequeue operations and don't try fancy stuff.
"since SQL Server 2005 introduced the OUTPUT clause, using tables as queues is no longer a hard problem". A great post on how to do this.
http://rusanu.com/2010/03/26/using-tables-as-queues/
I need mechanism whereby I can attempt
to select a single row from the table
and, if there isn't one, block until
there is (preferably for a specific
period of time).
You can loop and check for new rows every second:
while not exists (select * from QueueTable)
begin
wait for delay '00:01'
end
Disclaimer: this is not code I would use for a production system, but it does what you ask.
The previous commenter that suggested using Service Broker likely had the best answer. Service Broker allows you to essentially block while waiting for more input.
If Service Broker is overkill, you should consider a different approach to your problem. Can you provide more details of what you're trying to do?
Let me share my experiences with you in this area, you may find it helpful.
My team first used MSMQ transactional queues that would feed our asynchronous services (be they IIS hosted or WAS). The biggest problem we encountered was MS DTC issues under heavy load, like 100+ messages/second load; all it took was one slow database operation somewhere to start causing timeout exceptions and MS DTC would bring the house down so to speak (transactions would actually become lost if things got bad enough), and although we're not 100% certain of the root cause to this day, we do suspect MS DTC in a clustered environment has some serious issues.
Because of this, we started looking into different solutions. Service Bus for Windows Server (the on-premise version of Azure Service Bus) looked promising, but it was non-transactional so didn't suit our requirements.
We finally decided on the roll-your-own approach, an approach suggested to us by the guys who built the Azure Service Bus, because of our transactional requirements. Essentially, we followed the Azure Worker Role model for a worker role that would be fed via some queue; a polling-blocking model.
Honestly, this has been far better for us than anything else we've used. The pseudocode for such a service is:
hasMsg = true
while(true)
if(!hasMsg)
sleep
msg = GetNextMessage
if(msg == null)
hasMsg = false
else
hasMsg = true
Process(msg);
We've found that CPU usage is significantly lower this way (lower than traditional WCF services).
The tricky part of course is handling transactions. If you'd like to have multiple instances of your service read from the queue, you'll need to employ read-past/updlock in your sql, and also have your .net service enlist in the transactions in a way that will roll-back should the service fail. in this case, you'll want to go with retry/poison queues as tables in addition to your regular queues.
Related
I have data conversion and caching service running as self-hosted WCF service.
Now it uses database polling in constant short intervals to update its data.
I think it's unnecessary. The data can be changed only if one of the tables is changed, and when the data is changed depends on system users actions.
There is no problem in setting a trigger for specific tables, however I would need an action outside SQL-Server to update my cache. My WCF service could perform update when receiving specific URI via HTTP. So all I need is a command in table trigger which would send a request. Is it even possible?
I think about a hack I used back in the days with HTTP requests. I halted HTTP request response at server until data packet from somewhere else arrived. There was no delay between polling requests. I achieved fully asynchronous, "real-time" updates.
Maybe this approach is possible to apply with SQL? I think about a query which blocks termination until receives a signal. Well, it eventually times out, but it's good enough to try. Then - how to signal and wait in SQL? By locking and unlocking shared resource, like cursor or dummy table?
Any other options?
I need the cache update done at lowest possible frequency (because it's pretty expensive, so once per minute is great), but I need immediate update when the data is changed.
To answer your question, have you looked at xp_cmdshell?
https://msdn.microsoft.com/en-us/library/ms175046.aspx
However, the security/performance implications of such a decision could be non-trivial depending on your use case.
I was assigned to update existing system of gathering data coming from points of sale and inserting it into central database. The one that is working now is based on FTP/SFTP transmission, where the information is sent once a day, usually at night. Unfortunately, because of unstable connection links (low quality 2G/3G modems), some of the files appear to be broken. With just a few shops connected that way everything was working smooth, but along with increasing number of shops, errors became more often. What is worse, the time needed to insert data into central database is taking up to 12 - 14h (including waiting for the data to be downloaded from all of the shops) and that cannot happen during the working day as it would block the process of creating sale reports and other activities with the database - so we are really tight with processing time here.
The idea my manager suggested is to send the data continuously, during the day. Data packages would be significantly smaller, so their transmission and insertion would be much faster, central server would contain actual (almost real time) data and night could be used for long running database activities like creating backups, rebuilding indexes etc.
After going through many websites, I found that:
using ASMX web service is now obsolete and WCF should be used instead
WCF with MSMQ or System Messaging could be used to safely transmit data, where I don't have to care that much about acknowledging delivery of data, consistency, nodes going offline etc.
according to http://blogs.msdn.com/b/motleyqueue/archive/2007/09/22/system-messaging-versus-wcf-queuing.aspx WCF queuing is better
there are also other technologies for implementing message queue, like RabbitMQ, ZeroMQ etc.
And that is where I become confused. With so many options, do you have any pros and cons of these technologies?
We were using .NET with Windows Forms and SQL Server, but if it would be necessary, we could change to something more suitable. I am also a bit afraid of server efficiency. After some calculations, server would be receiving about 15 packages of data per second (peak). Is it much? I know there are many websites without serious server infrastructure, that handle hundreds of visitors online and still run smooth, but the website mainly uploads data to the client, and here we would download it from the client.
I also found somewhat similar SO question: Middleware to build data-gathering and monitoring for a distributed system
where DDS was mentioned. What do you think about introducing some middleware servers that would cope with low quality links to points of sale, so the main server would not be clogged with 1KB/s transmission?
I'd be grateful with all your help. Thank you in advance!
Rabbitmq can easily cope with thousands of 1kb messages per second.
As your use case is not about processing real time data, I'd say you should combine few messages and send them as a batch. That would be good enough in order to spread load over the day.
As the motivation here is not to process the data in real time, then any transport layer would do the job. Even ftp/sftp. As rabbitmq will work fine here, it's not the typical use case for it.
As you mentioned that one of your concerns is slow/unreliable network, I'd suggest to compress the files before sending them, and on the receiving end, immediately verify their integrity. Rsync or similar will probably do great job in doing that.
From what I understand, you have basically two problems:
Potential for loss/corruption of call data
Database write performance
The potential for loss/corruption of call data is being caused by a lack of reliability in the transmission of data from client to service.
And it's not clear what is causing the database contention/performance issues, beyond a vague reference to high volumes, so this answer will be more geared towards solving the first problem.
You have correctly identified the need for reliable asynchronous communication transport as a way to address the reliability issues in your current setup.
Looking at MSMQ to deliver this is a valid first step. MSMQ provides reliable communication via a store and forward messaging semantic which comes out of the box and requires very little in the way of configuration.
Unfortunately, while suitable for your needs, MSMQ relies on 2 things:
A reliable network protocol, and
A client service running on both sending and receiving machine.
From your description above, I don't believe 1 exists (the internet is not a reliable network), and you might well struggle with 2 - MSMQ only ships with Windows Server or business/enterprise versions of Windows on the desktop.(*see below...)
As a possible solution to the network reliability problem, you could use a WCF or a RESTful endpoint (using Nancy or WebApi) to expose a service operation(s) exposed over HTTP, which would accept the incoming calls from the client machines. These technologies are quite different, so you'll need to make sure you're making the correct choice early on.
WCF supports WS-ReliableMessaging from the SOAP 1.2 specification out of the box, which allows for reliable web service calls over http, however it's very config-heavy and not generally a nice framework to work with.
REST much simpler than WCF in .Net, is very lightweight and easy to use. However, for reliable delivery you would have to expose some kind of GET operation (in addition to a POST to allow the client to send data) to be called (within a reasonable time-frame) to verify the data was committed. The client would have to implement some kind of retry semantic if the result of the GET "acknowledgement" was negative.
Despite requiring two operations rather than one for the WCF route, I would favour the REST approach. I've done plenty of both and find REST services way nicer to work with.
(*) That's not to say that MSMQ wouldn't work in your ultimate solution, just that it would not be used to address the transmission reliability issue. However it could still be used to address another of your problems, that of database write contention. If you were to queue incoming requests once they came into the server, then these could be processed by an "offline" process, which could then perform the required database operations in a reliable manner. This could be done by using MSMQ transactional queues.
In response to comments:
99% messages are passed from shop to main server, but if some change
is needed (price correction, discounts etc.), that data has to be sent
to shop.
This kind of changes things. Had I understood from the beginning that you had a bidirectional requirement, and seeing as how you have managed to establish msmq communication, I would have nudged you towards NServiceBus, which is a really, really cool wrapper around MSMQ. The reason I would have done this is that you appear to have both a one way, and a publish-subscribe requirement, which is supported really nicely by NServiceBus.
I wish to create a queue where a lot of computers would be writing in but each computer will write only once in his entire life. What you think would be the best way to achieve that?
I have read about SQL Server queues, SQL Server tables used as queue or service broker infrastructure.
SQL Server table : pretty easy to create but I am afraid of the performance
Service broker : more complex infrastructure. It seems that you have to run a service on the sender and have a send queue which is useless in my case because because all of them only send one message in their entire life.
What solution would be the best in my case?
you don't have to create a service on each computer. Service Broker objects can be confined to one DB server. For example, if you have 100 computers that need to drop of a message, they will need a connection string to the database server and execute a stored procedure that would enqueue the said message.
that said, it seems like a Service Broker queue would be an overkill for this. A simple table would probably suffice, or even better an MSMSQ (which would eliminate the need to connect to a DB).
Our production code uses tables as queues. We don't really need the robustness of Service Broker, and all our code already connects to databases for other stuff anyway.
Our code doesn't need more than a few hundred transactions per second, and I've shown that our queue can achieve over 10k transactions per second, so I'm fairly happy with the performance.
Here's a great article describing how to design tables for use as queues: http://rusanu.com/2010/03/26/using-tables-as-queues/
I would not design your table without first giving it a read.
Our company is also contemplating an alternative queue strategy involving Redis that doesn't require disk access since we are considering a design that would require tens or hundreds of thousands of inserts a second, but don't necessarily care about losing the data in the event of a failure. I would also give those methods a consideration if you need the throughput.
Maybe the better way transform your whole system from "several writers and one reader" to "one writer and one reader"? I mean you may make some service (web or any other) who will receive requests to write and will be the only writer into your database. This is ordinary situation and has many standard solutions.
I'm working on a real-time application and building it on Azure.
The idea is that every user reports something about himself and all the other users should see it immediately (they poll the service every seconds or so for new info)
My approach for now was using a Web Role for a WCF REST Service where I'm doing all the writing to the DB (SQL Azure) without a Worker Role so that it will be written immediately.
I've come think that maybe using a Worker Role and a Queue to do the writing might be much more scalable, but might interfere with the real-time side of the service. (The worker role might not take the job immediately from the queue)
Is it true? How should I go about this issue?
Thanks
While it's true that the queue will add a bit of latency, you'll be able to scale out the number of Worker Role instances to handle the sheer volume of messages.
You can also optimize queue-reading by getting more than one message at a time. Since a single queue has a scalability target of 500 TPS, this lets you go well beyond 500 messages per second on reads.
You might look into a Cache for buffering the latest user updates, so when polling occurs, your service reads from cache instead of SQL Azure. That might help as the volume of information increases.
You could have a look at SignalR, it does not support farm scenarios out-of-the-box, but should be able to work with the use of either internal endpoint calls to update every instance, using the Azure Service Bus, or using the AppFabric Cache. This way you get a Push scenario rather than a Pull scenario, thus you don't have to poll your endpoints for potential updates.
An existing process changes the status field of a booking record in a table, in response to user input.
I have another process to write, that will run asynchronously for records with a particular status. It will read the table record, perform some operations (including calls to third party web services), and update the record's status field to indicate that processing is completed (or In Error, with an error count).
This operation sounds very similar to a queue. What are the benefits and tradeoffs of using MSMQ over a SQL Table in this situation, and why should I choose one over the other?
It is our software that is adding and updating records in the table.
It is a new piece of work (a Windows Service) that will be performing the asynchronous processing. This needs to be "always up".
There are several reasons, which were discussed on the Fog Creek forum here: http://discuss.fogcreek.com/joelonsoftware5/default.asp?cmd=show&ixPost=173704&ixReplies=5
The main benefit is that MSMQ can still be used when there is intermittant connectivity between computers (using a store and forward mechanism on the local machine). As far as the application is concerned it delivered the message to MSMQ, even though MSMQ will possibly deliver the message later.
You can only insert a record to a table when you can connect to the database.
A table approach is better when a workflow approach is required, and the process will move through various stages, and these stages need persisting in the DB.
If the rate at which booking records is created is low I would have the second process periodically check the table for new bookings.
Unless you are already using MSMQ, introducing it just gives you an extra platform component to support.
If the database is heavily loaded, or you get a lot of lock contention with two process reading and writing to the same region of the bookings table, then consider introducing MSMQ.
I also like this answer from le dorfier in the previous discussion:
I've used tables first, then refactor
to a full-fledged msg queue when (and
if) there's reason - which is trivial
if your design is reasonable.
Thanks, folks, for all the answers. Most helpful.
With MSMQ you can also offload the work to another server very easy by changing the location of the queue to another machine rather then the db server.
By the way, as of SQL Server 2005 there is built in queue in the DB. Its called SQL server Service Broker.
See : http://msdn.microsoft.com/en-us/library/ms345108.aspx
Also see previous discussion.
If you have MSMQ expertise, it's a good option. If you know databases but not MSMQ, ask yourself if you want to become expert in another technology; whether your application is a critical one; and which you'd rather debug when there's a problem.
I have recently been investigating this myself so wanted to mention my findings. The location of the Database in comparison to your application is a big factor on deciding which option is faster.
I tested inserting the time it took to insert 100 database entries versus logging the exact same data into a local MSMQ message. I then took the average of the results of performing this test several times.
What I found was that when the database is on the local network, inserting a row was up to 4 times faster than logging to an MSMQ.
When the database was being accessed over a decent internet connection, inserting a row into the database was up to 6 times slower than logging to an MSMQ.
So:
Local database - DB is faster, otherwise MSMQ is.
Instead of making raw MSMQ calls, it might be easier if you implement your sevice as a queued COM+ component and make queued function calls from your client application. In the end, the asynchronous service still uses MSMQ in the background, but your code will be much clearer and easier to use.
I would probably go with MSMQ, or ActiveMQ myself. I would suggest (presuming that you are considering MSMQ you are using windows, with MS technology) looking into WCF, or if you are using MS-SQL 2005+ having a trigger that calls into .net code to run your processing.
Service Broker was introduced in SQL 2005 and it is designed to be very quick at handling messages as the process is relatively simple (I believe its roots were in triggers). If you are concerned about scalability, in SQL 2008 they have released an independant processing executable to separate the processing from SQL Server (in standard Service Broker, everything is controlled by the SQL Server instances).
I would definitely consider using Service Broker over MSMQ but this is dependant on your SQL Development/DBA resources and their knowledge.
Besides of Mitch's answer, some other scenarios:
1. each of your message have its own due date to trigger the action, this can be done through MQ as well, but in this case I prefer to store it into db as it is more controllable;
2. subscriber needs to filter message and then process a portion of it, this can be done by LINQ too, depends on how complex the filter is, the db approach is better because I can use linq to EF do complex query easily;
3. For deployment, i want fully automated deployment process so that DB is a better choice for me. I am not a big fan of manual configurations.