Nathan Evans' Nemesis of the Moment

Azure Table Storage versus SQLite (on Azure Blob Store)

Posted in .NET Framework, Distributed Systems, Uncategorized by Nathan B. Evans on March 31, 2013

I’ve been trying to decide what storage platform I should use for my application.

It will be storing what are essentially ever-growing (but potentially prune-able past a certain age, say 1 to 3 years) transaction logs. Each record consists of four timestamp values (each 64-bits wide), three 32-bit integer values, and three text fields (two of which are generally of constricted length, say a max. of 256 characters) but one of a typically longer length but hopefully not more than about 1KB at worst case.

Having tried out SQLite on my local machine (which has an SSD), I managed to insert 60,000 of these records in about 1 second flat. I was impressed but cautious, because SQLite isn’t really a cloud-ready data store and it would require quite a bit of work in wrapping up with concurrency handling to make it work for what I’d need it to do. But I could not ignore that it was fast.

When I first read up about Azure Table Storage, I was a bit underwhelmed. It just seemed incredibly bloated and inefficient. It uses XML as its serialization transport. It uses HTTP/S as its network transport (and there is no fast low-level interface available like there is for Azure Service Bus). If you’ve ever used ProtoBuf’s, getting to know Azure Table Storage is a depressing experience. You can see the wastage but there is nothing you can do. Sure you can override the serialization to remove its reliance on reflection and shorten up the property names, but that’s only half the story.

I persisted anyway, and dived into Table Storage to give it a proper go and see what it could do.

I ran into a couple problems, mostly with the .NET Client API. I was submitting a batch of approx. 600 entities. It was returning back to me with a rather vague and puzzling exception:

Microsoft.WindowsAzure.Storage.StorageException was caught
  HResult=-2146233088
  Message=Unexpected response code for operation : 99
  Source=Microsoft.WindowsAzure.Storage
  StackTrace:
       at Microsoft.WindowsAzure.Storage.Core.Executor.Executor.ExecuteSync[T](StorageCommandBase`1 cmd, IRetryPolicy policy, OperationContext operationContext)
       at Microsoft.WindowsAzure.Storage.Table.TableBatchOperation.Execute(CloudTableClient client, String tableName, TableRequestOptions requestOptions, OperationContext operationContext)
       at Microsoft.WindowsAzure.Storage.Table.CloudTable.ExecuteBatch(TableBatchOperation batch, TableRequestOptions requestOptions, OperationContext operationContext)
       at Tests.WazTableSandbox.Write()

Nothing of any worth showed up on Google about this. I dug into it a bit further and noticed the extended exception information mentioned something about “InvalidInput” and “99:One of the request inputs is not valid.” Not really that useful still. Even Googling these gave me no clues as to what was wrong.

I read somewhere that Azure Table Storage batches are limited to 100 entities per batch. So I wrote a quick LINQ GroupBy to batch up my dataset by partition key (yes, that’s another requirement; batches of operations must all be for the same partition key). Fortunately, the exception went away once I was grouping them into batches of 100 correctly. Surely the .NET Client API deserves a better and more self-explanatory exception message for this edge case though? It’s blatantly going to be the first problem any developer encounters when trying to use CloudTable.ExecuteBatch().

With that solved, I continued with my tests.

My test data was batched up, by partition key, into these batch sizes: 26, 28, 22, 46, 51, 61, 32, 14, 46, 34, 31, 42, 59 and 8.

I then wrote some test code for SQLite that mirrored what I was doing with the Table Storage. I made sure to use a SQLite transaction per batch, so that each batch would be written as an atomic unit. I purposefully gave SQLite an advantage by “preparing” the command (i.e. pre-compiling the byte code for the SQL command).

I deployed my test program onto an Azure VM (“extra small”, if it matters?) and ran it. Here’s what came out:

WazTable
Executing batch of 26
Executing batch of 28
Executing batch of 22
Executing batch of 46
Executing batch of 51
Executing batch of 61
Executing batch of 32
Executing batch of 14
Executing batch of 46
Executing batch of 34
Executing batch of 31
Executing batch of 42
Executing batch of 59
Executing batch of 8
00:00:01.8756798

Sqlite
Executing batch of 26
Executing batch of 28
Executing batch of 22
Executing batch of 46
Executing batch of 51
Executing batch of 61
Executing batch of 32
Executing batch of 14
Executing batch of 46
Executing batch of 34
Executing batch of 31
Executing batch of 42
Executing batch of 59
Executing batch of 8
00:00:03.4291801

So although SQLite was massively faster on my local SSD-powered workstation. It was substantially slower (almost 2x) when running from the Azure VM (and hence on a blob store). This was a bit disappointing but it gives me confidence that I am using the right data storage tool for the job.

You may be wondering why I even considered SQLite as an option in the first place. Well, good question. I am still on the fence as to whether my application will be “full cloud” or just a half-way house that can be installed somewhere without any cloudy stuff involved. That’s why I wanted to investigate SQLite as it’s a standalone database. I might support both, in which case I would use SQLite for non-cloud deployments and Azure Table Storage for cloud deployments. I still find it disappointing how inefficient the Azure Table Storage has been designed. They really need to introduce a lower-level network transport like the one for Service Bus. And a better, XML-less, serialization format.

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Three gotchas with the Azure Service Bus

Posted in .NET Framework, Distributed Systems, Software Design by Nathan B. Evans on March 28, 2013

I’ve been writing some fresh code using Azure Service Bus Queues in recent weeks. Overall I’m very impressed. The platform is good, stable and the Client APIs (at least in the form of Microsoft.ServiceBus.dll that I’ve used) is quite modern in design and layout. It’s only slightly annoying that the Client APIs seem to use the old fashioned Begin/End async pattern that was perhaps more in vogue back in the .NET 1.0 to 2.0 days. Why not just return TPL Tasks?

However, there have been a few larger gotchas that I’ve discovered which can quite easily turn into non-trivial problems for a developer to safely work around. These are the sort of problems that can inherently change the way your application is designed.

Deferring messages via Defer()

I’m of the opinion that a Service Bus should take care of message redelivery mechanisms itself. On the most part, Azure Service Bus does this really well. But it supports this slightly bizarre type of return notification called deferral (invoked via a Defer() or BeginDefer() method). This basically sets a flag on the message internally so that it will never be implicitly redelivered by the queue to your application. But the message will fundamentally still exist inside the queue and you can even still Receive() it by asking for it by its SequenceId explicitly. That’s all good and everything but it leaves your application with a bigger problem. Where does it durably store those SequenceId‘s so that it knows what messages it has deferred? Sure you could hold them in-memory, that would be the naive approach and seems to be the approach taken by the majority of Azure books and documentation. But that is, frankly, a ridiculous idea and its insulting that authors in the fault-tolerant distributed systems space can even suggest such rubbish. The second problem is of course what sort of retry strategy do you adopt for that queue of deferred SequenceId‘s. Then you have to think about the transaction costs (i.e. money!) involved of whatever retry strategy you employ. What if your system has deferred hundreds of thousands of millions of messages? Consider that those deferred messages were outbound e-mails and they were being deferred because your mail server is down for 2 hours. If you were to retry those messages every 5 seconds, that is a lot of Service Bus transactions that you’ll get billed for.

One wonders why the Defer() method doesn’t support some sort of time duration or absolute point in time as a parameter that could indicate to the Service Bus when you actually want that message to be redelivered. It would certainly be a great help and I can’t imagine it would require that much work in the back-end for the Azure guys.

So how do you actually solve this problem?

For now, I have completely avoided the use of Defer() in my system. When I need to defer a message I will simply not perform any return notification for the message and I will allow the PeekLock to expire by its own accord (which the Service Bus handles itself). This approach has the following application design side affects:

  • The deferral and retry logic is performed by the Service Bus entirely. My application does not need to worry about such things and the complexities involved.
  • The deferral retry time is constant and is defined at queue description level. It cannot be controlled dynamically on a per message basis.
  • Your queue’s MaxDeliveryCount, LockDuration and DefaultTimeToLive parameters will become inherently coupled and will need to be explicitly controlled.
    (MaxDeliveryCount x LockDuration) will determine how long a message can be retried for and at what interval. If your LockDuration is 1.5 minutes and you want to retry the message for 1 day then MaxDeliveryCount = (1 day / 1.5 minutes) = 960.

This is a good stop-gap measure whilst I am iterating quickly. For small systems it can perhaps even be a permanent solution. But sooner or later it will cause problems for me and will need to be refactored.

I think the key to solving this problem is gaining better understanding over the reason why the message is being deferred in the first place, therefore providing you with more control. In my particular application it can only be caused when for instance an e-mail server is down or unreachable etc. So maybe I need some sort of watchdog in my application that (eventually) detects when the e-mail server is down and then actively stops trying to send messages, and indeed maybe even puts the brakes on actually Receive()‘ing messages from the queue in the first place. For those messages that have been received already then maybe there should be a separate queue called something like “email-outbox-deferred” (note the suffix). Messages queued on this would not actually be the real message but simply a pointer record that points back to the SequenceId of the real one on the “email-outbox” queue. When the watchdog detects that the e-mail server has come back up then it can start opening up the taps again. Firstly it would perform a Receive() loop on the “email-outbox-deferred” queue and attempt to reprocess those messages by following the SequenceId pointers back to the real queue. If it manages to successfully send the e-mail then it can issue a Complete() on both the deferred pointer message and the real message; to entirely remove it from the job queue. Otherwise it can Abandon() them both and the watchdog can start from square one by waiting to gain confidence of the e-mail servers health before retrying again.

The key to this approach is the watchdog. The watchdog must act as a type of valve that can open and close the Receive() loops on the two queues. Without this component you are liable to create long unconstrained loops or even infinite-like loops that will cause you to have potentially massive Service Bus transaction costs on your next bill from Azure.

I believe what I have described here is considered to be a SEDA or “Staged event-driven architecture“. Documentation of this pattern is a bit thin on the ground at the moment. Hopefully this will start to change as enterprise-level PaaS cloud applications gain more and more traction. But if anyone has any good book recommendations… ping me a message.

I’d be interested in learning more about message deferral and retry strategies, so please comment!

Transient fault retry logic is not built into the Client API

Transient faults are those that imply there is probably nothing inherently wrong with your message. It’s just that the Service Bus is perhaps too busy or network conditions dictate that it can’t be handled at this time. Fortunately the Client API includes a nice IsTransient boolean property on every MessagingException. Making good use of this property is harder than it first appears though.

All the Azure documentation that I’ve found makes use of (the rather hideous) Enterprise Library Transient Fault Block pattern. That’s all fine and good. But who honestly wants to be wrapping up every Client API action they do in that? Sure you can abstract it away again by yourself but where does it end?

It seems odd that the Client API doesn’t have this built in. Why when you invoke some operation like Receive() can’t you specify a transient fault retry strategy as an optional parameter? Or hell, why can’t you just specify this retry strategy at a QueueClient level?

I remain hopeful that this is something the Azure guys will fix soon.

Dead lettering is not the end

You may think that once you’ve dead lettered a message that you’ll not need to worry about it again from your application. Wrong.

When you dead letter a message it is actually just moved to a special sub-queue of your queue. If left untouched, it will remain in that sub-queue forever. Forever. Yes, forever. Yes, a memory leak. Eventually this will bring down your application because your queue will run into its memory limit (which can only be a maximum of 5GB). Annoyingly most developers are simply not aware of the dead letter sub-queues existence because it does not show up as a queue on the Server Explorer pane in Visual Studio. Bit of an oversight that one!

Having a human flush this queue out every now and then is not an acceptable solution for most systems. What if your system has a sudden spike in dead letters. Maybe a rogue system was submitting messages to your queues using an old serialization format or something? What if there were millions of these messages? Your application is going to be offline quicker than any human can react. So you need to build this logic into your application itself. This can be done by a watchdog process that keeps track of how many messages are being put onto the dead letter queue and actively ensures it is frequently pruned. This is very much a non-trivial problem.

Alternatively you can avoid the use of dead lettering entirely. This seems drastic but it may not be such a bad idea actually. You should consider if you actually care enough about retaining that carbon-copy of a message to keep it around as-is. Ask yourself whether just some simple and traditional trace/log output of the problem and approximate message content would be sufficient? Dead lettering is inherently a human concept that is analogous to “lost and found” or a “spam folder”. So perhaps with fully automated systems that desire as little human influence or administrative effort as possible then avoiding dead lettering entirely is the best choice.

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