Challenges from large scale computing at Google

From Greg Linden on a talk Google Fellow Jeff Dean gave last week at University of Washington Computer Science titled "Research Challenges Inspired by Large-Scale Computing at Google" : Coming away from the talk, the biggest points for me were the considerable interest in reducing costs (especially reducing power costs), the suggestion that the Google cluster may eventually contain 10M machines at 1k locations, and the call to action for researchers on distributed systems and databases to think orders of magnitude bigger than they often are, not about running on hundreds of machines in one location, but hundreds of thousands of machines across many locations.

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SQL Server 2008 Database Performance and Scalability

Microsoft SQL Server 2008 incorporates the tools and technologies that are necessary to implement relational databases, reporting systems, and data warehouses of enterprise scale, and provides optimal performance and responsiveness.
With SQL Server 2008, you can take advantage of the latest hardware technologies while scaling up your servers to support server consolidation. SQL Server 2008 also enables you to scale out your largest data solutions.

This white paper describes the performance and scalability capabilities of Microsoft® SQL Server® 2008 and explains how you can use these capabilities to:
* Optimize performance for any size of database with the tools and features that are available for the database engine, analysis services, reporting services, and integration services.
* Scale up your servers to take full advantage of new hardware capabilities.
* Scale out your database environment to optimize responsiveness and to move your data closer to your users.

Read the entire article about SQL Server 2008 Database Performance and Scalability at MyTestBox.com - web software reviews, news, tips & tricks.

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Useful Corporate Blogs that Talk About Scalability

Some intrepid company blogs are posting their technical challenges and how they solve them. I wish more would open up and talk about what they are doing as it helps everyone move forward. Here are a few blogs documenting their encounters with the bleeding edge:

Flickr

Digg

LinkedIn

Facebook

Amazon Web Services blog

Twitter blog

Reddit blog

Photobucket blog

Second Life blog

PlentyofFish blog

Joyent's Blog Any others that should be added?

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The Art of Capacity Planning: Scaling Web Resources

Update 3: The book was released! Find it on Amazon at The Art of Capacity Planning. Update 2: Maybe the iPhone can use a little capacity planning? What's Behind the iPhone 3G Glitches: One source says Apple programmed the Infineon chip to demand a more powerful 3G signal than the iPhone really requires. So if too many people try to make a call or go on the Internet in a given area, some of the devices will decide there's insufficient power and switch to the slower network—even if there is enough 3G bandwidth available. Update: To get a taste of what will be served, mySQL DBA has a nice post titled Capacity Planning, Architecture, Scaling, Response time, Throughput. You learn how to figure out when your application will break by building a 3rd order polynomial. Cool stuff! John Allspaw who is the Operations Engineering Manager at Flickr is about to publish a book with O'Reilly. There are not much details so far but it seems interesting and relevant to High Scalability. Allspaw combines personal anecdotes from many phases of Flickr's growth with insights from his colleagues in many other industries to give you solid guidelines for measuring your growth, predicting trends, and making cost-effective preparations. Topics include:

• Evaluating tools for measurement and deployment
• Capacity analysis and prediction for storage, database, and application servers
• Designing architectures to easily add and measure capacity
• Handling sudden spikes
• Predicting exponential and explosive growth
• How cloud services such as EC2 can fit into a capacity strategy

The Art of Capacity Planning: Scaling Web Resources is available for pre-order on amazon.com

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Strategy: Flickr - Do the Essential Work Up-front and Queue the Rest

This strategy is stated perfectly by Flickr's Myles Grant: The Flickr engineering team is obsessed with making pages load as quickly as possible. To that end, we’re refactoring large amounts of our code to do only the essential work up front, and rely on our queuing system to do the rest. Flickr uses a queuing system to process 11 million tasks a day. Leslie Michael Orchard also does a great job explaining the queuing meme in his excellent post Queue everything and delight everyone. Asynchronous work queues are how you scalably solve problems that are too big to handle in real-time. The process:

Identify the minimum feedback the client (UI, API) needs to know an operation succeeded. It's enough, for example, to update a client's view when a posting a message to a microblogging service. The client probably isn't aware of all the other steps that happen when a message is added and doesn't really care when they happen as long as the obvious cases happen in an appropariate period of time.

Queue all work not on the critical path to a job queueing system so the critical path remains unblocked. Work is then load balanced across a cluster and completed as resources permit. The more sharded your architecture is the more work can be done in parallel which minimizes total throughput time. This approach makes it much easier to bound response latencies as features scale.

Queues Give You Lots of New Knobs to Play With

As features are added data consumers multiply, so throwing a new task into a sequential process has a good chance of blowing latencies. Queueing gives much more control and flexibility over the performance of a system. With queues some advanced strategies you have at your disposal are:

Horizontal scaling. Add more processing resources to do more work in parallel.

Priority order processing. Paying customers, can be processed first, for example. Take measures to avoid starvation.

Aggregation. Work sitting on the same queue for the same user can be aggregated together so it can be processed as a batch.

Work canceling. A request later in the queue can cancel work earlier in the queue. These can just be dropped.

CPU limitting. When jobs have unbounded CPU time it destroys the latency for other jobs sitting in the queue. Bounding CPU limits on jobs evens out latency for everyone.

Low priority work dropping. Under load low priority jobs can be dropped. Just make you have background sweep processes that catch work that should have been done and redoes it.

Admission control. Under load clients can be told about when to retry. This is the best form of flow control, end-to-end flow with the client. We want to push back on work as high up the stack as we can. Stop the client from pushing work to you and you've accomplished something. Just having blind retries and timeouts puts immense pressure on the whole system. These ideas have been employed in embedded real-time systems forever and now it seems they'll move into web services as well.

What Can You do with Your Queue?

The options are endless, but here are some uses I found out in the wild:

Backfill jobs. Backfill is what Flickr calls asynchronous job that: alter database tables in preparation for a new feature; fix existing features; or other operation that touch a lot of accounts, photos, or groups. For example, a sharding approach means related data is spread through many different shards. To delete a user account would require visiting each shard to delete that users data. Each of those deletes would be queued to they could be done in parallel. Now lets say a bug prevented some of the user data from deleting. After the bug was fixed the user data for all the impacted user accounts would have to be scheduled to be deleted again.

Low latency funciton call router.

Scatter/gather calls in paralellel.

Defer expensive library calls.

Parellize database queries.

Job queue system for a cluster. Efficiently use all your pool of CPU power.

Sending scheduled mail merged emails.

Creating guest hosts

Put heavy code on backend instead of the web server.

Call a cron script to update topic hits and popular article hits.

Clean useless data from database because it's outdated.

Resize photos.

Run daily reports.

Update search indexes.

Speed up batch jobs by running them in parallel.

SpamAssassin spamtraps.

Queuing Implies an Event Driven State Machine Based Client Architecture

Moving to queuing has architecture implications. The client and server are nolonger connected in a direct request-response sort of way. Instead, the server continually sends events to clients. The client is event driven instead of request-response driven. Internally clients often simulates the reqest-response model even though Ajax is asynchronous. It might be better to drop the request-response illusion and just make the client an event driven state machine. An event can come from a request, or from asynchronous jobs, or events can be generated by others performing activities that a client should see. Each client has an event channel that the system puts events on for a client to consume. The client is responspible for making sense of the event in its current context and is capable of handling any event regardless of its original source.

Queuing Systems

If you are in the market for a queuing system take a look at:

Gearman - Open Source Message Queuing System

Amazon's SQS. The latencies for this service tend to be high and variable so it may not be appropriate for all tasks.

beanstalkd.

Apache ActiveMQ.

Spread Queue

Rabbit MQ

Open AMQ

The Schwartz

Starling

Simple MQ

Roll your own.

Related Articles

Flick Engineers Do it Offline by Myles Grant

Queue everything and delight everyone by Leslie Michael Orchard.

Gearman - Open Source Message Queuing System

GridGain: One Compute Grid, Many Data Grids

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Help a Scoble out. What should Robert ask in his scalability interview?

One of the cool things about Mr. Scoble is he doesn't pretend to know everything, which can be an deadly boring affliction in this field. In this case Robert is asking for help in an upcoming interview. Maybe we can help? Here's Robert's plight: I’m really freaked out. I have one of the biggest interviews of my life coming up and I’m way under qualified to host it. It’s on Thursday and it’s about Scalability and Performance of Web Services. Look at who will be on. Matt Mullenweg, founder of Automattic, the company behind WordPress (and behind this blog). Paul Bucheit, one of the founders of FriendFeed and the creator of Gmail (he’s also the guy who gave Google the “don’t be evil” admonishion). Nat Brown, CTO of iLike, which got six million users on Facebook in about 10 days. What would you ask?

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Paper: Scaling Genome Sequencing - Complete Genomics Technology Overview

Although the problem of scaling human genome sequencing is not exactly about building bigger, faster and more reliable websites it is most interesting in terms of scalability. The paper describes a new technology by the startup company Complete Genomics to sequence the full human genome for the fraction of the cost of earlier possibilities. Complete Genomics is building the world’s largest commercial human genome sequencing center to provide turnkey, outsourced complete human genome sequencing to customers worldwide. By 2010, their data center will contain approximately 60,000 processors with 30 petabytes of storage running their sequencing software on Linux clusters. Do you find this interesting and relevant to HighScalability.com?

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Scalability for Startups: How to Grow Up without Blowing Up

This is a useful post by Frank Mashraqi, Director of Business Operations & Technical Strategy for a top 50 website that delivers billions of page views per month.

Since scalability is considered a non-functional requirement, it is often overlooked in the hopes of decreasing time to market. Adding scalability down the road can decrease the time to market but only after assuming significant technical debt.

Balancing performance and scalability vs. fast iteration and cost efficiency can be a significant challenge for startups. The good news is that achieving this balance is not impossible.

Read the rest of the article here and view a presentation here.

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Paper: Scalability Design Patterns

I have introduced pattern languages in my earlier post on The Pattern Bible for Distributed Computing. Achieving highest possible scalability is a complex combination of many factors. This PLoP 2007 paper presents a pattern language that can be used to make a system highly scalable. The Scalability Pattern Language introduced by Kanwardeep Singh Ahluwalia includes patterns to:

• Introduce Scalability
• Optimize Algorithm
• Add Hardware
• Add Parallelism
  • Add Intra-Process Parallelism
  • Add Inter-Porcess Parallelism
  • Add Hybrid Parallelism
• Optimize Decentralization
• Control Shared Resources
• Automate Scalability

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Is MapReduce going mainstream?

Compares MapReduce to other parallel processing approaches and suggests new paradigm for clouds and grids

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