I have an application with couple of web servers that uses MemcacheD. How can i synchronize concurrent put to the cache? The value of the entry is list. Atomic append operation could have been helpful, but unfortunately memcahe doesn't support atomic append.
Seems as though anonymous users can edit old posts w/o any authentication. This post was loaded with spam/porn links. Now it is not. /anonymous
Release It! author Michael Nygard tells a tale of two web sites, both brought low by unexpectedly huge unbounded results sets that slowed down their sites to the speed of a Christmas checkout line. I've committed this error more than a few times. During testing the results sets are often small, so you don't see problems. Or when a product is new you don't have a lot of data so everything is fine, until some magic line is crossed and you get that dreaded 2AM fix it call. My most embarrassing bug of this type caused a rather spectacular failure at a customer site as the variance in response times was out of spec and this kicked in penalty clauses. What happened was the customer had a larger network than we could even test (customers always get the good stuff). I took a lock and went to get all the data. Because the result set was so much larger in their larger system I took the lock for many more milliseconds than I should have. Unknown to me a chunk of code on the critical path also was in the lock path and all hell broke loose. I had to change the logic to process the result set in fixed size deterministic chunks, releasing locks as I went. I even had to measure CPU usage and back off after a certain amount of CPU was used. But all was well again. I then hunted down every other place I made the same mistake. And there were a few. To solve this problem in general I developed an architecture supporting scheduling work by CPU usage. A common theme in many of the profiles on this site is protecting your system from requests that can bring down the system. Mailinator has a lot resource exhaustion problems and does a good job solving them. Ebay has an interesting strategy of doing as little work as possible in the database which leads them to do joins in application space. Which is exactly the opposite of this strategy's conclusion. But I think this may be going too far. With proper indexes performing selects in the database to minimize the result sets would seem to be a win as databases are good at this sort of thing. Yah, relational databases suck at doing top 10 type of logic, so calculate that on the fly and cache it. How can you bound results sets?
This is a question asked on the ycombinator list and there are some good responses. I gave a quick response, but I particularly like neilk's knock out of the park insightful answer:
Not surprisingly opinions on SimpleDB vary from it sucks, don't take my database, to it will change the world, who needs a database anyway? From a quick survey of the blogosphere, here's where SimpleDB stands at the moment:
Amazon has announced the limited beta of Amazon SimpleDB - a simple web services interface to create and store multiple data sets, query your data easily, and return the results. Together with the Simple Storage Service (S3), Elastic Compute Cloud (EC2) and other web services Amazon offers a complete utility computing platform. SimpleDB was the missing piece of AWS - the scalable structured database. Check out my blog entry: http://innowave.blogspot.com/2007/12/amazon-simpledb-scalable-cloud-database.html I was waiting for this one :-) Geekr
On the blogs.technet.com article on microsoft.com's infrastructure: The article reads like a blatant ad for it's own products, and is light on the technical side. The juicy bits are here, so you know what the fuss is about:
- Cytrix Netscaler (= loadbalancer with various optimizations)
- W2K8 + IIS7 and antivirus software on the webservers
- 650GB/day ISS log files
- 8-9GBit/s (unknown if CDN's are included)
- Simple network filtering: stateless access lists blocking unwanted ports on the routers/switches (hence the debated "no firewalls" claim).
Update 3: InfoQ's Big Architecture Up Front - A Case of Premature Scalaculation? twines several different threads on the topic together into a fine noose. Update 2: Kevin says the biggest problems he sees with startups is they need to scale their backend (no, the other one). Update: My bad. It's hard to sell scalability so just forget it. The premise of Startups and The Problem Of Premature Scalaculation and Don’t scale: 99.999% uptime is for Wal-Mart is that you shouldn't spend precious limited resources worrying about scaling before you've first implemented the functionality that will make you successful enough to have scaling problems in the first place. It's kind of an embodied life force model of system creation. Energy is scarce so any parasites siphoning off energy must be hunted down and destroyed so the body has its best chance of survival. Is this really how it works? If I ever believed this I certainly don't believe it anymore. The world has changed, even since 2005. Thanks to many books and papers on how to scale the knowledge of scaling isn't the scarce precious resource it once was. It's no longer knowledge tightly held by a cabal of experts until Nicolas Cage flies in and pries it out of their grasping dessicated fingers. Now any journeyman computerista can do a reasonable job at designing a scalable system. Not only has knowledge dissemination improved, but so have our tools. Drastically. At one time building a scalable system up front would have required buying and configuring a truck load of servers, building out a data center, configuring a spider's web of networks, and bootstrapping an equally nasty storage network. All extremely complicated and disaster prone. Now you can use services like Amazon's EC2/S3, 3tera's grid OS, Joyent to cut significant parts of all that complexity out of the system. While most of us toil away in anonymity and scaling problems are just a fond dream, when the webosphere does find you it does so with a crush. With a little thinking ahead Blue Origin was able to handle 3.5 million requests and 758 GBs in bandwidth in a single day using S3. Did that effort prevent other features from getting implemented? I seriously doubt it. Usually doing the right thing isn't harder if you know what is the right thing to do. And what if Blue Origin wouldn't have been able to scale? Could they have recovered from the opportunity lost of grabbing the iron when it's hot and when potential customers are interested? Ask Friendster. What do you think? Has most of the risk associated with up front scalability design been squeezed out? Is premature scalation still something to be avoided? Or have times changed and does doing the simplest thing that could possibly work now include worrying about scaling up front?
People sometimes wonder why Oracle isn't mentioned on this site more. Maybe it will now as Michael Nygard reports Oracle 11g now does read/write splitting with their Active Data Guard product. Average replication latency was 1 second and it's accomplished with standard Oracle JDBC drivers. They see a 250% increase in transactions per service for read-write service. And a 110% improvement in tps for read-only service was found. You see a change in hardware architecture with the new setup. They now recommend using a primary and multiple standby servers, a single controller per server, and a single set of disks in RAID1. Previously the recommendation was to have a primary and secondary server with two controllers per server and a set of mirrored disks per controller. The changes increase performance, availability, and hardware utilization. They also have a useful looking best practices document for High Availability called Maximum Availability Architecture (MAA).
Update: Facebook pulls a Microsoft and embraces and extends by opening their platform to other social sites like Bebo. Very smart and unexpected. More info at Facebook to let other sites access platform code. This month's regular Facebook Meetup was held at Google and the topic of the day was OpenSocial. For those of you with real lives, OpenSocial "provides a common set of APIs for social applications across multiple websites." Over 200 excited people, hoping to do very exciting things, and dreaming of making an exciting pile of money, watched an OpenSocial presentation put on by a couple of appropriately knowledgeable evangelists. I could feel my social graph being more successfully monetized with each passing minute. Normally the meetings are much smaller, but Google puts on a very nice spread, so I think people may have showed up to dine :-) Or they could have showed up to learn why and how they should code to the new uber social API. By the looks of the full plates and the sounds of energetic chatter, it was likely a bit of both. The crowd seemed skeptical, yet interested. The Facebook world is somewhat self satisfied and that comfy world was being disturbed. It might get ugly I thought, but unfortunately it stayed quite civil and informative. With my bread I had hoped for a bit of circus. My take on OpenSocial: code social application once, run anywhere. Code your social app using Google's gadget model and the social API and it will run on any conforming social network container. It's kind of like a concurrency model based on mobile threads instead of the more traditional message passing model. So your friend's profile app will work just as will on Ning as Orkut. Interestingly, there's a layer of indirection the social network container has to locally interpret what things like friends are. So your friends in SalesForce could mean people you've email once and friends in Ning could mean people you've marked as friends. There's a fairly minimal API of verbs and nouns at this point, but that will undoubtedly grow. They are taking a "do the simplest thing" approach. Or they could have simply needed to get something out to compete with Facebook. Important features like a security model, authentication model, sharing model, and advanced data types are TBD. Lots of tricky things still have to be specified. How do you establish identity across services, who can share what information, how do apps deal with different terms of services, and how they deal with different social network models? OpenSocial is a group of companies so you hear a lot of things like "we'll have to meet and decide that. Joe has a lot of good ideas on how that might be done." The same sort of stuff you hear with all the complex Java standards that everyone hates. Maybe some group will Spring into action and fix some of the problems that develop. What I don't quite understand is how social networks will distinguish themselves from each other with a common API? Using the standard your app will run anywhere so why should I choose a particular social graph provider? So services will have to differentiate by adding nonstandard features which leads to a horrible complex mess of a system. They were already talking about using reflection so you could discover what capabilities a container had. Oh boy. Sounds like a hard road for developers. From a scalability POV you must still host your own applications. So that's no different from Facebook. If you get a million users overnight you have to figure out how to make them scale. On the bright side there was a properties like data store you could use to store data in. The amount of data, types, query model, transaction model, locking model, SLAs, etc seemed open, but not managing state is a big win. From a scalable development POV, I can't help but think the drive towards differentiation will require special coding for each target container and you'll have to pick just a few containers to develop for (think browser wars times 100), but we'll see.