Comet has popularized asynchronous non-blocking HTTP programming, making it practically indistinguishable from reverse Ajax, also known as server push. This JavaWorld article takes a wider view of asynchronous HTTP, explaining its role in developing high-performance HTTP proxies and non-blocking HTTP clients, as well as the long-lived HTTP connections associated with Comet.
The RAD Lab (Reliable Adaptive Distributed Systems Laboratory) wants to leapfrog the Big Switch and create The Next Big Switch, skipping the cloud/utility evolutionary stage altogether. This hyper-evolutionary niche buster develops technology so advanced the cloud disperses and you can go back to building your own personal datacenters again. Where Google took years to create their datacenters, using a prefab Datacenter Operating System you might create your own in a long holiday weekend. Not St. Patrick's of course. Their vision: Enable one person to invent and run the next revolutionary IT service, operationally expressing a new business idea as a multi-million-user service over the course of a long weekend. By doing so we hope to enable an Internet "Fortune 1 million". How? By wizardry in the form of a “datacenter operating system” created from a pinch of "statistical machine learning (SML)" and a tincture of "recent insights from networking and distributed systems." But like most magics it's not so outlandish once you understand it:
Hi. I'm looking for a way to share files between EC2 nodes. Currently we are using glusterfs to do this. It has been reliable recently, but in the past it has crashed under high load and we've had trouble starting it up again. We've only been able to restart it by removing the files, restarting the cluster, and filing it up again with our files from backup. This takes ages, and will take even longer the more files we get. What worries me is that it seems to make each node a point of failure for the entire system. One node crashes and soon the entire cluster has crashed. The other problem is adding another node. It seems like you have to take down the whole thing, reconfigure to include the new node, and restart. This kind of defeats the horizontal scaling strategy. We are using 2 EC2 instances as web servers, 1 as a DB master, and 1 as a slave. GlusterFS is installed on the web server machines as well as the DB slave machine (we backup files to s3 from this machine). The files are mostly thumbnails, but also some larger images and media files. Does anyone have a good solution for sharing files between EC2 nodes? I like the ThruDB [http://trac.thrudb.org/] concept of using the local filesystem as a cache for S3, but I'm not sure if ThruDB is mature enough yet. Or maybe some kind of distributed filesystem built on top of git would work? Any ideas? Thanks! ~rvr
[Tim O'Reilly] Continuing my series of queries about how "Web 2.0" companies used databases, I asked Cal Henderson of Flickr to tell me "how the folksonomy model intersects with the traditional database. How do you manage a tag cloud?"
I am working on the design for my database and can't seem to come up with a firm schema. I am torn between normalizing the data and dealing with the overhead of joins and denormalizing it for easy sharding. The data is essentially music information per user: UserID, Artist, Album, Song. This lends itself nicely to be normalized and have separate User, Artist, Album and Song databases with a table full of INTs to tie them together. This will be in a mostly read based environment and with about 80% being searches of data by artist album or song. By the time I begin the query for artist, album or song I will already have a list of UserID's to limit the search by. The problem is that the tables can get unmanageably large pretty quickly and my plan was to shard off users once it got too big. Given this simple data relationship what are the pros and cons of normalizing the data vs denormalizing it? Should I go with 4 separate, normalized tables or one 4 column table? Perhaps it might be best to write the data in both formats at first and see what query speed is like once the tables fill up... Another potential issue would be the fact that inserts will be coming in batches of about 500 - 2000+ per user at a time which will be pretty intensive to pull off for the normalized table as there will need to be quite a few selects for each insert due to the fact that the artist, album or song may already be in the database or it may not requiring an insert. What do you all think?
Paper: Consistent Hashing and Random Trees: Distributed Caching Protocols for Relieving Hot Spots on the World Wide Web
Consistent hashing is one of those ideas that really puts the science in computer science and reminds us why all those really smart people spend years slaving over algorithms. Consistent hashing is "a scheme that provides hash table functionality in a way that the addition or removal of one slot does not significantly change the mapping of keys to slots" and was originally a way of distributing requests among a changing population of web servers. My first reaction to the idea was "wow, that's really smart" and I sadly realized I would never come up with something so elegant. I then immediately saw applications for it everywhere. And consistent hashing is used everywhere: distributed hash tables, overlay networks, P2P, IM, caching, and CDNs. Here's the abstract from the original paper and after the abstract are some links to a few very good articles with accessible explanations of consistent hashing and its applications in the real world. Abstract: We describe a family of caching protocols for distributed networks that can be used to decrease or eliminate the occurrence of hot spots in the network. Our protocols are particularly designed for use with very large networks such as the Internet, where delays caused by hot spots can be severe, and where it is not feasible for every server to have complete information about the current state of the entire network. The protocols are easy to implement using existing network protocols such as TCP/IP, and require very little overhead. The protocols work with local control, make efficient use of existing resources, and scale gracefully as the network grows. Our caching protocols are based on a special kind of hashing that we call consistent hashing. Roughly speaking, a consistent hash function is one which changes minimally as the range of the function changes. Through the development of good consistent hash functions, we are able to develop caching protocols which do not require users to have a current or even consistent view of the network. We believe that consistent hash functions may eventually prove to be useful in other applications such as distributed name servers and/or quorum systems. Other excellent resources for learning more about consistent hashing are at:
Update: Zdnet says Ozzie signals Microsoft’s surrender to the cloud. CD ROMs are to the internet as the internet is to the cloud and Microsoft aims to scratch and claw its way into this paradigm shift as well. The gloves are off. The tag line for Microsoft's new SQL Server Data Service is Your Data, Any Place, Any Time. Thems fighten' words. Microsoft is itch'n for a fight! Who will be Amazon's second? The service description: SQL Server Data Services (SSDS) are highly scalable, on-demand data storage and query processing utility services. Built on robust SQL Server database and Windows Server technologies, these services provide high availability, security and support standards-based web interfaces for easy programming and quick provisioning. Sounds like a fast uppercut aimed squarely at SimpleDB's jaw. As a developer what do you need to know?
It looks like in the near future I'll have a chance to interview the Elastra CEO. Elastra provides standard databases--MySQL, EnterpriseDB and PostgreSQL-- on top of EC2 and S3. They are selling aggressive pricing, expandable and contactable database resource usage in response to demand, and a simple management and operations interface to well known databases deployed in a cloud. Elastra could be an important option for developers looking for a more traditional cloudy database. I was wondering if you guys had any suggestions for questions you would like answered? What would you like to know about their service? What are you looking for in a cloudy database? What would stop you from adopting it or what would make you decide to adopt it? Any ideas you have would help a lot and will probably be better than anything I have.