They can blow through enormous amounts of data.
Hypertable, an open source column-based database modeled upon BigTable, is used by local search engine Zvents to write 1 billion cells of data per day, according to a presentation by Doug Judd (PDF document), a Zvents engineer.
Meanwhile BigTable, in conjunction with its sister technology, MapReduce, processes as much as 20 petabytes of data per day.
"Definitely, the volume of data is getting so huge that people are looking at other technologies," said SpringSource's Travis, whose "VPork" technology helps NoSQL users benchmark the performance of their database alternative.
They run on clusters of cheap PC servers.
PC clusters can be easily and cheaply expanded without the complexity and cost of "sharding," which involves cutting up databases into multiple tables to run on large clusters or grids.
Google has said that one of BigTable's bigger clusters manages as much as 6 petabytes of data across thousands of servers.
"Oracle would tell you that with the right degree of hardware and the right configuration of Oracle RAC (Real Application Clusters) and other associated magic software, you can achieve the same scalability. But at what cost?" asks Javier Soltero, CTO of SpringSource.
They beat performance bottlenecks.
By sidestepping the time-consuming toil of translating Web or Java apps and data into a SQL-friendly format, NoSQL architectures perform much faster, say proponents.
"SQL is an awkward fit for procedural code, and almost all code is procedural," said Curt Monash, an independent database analyst and blogger. For data upon which users expect to do heavy, repeated manipulations, the cost of mapping data into SQL is "well worth paying ... But when your database structure is very, very simple, SQL may not seem that beneficial."
Raffaele Sena, a senior computer scientist at Adobe Systems Inc., said that when Adobe relaunched its ConnectNow Web collaboration service a year and a half ago, it decided against using a relational database for just the reason raised by Monash.
Adobe uses Java clustering software from Terracotta Inc. to manage data in Java formats, which Sena says is key to boosting ConnectNow's performance two to three times over the prior version.
"The system would have been more complex and harder to develop using a relational database," he said.
Another project, MongoDB, calls itself a "document-oriented" database because of its native storage of object-style data.
While conceding that relational databases offer an unparalleled feature set and a rock-solid reputation for data integrity, NoSQL proponents say this can be too much for their needs.
Take Adobe's ConnectNow, which, even without a database, makes three copies of users' session data while they are online -- data that is mostly deleted after logoff, said Sena.
"We didn't need a database since the best representation of the data was already in memory," he said.