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A month ago I left Rackspace to start Riptano , a Cassandra support and services company. I was in the unusal position of being a technical person looking for a business-savvy co-founder. For whatever reason, the converse seems a lot more common . Maybe technical people tend to sterotype softer skills as being easy. But despite some examples to the contrary (notably for me, Josh Coates at Mozy ), I found that starting a company is too hard for just one person . Unfortunately, all of my fairly slim portfolio of business guys I'd like to co-found with were unavailable. So progress was slow, until Matt Pfeil heard that I was leaving Rackspace and drove to San Antonio from Austin to talk me out of it. Not only was he not successful in talking me out of leaving, but he ended up co-founding Riptano. And here we are, with a Riptano mini-faq. Isn't Cassandra mostly just a web 2.0 thing for ex-mysql shops? Although most of the early adopters fit this stereotype, we...

Cassandra has seen some impressive adoption success over the past months, leading some to conclude that Cassandra is the frontrunner in the highly scalable databases space (a subset of the hot NoSQL category ). Among all the attention, some misunderstandings have been propagated, which I'd like to clear up. Fiction : "Cassandra relies on high-speed fiber between datacenters" and can't reliably replicate between datacenters with more than a few ms of latency between them. Fact : Cassandra's multi-datacenter replication is one of its earliest features and is by far the most battle-tested in the NoSQL space. Facebook had Cassandra deployed on east and west coast datacenters since before open sourcing it. SimpleGeo's Cassandra cluster spans 3 EC2 availability zones , and Digg is also deployed on both coasts. Claims that this can't possibly work are an excellent sign that you're reading an article by someone who doesn't know what he's ta...

Cassandra is participating in the Google Summer of Code, which opened for proposal submission today . Cassandra is part of the Apache Software Foundation, which has its own page of guidelines up for students and mentors. We have a good mix of project ideas involving both core and non-core areas, from straightforward code bashing to some pretty tricky stuff, depending on your appetite. Core tickets aren't necessarily harder than non-core, but they will require reading and understanding more existing code. Non-core Create a web ui for cassandra : we have a (fairly minimal) command line interface, but a web gui is more user-friendly. There is the beginnings of such a beast in the Cassandra source tree at contrib/cassandra_browser [pretty ugly Python code] and a gtk-based one at http://github.com/driftx/chiton [also Python, less ugly]. First-class commandline interface : if you prefer to kick things old-school, improving the cli itself would also be welcome. Create a Cassandr...

There's been a lot of new articles about Cassandra deployments in the past month, enough that I thought it would be useful to summarize in a post. Ryan King explained in an interview with Alex Popescu why Twitter is moving to Cassandra for tweet storage, and why they selected Cassandra over the alternatives. My experience is that the more someone understands large systems and the problems you can run into with them from an operational standpoint, the more likely they are to choose Cassandra when doing this kind of evaluation. Ryan's list of criteria is worth checking out. Digg followed up their earlier announcement that they had taken part of their site live on Cassandra with another saying that they've now "reimplemented most of Digg's functionality using Cassandra as our primary datastore." Digg engineer Ian Eure also gave some more details on Digg's cassandra data model in a Hacker News thread. Om Malik quoted extensively from the Digg...

Several of the reports of the recently-concluded NoSQL Live event mentioned that I took a contrarian position on the "NoSQL in the Cloud" panel, arguing that traditional, bare metal servers usually make more sense. Here's why. There are two reasons to use cloud infrastructure (and by cloud I mean here "commodity VMs such as those provided by Rackspace Cloud Servers or Amazon EC2): You only need a fraction of the capacity of a single machine Your demand is highly elastic; you want to be able to quickly spin up many new instances, then drop them when you are done Most people looking at NoSQL solutions are doing it because their data is larger than a traditional solution can handle, or will be, so (1) is not a very strong motivation. But what about (2)? At first glance, cloud is a great fit for adding capacity to a database cluster painlessly. But there's an important difference between load like web traffic that bounces up and down frequently,...

Handling deletes in a distributed, eventually consistent system is a little tricky, as demonstrated by the fairly frequent recurrence of the question, " Why doesn't disk usage immediately decrease when I remove data in Cassandra ?" As background, recall that a Cassandra cluster defines a ReplicationFactor that determines how many nodes each key and associated columns are written to. In Cassandra (as in Dynamo ), the client controls how many replicas to block for on writes, which includes deletions. In particular, the client may (and typically will) specify a ConsistencyLevel of less than the cluster's ReplicationFactor, that is, the coordinating server node should report the write successful even if some replicas are down or otherwise not responsive to the write. (Thus, the "eventual" in eventual consistency: if a client reads from a replica that did not get the update with a low enough ConsistencyLevel, it will potentially see old data. Cassandra u...

Apache Cassandra 0.5.0 was released over the weekend, four months after 0.4. ( Upgrade notes ; full changelog .) We're excited about releasing 0.5 because it makes life even better for people using Cassandra as their primary data source -- as opposed to a replica, possibly denormalized, of data that exists somewhere else. The Cassandra distributed database has always had a commitlog to provide durable writes, and in 0.4 we added an option to waiting for commitlog sync before acknowledging writes, for cases where even a few seconds of potential data loss was not an option. But what if a node goes down temporarily? 0.5 adds proactive repair, what Dynamo calls "anti-entropy," to synchronize any updates Hinted Handoff or read repair didn't catch across all replicas for a given piece of data. 0.5 also adds load balancing and significantly improves bootstrap (adding nodes to a running cluster). We've also been busy adding documentation on operations in product...

I want to write about Cassandra performance tuning, but first I need to cover some basics: how to use vmstat, iostat, and top to understand what part of your system is the bottleneck -- not just for Cassandra but for any system. vmstat You will typically run vmstat with "vmstat sampling-period", e.g., "vmstat 5." The output looks like this: procs -----------memory---------- ---swap-- -----io---- -system-- ----cpu---- r b swpd free buff cache si so bi bo in cs us sy id wa 20 0 195540 32772 6952 576752 0 0 11 12 38 43 1 0 99 0 22 2 195536 35988 6680 575132 6 0 2952 14 959 16375 72 21 4 3 The first line is your total system average since boot; typically this will not be very useful, since you are interested in what is causing problems NOW. Then you will get one line per sample period; most of the output is self explanatory. The reason to start with vmstat is the "swap" section: si and s...