Clustering

Analysis of products and issues in database clustering. Relates subjects include:

August 26, 2012

How immediate consistency works

This post started as a minor paragraph in another one I’m drafting. But it grew. Please also see the comment thread below.

Increasingly many data management systems store data in a cluster, putting several copies of data — i.e. “replicas” — onto different nodes, for safety and reliable accessibility. (The number of copies is called the “replication factor”.) But how do they know that the different copies of the data really have the same values? It seems there are three main approaches to immediate consistency, which may be called:

I shall explain.

Two-phase commit has been around for decades. Its core idea is:

Unless a piece of the system malfunctions at exactly the wrong time, you’ll get your consistent write. And if there indeed is an unfortunate glitch — well, that’s what recovery is for.

But 2PC has a flaw: If a node is inaccessible or down, then the write is blocked, even if other parts of the system were able to accept the data safely. So the NoSQL world sometimes chooses RYW consistency, which in essence is a loose form of 2PC: Read more

July 25, 2012

Thoughts on the next releases of Oracle and Exadata

A reporter asked me to speculate about the next releases of Oracle and Exadata. He and I agreed:

My answers mixed together thoughts on what Oracle should and will emphasize (which aren’t the same thing but hopefully bear some relationship to each other ;)). They were (lightly edited):

July 19, 2012

A data distribution idea at Vertica and Clustrix

Yesterday I wrote:

Clustrix has one cool idea I haven’t heard from anybody else, which I’m calling index distribution. The idea is that each index can be distributed differently across the cluster …  i.e. on different distribution keys. Clustrix thinks that paying special attention to index distribution and movement is helpful to the performance of distributed joins.

While that’s true, I thought I’d heard something similar from Vertica; so I checked, and indeed I had. Vertica famously lets you store columns in different sort orders, in both reasonable senses:

It turns out those columns can also be distributed on different keys as well.

Related link

July 18, 2012

Clustrix 4.0 and other Clustrix stuff

It feels like time to write about Clustrix, which I last covered in detail in May, 2010, and which is releasing Clustrix 4.0 today. Clustrix and Clustrix 4.0 basics include:

The biggest Clustrix installation seems to be 20 nodes or so. Others seem to have 10+. I presume those disaster recovery customers have 6 or more nodes each. I’m not quite sure how the arithmetic on that all works; perhaps the 125ish count of nodes is a bit low.

Clustrix technical notes include: Read more

July 15, 2012

Memory-centric data management when locality matters

Ron Pressler of Parallel Universe/SpaceBase pinged me about a data grid product he was open sourcing, called Galaxy. The idea is that a distributed RAM grid will allocate data, not randomly or via consistent hashing, but rather via a locality-sensitive approach. Notes include:

The whole thing is discussed in considerable detail in a blog post and a especially in a Hacker News comment thread. There’s also an error-riddled TechCrunch article. Read more

July 15, 2012

Issues in regulatory compliance

From time to time, I hear of regulatory requirements to retain, analyze, and/or protect data in various ways. It’s hard to get a comprehensive picture of these, as they vary both by industry and jurisdiction; so I generally let such compliance issues slide. Still, perhaps I should use one post to pull together what is surely a very partial list.

Most such compliance requirements have one of two emphases: Either you need to keep your customers’ data safe against misuse, or else you’re supposed to supply information to government authorities. From a data management and analysis standpoint, the former area mainly boils down to:

The latter, however, has numerous aspects.

First, there are many purposes for the data retention and analysis, including but by no means limited to: Read more

June 19, 2012

“Enterprise-ready Hadoop”

This is part of a four-post series, covering:

The posts depend on each other in various ways.

Cloudera, Hortonworks, and MapR all claim, in effect, “Our version of Hadoop is enterprise-ready, unlike those other guys’.” I’m dubious.

That said, “enterprise-ready Hadoop” really is an important topic.

So what does it mean for something to be “enterprise-ready”, in whole or in part? Common themes in distinguishing between “enterprise-class” and other software include:

For Hadoop, as for most things, these concepts overlap in many ways. Read more

June 16, 2012

Metamarkets’ back-end technology

This is part of a three-post series:

The canonical Metamarkets batch ingest pipeline is a bit complicated.

By “get data read to be put into Druid” I mean:

That metadata is what goes into the MySQL database, which also retains data about shards that have been invalidated. (That part is needed because of the MVCC.)

By “build the data segments” I mean:

When things are being done that way, Druid may be regarded as comprising three kinds of servers: Read more

June 16, 2012

Metamarkets Druid overview

This is part of a three-post series:

My clients at Metamarkets are planning to open source part of their technology, called Druid, which is described in the Druid section of Metamarkets’ blog. The timing of when this will happen is a bit unclear; I know the target date under NDA, but it’s not set in stone. But if you care, you can probably contact the company to get involved earlier than the official unveiling.

I imagine that open-source Druid will be pretty bare-bones in its early days. Code was first checked in early in 2011, and Druid seems to have averaged around 1 full-time developer since then. What’s more, it’s not obvious that all the features I’m citing here will be open-sourced; indeed, some of the ones I’m describing probably won’t be.

In essence, Druid is a distributed analytic DBMS. Druid’s design choices are best understood when you recall that it was invented to support Metamarkets’ large-scale, RAM-speed, internet marketing/personalization SaaS (Software as a Service) offering. In particular:

Interestingly, the single-table/multi-valued choice is echoed at WibiData, which deals with similar data sets. However, WibiData’s use cases are different from Metamarkets’, and in most respects the WibiData architecture is quite different from that of Metamarkets/Druid.

Read more

June 3, 2012

Introduction to Cloudant

Cloudant is one of the few NoSQL companies with >100 paying subscription customers. For starters:

Company demographics include:

The Cloudant guys gave me some customer counts in May that weren’t much higher than those they gave me in February, and seem to have forgotten to correct the discrepancy. Oh well. The latter (probably understated) figures included ~160 paying customers, of which:

The largest Cloudant deployments seem to be in the 10s of terabytes, across a very low double digit number of servers.

Read more

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