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CAP: All content tagged as CAP in NoSQL databases and polyglot persistence

Rolling With Eventual Consistency or the Pros and Cons of a Dynamo Style Key-Value Store

Great educational post by Casey Rosenthal on Basho’s blog about the radically different approach of data modelling when using non-relational storage engines or non-queryable data models.

In a previous post I wrote about the different mindset that a software engineer should have when building for a key-value database as opposed to a relational database. When working with a relational database, you describe the model first and then query the data later. With a key-value database, you focus first on what you want the result of the query to look like, and then work backward toward a model.

A different way to look at it is that the advantage of the Dynamo’s style high availability key-value store doesn’t come for free. In the world of distributed systems there’s always a trade-off and you need to carefully choose each component of the architecture to match the requirements, but also be aware of the concenssions or complexity you’ll have to accept in other parts of the system.

Original title and link: Rolling With Eventual Consistency or the Pros and Cons of a Dynamo Style Key-Value Store (NoSQL database©myNoSQL)


Microsoft SQL Server 2012 High Availability Solutions

The recent announcement of the Microsoft SQL Server 2012 release emphasized the high availability features added to this version. Here is what I could find after some digging through the documentation:

  • AlwaysOn Failover Cluster Instances: As part of the SQL Server AlwaysOn offering, AlwaysOn Failover Cluster Instances leverages Windows Server Failover Clustering (WSFC) functionality to provide local high availability through redundancy at the server-instance level—a failover cluster instance (FCI). An FCI is a single instance of SQL Server that is installed across Windows Server Failover Clustering (WSFC) nodes and, possibly, across multiple subnets. On the network, an FCI appears to be an instance of SQL Server running on a single computer, but the FCI provides failover from one WSFC node to another if the current node becomes unavailable.

    This is explained in more detail on AlwaysOn Failover Cluster Instances (SQL Server).

  • AlwaysOn Availability Groups: The AlwaysOn Availability Groups feature is a high-availability and disaster-recovery solution that provides an enterprise-level alternative to database mirroring. Introduced in SQL Server 2012, AlwaysOn Availability Groups maximizes the availability of a set of user databases for an enterprise. An availability group supports a failover environment for a discrete set of user databases, known as availability databases, that fail over together. An availability group supports a set of read-write primary databases and one to four sets of corresponding secondary databases. Optionally, secondary databases can be made available for read-only access and/or some backup operations.

    More documentation about AlwaysOn Availability groups can be found here.

  • Database mirroring: This feature will be removed in a future version of Microsoft SQL Server.

  • Log shipping: SQL Server Log shipping allows you to automatically send transaction log backups from a primary database on a primary server instance to one or more secondary databases on separate secondary server instances.

    This is the well-known master-slave setup. More details can be found here.

Also worth checking the availability of these feature per SQL Server 2012 editions:

SQL Server 2012 Hgih Availability

Original title and link: Microsoft SQL Server 2012 High Availability Solutions (NoSQL database©myNoSQL)

Networks Never Fail

A reminder to those thinking that networks never fail and automation can solve everything. Christina Ilvento, on behalf of the App Engine team:

The root cause of the outage was a combination of two factors during a scheduled network maintenance in one of our datacenters. As part of the scheduled maintenance, network capacity to and from this datacenter was reduced. This alone was expected, and was not a problem. However, this maintenance exposed a previously existing misconfiguration in the system that manages network bandwidth capacity.

Ordinarily, the bandwidth management system helps isolate and prioritize traffic. When capacity is reduced because of maintenance, network failure, or due to an excess of normal traffic, the bandwidth management system keeps things running smoothly by throttling back the rate of low priority traffic. However, as mentioned, the bandwidth management system had a latent misconfiguration which did not show up until capacity was reduced due to the scheduled maintenance. This misconfiguration under-reported the available network capacity to and from the datacenter, causing the network modeler to believe that there was less overall capacity than actually existed.

The configuration error in the bandwidth management system, when combined with an expected reduction in capacity due to the scheduled maintenance, led the system to conclude that there was insufficient bandwidth available for current traffic demand to and from this datacenter. (In reality, there was more than sufficient excess capacity, as otherwise the maintenance would not have been allowed to go forward.) Because of this combination of misconfiguration and scheduled maintenance, a number of services were automatically blocked from sending network traffic. […]

The outage occurred because two independent systems failed at the same time, which resulted in mistakes in our usual escalation procedures which significantly impacted the duration of the outage.

Original title and link: Networks Never Fail (NoSQL database©myNoSQL)

Which NoSQL Databases Are Robust to Net-Splits?

Answered on Quora:

  • Dynamo (key-value)
  • Voldemort (key-value)
  • Tokyo Cabinet (key-value)
  • KAI (key-value)
  • Cassandra (column-oriented/tabular)
  • CouchDB (document-oriented)
  • SimpleDB (document-oriented)
  • Riak (document-oriented)

A couple of clarifications to the list above:

  1. Dynamo has never been available to the public. On the other hand DynamoDB is not exactly Dynamo
  2. Tokyo Cabinet is not a distributed database so it shouldn’t be in this list
  3. CouchDB isn’t a distributed database either, but one could argue that with its peer-to-peer replication it sits right at the border. On the other hand there’s BigCouch.

Original title and link: Which NoSQL Databases Are Robust to Net-Splits? (NoSQL database©myNoSQL)

Distributed System Reliability: It's About Operations, Not Architecture or Design

Jay Kreps1:

I have come around to the view that the real core difficulty of these systems is operations, not architecture or design. Both are important but good operations can often work around the limitations of bad (or incomplete) software, but good software cannot run reliably with bad operations. […] I really think there is really only one thing to talk about with respect to reliability: continuous hours of successful production operations.

  1. Jay Kreps: works for LinkedIn where he is the technical lead for the SNA team that handles search, social graph, data infrastructure, and recommendation systems. 

Original title and link: Distributed System Reliability: It’s About Operations, Not Architecture or Design (NoSQL database©myNoSQL)


Why DynamoDB Consistent Reads Cost Twice or What’s Wrong With Amazon’s DynamoDB Pricing?

Peter Bailis has posted an interesting article about the cost structure for Amazon DynamoDB reads— consistent reads are double the price of eventually consistent reads:

  1. The cost of strong consistency to Amazon is low, if not zero. To you? 2x.
  2. If you were to run your own distributed database, you wouldn’t incur this cost (although you’d have to factor in hardware and ops costs).
  3. Offering a “consistent write” option instead would save you money and latency.
  4. If Amazon provided SLAs so users knew how well eventual consistency worked, users could make more informed decisions about their app requirements and DynamoDB. However, Amazon probably wouldn’t be able to charge so much for strong consistency.

It is not the first time I’ve heard this discussion, but it is the first time I’ve found it in a detailed form. I have no reasons to defend Amazon’s DynamoDB pricing strategy, but:

  1. Comparing the costs of operating self hosted with managed highly available distributed databases seems to me to be out of place and cannot lead to a real conclusion.
  2. While consistent writes could be a solution for always having consistent reads, it would require Amazon to reposition the DynamoDB offer from a highly available database to something else. Considering Amazon has always explained their rationale for building highly available systems I find this difficult to believe it would happen.
  3. Getting back to the consistent vs eventually consistent reads, what one needs to account for is a combination of:

    • costs for cross data center access
    • costs for maintaining the request capacity SLA
    • costs for maintaining the request latency promise
    • penalty costs for not meeting the service commitment

    I agree thought it’s almost impossible to estimate each of these and decide if they lead or not to the increased consistent read price.

Original title and link: Why DynamoDB Consistent Reads Cost Twice or What’s Wrong With Amazon’s DynamoDB Pricing? (NoSQL database©myNoSQL)

Where Does Xeround Fit In The CAP Theorem?

Itamar Haber over the Xeround blog:

Q: Is Xeround Inconsistent? Xeround employs a set of majority-based algorithms to facilitate its reading and writing of data from/to multiple, distributed nodes. […] Via the use of these algorithms we are ensured that all access to the data is consistent so inconsistency is not an issue.

Q: Is Xeround Unavailable? There is no single point of failure in Xeround and every component that the system consists of is redundant and replaceable.

Q: Is Xeround Partitioning-Intolerant? Yes, to a certain extent it is.

After reading it, I got the same impression as VoltDB’s John Hugg who commented:

It sounds like you’ve gotten this backwards. According to you, in the face of a network event, the system becomes unavailable, but remains consistent. I think you have partition tolerance, but with reduced availability.

Instead of focusing strictly on the CAP characteristics of a distributed database, one should focus on what is the required behavior for their system and look for the database solution that offers them the guarantees they need.

Original title and link: Where Does Xeround Fit In The CAP Theorem? (NoSQL database©myNoSQL)


Eventual and Strong Consistency, Sloppy and Strict Quorums, and Other Lessons and Thoughts on Distributed Systems

Anything I’d write would just steal from your time to read and think about the email Joseph Blomstedt posted to the Riak list.

Original title and link: Eventual and Strong Consistency, Sloppy and Strict Quorums, and Other Lessons and Thoughts on Distributed Systems (NoSQL database©myNoSQL)

Card Payment Sytems and the CAP Theorem

On the surface it would appear that building such a system would be easy since the card vault can be implemented in a data store (either RDBMS or noSQL store) and the data stores schema could be simple, containing just the PAN, token and perhaps some timestamp information. There are plenty of companies that have attempted to build their own card vaults and many vendors offering commercial products. However we shall see later in this article that designing a card vault it requires a distributed data store and a decision is needed on which compromises of the CAP Theorem your system is willing to accept.

Firstly a small correction to the original post: instead of “partition tolerance is not an option”, read “partition tolerance is not optional”.

One of the most frequently asked question about NoSQL databases is “how do they handle transactions. Like in a banking system”. I’ve never developed a banking system, so I don’t know how those work. But I’d bet most of those asking haven’t worked on one either. So why not asking about the solution a NoSQL database would require for the system you are actually working on.

Original title and link: Card Payment Sytems and the CAP Theorem (NoSQL database©myNoSQL)


How to Achieve the High Availability Imperative

Dr. John Busch (founder, Chairman, and CTO of Schooner):

Tightly-coupled database architectures utilizing  parallel synchronous replication exploiting commodity multi-core servers can achieve 99.999% availability with full data integrity; unlimited scaling with exceptional performance and high data consistency; and greatly simplified administration including instantaneous, automatic fail-over and on-line scaling and upgrades.

Tightly-coupled architectures and high availability used in the same phrase.

Original title and link: How to Achieve the High Availability Imperative (NoSQL database©myNoSQL)


Comments on Urban Myths About NoSQL

Dan Weinreb comments on Michael Stonebraker’s Urban Myths about SQL (PDF) :

Dr. Michael Stonebraker recently posted a presentation entitled “Urban Myths about NoSQL”. Its primary point is to defend SQL, i.e. relational, database systems against the claims of the new “NoSQL” data stores. Dr. Stonebraker is one of the original inventors of relational database technology, and has been one of the most eminent database researchers and practitioners for decades.

In fact, Michael Stonebraker bashes everything that is not his current product—this GigaOm interview is the latest example.

For now, I’m filing this away until VoltDB is sold.

Original title and link: Comments on Urban Myths About NoSQL (NoSQL database©myNoSQL)