HBase and Consistency in CAP

Showing 1-10 of 10 messages
HBase and Consistency in CAP Mohit Anchlia 12/2/11 11:57 AM
Why is HBase consisdered high in consistency and that it gives up
parition tolerance? My understanding is that failure of one data node
still doesn't impact client as they would re-adjust the list of
available data nodes.
Re: HBase and Consistency in CAP Jean-Daniel Cryans 12/2/11 11:59 AM
No, data is only served by one region server (even if it resides on
multiple data nodes). If it dies, clients need to wait for the log
replay and region reassignment.

J-D

Re: HBase and Consistency in CAP Mohit Anchlia 12/2/11 12:01 PM
Where can I read more on this specific subject?

Based on your answer I have more questions, but I want to read more
specific information about how it works and why it's designed that
way.

Re: HBase and Consistency in CAP Jean-Daniel Cryans 12/2/11 12:03 PM
Get the HBase book:
http://www.amazon.com/HBase-Definitive-Guide-Lars-George/dp/1449396100

And/Or read the Bigtable paper.

J-D

Re: HBase and Consistency in CAP Ian Varley 12/2/11 12:15 PM
Mohit,

Yeah, those are great places to go and learn.

To fill in a bit more on this topic: "partition-tolerance" usually refers to the idea that you could have a complete disconnection between N sets of machines in your data center, but still be taking writes and serving reads from all the servers. Some "NoSQL" databases can do this (to a degree), but HBase cannot; the master and ZK quorum must be accessible from any machine that's up and running the cluster.

Individual machines can go down, as J-D said, and the master will reassign those regions to another region server. So, imagine you had a network switch fail that disconnected 10 machines in a 20-machine cluster; you wouldn't have 2 baby 10-machine clusters, like you might with some other software; you'd just have 10 machines "down" (and probably a significant interruption while the master replays logs on the remaining 10). That would also require that the underlying HDFS cluster (assuming it's on the same machines) was keeping replicas of the blocks on different racks (which it does by default), otherwise there's no hope.

HBase makes this trade-off intentionally, because in real-world scenarios, there aren't too many cases where a true network partition would be survived by the rest of your stack, either (e.g. imagine a case where application servers can't access a relational database server because of a partition; you're just down). The focus of HBase fault tolerance is recovering from isolated machine failures, not the collapse of your infrastructure.

Ian

Re: HBase and Consistency in CAP Mohit Anchlia 12/2/11 12:54 PM
Thanks for the overview. It's helpful. Can you also help me understand
why 2 region servers for the same row keys can't be running on the
nodes where blocks are being replicated? I am assuming all the
logs/HFiles etc are already being replicated so if one region server
fails other region server is still taking reads/writes.
Re: HBase and Consistency in CAP Ian Varley 12/2/11 1:42 PM
The simple answer is that HBase isn't architected such that 2 region servers can simultaneously host the same region. In addition to being much simpler from an architecture point of view, that also allows for user-facing features that would be difficult or impossible to achieve otherwise: single-row put atomicity, atomic check-and-set operations, atomic increment operations, etc.--things that are only possible if you know for sure that exactly one machine is in control of the row.

Ian

On Dec 2, 2011, at 2:54 PM, Mohit Anchlia wrote:

Thanks for the overview. It's helpful. Can you also help me understand
why 2 region servers for the same row keys can't be running on the
nodes where blocks are being replicated? I am assuming all the
logs/HFiles etc are already being replicated so if one region server
fails other region server is still taking reads/writes.

On Fri, Dec 2, 2011 at 12:15 PM, Ian Varley <iva...@salesforce.com<mailto:iva...@salesforce.com>> wrote:
Mohit,

Yeah, those are great places to go and learn.

To fill in a bit more on this topic: "partition-tolerance" usually refers to the idea that you could have a complete disconnection between N sets of machines in your data center, but still be taking writes and serving reads from all the servers. Some "NoSQL" databases can do this (to a degree), but HBase cannot; the master and ZK quorum must be accessible from any machine that's up and running the cluster.

Individual machines can go down, as J-D said, and the master will reassign those regions to another region server. So, imagine you had a network switch fail that disconnected 10 machines in a 20-machine cluster; you wouldn't have 2 baby 10-machine clusters, like you might with some other software; you'd just have 10 machines "down" (and probably a significant interruption while the master replays logs on the remaining 10). That would also require that the underlying HDFS cluster (assuming it's on the same machines) was keeping replicas of the blocks on different racks (which it does by default), otherwise there's no hope.

HBase makes this trade-off intentionally, because in real-world scenarios, there aren't too many cases where a true network partition would be survived by the rest of your stack, either (e.g. imagine a case where application servers can't access a relational database server because of a partition; you're just down). The focus of HBase fault tolerance is recovering from isolated machine failures, not the collapse of your infrastructure.

Ian


On Dec 2, 2011, at 2:03 PM, Jean-Daniel Cryans wrote:

Get the HBase book:
http://www.amazon.com/HBase-Definitive-Guide-Lars-George/dp/1449396100

And/Or read the Bigtable paper.

J-D

On Fri, Dec 2, 2011 at 12:01 PM, Mohit Anchlia <mohita...@gmail.com<mailto:mohita...@gmail.com>> wrote:
Where can I read more on this specific subject?

Based on your answer I have more questions, but I want to read more
specific information about how it works and why it's designed that
way.

On Fri, Dec 2, 2011 at 11:59 AM, Jean-Daniel Cryans <jdcr...@apache.org<mailto:jdcryans@apache.org>> wrote:
No, data is only served by one region server (even if it resides on
multiple data nodes). If it dies, clients need to wait for the log
replay and region reassignment.

J-D

On Fri, Dec 2, 2011 at 11:57 AM, Mohit Anchlia <mohita...@gmail.com<mailto:mohita...@gmail.com>> wrote:
Why is HBase consisdered high in consistency and that it gives up
parition tolerance? My understanding is that failure of one data node
still doesn't impact client as they would re-adjust the list of
available data nodes.


Re: HBase and Consistency in CAP Mohit Anchlia 12/2/11 1:48 PM
Thanks. I am having just bit of conflict in understanding how is
random node failure different than network partition? In both cases
there is an impact clearly visible to the user (time it takes to
failover and replay logs)?
Re: HBase and Consistency in CAP Andrew Purtell 12/3/11 12:32 AM
> From: Mohit Anchlia <mohita...@gmail.com>

> I am having just bit of conflict in understanding how is
> random node failure different than network partition? In both cases
> there is an impact clearly visible to the user (time it takes to
> failover and replay logs)?


I think you are conflating things a bit.
Partition tolerance in CAP, shorthanded, is the ability of a system to survive message loss (due to server failure, network problem, etc.). BigTable/HBase does this of course. A server failure or message loss does not toast the database.

Availability is the dimension of "CAP" that you are pondering here. ("time it takes to failover and replay logs") Recovery from message loss / server failure includes the time it takes to fail over and replay logs.

BigTable does trade some availability to achieve a stronger level of consistency than would be possible otherwise. The Google paper includes some discussion of this design rationale.


Best regards,

    - Andy


>________________________________
> From: Mohit Anchlia <mohita...@gmail.com>
>To: us...@hbase.apache.org
>Sent: Saturday, December 3, 2011 5:48 AM
>Subject: Re: HBase and Consistency in CAP

Re: HBase and Consistency in CAP Todd Lipcon 12/3/11 12:49 AM
You may also be interested in this article by my colleague Henry:
http://www.cloudera.com/blog/2010/04/cap-confusion-problems-with-partition-tolerance/

The short summary of the article is that CAP isn't "C, A, or P, choose
two," but rather "When P happens, choose A or C."

Partitions, like death and taxes, are unavoidable -- think of machine
death as just a partition of that machine out into the networking
equivalent of the afterlife. So it's up to the system designer to
decide if, when that happens, we give up availability or give up
consistency.

In HBase's case we choose consistency, so we have to give up some availability.

-Todd

--
Todd Lipcon
Software Engineer, Cloudera