Comments on: Cassandra Internals – Writing

By: Cassandra内核介绍–写操作 « a db thinker's home

Cassandra内核介绍–写操作 « a db thinker's home — Sun, 30 May 2010 04:03:46 +0000

[...] Cassandra内核介绍–写操作 by Mike Perham Translated by [...]

By: jametong

jametong — Sat, 29 May 2010 06:38:29 +0000

Oracle has use dbwr/lgwr/ckpt systm processes do similar jobs as Cassandra does. I refer it as change sync jobs to async jobs, and change random i/o to sequential i/o. Cassandra does it both in Commit log and SSTable.

By: Steve Lihn

Steve Lihn — Mon, 24 May 2010 01:55:53 +0000

Nice article. Just curious when you said “Unlike a database, Cassandra does not update data in-place on disk, nor update indices, so there’s no intensive synchronous disk operations to block the write.”, which database are you referring to? Oracle can modify data blocks in memory and only write the redo log to disk, then move on. It seems to be Cassandra is similar to what Oracle is doing in this respect, isn’t it?

By: OCTO talks ! » no:sql(eu) et NoSQL : qu’en est il?

OCTO talks ! » no:sql(eu) et NoSQL : qu’en est il? — Thu, 22 Apr 2010 22:00:16 +0000

[...] – NoSQL concerne l’utilisation de la donnée et nous rappelle qu’un point important – et je pense souvent oublié dans nos SIs traditionnels – est de comprendre comment la donnée est utilisée. Un système peut vivre avec de la donnée inconsistante. En fait, c’est déjà le cas si vous utilisez les mécanismes de réplication Master/Slave, du cache ou simplement l’utilisation de messages asynchrones. Juste se demander comment sera utiliser la donnée. Que se passera-t-il du point de vue du métier si mon opération (disons un retrait) est consistant mais que mon solde reste inconsistant pendant quelques secondes ou même quelques minutes? Ma donnée sera-t-elle accéder de manière concurrente? Y aura-t-il une forte probabilité de conflit sur cette donnée? En cas de conflit, de quelle stratégie de « read-repair » aurai-je besoin (un modèle type Vector Clock ou plus simplement l’approche timestamp de Cassandra)? Combien de temps y a t-il entre le moment ou j’écris ma donnée et le moment où elle est lue? Du coup, quelle est la probabilité de « Stale State »? S’il y a un une heure entre les deux opérations, la probabilité est faible… Ma donnée est-elle plutôt utilisée en écriture ou en lecture? Certains de ces systèmes ont été optimisés pour l’écriture. Par exemple, Cassandra réalise plus d’accès disque pour une lecture que pour une écriture. [...]

By: OCTO talks ! » no:sql(eu) and NoSQL: What’s the deal?

OCTO talks ! » no:sql(eu) and NoSQL: What’s the deal? — Thu, 22 Apr 2010 17:47:21 +0000

[...] – NoSQL is about data usages and reminds us that the most important – and I guess the forgotten part in our traditional systems – is to understand how data is used. A system can live with inconsistent data (in fact this is already the case if you are using database Master/Slave replication, caching or asynchronous approaches). This is not a big deal in most of the cases but we need to keep that in mind (I must admit that we, as architects, often forget about this concern since we have been using RDBMS and ACID transactions for a long time). What happens for the business if an operation (let’s say a withdrawal) is consistent but my balance stays inconsistent a couple of seconds or even a couple of minutes? Will my data be accessed in a highly concurrent manner and so do I have a high probability to get conflicts on that data? In case of conflicts, do I need complex conflict resolutions strategies for read-repair? more specifically, do I need a Vector Clock model or a much simpler but also efficient mechanism based on timestamps like in Cassandra? How many revisions of my objects do I need to keep? For instance, Riak stores revisions of objects that can be used for conflicts resolutions but the number of revisions you need to keep has to be correlated to the probability of having a conflict on that data. How much time do I have between the moment my data is written and the moment that same data is read? And what is the probability of being in an inconsistent state? If there is one hour between the write and the read, the probability is quite low. Is my data mainly used for reads or writes ? Some of these systems have been optimized for writes and less for reads. For instance Cassandra does many more disk accesses during a read than during a write [...]

By: Cheatsheet: 2010 04.01 ~ 04.07 - gOODiDEA.NET

Cheatsheet: 2010 04.01 ~ 04.07 - gOODiDEA.NET — Wed, 07 Apr 2010 02:20:09 +0000

[...] Cassandra Internals – Writing – Cassandra Write Operation Performance Explained [...]

By: Lenin Gali

Lenin Gali — Sat, 20 Mar 2010 09:30:29 +0000

Mike, great article. I hope you can write one about reading from cassandra and how that works. I am curious about your EC2 experience, if you can share some details on
1.what kind of performance are you getting, how many writes vs reads do you do per min?
2. have you used EBS or local disks and
3. what would you recommend from your experience to those who would like to try out cassandra on EC2?
4. Some one suggested to use Zookeeper for adding locks on top Cassandra? what are pros and cons of doing that?

By: Roger Schildmeijer

Roger Schildmeijer — Fri, 19 Mar 2010 18:34:39 +0000

Nice A4 summary, good work.

By: Mike Perham

Mike Perham — Wed, 17 Mar 2010 21:54:59 +0000

Chris, not every node has a copy of the data, just N nodes where typically N = 3. The fact that the write will eventually propagate to those three nodes makes Cassandra eventually consistent.

By: Chris R.

Chris R. — Wed, 17 Mar 2010 21:01:45 +0000

So, if each write only writes to N nodes, how do the other nodes eventually get updated so that they’re eventually consistent for reads?