MongoDB vs Cassandra

Question

I am evaluating what might be the best migration option   Currently  I am on a sharded MySQL  horizontal partition   with most of my data stored in JSON blobs  I do not have any complex SQL queries  already migrated away after since I partitioned my db    Right now  it seems like both MongoDB and Cassandra would be likely options  My situation    Lots of reads in every query  less regular writes Not worried about  massive  scalability More concerned about simple setup  maintenance and code Minimize hardware server cost

User · Answer

I haven t used Cassandra  but I have used MongoDB and think it s awesome   If you re after simple setup  this is it  You simply untar MongoDB and run the mongod daemon and that s it     it s running   Obviously that s only a starter  but to get you started it s easy

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I saw a presentation on mongodb yesterday  I can definitely say that setup was  simple   as simple as unpacking it and firing it up  Done   I believe that both mongodb and cassandra will run on virtually any regular linux hardware so you should not find to much barrier in that area    I think in this case  at the end of the day  it will come down to which do you personally feel more comfortable with and which has a toolset that you prefer  As far as the presentation on mongodb  the presenter indicated that the toolset for mongodb was pretty light and that there werent many  they said any really  tools similar to whats available for MySQL  This was of course their experience so YMMV  One thing that I did like about mongodb was that there seemed to be lots of language support for it  Python  and  NET being the two that I primarily use    The list of sites using mongodb is pretty impressive  and I know that twitter just switched to using cassandra

User · Answer

Lots of reads in every query  fewer regular writes  Both databases perform well on reads where the hot data set fits in memory   Both also emphasize join-less data models  and encourage denormalization instead   and both provide indexes on documents or rows  although MongoDB s indexes are currently more flexible   Cassandra s storage engine provides constant-time writes no matter how big your data set grows  Writes are more problematic in MongoDB  partly because of the b-tree based storage engine  but more because of the multi-granularity locking it does   For analytics  MongoDB provides a custom map reduce implementation  Cassandra provides native Hadoop support  including for Hive  a SQL data warehouse built on Hadoop map reduce  and Pig  a Hadoop-specific analysis language that many think is a better fit for map reduce workloads than SQL    Cassandra also supports use of Spark   Not worried about  massive  scalability  If you re looking at a single server  MongoDB is probably a better fit  For those more concerned about scaling  Cassandra s no-single-point-of-failure architecture will be easier to set up and more reliable   MongoDB s global write lock tends to become more painful  too   Cassandra also gives a lot more control over how your replication works  including support for multiple data centers   More concerned about simple setup  maintenance and code  Both are trivial to set up  with reasonable out-of-the-box defaults for a single server  Cassandra is simpler to set up in a multi-server configuration since there are no special-role nodes to worry about   If you re presently using JSON blobs  MongoDB is an insanely good match for your use case  given that it uses BSON to store the data  You ll be able to have richer and more queryable data than you would in your present database  This would be the most significant win for Mongo

User · Answer

I ve used MongoDB extensively  for the past 6 months   building a hierarchical data management system  and I can vouch for both the ease of setup  install it  run it  use it   and the speed  As long as you think about indexes carefully  it can absolutely scream along  speed-wise   I gather that Cassandra  due to its use with large-scale projects like Twitter  has better scaling functionality  although the MongoDB team is working on parity there  I should point out that I ve not used Cassandra beyond the trial-run stage  so I can t speak for the detail   The real swinger for me  when we were assessing NoSQL databases  was the querying - Cassandra is basically just a giant key value store  and querying is a bit fiddly  at least compared to MongoDB   so for performance you d have to duplicate quite a lot of data as a sort of manual index  MongoDB  on the other hand  uses a  query by example  model   For example  say you ve got a Collection  MongoDB parlance for the equivalent to a RDMS table  containing Users  MongoDB stores records as Documents  which are basically binary JSON objects  e g        FirstName   John      LastName   Smith      Email   john smith com      Groups    Admin    User    SuperUser       If you wanted to find all of the users called Smith who have Admin rights  you d just create a new document  at the admin console using Javascript  or in production using the language of your choice         LastName   Smith      Groups   Admin         and then run the query  That s it  There are added operators for comparisons  RegEx filtering etc  but it s all pretty simple  and the Wiki-based documentation is pretty good

User · Answer

I m probably going to be an odd man out  but I think you need to stay with MySQL  You haven t described a real problem you need to solve  and MySQL InnoDB is an excellent storage back-end even for blob json data    There is a common trick among Web engineers to try to use more NoSQL as soon as realization comes that not all features of an RDBMS are used  This alone is not a good reason  since most often NoSQL databases have rather poor data engines  what MySQL calls a storage engine     Now  if you re not of that kind  then please specify what is missing in MySQL and you re looking for in a different database  like  auto-sharding  automatic failover  multi-master replication  a weaker data consistency guarantee in cluster paying off in higher write throughput  etc

User · Answer

Why choose between a traditional database and a NoSQL data store  Use both  The problem with NoSQL solutions  beyond the initial learning curve  is the lack of transactions -- you do all updates to MySQL and have MySQL populate a NoSQL data store for reads -- you then benefit from each technology s strengths  This does add more complexity  but you already have the MySQL side -- just add MongoDB  Cassandra  etc to the mix    NoSQL datastores generally scale way better than a traditional DB for the same otherwise specs -- there is a reason why Facebook  Twitter  Google  and most start-ups are using NoSQL solutions  It s not just geeks getting high on new tech

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