What is the difference between Cassandra and CouchDB?
I'm looking at both projects and I can't really see the difference
from Cassandra Site:
Cassandra is a highly scalable, eventually consistent, distributed, structured key-value store...Cassandra is eventually consistent. Like BigTable, Cassandra provides a ColumnFamily-based data model richer than typical key/value systems.
from CouchDB Site:
Apache CouchDB is a distributed, fault-tolerant and schema-free document-oriented database accessible via a RESTful HTTP/JSON API.
That said, I see the specific differences between each project as: access methods, written languages, etc. but to put AN EXAMPLE, when you talk about SOLR or Sphinx you know both are indexers with big differences but at the end are indexers.
Can I say here that Cassandra and CouchDB are non-relational databases that in some cases one can replace the other?
Answer
CouchDB is a document store. You put documents (JSON objects) in it and define views (indexes) over them. The objects can be arbitrarily complex with potentially deep structure. Further, they are not constrained to following some consistent schema.
Cassandra is a ragged-table key-value store. It just stores rows, each of which has a set of named columns grouped in to families with values. It sounds quite close to BigTable; BigTable doesn't require each row to have the same structure (unlike an SQL database). The values may have some structure, but this kind of store doesn't know anything about that -- they're just strings/byte sequences.
Yes, they are both non-relational databases, and there is probably a fair amount of overlap in their applicability, but they do have distinctly different data organization models. Each can probably be forced into emulating the other, but each model will map best to a different set of problems.