CAP Theorem
1. Consistency
Weak consistency
After a W, R may or may not see it: VoIP, video chat, realtime.
Memcached
Eventual Consistency - Data is replicated asynchronously
Highly available systems with NoSQLDB: after a W, R will eventually see it: Email or DNS.
Strong consistency - transaction based systems
After W, R will see it: file systems and RDBMSes.
Google I/O 2009 - Transactions Across Datacenters.. - YouTube
2. Availability
Fail-over
Active-passive (master-slave) failover
Active-active (master-master) failover
Potential data loss
3. Replication
4. DBs
DB types:
- CP
- AP
- CA
System Properties Comparison Cassandra vs. CouchDB vs. MongoDB
SQL vs. NoSQL Databases: What's the Difference?
| Name | Type | Features | Tech |
|---|---|---|---|
| MongoDB | CPa | BSON(Binary JSON) doc, single-master arch, SQL query via Connector | C++ |
| Cassandra | cAP | wide-column, master-less arch, repair functionality, SQL-like support | Java |
| CouchDB | cAP | JSON | Erlang |