I have come across many NoSQL databases and SQL databases. There are varying parameters to measure the strength and weaknesses of these databases and scalability is one of them. What is the difference between horizontally and vertically scaling these databases?
10 Answers
Scaling horizontally ===> Thousands of minions will do the work together for you.
Scaling vertically ===> One big hulk will do all the work for you.
The accepted answer is spot on the basic definition of horizontal vs vertical scaling. But unlike the common belief that horizontal scaling of databases is only possible with Cassandra, MongoDB, etc I would like to add that horizontal scaling is also very much possible with any traditional RDMS; that too without using any third party solutions.
I know of many companies, specially SaaS based companies that do this. This is done using simple application logic. You basically take a set of users and divide them over multiple DB servers. So for example, you would typically have a "meta" database/table that would store clients, DB server/connection strings, etc and a table that stores client/server mapping.
Then simply direct requests from each client to the DB server they are mapped to.
Now some may say this is akin to horizontal partitioning and not "true" horizontal scaling and they will be right in some ways. But the end result is that you have scaled your DB over multiple Db servers.
The only difference between the two approaches to horizontal scaling is that one approach (MongoDB, etc) the scaling is done by the DB software itself. In that sense you are "buying" the scaling. In the other approach (for RDBMS horizontal scaling), the scaling is built by application code/logic.
Adding lots of load balancers creates extra overhead and latency and that is the drawback for scaling out horizontally in nosql databases. It is like the question why people say RPC is not recommended since it is not robust.
I think in a real system we should use both sql and nosql databases to utilize both multicore and cloud computing capabilities of today's systems.
On the other hand, complex transactional queries has high performance if sql databases such as oracle being used. NoSql could be used for bigdata and horizontal scalability by sharding.
You have a company and there is only 1 worker but you got 1 new project at that time you hire new candidate -- this is horizontal scaling. where new candidate is new machines and project is new traffic/calls to your api's.
Where as 1 project with an IIT/NIT guy handling all request to your api/traffic. If any time more request to your api's then fire him and replacing him with a high IQ NIT/IIT guy -- this is vertical scaling.
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