TL;DR
Use horizontal scaling if you can, it’s usually preferable to vertical scaling. However, if you need run large monolithic apps, then vertical scaling is probably your best bet, although you should try to cut back on app size and resource utilization ASAP.
In slightly more detail, I would propose the following pros-and-cons list of horizontal vs. vertical scaling:
| Dimension of Comparison | Horizontal Scaling | Vertical Scaling |
|---|---|---|
| Cost | less expensive (through commodity hardware components) | more expensive (specialized servers) |
| Availability of IT Resources | IT resources instantly available | IT resources normally instantly available |
| Replication and Backup | nodes that can often serve as backup for one another | other means will need to be found to replicate and backup data |
| Scaling Limitations | often automated and not limited by hardware capacity | often manual and limited by maximum hardware capacity |
| Failover | usually builtin with nodes that can easily substitute for other nodes | often a single point of failure |
| Ability to Run Large Applications | App size and utilization is usually limited by smaller size hosts | can often run large monolithic applications |
My justification for the proposal of the above pros-and-cons list is the long explanation below, not the short abstract above.
So I’ve just started reading a book called “Cloud Computing: Concepts, Technology & Architecture” (CCCTA). So far the book is quite good. It got mixed reviews on Amazon, but from what I’m reading so far, it contains useful basic info on cloud theory. One of CCCTA’s sections describes pros and cons for horizontal scaling vs. vertical scaling. Frankly, I largely disagree with the author’s pros-and-cons list. The pros and cons of horizontal and vertical scaling can be found on page 38, in a table (Table 3.1). The table contains the following data:
| Horizontal Scaling | Vertical Scaling |
|---|---|
| less expensive (through commodity hardware components) | more expensive (specialized servers) |
| IT resources instantly available | IT resources normally instantly available |
| resource replication and automated scaling | additional setup is normally needed |
| additional IT resources needed | no additional IT resources needed |
| not limited by hardware capacity | limited by maximum hardware capacity |
Some points about this table:
- I agree with the first row. Horizontal scaling scaling is generally cheaper than vertical scaling.
- I also agree with the second row. Vertical scaling can sometimes require more specialized hosts and consequently more specialized IT resources that may not be immediately available.
- Row three is where I start to disagree with the author. True, horizontal scaling does often come with resource replication and automated scaling. But the statement “additional setup is normally needed” for vertical scaling (while probably true) is largely moot. What really matters are the IT resources which may not be immediately available (see row 2). Furthermore, this row misses the point that while resource replication and automated scaling are often benefits of horizontal scaling, they aren’t benefits of vertical scaling. Replication and backups still need to be thought of separately, because you don’t have inbuilt backups in your cluster/group of nodes. Furthermore, scaling is manual, not automatic, since you always need to provision larger hosts.
- The fourth row, is IMO, incorrect. We need additional IT resources for horizontal scaling but not for vertical scaling… Huh? Don’t we need additional IT resources to setup and provision and switchover to the new larger host? Seems like the amount of labor involved for vertical scaling is at least as much as horizontal scaling.
- As for the fifth row of the table, I agree with it. The limits of scale for horizontal scaling is, in my experience, much larger than the scaling limits of vertical scaling.
If this table was correct and complete, I don’t think that anybody would ever choose vertical scaling, because based solely on the above table, vertical scaling has no advantages over horizontal scaling. But, people do choose vertical scaling, and for good reason. The reason vertical scaling is sometimes chosen is because for all the benefits that horizontal scaling offers (and IMO Table 3.1 of CCCTA is largely correct about those benefits), not all the apps we have to deal with scale out horizontally. For big, monolithic, resource hogging apps, sometimes vertical scaling is the best choice. The high business value of some of these applications can dramatically outweigh the cost of the specialized hardware. Sure, ideally, we wouldn’t want behemoth apps that are a pain to maintain, but sometimes it’s just the best course of action. For example, mission critical databases are often such apps that demand vertical scaling. Sure, you could switch those DBs over to a horizontally scalable NoSQL solution, but oftentimes you can’t. When your Oracle DB contains real-time critical data related to bookings, and a plethora of apps access that DB directly, and you need to scale up, vertically scaling that DB might just be the fastest and cheapest thing to do (when you factor in the cost of changing all applications that connect to that DB). That being said, IMO horizontal scaling is the best longterm strategy, whereas vertical scaling might be what you need in the short-term.
Outside of table, this section of CCCTA also contains another “con” of vertical scaling that I really disagree with. On page 37, it reads:
Vertical scaling is less common in cloud environments due to the downtime required while the replacement is taking place.
IMO, this statement is wrong for two reasons:
- Vertical scaling doesn’t necessarily need to require any downtime. You put your old smaller host behind an LB, then setup the new larger host, then switch traffic over to the new host at the LB level. Poof… near-zero downtime. Of course, there are exceptions to this rule, but in my experience, they are few and far between.
- Vertical scaling is less common in cloud environments not because of downtime, but because of cost and resiliency. Bigger hosts cost more money. Like, a lot more money. And, with vertical scaling, you still have a single point of failure. Moreover, you can only scale up vertically so far, before host costs become either prohibitive, or simply unavailable.
With all that said, CCCTA is still IMO a decent book so far. I just take issue with this individual section. For the reasons described above in my “long explanation”, I would revise Table 3.1 in CCCTA to be my own table of reasons given at the start of this post.