Performance != Scalability

Performance and scalability are often conflated, sometimes equated, but only somewhat related.  In web services platforms, both characteristics are business critical.  But while each exerts influence over the other, they are highly distinct and have profoundly different business impacts.

Performance & Scalability Revealed

Using a web services platform as context, performance and scalability can be defined as follows:

particlePerformance is a measure of speed from a caller’s perspective; the time it takes for the platform to accept a request, process it, and return a response.  We measure response time at the platform’s front door (i.e., excluding Internet latencies, etc.).  From a platform perspective, performance is also a measure of throughput; the number of operations that can be completed per unit time.  This is a function both of response time and concurrency.

Scalability is an observation about the trend in performance under increasing demands.  For example scalability can be characterized as the rate of response time degradation relative to increasing platform traffic.  Alternately, scalability can be viewed as the increase in throughput attained via an increase in system resources (e.g., if I double my server capacity, will I actually get twice the throughput?).

Note that this definition of scalability is irrespective of absolute performance.  For example, a slow operation may be considered scalable if a 10X increase in load results in only a 1.2X increase in response time.  Conversely, a lightning fast operation may require a resource that can only be accessed serially, thereby causing a concurrency bottleneck and thus may not be scalable.

Clearly performance and scalability are related, but are not equivalent.  In practice however, the faster an operation’s response time, the less system resources it consumes.  This means that it reduces load on other operations and is less affected by the load of others.  Both effects can positively impact throughput and thus scalability; doing more per unit system resource.


In our practical universe, scalability and performance are highly related through economics.  Per the previous point, the faster an operation’s response time, the less system resources it consumes.  Generally speaking, this translates into higher throughput by a given set of physical resources.  Thus, the higher the performance, the lower the rate of capital expenditures required to handle a given rate of traffic growth.  Therefore, scalability and performance both contribute to economic scalability; doing more per unit cost.

Put another way, both higher performance and higher scalability reduce the cost of scaling.

So Who Cares?

Performance as response time is business critical because it directly impacts user experience, without which there is no business.  Our users and our business partners care.

Performance as throughput is business critical because it directly impacts system expenditures and the ongoing total cost of ownership.  I just made the CFO happy.

Scalability is business critical for avoiding infrastructure meltdowns in the face of market success and is one of technology’s key contributions to profitability.  Clearly we all care about this one.

Finally, an early understanding of scalability characteristics is critical to aligning engineering investments with company growth.  Unlike performance tuning, scalability limitations are rarely fixed quickly since they are often a function of architecture rather than implementation.  Many resource constrained startups choose functionality over scalability for good and obvious reasons.  However, the sooner an engineering team can target some serious focus on scalability, the fewer techies will be seeing 3am from the office.


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