An API or an SDK? The difference should be clear, but somehow it isn’t. The two terms are used interchangeably by tech novices, senior developers, and everyone in between.

Luckily for you, today I’m going to make the difference crystal clear. So if that’s what you’re here for, join me as I explore the SDK vs API confusion in such a way that will allow you to know, by the end, which to use when.

API stands for Application Programmable Interface, and SDK stand for Software Development Kit. You probably knew that already, but I promised to be crystal clear. And in that spirit, let me add this: APIs are the gateways for applications to access other applications. They’re like your smartphone screen, which gives you access to everything on the device. SDKs, for their part, are a more complete set of tools built for a platform that can include an API, documentation, samples, and everything else that you’ll need to build something. Both an API and an SDK enable interfacing with another application and building on it.

sdk vs api

SDK Is Tooling; API Is Specification

If that was not clear enough, let’s look at it from another perspective. Recall the parts of a strong cover letter: a greeting, an opening, a hook, skills, and a close. It’s a simple yet efficient structure for powerful writing.  Now imagine a folder on your PC that has documents containing ten samples of each of the previously mentioned sections.

The former is a guideline that specifies how to write your cover letter correctly. It’s like an API. Our job application kit, the folder with dozens of documents in it, is like an SDK.

I love this analogy because it illustrates APIs and SDKs nicely while showing the strength of each in a manner that is simple to understand. But there’s no better way to demonstrate the differences between the two than by using some real-world examples. Before we get to that, though, let’s look a little more at APIs and SDKs.

Characteristics of a Good SDK

  • Easy to use and to build upon
  • Well-defined functionality organized appropriately
  • Proper documentation of how all the code works
  • Compatibility with other SDKs
  • Does not consume excessive CPU, battery, or data

Characteristics of a Good API

  • Intuitive
  • A simple, consistent, and precise naming convention for resources and endpoints
  • Minimalistic for ease of understanding
  • Clear and simple semantics
  • All relevant functionality included
sclayr pull quote sdk vs api

SDK vs API by Example

Let’s look at Stripe’s SDK and API, which are commonly used applications that process payments. Stripe’s API is quite vast and well documented, but for the sake of simplicity, consider the following “GET /v1/balance” endpoint.

curl https://api.stripe.com/v1/balance \
  -u sk_test_4eC39HqLyjWDarjtT1zdp7dc:

This endpoint makes use of the GET HTTP verb. It allows an application to retrieve the balance of a Stripe user. If we contrast this with the corresponding Python SDK implementation, we clearly see that it’s easier to use the API than the SDK, which is the whole raison d’être of the former.

import stripe
stripe.api_key = "sk_test_4eC39HqLyjWDarjtT1zdp7dc"

stripe.Balance.retrieve()

Another good example is the Facebook SDK vs its API. The SDK allows for the creation of applications in the Facebook ecosystem, while the API enables the generation and retrieval of Facebook activity. Both the SDK and the API have created a whole lot of possibilities that businesses can leverage to optimize their benefits on Facebook’s platform.

For example, the following script makes use of the Facebook Marketing API to carry out customized bidding tailored to specific budgets.

use FacebookAds\Object\AdSet;
use FacebookAds\Object\Fields\AdSetFields;
use FacebookAds\Object\Values\BillingEvents;
use FacebookAds\Object\Values\OptimizationGoals;

$adset = new AdSet(null, 'act_');
$adset->setData(array(
  AdSetFields::NAME => 'My Ad Set for oCPM',
  AdSetFields::BILLING_EVENT => BillingEvents::IMPRESSIONS,
  AdSetFields::OPTIMIZATION_GOAL => OptimizationGoals::LINK_CLICKS,
  AdSetFields::BID_AMOUNT => 150,
  AdSetFields::CAMPAIGN_ID => ,
  AdSetFields::DAILY_BUDGET => 1000,
  AdSetFields::TARGETING => array(
    'geo_locations' => array(
      'countries' => array(
        'US'
      ),
   ),
  ),
));

$adset->create(array(
  AdSet::STATUS_PARAM_NAME => AdSet::STATUS_PAUSED,
));

NB: Here at Scalyr, we have a well-documented API to allow your applications to interface directly with and control your resources on our platform.

When to Use Which

Design and code implementation choices are key in any software development cycle. That’s why, I suppose, you are here scouring the Internet for answers—and I assure you, you will get them. To be as concise as possible, we will base our comparison on the following factors: flexibility, ease of use, time, integration, and resilience. These will nail down the basics well enough to properly understand APIs and SDKs once and for all.

Flexibility

In the same way the cover letter outline allows for a certain originality, an API does the same. Its bare-bones nature means you have the leeway to play around and build highly customized functionalities that suit your use case. SDKs are lacking in this regard, especially since a new one has to be specially crafted for a given language.

Ease of Use

As illustrated by the Stripe example, SDKs are designed to ease the usage of the platform for which they are built. This is why they are niche-oriented. Stripe, for example, offers its SDK in Python, PHP, Ruby, Java, Node, Go, and . NET. This means that even teams of developers with average to low experience with the platform can quickly learn to use an extensive SDK more easily than an extensive API.

Quick Go-to-Market

SDKs allow for shorter development times as a direct consequence of their ease of use. With components like authentication and security handled to a certain extent in the SDK, it can be relatively simple to get the ball rolling faster.

SDK vs API Integration

Integrating an SDK is generally easier since the platform developer does all the dirty work for you behind the scenes. This usually means the quality of your project code gets a boost since the SDK most likely incorporates best coding practices. However, you might still want to consider an API if your application is sensitive to third-party packages and their size.

Resilience

All SDKs are APIs, but not all APIs are SDKs. Which means that platforms with an SDK are naturally more resilient simply because the developers worked on the API twice. What this means for you when you’re trying to make a choice is that the SDK is most likely more stable since more hours were put into it. You don’t have to worry as much about changes here and there that might require regular updates or fixes.

sclayr pull quote sdk vs api

Now You Know

An SDK is a grouping of APIs that allow you to do most if not all actions needed for creating your application. In addition, an SDK may include other things for developing on the platform it’s designed for. An API, on the other hand, is just a series of specifications.

Clearly there is an overlap between APIs and SDKs. However, depending on the purpose, characteristics, and use cases of your application, you may find yourself inclined toward one more than the other.

Now that you have a better sense of the differences, I’m confident that your choices will be smarter and drive your project forward in the right direction. Just keep in mind that APIs will give you finesse and allow for more customization while SDKs take a more integrated approach that can help you get going faster and save you time and cost.

This post was written by Boris Bambo. Boris is a backend engineer who’s passionate about cloud-enabled applications and loves using tech to bring ideas to life.

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