Although Swift is generally known to be a client-side language, it is fast becoming the language for your server-side needs. Swift is fast, type-safe, modern and contains a vast set of libraries that reduce the effort of writing code.

Swift is now available as a server-side language on the zSeries mainframe with the Swift on z/OS Beta Program and Swift for LinuxOne offerings.


This article demonstrates how to call REST API web services via Swift. Typical REST API calls are performed via an HTTP request which respond with a JSON output. REST API calls are becoming the standard way to expose interfaces across services. IBM provides many REST services such as z/OS Connect APIs, IBM’s Weather Channel REST API, and much more.

This example will demonstrate how to retrieve the temperature of a given city and state using the Weather Channel API and Swift.

To reproduce this example, you must register at to generate a weather API key. Developer API keys are free for a limited number of API requests.

Perform a HTTP Request on a REST API

The swift language ships with many useful collections and libraries that aid your development.

Initially, what we will do is create a dictionary of cities and their states/provinces as such:

var cities : [String: String] = ["New_York": "NY", "San_Francisco": "CA", "Toronto": "Ontario"] 

Swift allows you to easily loop through a dictionary using for(key, value) construct as such:

for (city, state) in cities 

For each iteration, we will make a URL Request to the REST API endpoint using the following string:

let urlString =\(weather_key)/conditions/q/\(state)/\(city).json 

Swift using string interpolation to replace the weather_key, state, and city variables in the string above.

The following Swift code will implement the REST API call in Swift

import Foundation

let weather_key = "ENTER_YOUR_KEY_HERE"

var cities : [String: String] = ["New_York": "NY", "San_Francisco": "CA", "Toronto": "Ontario"]
for (city, state) in cities {
    let cond = NSCondition()
    let urlString = "\(weather_key)/conditions/q/\(state)/\(city).json"
    let url = URL(string: urlString)!
    let getRequest = URLRequest(url: url)
    let session = URLSession(configuration: .default, delegate: nil, delegateQueue: nil)
    let task = session.dataTask(with: getRequest) { data, response, error in
            if let data = data {                                                                                                                                                                                            if let response = response as? HTTPURLResponse , 200...299 ~= response.statusCode {
                    let json1 = try? JSONSerialization.jsonObject(with: data, options: .allowFragments) as! [String:Any]
                    if let json = json1 {
                        let obs = json["current_observation"] as! [String:Any]
                        let temperature = obs["temperature_string"]!
                        print("The temperoature at \(city) is \(temperature)")
                } else {
                    print("Error \(response)");

The JSONSerialization.jsonObject parses the data into dictionary and then we access the temperature_string in the dictionary and display it to the console.

To compile and run use: swiftc rest.swift && ./rest

You should receive a response as follows:

The temperature at Toronto is 19.8 F (-6.8 C) 
The temperature at New_York is 30.7 F (-0.7 C) 
The temperature at San_Francisco is 30.7 F (-0.7 C) 


Swift on zOS:
IBM Toolkit for Linux on z
Weather API:

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