Multiple web apps

Introduction

This tutorial walks you through creating multiple web applications that will share a common web server. We’ll cover the following steps:

1. Creating a web app.
2. Creating multiple web apps and “sharing” the first web app’s service plan and Application Insights instances.
3. How to use F#‘s list comprehensions to rapidly create multiple websites.

Creating a single web app

Create a standard web app as normal:

let primaryWebApp = webApp {
    name "primarywebapp"
    sku WebApp.Sku.F1
}

Creating secondary web apps

Create a second web app, but this time link to the service plan that is part of the first web app:

let secondaryWebApp = webApp {
    name "secondarywebapp"
    link_to_service_plan primaryWebApp.ServicePlanName
    link_to_app_insights primaryWebApp.AppInsightsName
}

You can now add both web apps to the arm { } block for deployment:

let template = arm {
    location Location.NorthEurope
    add_resource primaryWebApp
    add_resource secondaryWebApp
}

Creating dedicated Service Plan and App Insights instances

Rather than “piggy back” on a “primary” web app, you can also opt to create a dedicated service plan and app insights instances and configure all web apps to use them. This is a slightly more verbose option, but you may find it clearer, and as we’ll see shortly, it can sometimes be useful to declare these instances outside of the web app:

let plan = servicePlan {
    name "theFarm"
    sku WebApp.Sku.F1
}

let ai = appInsights {
    name "insights"
}

let aWebApp = webApp {
    name "primarywebapp"
    link_to_service_plan plan
    link_to_app_insights ai
}

let anotherWebApp = webApp {
    name "secondarywebapp"
    link_to_service_plan plan
    link_to_app_insights ai
}

As you are creating the plan and AI instances yourself, you also need to remember to add them to the arm { } block!

Rapidly creating multiple web apps

F# has excellent support for working with collections of data, including creating data. Let’s assume we wanted to create four web apps, each with a name “mywebapp-{index}” e.g. “mywebapp-1” etc. We can use F#‘s list comprehensions to create four web apps quickly and easily.

let webApps : IBuilder list = [
    for i in 1 .. 4 do
        webApp {
            name ("mywebapp-" + string i)
            link_to_service_plan plan
            link_to_app_insights ai
        }
]

The key parts to note are:

1. Use of [ ], which in F# signify a list of some data.
2. Use of for .. in .. do syntax to iterate over numbers 1 to 4, assigning each value to i.
3. Creating unique names for each web app using simple string concatenation.
4. An explicit type annotation (: IBuilder list). Without getting into too much detail, this is needed because F# is somewhat stricter about implicit type conversions than other languages, particularly around list contra/covariance.

List comprehensions in F# are very powerful. You can use this approach with a specific set of names that are themselves a list as well e.g.

let planets = [ "jupiter"; "mars"; "pluto"; "venus" ]
let webApps = [
    for planet in planets do
        ...
]

Adding multiple resources to the template

Once you have created a list of web apps, you can add them all at once to the ARM builder using the add_resources keyword:

let template = arm {
    ...
    add_resources webApps
}