By the end of this lesson you will be able to
- Compare a web app with an installed app on updates, offline use and storage.
- Explain how synchronisation keeps copies matching and what a conflict is.
- Describe SaaS, PaaS and IaaS in plain language with an example of each.
- Say who is responsible for what under each service model.
Many of the programs you use today do not live on your computer at all. They live in a browser tab. At work you may also hear three labels, SaaS, PaaS and IaaS, used as though everybody knows them. They are less frightening than they look: each simply tells you how much of the work the provider does for you and how much is left to you. This lesson explains the difference and what it means for choosing and trusting services.
Web apps and installed apps
An installed app is a program you download and put on your computer or phone: LibreOffice, a photo editor, a game. It runs on your device using its own processor and memory, and its files are usually on its disk. A web app runs on a server and appears in your Browser. Your device mainly shows the pages and sends your clicks and typing. Online email and online word processors are web apps.
Some web apps can be installed from the browser as a progressive web app: it gets its own icon and window, may keep some data for offline use, and is still the same web app underneath. Whether this is offered depends on the browser and the site.
| Feature | Installed app | Web app |
|---|---|---|
| How you get it | Download and install | Open an address and sign in |
| Updates | You or the system must install them | Happen on the server; you always get the latest |
| Offline | Usually works fully | Usually needs internet unless offline mode is set up |
| Where files live | On your device by default | On the provider's servers |
| Works on | Only systems it was written for | Any device with a modern browser |
| Power | Can use your whole computer | Limited by the browser and your connection |
| Risk if provider stops | Keeps working as installed | May stop working or lose features |
- Download once
- Uses your processor
- Files on your disk
- You install updates
- Open an address
- Browser only displays
- Data on provider's servers
- Updates happen for you
Read this figure as text
Two columns showing an installed app running on your device and a web app running on a server with your browser as display.
Two columns. Installed app: you download it once; it runs on your computer's own processor; your files sit on your disk; updates are installed on your computer. Web app: you open an address in the browser; the program runs on the provider's server; the browser shows pages and sends your clicks; your data is stored on the server; updates happen there automatically.
Keeping copies in step
Many services keep the same data on a server, your laptop and your phone. To keep them matching they use synchronisation: when you change something on one device, the change is sent to the server and then to your other devices. This is why a contact you add on your phone appears on your laptop a few moments later.
Trouble starts when the same item is changed on two devices while one is offline. When both come back online the service has two different versions and must decide what to do. It might keep the newest, keep both, or ask you. Keeping both produces the conflicted copy files you sometimes see, which is a sync conflict. Avoid it by letting a device finish syncing before you edit the same file elsewhere, and by using shared online documents when several people edit at once.
Read this figure as text
A sequence between phone, server and laptop showing an edit being sent to the server and then to the laptop.
Three actors: Phone, Server and Laptop. 1: you edit a note on the phone. 2: the phone sends the change to the server. 3: the server stores it as the newest version. 4: the laptop asks the server whether anything is new. 5: the server sends the changed note to the laptop. If the laptop were offline, step 4 would wait until it reconnects.
SaaS, PaaS and IaaS
Cloud providers sell their services at different levels. Picture making pizza. You could rent only a kitchen and bring everything else (IaaS), rent a kitchen with ready-prepared dough and sauce so you only add toppings and bake (PaaS), or simply order a finished pizza (SaaS). The more the provider does, the less you control and the less you have to look after.
- SaaS, Software as a Service: you use finished software through a browser or app. The provider runs everything. Examples: online email, an online word processor, a video meeting service.
- PaaS, Platform as a Service: the provider runs the servers and operating system; you supply your own application and data. Example: a service where a developer uploads a website's code and the platform runs it.
- IaaS, Infrastructure as a Service: you rent raw computing power, storage and networking, usually as virtual machines, and set up the operating system and software yourself. Example: renting a virtual server and installing your own tools on it.
Higher layers are yours in IaaS; the provider takes more in PaaS and SaaS
Read this figure as text
A layered diagram of a technology stack from application down to hardware, showing the provider's share growing from IaaS to PaaS to SaaS.
A stack of five layers from top to bottom: your data, the application, the runtime and operating system, the virtual machines and storage, and the physical hardware in the data centre. With IaaS the provider looks after the bottom two layers and you look after the other three. With PaaS the provider looks after the bottom three and you look after your application and your data. With SaaS the provider looks after everything except your own data and how you use it.
| Model | You use or manage | Provider manages | Everyday example |
|---|---|---|---|
| SaaS | Your content and settings | Everything else | Online email |
| PaaS | Your application and data | Servers, operating system, runtime | A host that runs your website's code |
| IaaS | Operating system, software, data | Hardware, network, virtualisation | A rented virtual server |
As a learner you will mostly meet SaaS. PaaS and IaaS matter when you build and run your own services, for example when a site in the earlier project outgrows a free static host. Knowing the labels lets you read a provider's pricing page and understand what you are being offered.
Common mistakes
✖ Instead of: Assuming a web app works on a plane.
✔ Do this: Check whether it has an offline mode and turn it on beforehand, or keep a local copy of what you need.
✖ Instead of: Editing the same synced file on two devices at once.
✔ Do this: Let one device finish syncing before using another, or use a shared online document.
✖ Instead of: Believing that SaaS means the provider handles security for you.
✔ Do this: Use strong passphrases, multifactor authentication and careful sharing settings yourself.
✖ Instead of: Mixing up PaaS and IaaS.
✔ Do this: Ask: do I manage the operating system? If yes, it is IaaS. If I only supply my application, it is PaaS.
If something goes wrong
- A web app shows a blank page or keeps loading.
- Refresh with F5, check other websites work, and try a private window to rule out stored data. If it is still broken, check the provider's status page.
- Changes I made offline have disappeared.
- Check the service's version history or recycle bin and look for a conflicted copy file next to the original. Reconnect and wait for syncing to finish before editing again.
- Two devices show different versions of the same file.
- Make sure both are online and signed in to the same account, and that sync is not paused. Compare the version history and keep the newest, then copy anything missing across.
- The app says my storage is full and will not sync.
- Free space by deleting what you do not need and emptying the bin, then restart sync. Consider moving big old files to a local backup.
Key terms
- SaaS
- Software you use through a browser or app while the provider runs and maintains everything behind it.
- PaaS
- A service where the provider runs the servers and system and you only supply your own application and data.
- IaaS
- A service where you rent raw computing power, storage and networking and set up the software yourself.
- Web app
- A program you use inside a web browser, so it works on any computer with a browser and an internet connection.
- sync conflict
- A sync conflict happens when the same file is changed in two places before they could sync, so the service keeps both versions.
- Sync
- Keeping the same files up to date on more than one device or place.
Practise it for real
Reading is not the same as being able to do it. Complete these on your own computer, then tick only what you have actually done. These ticks are your own honest record: nothing checks them for you.
Classify the services you use
List at least six digital services you use, such as email, a video meeting tool or a photo service. For each, write whether it is a web app, an installed app or both, whether it works offline, and whether it is SaaS, PaaS or IaaS from your point of view.
Challenge: test a sync safely
Using a notes, calendar or document service you already use, add a harmless test item such as a note called Sync test on one device. Time how long it takes to appear on another device or browser tab. Delete it afterwards. Write down what you learned about how sync behaves.
Knowledge check
6 questions. You need 80% to pass. Mistakes here are part of learning, and you can retry as often as you like.
Summary
- Installed apps run on your device; web apps run on a server and you view them in a browser.
- Sync keeps copies matching through a server; a conflict happens when two devices change the same item separately.
- SaaS is finished software, PaaS runs your application, IaaS rents raw machines.
- The more the provider manages, the less you control and the less you maintain.
- You always remain responsible for your account security and your sharing choices.
Checklist: can you do these?
Marking a lesson as read does not prove you can do it. Skills count as demonstrated when you pass the quiz (not yet) and complete the practical tasks (not yet).
Related lessons
References and further reading
Official sources for details that can change between versions.
- The NIST definition of cloud computing (NIST)
- Progressive web apps (MDN Web Docs)
- Understanding cloud services (National Cyber Security Centre (UK))