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I know more or less how Push works, but why is there a Push on top (in Settings → Accounts & Passwords → Fetch New Data) then other options for iCloud, Outlook, Gmail etc?.
Is any of these related to My Photo Stream or iCloud Photo Library? Do these use Push or Fetch too?
The top push toggle allows you to override any accounts that have push set up. The data will then be fetched from the servers according to your fetch settings at the bottom.
iCloud and Outlook will be fetched with your current settings because you’ve essentially disabled push from any of your accounts (which saves battery and cell data). If you turn it on, they will both be pushed immediately (meaning your phone is constantly “listening” to the server for any updates) because that is how you set them on an individual level (the override is now off).
If you click on iCloud/Outlook you’ll see this explained when push is selected:
“If Push is not available [because you toggled it off], the Fetch schedule will be used.”
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In the comments of Nimesh’s answer you explain a flawed experiment. When you opened iCloud Drive on your phone, it fetched immediately. The way to test would be to send yourself an email from another device to either your Outlook or iCloud account and see how long it takes to get a notification,
If you have it set to fetch (check the server) every 30 minutes, it should come within 30 (not at 30 exactly, as it may be closer to the next time it checks the server).
Push, on the other hand, would be instant.
Of course the actual server changes immediately when you send an email etc but fetch vs push refers to how often it checks for those changes on your iPhone without you opening it “manually”. In practice, this is really only visible in terms of notifications (also whether the data would already be populated when you open the app).
Not all services support Push. iCloud mail and Outlook are integrated with Apple services and Apple servers support pushing the content to iOS device. Not the case with other email providers like Gmail where the only resort is to fetch automatically or manually.
The fetch section allows one to configure fetching schedule for services which doesn't support Push. It also configures the fetching schedule for services supporting Push for the case when Push is turned off by the user.
Thanks for you answers, you've clarified it very well.
Please allow me to sum this up and correct me if I'm wrong:
The Push and Fetch settings in iOS settings determine how often your device will check with service providers for updates for Mail, Calendar, Contacts, Notes, etc (with updates I don’t mean “software updates”, it’s just basically asking if there’s any new email, note, etc). The reason for their existence is for the user to be able to decide how much battery, cellular (or WI-FI) data and other system resources (software and hardware workload) will be used for these services. So if you’re lost in an airport and you’re concerned about the iPhone battery running out, you can set up these services to any of these Fetch options, or just put your iPhone into Low Power mode, which will set these to Fetch to save battery.
These settings are set up in Settings > Accounts & Passwords > Fetch new data (this is for iOS 11.3).
Here’s how these settings operate:
Fetch queries the servers as often as you set it up, every 15 minutes, 30 minutes…. Each fetch takes a toll on system resources, so the longer the interval, the less of these resources will be consumed.
Push is sort of “fetch all the time”, not sure what it does exactly, if it’s in fact querying the servers every 5 seconds or what, but it’s asking all the time if there’s any new email, contact, etc and it’s pretty instantaneous. This of course consumes more battery and system resources, as the device is querying the different servers all the time asking for news.
Fetch can be also set up as “automatically”, meaning, “query the servers automatically whenever I put the iOS on charge if it’s connected to a WI-FI network”. So if for example, you only plug in your iPhone before going to bed every day, that's when any new emails etc will be checked for, provided it's connected to WI-FI.
Fetch can also be set up as “manually”, meaning, whenever you open the app (such as the Mail app).
You can set up Push or Fetch separately for each of these services and then you can override all of them at once by turning off “Push” at the top of the screen, which means that Fetch will be used for all services.
