Ahead-of-time vs on-demand
There are two options when downloading an offline map from a web map:
- Ahead-of-time: Select a predefined area of a web map.
- On-demand: Specify a custom area of a web map.
Both options require that the web map is enabled for offline use.
The capabilities of an offline map are the same whether it is downloaded using the ahead-of-time approach or the on-demand approach.
To use this option, you request a custom geographic area of a web map as an offline map. The ArcGIS Runtime API generates and downloads an offline map which can be used without a network connection.
On-demand offline maps are flexible because a user can request any area to download, but this means that each offline map must be generated at the time it is requested. This introduces a delay before the offline map can be downloaded, which impacts the offline application's user experience. See Optimize an offline map for ways to mitigate this delay.
An on-demand offline map can only be downloaded once by the offline application that requested it. It is not hosted for later download. If another offline application needs to download the same geographic area, it must request a new offline map.
This is called the on-demand approach because the offline map is created only when needed.
To use this option, the owner of a web map must first define geographic areas of the web map to be packaged as offline maps. You can then download one of these offline maps without waiting for it to be generated.
A web map can have up to 16 offline map areas defined. The packaging of offline maps from these areas occurs on a schedule. The area and packaging schedule for each offline map are defined in the web map's item settings page by the web map's owner.
Ahead-of-time offline maps are quick to download and start using because they are generated before they are needed. They are hosted alongside the web map, ready for download, so the user experience is similar to downloading any other file or resource.
An ahead-of-time offline map remains available for download until the owner of the web map removes its offline map area definition, and it can be downloaded multiple times by different offline applications.
This is called the ahead-of-time approach because the owner of the web map plans and defines these offline maps before they are needed.
Offline maps, created ahead of time, also support update packages, which can be useful for applications that cover large areas, are updated frequently, or support many mobile workers. To learn more, see Update packages.
If your users work in well defined geographic areas which are known in advance (for example in a selection of parks, along a specific highway, or in the area covered by a utility network), then ahead-of-time offline maps are often a good solution.
If you do not know which area of a map your users will need to use offline, then on-demand offline maps are typically a better option.