What order is `on_initialize` invoked across different modules in a Substrate runtime?

Question

I need to be sure that the on_initialize handler for a specific module runs before the same handler before all other modules in my runtime.

a) How do ensure this?

b) Is there some compile or runtime check I can enforce to absolutely guarantee that this will be respected?

Answer 1

The 'on_initialize' function for each Substrate runtime module is called via the Executive module, which handles all of the top-level stuff; essentially just executing blocks/extrinsics.

Every time a block is executed (execute_block), first initialize_block is called which ultimately calls the 'on_initalize' block for theAllModules type:

srml/executive/src/lib.rs

<AllModules as OnInitialize<System::BlockNumber>>::on_initialize(*block_number);

The AllModules type is a tuple of the different module identifiers in your runtime. It generated by the construct_runtime! macro and lists the modules in the order you defined them in the macro