How to implement an in memory store for extrinsic scope variable?

[xlc]

I need a place to store some values that are global accessible by multiple modules. The lifetime of this store does not need to exceeds the execution of extrinsic. i.e. it get cleared between each time an extrinsic is applied.

Basically I want to setup a Map<Vec<u8>, Vec<u8>> before this line and make it accessible during the extrinsic execution. May be some helper methods/macros to ensure type safety.

substrate/srml/executive/src/lib.rs

Line 186 in c4ca27f

let r = f.dispatch(s.into());

I can't find a way to implement it without some kind of global variables. I am not sure if lazy_static is allowed in the runtime. The only other solution I can think of is add new method in Externalities

Related comments:
#1924 (comment)
#1815 (comment)

[gavofyork]

Any data that needs to persist outside of apply_extrinsic (a single dispatchable Call), must be stored in a storage item.