sharedService.ts

class MySuperService {
   cache: {
      [key:string] : Observable<IMyEntity>
   } = {};

   getItem(id number){
      let promise = this.cache[id];
      if (!promise){
          promise = this._http.get<IMyEntity>('/entities');
          this.cache[id] = promise; 
      }

      return promise;
   }
}