As developers, we often find ourselves in the throes of complex scenarios that put our skills to the test. One such puzzle I encountered in my Angular journey involved dealing with multiple dispatches of the same action, triggering an effect that made HTTP requests using a service. This conundrum led me to explore the nuances of the switchMap and mergeMap operators within NGRX effects. In this post, I\’ll share my insights into this issue and how I found solace in mergeMap.

The Challenge: Dispatching the same action multiple times
#

Imagine this: you’re working on an Angular application integrated with NGRX for state management. You have an action that’s fired whenever a user interacts with your app, such as searching for data. Each time this action is dispatched, it triggers an effect that communicates with a service to make HTTP requests and fetch the required data.

Seems straightforward, right? But here’s the twist: due to certain user interactions or event triggers, you might end up invoking the same action in rapid succession. Now, NGRX’s default behavior with the switchMap operator might not play out as expected.

The Pitfall of switchMap
#

The switchMap operator is incredibly powerful when you want to switch to a new observable whenever the source observable emits. This is perfect for scenarios where you only care about the latest action, canceling ongoing operations to prioritize the newest one. However, in the scenario where you\’re dispatching the same action multiple times in quick succession, the switchMap operator can turn into a pitfall.

When you’re using switchMap and a new action comes in while an ongoing operation is in progress, the operator cancels the ongoing operation and switches to the new one. In the context of HTTP requests, this can lead to canceled requests and incomplete data fetching. If you\’re relying on each request\’s data, this behavior can be a major headache.

Embracing mergeMap for Parallel Processing
#

Enter mergeMap, the operator that thrives in parallel processing. Unlike switchMap, mergeMap doesn\’t cancel ongoing operations when a new source value arrives. It runs all inner observables concurrently and merges their results into a final output. This is precisely what we need when dealing with multiple dispatches of the same action.

By utilizing mergeMap, you ensure that each dispatched action triggers its HTTP request independently. Even if subsequent actions arrive before ongoing operations complete, mergeMap keeps them running side by side. As a result, your HTTP requests don\’t get canceled prematurely, and you obtain the complete data you\’re looking for.

The Solution: Leveraging mergeMap
#

In my journey through this challenge, I discovered that swapping out switchMap for mergeMap in my NGRX effects made all the difference. It brought stability to the data fetching process, allowing the app to handle rapid-fire dispatches of the same action gracefully.

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12

searchResults$ = createEffect(() =>  
  this.actions$.pipe(    
    ofType(SearchActions.search),    
    mergeMap((action) =>      
      this.searchService.search(action.query).pipe(        
        map((results) => SearchActions.searchSuccess({ results })),        
        catchError((error) => of(SearchActions.searchFailure({ error })))      
      )    
    )  
  ),  
  { dispatch: true }
);

Final Thoughts
#

Navigating the seas of NGRX effects in Angular can be both rewarding and challenging. The choice between switchMap and mergeMap hinges on the nature of your use case. If you find yourself in a situation where multiple dispatches of the same action wreak havoc on your HTTP requests, mergeMap might be the lighthouse guiding you to calmer waters.

The moral of the story? Don’t shy away from exploring the depths of operators like mergeMap to find the right fit for your specific scenario. By understanding their behaviors and embracing them judiciously, you\’ll steer your Angular ship through even the stormiest of coding waters. Happy coding and smooth sailing!