Added the GET /api/promos/{id} route with validations

src/main/java/com/nisum/ascend/promotions/controller/PromotionsController.java

 package com.nisum.ascend.promotions.controller;
 
 import com.nisum.ascend.promotions.dto.PromotionDto;
+import com.nisum.ascend.promotions.exception.PromotionNotFoundException;
 import com.nisum.ascend.promotions.model.Promotion;
 import com.nisum.ascend.promotions.repository.PromotionRepository;
 import com.nisum.ascend.promotions.service.PromotionService;
@@ -28,9 +29,10 @@ public class PromotionsController {
     }
 
     @GetMapping("/{id}")
-    public Mono<ResponseEntity<PromotionDto>> getPromotionById(@PathVariable String id){
-        //TODO: get promo by ID
-        return null;
+    public Mono<PromotionDto> getPromotionById(@PathVariable String id){
+        return promotionService
+                .findPromoById(id)
+                .onErrorMap(throwable -> new PromotionNotFoundException(id));
     }
 
     @PostMapping()

src/main/java/com/nisum/ascend/promotions/exception/PromotionNotFoundException.java

+package com.nisum.ascend.promotions.exception;
+
+public class PromotionNotFoundException extends RuntimeException {
+    public PromotionNotFoundException(String id) {
+        super("The promotion with the ID of " + id + " was not found.");
+    }
+}

src/main/java/com/nisum/ascend/promotions/exception/RestWebExceptionHandler.java

+package com.nisum.ascend.promotions.exception;
+
+import org.springframework.core.annotation.Order;
+import org.springframework.http.HttpStatus;
+import org.springframework.stereotype.Component;
+import org.springframework.web.server.ServerWebExchange;
+import org.springframework.web.server.WebExceptionHandler;
+import reactor.core.publisher.Mono;
+
+@Component
+@Order(-2)
+class RestWebExceptionHandler implements WebExceptionHandler {
+
+    @Override
+    public Mono<Void> handle(ServerWebExchange exchange, Throwable ex) {
+        if (ex instanceof PromotionNotFoundException) {
+            exchange.getResponse().setStatusCode(HttpStatus.NOT_FOUND);
+
+            // marks the response as complete and forbids writing to it
+            return exchange.getResponse().setComplete();
+        }
+        return Mono.error(ex);
+    }
+}
\ No newline at end of file

src/main/java/com/nisum/ascend/promotions/repository/PromotionRepository.java

@@ -5,8 +5,11 @@ import com.nisum.ascend.promotions.model.Promotion;
 import org.springframework.data.mongodb.repository.ReactiveMongoRepository;
 import org.springframework.stereotype.Repository;
 import reactor.core.publisher.Flux;
+import reactor.core.publisher.Mono;
 
 @Repository
 public interface PromotionRepository extends ReactiveMongoRepository<Promotion, String> {
     Flux<Promotion> findByProductSku(String sku);
+
+    Mono<Promotion> findByPromotionId(String promoId);
 }

src/main/java/com/nisum/ascend/promotions/service/PromotionService.java

@@ -5,6 +5,7 @@ import com.nisum.ascend.promotions.repository.PromotionRepository;
 import org.springframework.beans.factory.annotation.Autowired;
 import org.springframework.stereotype.Service;
 import reactor.core.publisher.Flux;
+import reactor.core.publisher.Mono;
 
 @Service
 public class PromotionService {
@@ -16,6 +17,11 @@ public class PromotionService {
         return promotionRepository.findAll().map(PromotionDto::generateDtoFromPromotion);
     }
 
+    public Mono<PromotionDto> findPromoById(String promoId) {
+        return promotionRepository.findByPromotionId(promoId)
+                .map(PromotionDto::generateDtoFromPromotion);
+    }
+
     public Flux<PromotionDto> findPromotionsByProductSku(String sku){
         return promotionRepository.findByProductSku(sku).map(PromotionDto::generateDtoFromPromotion);
     }