Explicitly set MongoDB connection parameters (#4097)

frameworks/Java/dropwizard/hello-world-mongo.yml

@@ -22,3 +22,4 @@ mongo:
   host: tfb-database
   port: 27017
   db: hello_world
+  connections: 256

frameworks/Java/dropwizard/src/main/java/com/example/helloworld/config/MongoClientFactory.java

@@ -1,14 +1,18 @@
 package com.example.helloworld.config;
 
-import com.fasterxml.jackson.annotation.JsonProperty;
-import com.mongodb.MongoClient;
-import org.hibernate.validator.constraints.NotEmpty;
-
 import javax.validation.constraints.Max;
 import javax.validation.constraints.Min;
-import java.net.UnknownHostException;
+
+import org.hibernate.validator.constraints.NotEmpty;
+
+import com.fasterxml.jackson.annotation.JsonProperty;
+import com.mongodb.MongoClient;
+import com.mongodb.MongoClientOptions;
+import com.mongodb.ServerAddress;
 
 public class MongoClientFactory {
+	private static final int MAX_DB_REQUEST_CONCURRENCY = 512;
+	
     @JsonProperty
     @NotEmpty
     private String host;
@@ -21,6 +25,11 @@ public class MongoClientFactory {
     @JsonProperty
     @NotEmpty
     private String db;
+    
+    @JsonProperty
+    @Min(1)
+    @Max(4096)
+    private int connections;
 
     public String getHost() {
         return host;
@@ -34,7 +43,15 @@ public class MongoClientFactory {
         return db;
     }
 
+    public int getConnections() {
+    	return connections;
+    }
+    
     public MongoClient build() {
-        return new MongoClient(host, port);
+    	MongoClientOptions.Builder options = MongoClientOptions.builder();
+        options.connectionsPerHost(connections);
+        options.threadsAllowedToBlockForConnectionMultiplier(
+            (int) Math.ceil((double) MAX_DB_REQUEST_CONCURRENCY / connections));
+        return new MongoClient(new ServerAddress(host, port), options.build());
     }
 }