Constraints

To improve the stability and security of DB instances, there are some constraints on the use of DDS. For details, see Table 1.

**Table 1** Function constraints
Operation	Constraints
Connecting to a DB instance	When connecting to a DB instance over private networks, you need to bind an EIP to the prepared ECS and ensure that the DB instance and the ECS are in the same region, AZ, and VPC subnet. By default, DDS is not accessible from ECSs that are not in the same security group. If the ECS is not in the same group, you need to add an inbound rule to enable access. The default DDS port is 8635, but this port can be modified if necessary.
Deployment	ECSs in which DB instances are deployed are not visible to you. Your applications can access the database only through an IP address and port.
Obtaining permissions of user rwuser	Only the rwuser user permissions are provided on the instance creation page. For details about the related commands, see Which Commands are Supported or Restricted by DDS?
Setting database parameters	Most database parameters in the parameter groups you created can be modified. For details, see section Editing a Parameter Group.
Migrating data	You can use command line tools, including mongoexport and mongoimport, to migrate data. For details, see section Migrating Data.
Storage engine	Currently, DDS supports the WiredTiger storage engine.
Restarting a DB instance or a node	DB instances cannot be restarted using commands. They must be restarted on the RDS console.
Viewing DDS backup files	Currently, backup files cannot be downloaded from the DDS console.