Constraints

Before using DDS, you must familiarize yourself with the constraints of different types of DDS DB instances to help you select the instance architectures that best suit your business scenarios.

**Table 1** Constraints on cluster instances
Function	Constraints
Instance Deployment	Cluster DB instances cannot be deployed in all regions and AZs.
Database Minor Version Upgrade	The instance version must be 3.4, 4.0, or 4.2. If the instance status is abnormal or the instance is being operated, the upgrade cannot be performed. The upgrade cannot be performed if there are abnormal instance nodes.
Instance Modifications	Scaling Up Storage Space NOTE: Only shard nodes can be scaled up. dds mongos and config nodes cannot be scaled up. Storage space can only be scaled up. It cannot be scaled down. If the storage space is about to be used up, the database is set to read-only, and data cannot be modified. The maximum storage space of a single shard is 2000 GB. If the storage space exceeds 2000 GB, scale-out will fail. Adding and Reverting Instance Nodes NOTE: You can add nodes for a DB instance only when the DB instance is normal. You can roll back a node only when the node fails to be added. Changing a DB Instance Class NOTE: When the instance class is being changed, a primary/secondary switchover may occur once or twice and the database connection will be interrupted each time for up to 30s. You are advised to change the class during off-peak hours to reduce impacts and ensure that the service system of your client can reconnect to the database if the connection is interrupted.
Data Backups	Physical and snapshot backup methods are supported.
Data Restorations	Backup data can be restored to a new DB instance. Backup data can be restored to the original DB instance. Backup data can be restored to a specified point in time. Backup data can be restored to an on-premises database.
Data Security	SSL encryption can be set. NOTE: Enabling or disabling SSL will cause instances to restart. Exercise caution when performing this operation.
Data Migration	DDS provides multiple migration schemes to migrate MongoDB databases in different service scenarios. For details, see Migration Scheme Overview. Migrating data using the export and import tools Migrating data from other cloud MongoDB to DDS Migrating data from on-premises MongoDB to DDS Migrating data from ECS-hosted MongoDB to DDS Migrating data from DDS to MongoDB
Logs	Error logs, slow query logs, and audit logs are supported.
Billing	DDS allows you to pay only for what you use. There is no minimum fee requirement. For more information, see Pricing Details.
Connections	You can enable shard and config IP addresses. You can change a private IP address of a DB instance. You can change a database port. NOTE: The shard node port is 8637, and the config node port is 8636, which cannot be changed. You can apply for and modify a private domain name for database connections.
Recycle Bin	Instances in the recycle bin are retained for 7 days by default. Up to 100 instances can be moved to the recycle bin. Once the recycle bin is full, you can still delete instances, but they cannot be placed in the recycle bin.

**Table 2** Constraints on replica set instances
Function	Constraints
Instance Deployment	Replica set DB instances cannot be deployed in all regions and AZs.
Database Minor Version Upgrade	The instance version must be 3.4, 4.0, or 4.2. If the instance status is abnormal or the instance is being operated, the upgrade cannot be performed. The upgrade cannot be performed if there are abnormal instance nodes. Read replicas do not support minor version upgrade.
Instance Modifications	Scaling Up Storage Space NOTE: Storage space can only be scaled up. It cannot be scaled down. If the storage space is about to be used up, the database is set to read-only, and data cannot be modified. Adding and Reverting Instance Nodes NOTE: You can add nodes for a DB instance only when the DB instance is normal. You can roll back a node only when the node fails to be added. Deleting Instance Nodes or Read Replicas NOTE: Nodes cannot be deleted from instances that have abnormal nodes. Changing a DB Instance Class NOTE: When the instance class is being changed, a primary/secondary switchover may occur once or twice and the database connection will be interrupted each time for up to 30s. You are advised to change the class during off-peak hours to reduce impacts and ensure that the service system of your client can reconnect to the database if the connection is interrupted.
Data Backups	Physical and snapshot backup methods are supported.
Data Restorations	Backup data can be restored to a new DB instance. Backup data can be restored to the original DB instance. Backup data can be restored to a specified point in time. Backup data can be restored to an on-premises database.
Data Security	SSL encryption can be set. NOTE: Enabling or disabling SSL will cause instances to restart. Exercise caution when performing this operation.
Data Migration	DDS provides multiple migration schemes to migrate MongoDB databases in different service scenarios. For details, see Migration Scheme Overview. Migrating data using the export and import tools Migrating data from other cloud MongoDB to DDS Migrating data from on-premises MongoDB to DDS Migrating data from ECS-hosted MongoDB to DDS Migrating data from DDS to MongoDB
Logs	Error logs, slow query logs, and audit logs are supported. Read replicas do not support slow query logs and error logs.
Billing	DDS allows you to pay only for what you use. There is no minimum fee requirement. For more information, see Pricing Details.
Connections	You can configure access across CIDR blocks. You can change a private IP address of a DB instance. You can change a database port. NOTE: The default port is 8635. You can change the port after the instance is created. You can apply for and modify a private domain name for database connections.
Recycle Bin	Instances in the recycle bin are retained for 7 days by default. Up to 100 instances can be moved to the recycle bin. Once the recycle bin is full, you can still delete instances, but they cannot be placed in the recycle bin.

**Table 3** Constraints on single node instances
Function	Constraints
Instance Deployment	Single node DB instances cannot be deployed in all regions and AZs.
Database Minor Version Upgrade	Not supported.
Instance Modifications	Scaling Up Storage Space NOTE: Storage space can only be scaled up. It cannot be scaled down. If the storage space is about to be used up, the database is set to read-only, and data cannot be modified. Changing a DB Instance Class NOTE: When the instance class is being changed, the database connection will be interrupted for 5 to 10 minutes. You are advised to change the class during off-peak hours to reduce impacts and ensure that the service system of your client can reconnect to the database if the connection is interrupted.
Data Backups	Logical and snapshot backup methods are supported.
Data Restorations	Backup data can be restored to a new DB instance. Backup data can be restored to the original DB instance. Backup data can be restored to an on-premises database.
Data Security	SSL encryption can be set. NOTE: Enabling or disabling SSL will cause instances to restart. Exercise caution when performing this operation.
Data Migration	DDS provides multiple migration schemes to migrate MongoDB databases in different service scenarios. For details, see Migration Scheme Overview. Migrating data using the export and import tools Migrating data from other cloud MongoDB to DDS Migrating data from on-premises MongoDB to DDS Migrating data from ECS-hosted MongoDB to DDS Migrating data from DDS to MongoDB
Logs	Error logs, slow query logs, and audit logs are supported.
Billing	DDS allows you to pay only for what you use. There is no minimum fee requirement. For more information, see Pricing Details.
Connections	You can change a private IP address of a DB instance. You can change a database port. NOTE: The default port is 8635. You can change the port after the instance is created. You can apply for and modify a private domain name for database connections.
Recycle Bin	Instances in the recycle bin are retained for 7 days by default. Up to 100 instances can be moved to the recycle bin. Once the recycle bin is full, you can still delete instances, but they cannot be placed in the recycle bin.