Read Replica
To improve the read capability of the primary node, DDS provides read replicas that can be accessed independently, which is good for handling read-intensive workloads and reducing memory pressure on the primary node. You can also create one or more read replicas to process a large number of read requests and improve throughput.
Differences Between Read Replica and Secondary Node
Node |
Description |
Application Scenario |
---|---|---|
Secondary nodes |
The secondary node ensures high availability. If a primary node fails, the system automatically switches traffic to the secondary node. If the primary node is faulty, each secondary node may be elected as a new primary node to execute data write requests. |
DDS provides an HA connection address to connect to both the primary and secondary nodes and separate read and write operations. In this way, a DDS instance can handle heavy read workloads with ease. In addition, the impact of node faults on services is wakened while the performance is improved. |
Read replicas in a replica set instance |
A read replica in a replica set instance synchronizes data only from the primary node. It does not ensure high availability and will not be elected as the primary node. A read replica can be accessed through an independent connection. It will not interfere with connections between your applications and the primary and secondary nodes of a replica set instance. |
Read replicas are useful when applications need to read large amounts of data from existing instances. |
Read replicas in a cluster instance |
Read replicas in a cluster instance ensure high availability. If a read replica fails, the system automatically switches workloads to another available read replica or secondary node. A read replica in a cluster instance can be accessed through an independent connection. It will not interfere with connections between your applications and the primary and secondary nodes of a cluster instance. All read replicas can be connected using a unified address, so you can expand the read capability of your applications by adding read replicas without modifying the connection address. A read replica in a cluster instance will not be elected as the primary node. |
Read replicas are for scenarios where applications need to read large amounts of data from existing instances, read request load balancing is required, and read replica HA is required. |
Constraints
- Only replica set instances of versions 3.4, 4.0, 4.2, 4.4 and 5.0 and cluster instances of version 3.4 can have read replicas.
- Instances that only have read replicas cannot be created.
- Read replicas are used only for processing read requests and do not participate in primary/secondary node election.
- A maximum of five read replicas can be added to a replica set instance. For details, see Adding Read Replicas to a Replica Set Instance.
- Up to five read replicas can be added to each shard of a cluster DB instance.
- Data is asynchronously replicated between the primary node and the read replica, and this process will delay by a few seconds.
Functions
- You can change the number of read replicas to meet fluctuating service demands and reduce service costs.
- The specifications of a read replica are the same as those of the primary node. Data is automatically synchronized to the read replica.
- Resources on the primary node are not occupied. Adding or deleting read replicas does not affect services on the primary node.
- Read replicas can be connected independently and do not interfere with existing primary and secondary nodes. For details about read replicas in a replica set instance, see Connecting to Read Replicas Using Mongo Shell.
- You can monitor the performance of read replicas.
Billing
- After a read replica is added to the current instance, you will be billed for using the VM and disks of the read replica.
- The price of a read replica is equal to that of a single node in a replica set instance or a shard node in a cluster instance. For example, if the price of a three-node replica set instance is USD $3,000, the price of a read replica is USD $1,000.
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