What Is Single-Active/Dual-Active Disaster Recovery?

With the rapid development of information technologies, data and information play an increasingly important role in modern enterprises. Loss and damage of data will cause inestimable losses to enterprises. How to defend against large-scale disasters has drawn increasing attention. Currently, remote disaster recovery (DR) is the only feasible solution. The backup and restoration of key data is an important part of the routine operation and maintenance of the system.

The dual-AZ, HA instances of RDS can meet the requirements of intra-city disaster recovery. DRS provides cross-region and cross-cloud DR capabilities, including single-active DR and dual-active DR.

Currently, RDS for MySQL instances support the single-active or dual-active DR. If both sides are RDS for MySQL instances, cross-region DR can be performed.

Single-Active DR

In single-active DR mode, one active database and one standby database are deployed. When a disaster occurs, the DR database functions as the service database to ensure service continuity. DRS supports active/standby switchover. Before a switchover, services are running properly in the service database and data is synchronized to the DR database in real time. In this case, data cannot be written into the DR database. After an active/standby switchover, the DR database becomes readable and writable, services can be switched to the DR database, and data cannot be written to the service database.

Figure 1 Single-active DR

Dual-Active DR

The dual-active DR mode is used in scenarios where the two databases work in active/standby mode and share services. Dual-active DR contains two roles, active database 1 and active database 2. Before performing dual-active DR, you need to determine the RDS role in the current cloud (region). A complete dual-active DR is performed in two directions, one in the forward direction and the other in the backward direction. The two directions must be created in sequence. At the beginning, active database 1 is readable and writable, and active database 2 is read-only. The backward DR can be started only after the initial data is fully synchronized from active database 1 to active database 2 in the forward direction. In this case, both active database 1 and active database 2 are readable and writable, and incremental data is continuously synchronized to active database 2 and active database 1 in the forward and backward directions, respectively.

Figure 2 Dual-Active DR

Features and constraints:

The dual-active DR deployment poses strict requirements on the procedure. Perform the following steps to ensure that the dual-active DR task can be successfully deployed.
1. Create a DR task. After the creation is complete, two subtasks are generated, that is, the forward DR task and reverse DR task. In this case, the reverse DR task is in the configuration state.
2. When the forward DR task is in the DR state (the reverse task is displayed in the Operation column), configure and start the reverse task.
  On the Disaster Recovery Management page, select the backward DR task and click Edit in the Operation column. The Create Disaster Recovery Task page is displayed. Continue to create the backward task.
  
  You are advised to perform the verification on the active database 2 and start the backward task after the expected result is met.
  Figure 3 Forward and backward DR task