Currently, TencentDB for MariaDB supports the intra-city 2-DC active/active scheme, which has the following main features:
Intra-city 2-DC deployment
2-DC writability: If your servers are deployed in different subnets of two DCs, you can connect to the database and write data to it from any server in either DC.
Automatic failover and recovery
Unique access IP of both DCs
However, the intra-city 2-DC active-active scheme alone cannot implement disaster recovery at the business system level. Actually, it is easy to switch a single system/module to an intra-city disaster recovery DC, but the complicated correlation among and configurations in enterprise-level system businesses are challenges for the 2-DC scheme.
Therefore, to build an active-active business system, the business must allow both DCs to run in real time and interwork with each other throughout all phases of design, use, management, and system upgrade, so that the business can be quickly resumed with little to no modification in case of failures. This is also the goal of designing TencentDB for MariaDB's intra-city 2-DC active-active scheme. It allows business systems in both DCs to fully read from and write to the database system over the local network while ensuring strong database consistency.
Design Standard
The active-active feature of TencentDB for MariaDB is designed based on "GB/T 20988-2007 Information Security Technology - Disaster Recovery Specifications for Information System". For a single database module:
RTO ≤ 60 seconds
RPO ≤ 5 seconds
Failover time ≤ 5 seconds
Failure detection time ≤ 30 seconds
This means that it takes about 40 seconds to complete failover after a failure occurs (including failure detection time).
Risk warning: When performing tests in a production environment, make sure that the business system has an automatic database reconnection mechanism. The business system usually has multiple modules, and each module may be associated with multiple data sources; therefore, the more complex the system, the longer the recovery time.
Applicable Regions of Condition Keys
Supported items
Instance Version:
Standard Edition: One primary and one replica (two nodes) or one primary and two replicas (three nodes);
Finance Edition: One primary and one replica (two nodes) or one primary and two replicas (three nodes);
Network requirement: VPC only
Supported regions:
Beijing (Beijing Zone 1, Beijing Zone 3)
Shanghai Finance (Finance Zone 1, Finance Zone 2)
Shenzhen Finance (Finance Zone 1, Finance Zone 2)
Pricing
Dual-AZ and single-AZ schemes are offered at the same price. For more information, see Billing Overview.
If the primary and replica AZs are the same, the single-AZ deployment scheme is used.
If the primary and replica AZs are different, the intra-city 2-DC deployment scheme is used.
Note:
The primary AZ is the zone where your primary server is located. The database should be deployed in the same VPC subnet as the primary server to reduce access delay. A replica AZ is the zone where a replica node of the database is located. For the one-primary-two-replica architecture (three nodes), two nodes will be deployed in the primary AZ. For the one-primary-one-replica architecture (two nodes), one node will be deployed in the primary AZ.
If intra-city 2-DC policy is required for the finance cloud cage solution, an intra-city 2-DC cage solution needs to be built first. For more information, contact your sales rep and architect.
Viewing details of instance AZs
Log in to the TencentDB for MariaDB console and click an instance ID or Manage in the Operation column to enter the instance details page and view the information.
Primary/replica switch
To switch the primary node from one AZ to another, click Primary-Replica Switch on the instance details page in the console. As this operation is highly risky, the IP address of the login account must be verified. The switch process may cause a momentary disconnection from the database (≤ 1 second). Make sure that your business has a database reconnection mechanism. Frequent switching may result in business system exceptions or even data exceptions.
How It Works
By integrating the highly available primary/replica architecture of TencentDB for MariaDB with virtual IP drifting of VPC AZ, simultaneous reads from and writes to two DCs can be implemented. This architecture has the following features:
Proxy modules are deployed in a hybrid manner on the frontend of each TencentDB for MariaDB database node, which are responsible for routing data requests to corresponding database nodes.
Cross-region VPC gateway is deployed on the frontend of the proxy module to support virtual IP drifting.
Taking data writes as an example: as shown above, if the business server is deployed in AZ A, the VPC gateway will forward the data request to the proxy gateway in AZ A which will then transparently forward it to the primary node. If the business server is deployed in AZ B, the VPC gateway will forward the data request to proxy gateway in AZ B which will then forward it to the primary node over Tencent Cloud's BGP network.
No matter whether it is a read or write request, the entire process is imperceptible to the business. In case of database exception, the database cluster will be processed as follows:
1. If both the primary and proxy fail, the cluster will automatically promote the optimal replica to the new primary. The system will notify the VPC to modify the association between the virtual and physical IPs. The business will only perceive that some write requests are disconnected.
2. If the primary fails but the proxy is normal, the cluster will automatically promote the optimal replica to the new primary. The proxy will block requests until primary/replica switch is completed. In this case, the business will only perceive that some requests time out.
3. If the replica fails (no matter whether the proxy fails), if read/write separation is enabled, an operation will be performed according to the read-only policy of the preconfigured read-only account (there are three types of policies).
4. If AZ A completely fails, while the VPC and database are still working in AZ B, the replica2 node will be automatically promoted to the primary node. The read/write policy of the node will be adjusted according to the strong sync policy, and the VPC IP will drift to AZ B. In this case, the cluster will try to recover the node in AZ A. If the node cannot be recovered within 30 minutes, at least one replica node will be automatically created on node B. As there is an IP drifting policy, no database configuration modification is required of the business.
5. If DC B completely fails, it is equivalent to the failure of a replica node in the TencentDB for MariaDB cluster, and the failure can be processed in the same way as described in item 3 above.
FAQs
Compared with intra-city 1-DC, will the intra-city 2-DC scheme cause a decrease in performance?
Based on the strong sync replication scheme, as the cross-DC delay is slightly larger than that between devices in the same DC, the speed of SQL response will drop by about 5% in theory.
Is it possible for a primary node to switch from the primary AZ to the replica AZ?
Yes. You can ignore this if it does not affect the use of your business. If you are concerned about the impact, you can switch back by using the primary/replica switch feature in the console during off-peak hours.
How do I know that primary/replica switch is performed in the database cluster?
Log in to the Cloud Monitor console, select Alarm Policy on the left sidebar, and click Create. On the displayed page, select TencentDB for MariaDB as the Policy Type and configure an alarm on primary/replica switch.
If part of the read or write requests are handled by the replica AZ, the network delay will cause a decrease in performance, but I need the intra-city 2-DC feature. What should I do?
You can submit a ticket indicating the instance ID, deployment scheme of your servers in the AZ, and the ratio between read and write requests. Tencent Cloud DBA can help you adjust the dual-AZ loading mechanism to minimize the number of read and write requests sustained by the replica AZ.
What should I do if I want to change from the 1-DC architecture to intra-city 2-DC architecture?
Check whether the intra-city 2-DC scheme is supported in your region. It is now available in Beijing, Shanghai Finance, and Shenzhen Finance regions. Then, submit a ticket indicating the information of the account to be adjusted, instance ID, two AZs to be used, and recommended Ops time. Tencent Cloud staff will conduct an audit. If your request is eligible, the operation can be performed; otherwise, it will be rejected.
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