- Release Notes and Announcements
- User Guide
- Product Introduction
- Purchase Guide
- Getting Started
- Operation Guide
- Access Management
- Instance Management
- Node Management
- Version Upgrade
- Monitoring
- Backup and Rollback
- Data Security
- SSL Authentication
- Database Management
- Multi-AZ Deployment
- Public Network Access
- Disaster Recovery/Read-Only Instances
- Parameter Configuration
- Recycle Bin
- Task Management
- Performance Optimization
- Data Migration Guide
- Best Practice
- Optimizing Indexes to Break Through Read/Write Performance Bottlenecks
- Troubleshooting Mongos Load Imbalance in Sharded Cluster
- Troubleshooting Excessive Slow Queries
- Fixing High CPU Utilization in MongoDB Instance Based on DBbrain
- Troubleshooting Excessive Connections
- Troubleshooting High Request Latency Based on Slow Log
- Troubleshooting High CPU Utilization
- Considerations for Using Shard Clusters
- Sample of Reading and Writing Data in MongoDB Instance
- Export and Import
- Troubleshooting MongoDB Connection Failures
- Performance Fine-Tuning
- Ops and Development Guide
- Troubleshooting
- API Documentation
- History
- Introduction
- API Category
- Making API Requests
- Instance APIs
- RenameInstance
- OfflineIsolatedDBInstance
- ModifyDBInstanceSpec
- IsolateDBInstance
- DescribeSpecInfo
- DescribeDBInstances
- DescribeClientConnections
- CreateDBInstanceHour
- AssignProject
- RenewDBInstances
- DescribeSlowLogs
- DescribeSlowLogPatterns
- InquirePriceRenewDBInstances
- InquirePriceModifyDBInstanceSpec
- InquirePriceCreateDBInstances
- DescribeDBInstanceDeal
- DescribeSecurityGroup
- DescribeInstanceParams
- ModifyDBInstanceSecurityGroup
- ModifyDBInstanceNetworkAddress
- Backup APIs
- Account APIs
- Task APIs
- Other APIs
- Data Types
- Error Codes
- Instance Connection
- FAQs
- Service Agreement
- Glossary
- Contact Us
- Release Notes and Announcements
- User Guide
- Product Introduction
- Purchase Guide
- Getting Started
- Operation Guide
- Access Management
- Instance Management
- Node Management
- Version Upgrade
- Monitoring
- Backup and Rollback
- Data Security
- SSL Authentication
- Database Management
- Multi-AZ Deployment
- Public Network Access
- Disaster Recovery/Read-Only Instances
- Parameter Configuration
- Recycle Bin
- Task Management
- Performance Optimization
- Data Migration Guide
- Best Practice
- Optimizing Indexes to Break Through Read/Write Performance Bottlenecks
- Troubleshooting Mongos Load Imbalance in Sharded Cluster
- Troubleshooting Excessive Slow Queries
- Fixing High CPU Utilization in MongoDB Instance Based on DBbrain
- Troubleshooting Excessive Connections
- Troubleshooting High Request Latency Based on Slow Log
- Troubleshooting High CPU Utilization
- Considerations for Using Shard Clusters
- Sample of Reading and Writing Data in MongoDB Instance
- Export and Import
- Troubleshooting MongoDB Connection Failures
- Performance Fine-Tuning
- Ops and Development Guide
- Troubleshooting
- API Documentation
- History
- Introduction
- API Category
- Making API Requests
- Instance APIs
- RenameInstance
- OfflineIsolatedDBInstance
- ModifyDBInstanceSpec
- IsolateDBInstance
- DescribeSpecInfo
- DescribeDBInstances
- DescribeClientConnections
- CreateDBInstanceHour
- AssignProject
- RenewDBInstances
- DescribeSlowLogs
- DescribeSlowLogPatterns
- InquirePriceRenewDBInstances
- InquirePriceModifyDBInstanceSpec
- InquirePriceCreateDBInstances
- DescribeDBInstanceDeal
- DescribeSecurityGroup
- DescribeInstanceParams
- ModifyDBInstanceSecurityGroup
- ModifyDBInstanceNetworkAddress
- Backup APIs
- Account APIs
- Task APIs
- Other APIs
- Data Types
- Error Codes
- Instance Connection
- FAQs
- Service Agreement
- Glossary
- Contact Us
Basic Concepts
Read-only instance
TencentDB for MongoDB allows you to create one or multiple read-only instances in the source instance AZ or another AZ based on the cluster architecture and storage engine of the source instance. The data in the source instance will be automatically synced to read-only instances, which are granted the read-only permission. In this way, the read requests of the source instance can be distributed to read-only instances to increase the read/write performance and application throughput.
Disaster recovery instance
TencentDB for MongoDB allows you to create one or multiple disaster recovery instances in another region based on the cluster architecture and storage engine of the source instance. The data in the source instance will be automatically synced to disaster recovery instances, which are granted the read-only permission. When the region of the source instance is disconnected due to a force majeure event such as power outage or network issue, a disaster recovery instance can be promoted to primary instance to implement cross-region disaster recovery and quickly sustain business needs. This helps you improve the business continuity at low costs and guarantee the data reliability.
Differences between read-only instance and disaster recovery instance
Both read-only and disaster recovery instances are built based on the cluster architecture and storage engine of the source instance. They have the following differences:
Difference | Description | Read-Only Instance | Disaster Recovery Instance |
Architecture type | The system architecture of a read-only or disaster recovery instance cluster, which can be replica set or sharded cluster but not single-node. | Same as that of the source instance. | Same as that of the source instance. |
Cross-region | Whether a read-only or disaster recovery instance can be created in another region based on the source instance. | No | Yes |
Cross-AZ | Whether a read-only or disaster recovery instance can be created in another AZ in the current region based on the source instance. | Yes | Yes |
Database version | The compatible MongoDB version, including v4.4, v4.2, v4.0, v3.6, and v3.2. v3.2 is no longer for sale. | Same as that of the source instance and cannot be upgraded. | Same as that of the source instance and cannot be upgraded. |
Storage engine | The storage engine, which is WiredTiger by default. | Same as that of the source instance. | Same as that of the source instance. |
Instance specifications | The CPU, memory, and disk capacity requirements of a read-only or disaster recovery instance to guarantee the service capacity. | Cannot be lower than those of the source instance. | Cannot be lower than those of the source instance. |
Data write | Data write and database creation and deletion | No | No |
Backup and rollback | Data backup and restoration. | No | No |
Account management | Database access account creation and deletion. | No | No |
Manual disassociation from the source instance | Whether you can manually disassociate a read-only or disaster recovery instance from the source instance in the console. | No. Only after the source instance is terminated can a read-only instance be disassociated from it automatically. After disassociation, the read-only instance will be promoted to general instance and can be read/written normally. | Yes. You can promote a disaster recovery instance to primary instance, and it will become a general instance that can be read/written normally to quickly sustain the business needs. |
AZ upgrade | Whether the single-AZ deployment of a read-only or disaster recovery instance can be upgraded to multi-AZ deployment. | Yes | Yes |
How It Works
Read-only and disaster recovery instances can be understood as a data sync service, which continuously syncs the existing and incremental data (including databases, collections, indexes, and documents) of the source instance to the target read-only or disaster recovery instance and grants the target instance only the read-only permission to relieve the pressure on the source cluster.
Once a read-only or disaster recovery instance is created, full data sync will be initiated, and data sync operations will be logged as oplog. After full data sync is completed, the oplog can be replayed to keep incrementally syncing changed or added data. This works similarly to primary-secondary sync in a replica set. The entire process has two stages:
Full sync stage, where full data is synced. Before it starts, the latest oplog timestamp of the source cluster will be recorded. After it starts, the metadata, indexes, and data in all collections in the source cluster will be read and concurrently inserted to collections with the same name in the target. The duration of full sync is proportional to the data volume of the source cluster.
Incremental sync stage, which follows the full sync stage. During this stage, the oplog of the source cluster will be pulled based on the oplog timestamp recorded when the full sync stage starts and then replayed in the target.
Latency description
Due to the delay in data sync, the real-timeness of data sync for read-only instances may not be guaranteed. If your business requires read/write separation and high real-timeness, we recommend that your business read the secondary nodes of the primary instance. For more information, see Connecting to TencentDB for MongoDB Instance. You can log in to the console and go to the RO/DR page to view the status and latency of sync from the primary instance to the target instance.
Performance optimization
Similar to concurrent replay of MongoDB replica set oplogs, the read-only/disaster recovery data sync service pulls the oplogs to the cache, parses them concurrently, hashes them by collection name to ensure that the oplogs are sequential at the collection level, linearly hashes each oplog segment by document ID to assign oplogs of the same document to the same thread, and then concurrently sends the threads to the target instance. This guarantees the incremental data sync performance and keeps the sync latency within seconds.
Data security
During incremental sync, the sync service persistently stores the currently synced latest oplog timestamp, and the replay process of the sync service is idempotent. Therefore, the sync service supports checkpoint restart during the incremental sync stage. Even if a failure occurs in the source or target cluster, data security issues won't occur in the sync service.
During incremental data sync, if a primary/secondary switch occurs in the target cluster due to a disk failure or network issue, data may get lost. To address this problem, TencentDB for MongoDB adds an oplog that records the sync progress. The data sync service regularly inserts a sync progress record into the oplog transactions of the target cluster. After a new primary node takes effect, it will locate the latest record in its oplog transactions and sync the data again to prevent data loss.
Sync stability
Each read-only or disaster recovery instance is sustained by a separate data sync service, and each data sync service uses distributed locks and the lease mechanism to guarantee the service uniqueness and availability, monitors the sync task in real time, and fine-tunes the instance regularly to guarantee the data sync stability and reliability.
Impact and Limits
Impact on the source cluster
The impact of the read-only and disaster recovery data sync service is only limited to secondary nodes, and data will be pulled from a secondary node (hidden one preferably) in the source cluster.
During full data sync, the
getMore request is used to continuously pull data.During incremental data sync, the
getMore request is used to continuously pull oplogs.During both the full and incremental data sync stages, the sync service will create a cursor for sequential reads on the secondary node to mark the read progress, which has little impact on the secondary node.
Use limits
Read-only and disaster recovery instances belong to the source instance and cannot exist independently.
Read-only and disaster recovery instances will be unwritable after being created.
After the source instance is terminated, the system will automatically disconnect the sync service and promote a read-only instance to general instance for normal read/write.
Database version: For sharded clusters, read-only and disaster recovery instances can be on versions 4.0, 4.2, and 4.4. For replica sets, they can be on versions 3.2, 3.6, 4.0, 4.2, and 4.4. If the current instance has a read-only or disaster recovery instance, the database versions of both the current instance and the read-only or disaster recovery instance cannot be upgraded.
Quantity: You can create up to three read-only instances and up to three disaster recovery instances for an instance.
Cluster architecture and storage engine: The cluster architecture and storage engine of a read-only or disaster recovery instance are always the same as those of the source instance and cannot be changed.
Account management: When a read-only or disaster recovery instance is created, the account information in the source instance will be synced to it automatically. You cannot create or delete accounts in it. If the access account or password in the source instance is changed, the account won't be automatically synced to it. You need to manually modify the account information in it during connection; otherwise, an error will occur, and connection will fail.
Due to network isolation, you cannot create disaster recovery instances in finance zones for source instances in general regions, and vice visa.
Backup and rollback: They are not supported for both read-only and disaster recovery instances.
Data migration: Migrating data to read-only or disaster recovery instances is not supported.
Sync limits
Read-only or disaster recovery instance sync in a replica set is implemented by parsing oplogs, and all DDL operations are supported.
Read-only or disaster recovery instance sync in a sharded cluster is implemented by parsing change streams. A change stream covers an oplog with a layer of application and provides an API to push data in real time. In addition to basic CRUD operations, DDL operations related to database/collection structures and indexes are also supported for the pushed data, including
createIndexes, drop, rename, dropDatabase, create, createIndexes, dropIndexes, collMod, and convertToCapped.Billing Overview
The billing mode of a read-only or disaster recovery instance is the same as that of the source instance. You can select a billing mode based on your business needs. The pay-as-you-go billing mode is supported, and billable items include compute and storage resources. For more information, see Billing Overview.
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