This topic describes the terms that are used in PolarDB.
Region
The geographic area of a data center where a PolarDB cluster is deployed.
Zone
A geographical area within a region. A zone has an independent power supply and network. The network latency between instances within the same zone is lower than that between different zones.
Cluster
PolarDB uses a multi-node cluster architecture. A cluster has one primary node and multiple read-only nodes. A PolarDB cluster can be deployed across zones but not across regions.
Global database network (GDN)
A network that consists of multiple PolarDB clusters that are distributed across regions around the world. All clusters in the network synchronize with each other to reach data consistency. For more information, see Create a GDN.
Primary cluster
In each GDN, only one cluster is granted the read and write permissions. This cluster is called the primary cluster.
Secondary cluster
In each GDN, such cluster synchronizes data from the primary cluster.
Node
A PolarDB cluster consists of multiple physical nodes. Nodes in a cluster fall into two types. Both node types are equivalent and have the same specifications. These two types of nodes are known as primary nodes and read-only nodes.
Primary node
Each PolarDB cluster contains one primary node, which is a read/write node.
Read-only node
You can add up to 15 read-only nodes to a PolarDB cluster.
Cluster zone
A zone where the data in the cluster is distributed. The data in the cluster is automatically replicated in two zones for disaster recovery. You can migrate nodes only within these zones.
Primary zone
The zone where the primary node of a PolarDB cluster is deployed.
Failover
During a failover, a read-only node becomes the primary node. For more information, see Automatic failover and manual failover.
Node type
The specification of a node in PolarDB cluster. For example, the specification can be 8 CPU cores and 64 GB memory. For more information, see Compute node specifications of PolarDB Enterprise Edition.
Endpoint
An endpoint is a domain name used to access a PolarDB cluster. Each cluster provides multiple endpoints. Each endpoint can be used to connect to one or more nodes. For example, requests received by the primary endpoint are forwarded to the primary node. Multiple endpoints of a cluster enable read/write splitting. The endpoints can be used to connect to the primary node and read-only nodes. An endpoint contains the attributes of database connections, such as the read/write mode, node list, load balancing, and consistency levels.
Address
An address is the carrier of an endpoint on different networks. An endpoint may support a internal-facing address and an Internet-facing address. An address contains network attributes, such as the domain name, IP address, VPC, and vSwitch.
Primary endpoint
The endpoint of the primary node. If a failover occurs, the system automatically points the endpoint to the new primary node.
Cluster endpoint
A cluster endpoint can be used to access all nodes in a cluster. You can enable read-only or read/write mode for a cluster endpoint. In the dialog box for configuring cluster endpoints, you can also configure cluster settings such as auto scaling, read/write splitting, load balancing, and the consistency level. For more information, see Primary endpoints, cluster endpoints, and custom endpoints.
Eventual consistency
By default, eventual consistency is enabled in read-only mode. PolarDB clusters can deliver the best performance based on eventual consistency. For more information, see Eventual consistency..
Session consistency
Session consistency is also known as causal consistency. It is the default consistency option in read/write mode. Session consistency guarantees the read consistency at the session level to meet the requirements in most scenarios. For more information, see Session consistency..
Global consistency
Global consistency is also known as strong consistency, cross-session consistency, and highest-level consistency. It ensures the session consistency, but increases the workload on the primary node. When the replication latency between the primary node and read-only nodes is high, we recommend that you do not use global consistency. For more information, see Global consistency.
Transaction splitting
A configuration item of cluster endpoints. To reduce the load on the primary node, enable transaction splitting. This way, the read requests in a transaction are forwarded to read-only nodes without compromising data consistency. For more information, see Features.
Offload reads from primary node
A configuration item of cluster endpoints. After this feature is enabled, SQL query statements are sent to read-only nodes without compromising data consistency. This reduces the load on the primary node to ensure the stability of the primary node. For more information, see Features.
Private address
You can use PolarDB together with Alibaba Cloud DNS PrivateZone to reserve the domain name of your original database. This ensures that each internal-facing address of the primary endpoint and cluster endpoint of a PolarDB cluster can be associated with a private domain name. This private domain name takes effect only in the specified virtual private cloud (VPC) within the current region. For more information, see Private domain names.
Snapshot backup
PolarDB only allows you to back up data by creating snapshots. For more information, see Data backup.
Level-1 backup
A backup file that is locally stored in the cluster is a level-1 backup. Level-1 backups are stored in the distributed storage system that is associated with a cluster. These backups allow you to back up and restore data in a quick manner. However, level-1 backups have high costs. For more information, see Data backup.
Level-2 backup
Level-2 backups are backup files that are stored in on-premises storage media. All data in level-2 backups is archived from level-1 backups and can be permanently stored. Level-2 backup have low costs. However, a long time is required to restore data from level-2 backups. For more information, see Data backup.
Log backup
A log backup stores the redo logs of a database for point-in-time recovery (PITR). Log backups help avoid data loss caused by accidental operations. Log backups must be retained for at least seven days. Log backups are stored in on-premises storage and are cost-effective. For more information, see Data backup.
Storage plan
PolarDB provides storage plans to reduce the storage costs of clusters. The storage capacity of a PolarDB cluster is automatically scaled based on the amount of the data in the cluster. You do not need to manually specify the storage capacity. You are charged only for the used storage. To store large volumes of data, we recommend that you use storage plans to reduce costs. For more information, see Purchase a storage plan.
Storage capacity
Storage capacity is used to store cluster data files, index files, log files, and temporary files. Log files include online and archived logs.
After you purchase a PolarDB cluster, the system automatically creates the files that are required for regular database operations. These files include the preceding files and consume some storage.
Cluster Edition
PolarDB for MySQL Cluster Edition is recommended because it is developed based on an architecture in which compute is decoupled from storage. The number of compute nodes can be increased from 2 to a maximum of 16. For more information, see Cluster Edition.
Smart-SSD
Smart-SSD is a high compression technique at the physical SSD disk level. Smart-SSD disks are embedded with a dedicated FPGA or ASIC compression chip, which decompresses and compresses the data in real time during the data read/write process and greatly reduces data storage costs. Smart-SSD disks are optimized particularly for PolarDB scenarios and can provide almost the same performance as non-compressed standard hard disks. Smart-SSD disks provide access interfaces of standard disks that can be accessed by upper-layer applications to avoid the complex adaptation of applications. This technique is used in PolarDB PSL4 disks.
Hot standby storage cluster
A hot standby storage cluster is deployed in the secondary zone or in a different data center in the same zone of the region where a PolarDB cluster is deployed. The hot standby storage cluster uses independent storage for hot standby of the PolarDB cluster. When the PolarDB cluster or the primary zone is unavailable, the hot standby storage cluster quickly becomes the primary node to perform read and write operations on the PolarDB cluster. If you disable the hot standby storage cluster feature, the SLA of the PolarDB cluster is degraded.