Last Updated: Jul 25, 2017

Term Explanation
Region Region refers to the geographical location of the server for a user-purchased MongoDB instance. You can specify the region when activating the MongoDB instance. Currently, the region cannot be modified once the instance is purchased. When purchasing a MongoDB instance, it must be used with an Alibaba Cloud ECS instance. MongoDB only supports intranet access, so the selected region must be the same as that of the ECS instance.
Zone Zone refers to the physical zones with separate power supplies and networks in the same region. Intranet communication can take place between zones, but network latency is lower within a zone. Fault isolation can be performed between zones. Single-zone refers to when three nodes in the MongoDB instance replica set are located in the same zone. If ECS and MongoDB instances are deployed in the same zone, the network latency will be lower.
Instance A MongoDB instance, or simply an instance, is the basic unit of the MongoDB service purchased by users. The instance is the operating environment for ApsaraDB for MongoDB and exists as a separate process on the host. Users can use the console to create, modify, and delete MongoDB instances. Instances are mutually independent and their resources are isolated. They do not compete for CPU, memory, IO, or other resources. Each instance has its own features, such as database type and version. The system has corresponding parameters to control instance behavior.
Memory The maximum memory that can be used by an ApsaraDB for MongoDB instance.
Disk capacity Disk capacity is the disk size the user selects when purchasing the MongoDB instance. The disk capacity occupied by the instance includes set data and the space required for normal instance operation, such as the system database, database rollback log, redo log, and index. Ensure that the disk capacity is sufficient for the MongoDB instance to store data, otherwise, the instance may be locked. If insufficient disk space causes the instance to be locked, the user can purchase a larger disk to unlock the instance.
IOPS IOPS is the maximum number of block device reads/writes per second, measured in units of 4 KB.
CPU cores The maximum computing power of the instance. One core CPU has at least 2.3 GHz hyperthreading (Intel Xeon series Hyper-Threading) computing power.
Connections The number of TCP connections between clients and the MongoDB instance. If the client uses a connection pool, the connections between the client and MongoDB instance will be persistent. Otherwise, they will be short connections.
Cluster version ApsaraDB for MongoDB supports cluster versions. You can purchase multiple mongos and shard nodes and combine them with a single ConfigServer to form a cluster version. This allows you to easily create a MongoDB distributed database system.
Mongos Mongos are MongoDB’s cluster request portals. All requests must be coordinated through mongos that act as request distribution centers. They are responsible for forwarding data requests to the corresponding shard servers. You can use multiple mongos as request portals, so that, if one goes offline, MongoDB requests can still be processed.
Shard Shards are parts of MongoDB clusters. A single shard is composed of a three-node replica set, which ensures each shard has high availability. You can purchase multiple shards to scale the read/write performance and storage space of the product to suit your application performance and storage requirements, thus achieving a distributed database system.
ConfigServer The ConfigServer stores all database metadata (route, shard, etc.) configuration mongos, which are not stored on shard servers or in data routing information, but only cached in the memory. When mongos are started for the first time or shut down and then restarted, they load configuration information from the ConfigServer. If the ConfigServer information changes, all mongos are notified to update their statuses. This way, the mongos always have correct routing information. The ConfigServer stores shard route metadata. As there are high requirements for service availability and data reliability, ApsaraDB for MongoDB uses three-node replica sets to comprehensively ensure the reliability of the ConfigServer’s services.
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