Services or capabilities described in Amazon Web Services documentation might vary by Region. To see the differences applicable to the China Regions, see Getting Started with Amazon Web Services in China (PDF).

MemoryDB core components

Following, you can find an overview of the major components of a MemoryDB deployment.

Clusters

A cluster is a collection of one or more nodes serving a single dataset. A MemoryDB dataset is partitioned into shards, and each shard has a primary node and up to 5 optional replica nodes. A primary node serves read and write requests, while a replica only serves read requests. A primary node can failover to a replica node, promoting that replica to the new primary node for that shard. MemoryDB runs Redis as its database engine, and when you create a cluster, you specify the Redis version for your cluster. You can create and modify a cluster using the Amazon CLI, the MemoryDB API, or the Amazon Web Services Management Console.

Each MemoryDB cluster runs a Redis engine version. Each Redis engine version has its own supported features. Additionally, each Redis engine version has a set of parameters in a parameter group that control the behavior of the clusters that it manages.

The computation and memory capacity of a cluster is determined by its node type. You can select the node type that best meets your needs. If your needs change over time, you can change node types. For information, see Supported node types.

You run a cluster on a virtual private cloud (VPC) using the Amazon Virtual Private Cloud (Amazon VPC) service. When you use a VPC, you have control over your virtual networking environment. You can choose your own IP address range, create subnets, and configure routing and access control lists. MemoryDB manages snapshots, software patching, automatic failure detection, and recovery. There's no additional cost to run your cluster in a VPC. For more information on using Amazon VPC with MemoryDB, see MemoryDB and Amazon VPC.

Many MemoryDB operations are targeted at clusters:

For more detailed information, see the following related topics:

Nodes

A node is the smallest building block of a MemoryDB deployment and runs using an Amazon EC2 instance. Each node runs the Redis version that was chosen when you created your cluster. A node belongs to a shard which belongs to a cluster.

Each node runs an instance of the engine at the version chosen when you created your cluster. If necessary, you can scale the nodes in a cluster up or down to a different type. For more information, see Scaling .

Every node within a cluster is the same node type. Multiple types of nodes are supported, each with varying amounts of memory. For a list of supported node types, see Supported node types.

For more information on nodes, see Managing nodes.

Shards

A shard is a grouping of one to 6 nodes, with one serving as the primary write node and the other 5 serving as read replicas. A MemoryDB cluster always has at least one shard.

MemoryDB clusters can have up to 500 shards, with your data partitioned across the shards. For example, you can choose to configure a 500 node cluster that ranges between 83 shards (one primary and 5 replicas per shard) and 500 shards (single primary and no replicas). Make sure there are enough available IP addresses to accommodate the increase. Common pitfalls include the subnets in the subnet group have too small a CIDR range or the subnets are shared and heavily used by other clusters.

A multiple node shard implements replication by having one read/write primary node and 1–5 replica nodes. For more information, see Understanding MemoryDB replication.

For more information on shards, see Working with shards.

Parameter groups

Parameter groups are an easy way to manage runtime settings for Redis on your cluster. Parameters are used to control memory usage, item sizes, and more. A MemoryDB parameter group is a named collection of engine-specific parameters that you can apply to a cluster, and all of the nodes in that cluster are configured in exactly the same way.

For more detailed information on MemoryDB parameter groups, see Configuring engine parameters using parameter groups.

Subnet Groups

A subnet group is a collection of subnets (typically private) that you can designate for your clusters running in an Amazon Virtual Private Cloud (VPC) environment.

When you create a cluster in an Amazon VPC, you can specify a subnet group or use the default one provided. MemoryDB uses that subnet group to choose a subnet and IP addresses within that subnet to associate with your nodes.

For more detailed information on MemoryDB subnet groups, see Subnets and subnet groups.

Access Control Lists

An Access control list is a collection of one or more users. Access strings follow the Redis ACL rules to authorize user access to Redis commands and data.

For more detailed information on MemoryDB Access Control Lists, see Authenticating users with Access Control Lists (ACLs).

Users

A user has a user name and password, and is used to access data and issue commands on your MemoryDB cluster. A user is a member of an Access Control List (ACL), which you can use to determine permissions for that user on MemoryDB clusters. For more information, see Authenticating users with Access Control Lists (ACLs)