Oracle flex Cluster
Oracle RAC 12c introduced two new features. They are Hub Nodes and Leaf Nodes：
Hub Nodes are similar to Oracle Grid Infrastructure nodes in an Oracle Clusterware standard Cluster configuration: they are tightly connected, and have direct access to shared storage. In an Oracle Flex Cluster configuration, shared storage can be provisioned to Leaf Nodes independent of the Oracle Grid Infrastructure.
Leaf Nodes are different from standard Oracle Grid Infrastructure nodes, in that they do not require direct access to shared storage, but instead request data through Hub Nodes. Hub Nodes can run in an Oracle Flex Cluster configuration without having any Leaf Nodes as cluster member nodes, but Leaf Nodes must be members of a cluster that includes at least one Hub Node.
Difference between Hub Nodes and Leaf Nodes
Hub Nodes:
Direct access to the shared storage
They can access the Oracle Cluster Registry (OCR) and Voting Disk (VD) directly We can run an ASM instance on them.
We can run database instance on them as wellLeaf Nodes:
It's part of the cluster as well, but cannot direct access to the shared storage
They connected to the cluster via the Hub Node that is linked to
It does run Grid Infrastructure, but cannot run as ASM/Database instances. We can deploy Fusion Middleware ,weblogic on them
FLEX Cluster总结：
中心节点：和之前的版本一样，它们通过专用网络相互连接，可以直接访问共享存储。这些节点可以直接访问 Oracle 集群注册表 (OCR) 和表决磁盘 (Voting Disks)，最多可以有64个中心节点(Hub Nodes)
叶节点：这些节点彼此不互连，也不能像中心节点一样访问共享存储。每个叶节点与所连接的中心节点通信，并通过所连接的中心节点连接到集群，这些节点不能运行ASM/RDBMS，我们可以部署应用在这些叶节点上。应用可以通过叶节点访问数据库，而不必在叶节点上运行 ASM 实例。通过中心节点连接到数据库使得它对应用透明。
Oracle flex ASM
Oracle Flex ASM is an ASM instance which will be running in the cluster based on cardinality defined. The default cardinality is three, but you can change it using srvctl modify asm command. It means that ASM is not required to run on each node in the cluster.In previous release, ASM will be running on each node in the cluster and ASM clients (Database,ACFS,ADVM) can only access ASM by using an ASM on the same host.
With Oracle Flex ASM, the clients can connect to remote ASM using network connection (ie ASM network ).If a server running an ASM instance fails, Oracle Clusterware will start a new ASM instance on a different server to maintain the cardinality. If a 12c database instance is using a particular ASM instance, and that instance is lost because of a server crash or ASM instance failure, then the Oracle 12c database instance will reconnect to an existing ASM instance on another node. These features are collectively called Oracle Flex ASM.
FLEX ASM总结：
在 Oracle 12c 之前，对于要使用 ASM 的数据库实例来说，所有节点上的 ASM 实例必须已处于运行状态，才能启动数据库实例。如果 ASM 实例未运行，使用这些ASM实例的数据库实例不能启动。随着 Oracle 12c 的推出，Oracle Flex ASM 这个特性解除了上述限制，它的一个主要特性是故障切换到集群中的其他节点。实质上是一个中心和叶架构，Oracle Clusterware 通过一个替代ASM 实例将故障节点的连接将无缝转移到另一个成员节点。在给定集群中运行的 ASM 实例数被称作 ASM 基数，默认值为 3。
Note: If you choose to install an Oracle Flex Cluster, Oracle Flex ASM is enabled by default because an Oracle Flex Cluster requires Oracle Flex ASM.
