Leaf node connects with hub nodes to access database files. Because Hub node is having access to ASM Instance.
Also as you know private interconnect is required to get data from all nodes which provides consistent data using cache fusion.
This is not correct: a leaf node never accesses database files, it cannot run a database instance. Cache fusion is therefore irrelevant too.
@OP, a leaf node is part of the cluster: the interconnect is used by the clusterware processes whether the node is a hub or a leaf.
Leaf node does not have database instance is new to me. Because if you see oracle document it is mentioned below.
"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"
My question is if leaf node cannot run database instance then why it requesting data from hub nodes. Can you please provide details.
I found below details.
Oracle RAC database instance can also run on Leaf nodes. Since these Leaf nodes do not have direct access to the shared storage, the RAC database instances on the Leaf nodes will need to get the database through the Hub nodes. The technology to implement this remote storage access is another new Oracle 12c feature known as Oracle Flex ASM. From here, we can see that Flex Clusters require Flex ASM to enable storage access for Oracle database instances on Leaf nodes. For this reason, when you configure a Flex Cluster, Oracle Flex ASM is automatically enabled.
When quoting rubbish like that, you should give the source, because then people can laugh at the author. At the moment I can laugh only at people who believe it.
It is not only an amazing mis-understanding of flex cluster architecture, but also proves that the author has never actually tried to do it.
Oracle Certified Master DBA
Please read page 99 on Expert Oracle RAC 12c book written by Riyaj Shamsudeen, Syed Jaffar Hussain, Kai Yuhttp://books.google.com/books?id=gbp5AAAAQBAJ&pg=PA99&lpg=PA99&dq=Oracle+RAC+database+instance+can+also+run+on+Leaf+nodes&source=bl&ots=6dcO6ZfcXa&sig=eX3APGxY3KakhKEUXDWc-69w6EA&hl=en&sa=X&ei=cieeUoyMDYv7kQfg94CQDA&ved=0CFAQ6AEwBQ
Yes, I've already found that. I've put a review on amazon.com about the book, and sent a note to Apress for the errata. It is worrying that people can get the ACE qualification when they do not understand how ASM works.
Even if YOU do not understand ASM, you can test it: try to start an RDBMS instance on a leaf node. You'll soon find out.
I agree with you... it's sad.
Basic concept about Oracle Flex Cluster
Oracle Flex Cluster relaxes the tightly coupled nature of Clusterware and permits the collocation of Application and RDBMS in a common Oracle Flex Cluster.
In Oracle Flex Cluster, the database tier, composed of HUB servers, preserver the tightly couple configuration, while the application tier, composed of LEAF servers is loosely coupled and does not required shared storage and peer-to-peer communication with other server in the cluster
Oracle Flex Cluster 12c provides:
Hub Servers to Database tier (RDBMS and Application can run here)
Leaf Servers to Application tier (Only Application can run here)
Please watch this: