Ozone Manager (OM) is the namespace manager for Ozone.
This means that when you want to write some data, you ask Ozone Manager for a block and Ozone Manager gives you a block and remembers that information. When you want to read that file back, you need to find the address of the block and Ozone Manager returns it you.
Ozone Manager also allows users to organize keys under a volume and bucket. Volumes and buckets are part of the namespace and managed by Ozone Manager.
Each ozone volume is the root of an independent namespace under OM. This is very different from HDFS which provides a single rooted file system.
Ozone's namespace is a collection of volumes or is a forest instead of a single rooted tree as in HDFS. This property makes it easy to deploy multiple OMs for scaling.
OM maintains a list of volumes, buckets, and keys. For each user, it maintains a list of volumes. For each volume, the list of buckets and for each bucket the list of keys.
Ozone Manager will use Apache Ratis(A Raft protocol implementation) to replicate Ozone Manager state. This will ensure High Availability for Ozone.
The relationship between Ozone Manager and Storage Container Manager is best understood if we trace what happens during a key write and key read.
To write a key to Ozone, a client tells Ozone manager that it would like to write a key into a bucket that lives inside a specific volume. Once Ozone Manager determines that you are allowed to write a key to the specified bucket, OM needs to allocate a block for the client to write data.
To allocate a block, Ozone Manager sends a request to Storage Container Manager (SCM); SCM is the manager of data nodes. SCM picks three data nodes into which client can write data. SCM allocates the block and returns the block ID to Ozone Manager.
Ozone manager records this block information in its metadata and returns the block and a block token (a security permission to write data to the block) to the client.
The client uses the block token to prove that it is allowed to write data to the block and writes data to the data node.
Once the write is complete on the data node, the client will update the block information on Ozone manager.