What is Apache Zookeeper?
- Hadoop Zookeeper is a machine that provides distributed configuration service for distributed application
- Used for Synchronization, serialisation and coordination
- Uses a data model like directory tree and very easy to program
- Zookeeper stores information in tree like structure
- It has a shared hierarchical namespace looks like standard file system.
- The namespace has data nodes called Znodes.default size is 1MB.
- Data is kept in memory
- A file can store data and reference to other Znodes.
- Zookeeper is fast and can handle high loads
- Zookeeper is relaible
- Zookeeper is highly available and no single point of failure
- The client connects through TCP connection and keep getting heartbeat. if one server is not available then connect to a different server
- Zookeeper flags each update with a number
- Zookeeper performance is best where reads are more common than write
Install Zookeeper from below link:
Zookeeper data flags:
- The path name will have increasing counter relative to the parent appended
- The Znode will be deleted if the session is closed or times out or if explicitly deleted
- Zookeeper provides a feature to client to receive notification if anything changed like configuration data , connection fail etc.
- It is not automatic and needs to be triggered by the client for first time
- Client should verify the state after receiving notification
Zookeeper Stat Structure:
|czxid||The zxid of the change due to which the znode is created|
|mzxid||The zxid of the change that last modified this znode.|
|ctime||the time in millisecond when znode was created|
|mtime||the time in millisecond when znode was last modified|
|version||The number of changes to the data of this znode|
|cversion||The number of changes to the children of this znode|
|aversion||The number of changes to the ACL of this znode|
|ephemeralowner||The sessionid of the owner of znode|
|datalength||The length of data fields in znode|
|numchildren||The no of children for this znode|
- All servers store a copy of data
- A leader server is selected at startup
- Followers service clients,all updates go through leader server
- Update responses are sent when changes applied to majority of servers
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