What is Helix?

It  used for the automatic management of partitioned, replicated and distributed resources hosted on a cluster of nodes. Helix automates reassignment of resources in the face of node failure and recovery, cluster expansion, and reconfiguration. Modeling a distributed system as a state machine with constraints on states and transitions.

Terminologies

• Node :  A single machine
• Cluster: Set of Nodes   
• Resource : A logical entry (e.g.    database, index, task)
• Partition: Subset of the resource  (Each subtask is referred to as a partition)
• Replica: Copy of a Partition State  (e.g Master, Slave). It increase the availability of the system
• State: Describes the role of a replica (Each node in the cluster has its own Current State)
• State Machine and Transitions: An action that allows a replica to move from one state to another, thus changing its role. ( e.g    Slave    -->    Master )  
• spectators: the external clients. Helix provides an External View that is an aggregated view of the current state across all nodes.
• Current State: represents resource's actual state at a participating node.
  - INSTANCE_NAME: Unique name representing the process
  - SESSION_ID: ID that is automatically assigned every time a process joins the cluster
• Rebalancer: The core component of Helix is the Controller which runs the Rebalance algorithm on every cluster event.
• Dynamic Ideal State: Helix powerful is that Ideal State can be changed dynamically. It is adjusting the ideal state. Whenever a cluster event occurs, Helix can operate in one of three modes

1. FULL_AUTO
2. SEMI_AUTO
3. CUSTOMIZED

Cluster events can be one of the following:

• Nodes start and/or stop
• Nodes experience soft and/or hard failures
• New nodes are added/removed

[1] http://helix.apache.org/Concepts.html