Documentation/learning: Glossary update

Documentation/learning/glossary.md

@@ -2,15 +2,17 @@
 
 This document defines the various terms used in etcd documentation, command line and source code.
 
-## Node
+## Alarm
 
-Node is an instance of raft state machine.
+The etcd server raises an alarm whenever the cluster needs operator intervention to remain reliable.
 
-It has a unique identification, and records other nodes' progress internally when it is the leader.
+## Authentication
 
-## Member
+Authentication manages user access permissions for etcd resources.
+
+## Client
 
-Member is an instance of etcd. It hosts a node, and provides service to clients.
+A client connects to the etcd cluster to issue service requests such as fetching key-value pairs, writing data, or watching for updates.
 
 ## Cluster
 
@@ -18,6 +20,42 @@ Cluster consists of several members.
 
 The node in each member follows raft consensus protocol to replicate logs. Cluster receives proposals from members, commits them and apply to local store.
 
+## Compaction
+
+Compaction discards all etcd event history and superseded keys prior to a given revision. It is used to reclaim storage space in the etcd backend database.
+
+## Election
+
+The etcd cluster holds elections among its members to choose a leader as part of the raft consensus protocol.
+
+## Endpoint
+
+A URL pointing to an etcd service or resource.
+
+## Key
+
+A user-defined identifier for storing and retrieving user-defined values in etcd.
+
+## Key range
+
+A set of keys containing either an individual key, a lexical interval for all x such that a < x <= b, or all keys greater than a given key.
+
+## Keyspace
+
+The set of all keys in an etcd cluster.
+
+## Lease
+
+A short-lived renewable contract that deletes keys associated with it on its expiry.
+
+## Member
+
+A logical etcd server that participates in serving an etcd cluster.
+
+## Modification Revision
+
+The first revision to hold the last write to a given key.
+
 ## Peer
 
 Peer is another member of the same cluster.
@@ -26,10 +64,34 @@ Peer is another member of the same cluster.
 
 A proposal is a request (for example a write request, a configuration change request) that needs to go through raft protocol.
 
-## Client
+## Quorum
+
+The number of active members needed for consensus to modify the cluster state. etcd requires a member majority to reach quorum.
+
+## Revision
+
+A 64-bit cluster-wide counter that is incremented each time the keyspace is modified.
+
+## Role
+
+A unit of permissions over a set of key ranges which may be granted to a set of users for access control.
+
+## Snapshot
+
+A point-in-time backup of the etcd cluster state.
+
+## Store
+
+The physical storage backing the cluster keyspace.
+
+## Transaction
+
+An atomically executed set of operations. All modified keys in a transaction share the same modification revision.
+
+## Key Version
 
-Client is a caller of the cluster's HTTP API.
+The number of writes to a key since it was created, starting at 1. The version of a nonexistent or deleted key is 0.
 
-## Machine (deprecated)
+## Watcher
 
-The alternative of Member in etcd before 2.0
+A client opens a watcher to observe updates on a given key range.