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From GitBox <...@apache.org>
Subject [GitHub] [zookeeper] enixon commented on a change in pull request #1045: ZOOKEEPER-3400: Add documentation on local sessions
Date Wed, 04 Sep 2019 18:48:56 GMT
enixon commented on a change in pull request #1045: ZOOKEEPER-3400: Add documentation on local
URL: https://github.com/apache/zookeeper/pull/1045#discussion_r320917719

 File path: zookeeper-docs/src/main/resources/markdown/zookeeperProgrammers.md
 @@ -503,6 +503,85 @@ new list and failing to connect, the client moves back to the normal
mode of ope
 an arbitrary server from the connectString and attempts to connect to it. If that fails,
it will continue
 trying different random servers in round robin. (see above the algorithm used to initially
choose a server)
+**local session**.Added in 3.5.0, mainly implemented by [ZOOKEEPER-1147](https://issues.apache.org/jira/browse/ZOOKEEPER-1147).
+- Background:The create and close of session is costly in zookeeper, especially when these
operations of sessions need a
+quorum confirmation by leader, which we call this session type: global session. it will be
a bottle neck for ZooKeeper ensemble
+when it wants to handle thousands of client connections.
+So after 3.5.0, we introduce a new type of session: local session which doesn't have a full
functionality of a normal(global) session.
+,this feature will be available by turning on *localSessionsEnabled*.
+when *localSessionsUpgradingEnabled* is disable:
+- Local sessions cannot create ephemeral nodes
+- Once a local session is lost, users cannot re-establish it using the session-id/password.
The session and its watches are gone for good.
+- When a local session connects, the session info is only maintained on the zookeeper server
that it is connected to. The leader is not aware of the creation of such a session and
+there is no state written to disk.
+- The pings, expiration and other session state maintenance are handled by the server which
current session is connected to.
+when *localSessionsUpgradingEnabled* is enable:
+- A local session can upgrade to the global session automatically.
+- When a new session is created it is saved locally in a wrapped *LocalSessionTracker*. It
can subsequently be upgraded
+to a global session as required(e.g. create ephemeral nodes).If an upgrade is requested the
session is removed from local
+ collections while keeping the same session ID.
+- Currently, Only the operation:*create ephemeral node* needs a session upgrade from local
to global.
+The reason is that the create of ephemeral node depends heavily on a global session.if local
session can create ephemeral
+node without upgrading to global session, it will cause the data inconsistency between different
 Review comment:
   The leader also needs to know about the lifespan of a session in order to clean up ephemerals
on close/expiry. This requires a global session as the local session is tied to its particular

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