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From GitBox <...@apache.org>
Subject [GitHub] [zookeeper] eolivelli commented on a change in pull request #1045: ZOOKEEPER-3400: Add documentation on local sessions
Date Fri, 27 Sep 2019 21:54:26 GMT
eolivelli commented on a change in pull request #1045: ZOOKEEPER-3400: Add documentation on
local sessions
URL: https://github.com/apache/zookeeper/pull/1045#discussion_r329263687

 File path: zookeeper-docs/src/main/resources/markdown/zookeeperProgrammers.md
 @@ -503,6 +503,88 @@ 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 creation and closing of sessions are costly in ZooKeeper because they need
quorum confirmations,
+  they become the bottleneck of a ZooKeeper ensemble when it needs 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.
+  Note: Losing the tcp connection does not necessarily imply that the session is lost. If
the connection can be reestablished with the same zk server
+  before the session timeout then the client can continue (it simply cannot move to another
+- 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 be upgraded 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 creation 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
+The leader also needs to know about the lifespan of a session in order to clean up ephemeral
nodes on close/expiry.
+This requires a global session as the local session is tied to its particular server.
+- A session can be both a local and global session during upgrade, but the operation of upgrade
cannot be called concurrently by two thread.
+- *ZooKeeperServer*(Standalone) uses *SessionTrackerImpl*; *LeaderZookeeper* uses *LeaderSessionTracker*
which holds
+  *SessionTrackerImpl*(global) and *LocalSessionTracker*(if enable); *FollowerZooKeeperServer*
and *ObserverZooKeeperServer*
+  use *LearnerSessionTracker* which holds *LocalSessionTracker*.
+    The UML Graph of Classes about session:
+    ```
+    +----------------+     +--------------------+       +---------------------+
+    |                | --> |                    | ----> | LocalSessionTracker |
+    | SessionTracker |     | SessionTrackerImpl |       +---------------------+
+    |                |     |                    |                              +-----------------------+
+    |                |     |                    |  +-------------------------> | LeaderSessionTracker
+    +----------------+     +--------------------+  |                           +-----------------------+
+               |                                   |
+               |                                   |
+               |                                   |
+               |           +---------------------------+
+               +---------> |                           |
+                           | UpgradeableSessionTracker |
+                           |                           |
+                           |                           | ------------------------+
+                           +---------------------------+                         |
+                                                                                 |
+                                                                                 |
+                                                                                 v
+                                                                               +-----------------------+
+                                                                               | LearnerSessionTracker
+                                                                               +-----------------------+
+    ```
+- Q&A
+ - *What's the reason for having the config option to disable local session upgrade?*
+     - In a large deployment which wants to handle a very large number of clients, we know
that clients connecting via the observers
 Review comment:
   I can't understand this line

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