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From an...@apache.org
Subject [zookeeper] branch master updated: ZOOKEEPER-3400: Add documentation on local sessions
Date Thu, 10 Oct 2019 11:13:40 GMT
This is an automated email from the ASF dual-hosted git repository.

andor pushed a commit to branch master
in repository https://gitbox.apache.org/repos/asf/zookeeper.git

The following commit(s) were added to refs/heads/master by this push:
     new 92fb050  ZOOKEEPER-3400: Add documentation on local sessions
92fb050 is described below

commit 92fb05092aef6eb580e4259efc332f6f7cbdd994
Author: maoling <maoling199210191@sina.com>
AuthorDate: Thu Oct 10 13:13:33 2019 +0200

    ZOOKEEPER-3400: Add documentation on local sessions
    - more details in the [ZOOKEEPER-3400](https://issues.apache.org/jira/browse/ZOOKEEPER-3400)
    Author: maoling <maoling199210191@sina.com>
    Reviewers: andor@apache.org
    Closes #1045 from maoling/ZOOKEEPER-3400 and squashes the following commits:
    e961192cb [maoling] fix the Space and capital grammar nits
    9307087ce [maoling] fix some grammar nits reviewed by jhuan31 & add some notes reviewed
by enixon
    1963eec22 [maoling] ZOOKEEPER-3400:Add documentation on local sessions
 .../src/main/resources/markdown/zookeeperAdmin.md  |  7 ++
 .../resources/markdown/zookeeperProgrammers.md     | 82 ++++++++++++++++++++++
 2 files changed, 89 insertions(+)

diff --git a/zookeeper-docs/src/main/resources/markdown/zookeeperAdmin.md b/zookeeper-docs/src/main/resources/markdown/zookeeperAdmin.md
index 7749fcb..b0f08fe 100644
--- a/zookeeper-docs/src/main/resources/markdown/zookeeperAdmin.md
+++ b/zookeeper-docs/src/main/resources/markdown/zookeeperAdmin.md
@@ -1168,6 +1168,13 @@ As an example, this will enable all four letter word commands:
     so as to prevent unexpected additional load on the voting peers during
     the process. Defaults to 200 ms.
+* *localSessionsEnabled* and *localSessionsUpgradingEnabled* :
+   **New in 3.5:**
+   Optional value is true or false. Their default values are false.
+   Turning on the local session feature by setting *localSessionsEnabled=true*. Turning on
+   *localSessionsUpgradingEnabled* can upgrade a local session to a global session automatically
as required (e.g. creating ephemeral nodes),
+   which only matters when *localSessionsEnabled* is enabled.
 <a name="sc_authOptions"></a>
 #### Encryption, Authentication, Authorization Options
diff --git a/zookeeper-docs/src/main/resources/markdown/zookeeperProgrammers.md b/zookeeper-docs/src/main/resources/markdown/zookeeperProgrammers.md
index 21c8a9a..26e4aa5 100644
--- a/zookeeper-docs/src/main/resources/markdown/zookeeperProgrammers.md
+++ b/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
+    which is supposed to be local session only. So this is more like a safeguard against
someone accidentally creates lots of ephemeral nodes and global sessions.
+ - *When is the session created?*
+     - In the current implementation, it will try to create a local session when processing
*ConnectRequest* and when
+     *createSession* request reaches *FinalRequestProcessor*.
+ - *What happens if the create for session is sent at server A and the client disconnects
to some other server B
+    which ends up sending it again and then disconnects and connects back to server A?*
+     - When a client reconnects to B, its sessionId won’t exist in B’s local session
tracker. So B will send validation packet.
+     If CreateSession issued by A is committed before validation packet arrive the client
will be able to connect.
+     Otherwise, the client will get session expired because the quorum hasn’t know about
this session yet.
+     If the client also tries to connect back to A again, the session is already removed
from local session tracker.
+     So A will need to send a validation packet to the leader. The outcome should be the
same as B depending on the timing of the request.
 <a name="ch_zkWatches"></a>
 ## ZooKeeper Watches

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