Author: thomasm
Date: Thu Jan 23 11:39:28 2014
New Revision: 1560644
URL: http://svn.apache.org/r1560644
Log:
OAK-759 MongoMK: read from a secondary member when possible (document how to
change read/write mode at runtime)
Modified:
jackrabbit/oak/trunk/oak-doc/src/site/markdown/mongomk.md
Modified: jackrabbit/oak/trunk/oak-doc/src/site/markdown/mongomk.md
URL:
http://svn.apache.org/viewvc/jackrabbit/oak/trunk/oak-doc/src/site/markdown/mongomk.md?rev=1560644&r1=1560643&r2=1560644&view=diff
==============================================================================
--- jackrabbit/oak/trunk/oak-doc/src/site/markdown/mongomk.md (original)
+++ jackrabbit/oak/trunk/oak-doc/src/site/markdown/mongomk.md Thu Jan 23
11:39:28 2014
@@ -24,6 +24,19 @@ content.
Content Model
-------------
+The repository data is stored in two collections: the `nodes` collection for
node data,
+and the `blobs` collection for binaries. There is a third collection,
`clusterNodes`,
+which contains metadata of all cluster nodes. The data can be viewed using the
+MongoDB shell:
+
+ > show collections
+ blobs
+ clusterNodes
+ nodes
+
+Node Content Model
+------------------
+
The `MongoMK` stores each node in a separate MongoDB document and updates to
a node are stored by adding new revision/value pairs to the document. This way
the previous state of a node is preserved and can still be retrieved by a
@@ -325,6 +338,10 @@ Each MongoMK instance connecting to same
### Renew Cluster Id Lease
+Each cluster node uses a unique cluster node id, which is the last part of the
revision id.
+Each cluster node has a lease on the cluster node id, as described in the
section
+[Cluster Node Metadata](#Cluster_Node_Metadata).
+
### Background Document Split
MongoMK periodically checks documents for their size and if necessary splits
them up and
@@ -348,6 +365,60 @@ Pending Topics
### Conflict Detection and Handling
+Cluster Node Metadata
+---------------------
+
+Cluster node metadata is stored in the `clusterNodes` collection. There is one
entry
+for each cluster node that is running, and there are entries for cluster nodes
that were
+ran. Old entries are kept so that if a cluster node is started again, it gets
the same
+cluster node id as before (which is not strictly needed for consistency, but
nice for
+support, if one would want to find out which change originated from which
cluster node).
+
+Each running cluster node updates the lease time of the cluster node id once
every minute,
+to ensure each cluster node uses a different cluster node id.
+
+ > db.clusterNodes.find().pretty()
+
+ {
+ "_id" : "1",
+ "_modCount" : NumberLong(2),
+ "leaseEnd" : NumberLong("1390465250135"),
+ "instance" : "/Users/test/jackrabbit/oak/trunk/oak-jcr",
+ "machine" : "mac:20c9d043f141",
+ "info" : "...pid: 11483, uuid:
6b6e8e4f-8322-4b19-a2b2-de0c573620b9 ..."
+ }
+ {
+ "_id" : "2",
+ "_modCount" : NumberLong(2),
+ "leaseEnd" : NumberLong("1390465252206"),
+ "instance" : "/Users/mueller/jackrabbit/oak/trunk/oak-jcr",
+ "machine" : "mac:20c9d043f141",
+ "info" : "...pid: 11483, uuid:
28ada13d-ec9c-4d48-aeb9-cef53aa4bb9e ..."
+ }
+
+The `_id` is the cluster node id of the node, which is the last part of the
revision id.
+The `leaseEnd` is updated once per minute by running cluster nodes. It is the
number
+of milliseconds since 1970.
+The `instance` is the current working directory.
+The `machine` is the lowest number of the network addresses,
+or a random uuid if this is not available.
+The `info` contains the same info as a string, plus additionally the process id
+and the uuid.
+
+### Changing the Read Preference and Write Concern at Runtime
+
+The read preference and write concern of all cluster nodes can be changed at
runtime
+without having to restart the instances, by setting the property
`readWriteMode` of
+this collection. All cluster nodes will pick up the change within one minute
+(when they renew the lease of the cluster node id). This is a string property
with the
+format `'read:<readPreference>, write:<writeConcern>'` (please note the space
after
+the comma, and no spaces before and after the colon). The following shell
command will
+set the read preference to `primary` and the write concern to `majority` for
all
+cluster nodes:
+
+ > db.clusterNodes.update({},
+ {$set: {readWriteMode:'read:primary, write:majority'}},
+ {multi: true})
License
-------
