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Subject svn commit: r1510938 - /directory/site/trunk/content/mavibot/index.mdtext
Date Tue, 06 Aug 2013 12:45:51 GMT
Author: pamarcelot
Date: Tue Aug  6 12:45:51 2013
New Revision: 1510938

Rollback of Test.


Modified: directory/site/trunk/content/mavibot/index.mdtext
--- directory/site/trunk/content/mavibot/index.mdtext (original)
+++ directory/site/trunk/content/mavibot/index.mdtext Tue Aug  6 12:45:51 2013
@@ -16,4 +16,40 @@ Notice: Licensed to the Apache Software 
     specific language governing permissions and limitations
     under the License.
\ No newline at end of file
+# Apache Mavibot™
+Mavibot is a MVCC B+Tree implementation in Java.
+## Btree basics
+A *Btree* is a data structure that stores _<Key, Value>_ tuples in a tree, with the
guarantee that the tree will be ordered, and that the depth of the tree is the same for all
the leaves. A *Btree* has nodes and leaves (with the only exception of a *Btree* with only
a root page). The nodes are used to route to the underlying values, and have children. Leaves
don't have children.
+Nodes and leaves have a maximum number of elements stored into them, and when they are full,
they are split. If the split is done on a leaf, we may have to reorganize the tree so that
either we can move some elements up and keep the tree at the current height, or we may have
to reorganize the full tree so that all the leaves are at the same level, which will then
be one deeper (if we added some value) than the tree before the split.
+## Btree vs B+Tree
+The difference between those two data structures is that *Btree* store values in the nodes,
when *B+Tree* do store all the values in leaves.
+At first glance, we can say that finding a value in a *Btree* will be faster, as we may not
go down to the leaves to find it. OTOH, a *B+Tree* has many advantages, but the two major
advantages are :
+* we don't need to go up in the tree to browse the tree when searching for more than one
value, we can just read the leaves, as they are chained.
+* We will have smaller nodes, so we can cache more of the tree pages than if we have values
in the nodes.
+Those two big advantages make the *B+Tree* more interesting to use than the simpler *Btree*.
+(See [Wikipedia page on B+tree]( and [Wikipedia page
on Btree]( )
+## MVCC
+[MVCC]( (Multi Version Concurrency
Control) is a way to provide concurrent access to the *Btree* (it's extensively used in many
other areas, like programming languages and transactional memory). The main idea is to create
a new version of the tree each time we do a modification. It also allows the reorganization
of the data on the fly, but this is an extra benefit.
+The way it works is that when you do a search on the tree, you first acquire the current
revision. Even if the search is taking a while, because it fetches many values, the tree will
remain unchanged for the selected revision
+Any modification done on the tree will first create a new revision, and the modified pages
will first be copied, so that the previous versions will still be available for any search
operation being executed at the same time.
+It has three direct consequences :
+ * first, a search will always return 'outdated' values, in the way that new data won't be
returned, as they will be stored in a version which is newer.
+ * Second, and more important, we don't need any lock to access the data when doing a search,
as there is no possible modification on a versioned tree.
+ * Third, concurrent modifications are thus limited, as we want to be sure that we don't
override some modification done by another thread. They are ways to mitigate this constraints,
but in most of the case, it's acceptable.

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