Created: 2012-03-23 00:05
Updated: 2017-03-20 22:46
License: agpl-3.0

If you are in a hurry to get started, see the Python Quickstart or the C++ Quickstart depending on your language of choice.

What is an Aggregate B-tree

An Aggregate B-tree is an extension of the B+ Tree. The B+ Tree itself is powerful data structure that allows fast storage and lookup of key-value pairs, walking keys in order, or finding the nearest key, and forms the basis of most database indexes. Unlike many B-tree implementions, this implementaion supports an idea called 'shadowing', which means that it's a trival operation to 'clone' the tree. This allows one thread to modify a tree while another continues to see a stable version, or to keep many old versions with little overhead. The code in this libary is thread safe, and crash safe. Even if a process crashes in the middle of an operation, after it restarts, the tree will be in a stable state. In addition, the abtree libaray allows multiple trees to be used in a transactional way, providing ACID guarantees (see section on transactions)

Aggregation adds another level of features. Specifically, it's also possible to quickly compute 'aggregates', such as totals, maximums, minimums and averages over any range of keys. It's also possible quickly move through the tree based on these aggregations. Aggregations also allow more non-obvious abilities, such as putting ranges into a tree, and quickly finding all the overlapping ranges, quickly jumping of a specific number of records, seeing only 'high-priority' records, or even storing 3 dimensional points and quickly finding all of the points within an arbitrary box! The feature of aggregation can also be used to implement map-reduce like functionality, and in fact, an aggregate b-tree is the basis of some of the features of couchdb. While the current version of the library provides the direct raw interface needed to implement many of these specialized indexes, futures versions will attempt to provide wrappers to simplify common cases (such as spatial indexes).

Currently there are two independent API's to the libaray, a C++ API, and a python API. Both are meant to be run from a single process (although multiple threads are supported), and both are allow for either disk based storages, or in memory storage. Future versions supporting a distributed, fault tolerant version are in the works, but too early to meaningfully release. When using disk based storage, the libary writes to a 'store', which is a set of files in any directory chosen by the user. Multiple tree which are part of a transaction must be in the same store (see transactions and storage for details).

Using abtree in C++

## C++ Quickstart ### Install boost The abtree library uses the boost C++ libraries. For most linux distro, boost can be installed using the package manager. For example, in debian or ubuntu, you can simpley run `sudo apt-get install libboost-dev`. For macports, it's simply `sudo port install boost`. Since the abtree library only uses the header file from boost, not the actual libraries, you can also just untar a boost package from and copy the headers to your system include path. ### Build and install abtree ```` git clone cd aggregate_btree autoreconf -fi mkdir build && cd build ../configure make sudo make install ```` ### C++ Tutorial The libary makes the aggregate btree look like an STL collection with some extra behaviors. These is currently no documentation, but please see the examples in test/\*.cpp for a rough idea of the usage. ## C++ API

TODO: Write this

Using abtree in Python

## Python Quickstart ### Build and Install abtree
git clone
cd aggregate_btree
sudo python ./ install

Python Tutorial

An abtree Table looks like a very special python dictionary, which may optionally be backed by some files on disk. The simpliest use case would be as follows

import abtree

t = abtree.Table()  #  Create an empty in memory table
t['foo'] = 6
t['bar'] = 5
t['baz'] = 10
t['quux'] = 10

The table supports a superset of the operations of a python dictionary. In fact it's a MutableMapping However, unlike a dictionary, it is always in sorted order, by key, so using the previous example:

>>> print t.keys() 
['bar', 'baz', 'foo', 'quux']

Note that the default ordering for a table is that of cmp(), but this can be overridden. Unlike a normal dictionary, Tables are also slicable, by key, which is a constant time operation. That means it is possible to grab any range of the table. For example:

>>> t = abtree.Table()
>>> t['a'] = 3
>>> t['b'] = 10
>>> t['c'] = 4
>>> t['d'] = 5
>>> t['e'] = 10
>>> t['f'] = 6
>>> t['g'] = 6
>>> print t['b':'f']
{'b': 10, 'c': 4, 'd': 5, 'e': 10}
>>> print t['bar':'foo']
{'c': 4, 'd': 5, 'e': 10, 'f': 6}
>>> print t[:'c']
{'a': 3, 'b': 10}
>>> print t['f':]
{'f': 6, 'g': 6}

Note that just like regular slicing, the first element is the inclusive start, and the second element is the exclusive end, and that either can be left out. That is, the elements include all of those such that: start <= key < end. The output of a slicing operation is a range limit reference to the original map, meaning that changes can be made to it, however changes made to elements outside of the range will be ignore. For example:

>>> r1 = t['b':'e']
>>> r1['b'] = 6   # This will overwrite b in t
>>> r1['f'] = 12  # No effect (out of r1's range)
>>> print t['b']  # Shows altered value
>>> print t['f']  # Shows original value

Calling the 'keys()', 'value()', or 'items()' function of a Table returns a special immutable sequence. All sequences are lazy, that is unless looked at they will not do any work. They are guarentted to be ordered by the key, and moreover, they can be sliced in log(N) time. This means that it is quick to find the Nth item in a table, even for a large N. Also interesting, the resulting sequences will 'see' updates to the underlying table.

>>> k = t.keys()  # Get the keys from the example table
>>> print k   # Check what we have
['a', 'b', 'c', 'd', 'e', 'f', 'g']
>>> ks = k[1::2]  # Make a slice, element 1 - end, every other element
>>> print ks  # See what it looks like
['b', 'd', 'f']
>>> del t['c']  # Now we remove an element
>>> print ks   # See that even our complex slice updated
['b', 'e', 'g']

What is you don't want this 'updating' behavior? Well, actually, the internal implementation of Tables is 'copy on write', which means that it is 'free' to copy a table, even a huge one with millions of entries. Copys remain unaltered when the original is changes, as to sequences or slices based on the copy

>>> t2 = t  # Make another reference to t (see's updates of t)
>>> t3 = t.copy()  # Make a logical 'copy' of t (doesn't see updates).  Note this doesn't do any work
>>> del t['a']  # Make a change to t, and t2, but not t3
>>> del t2['e']  # t2 also effects t, they are one in the same
>>> del t3['c']  # t3 operates independantly 
>>> print t.keys() # t say two of the changes  
['b', 'c', 'd', 'f', 'g']
>>> print t2.keys()  # t2 looks just like t
['b', 'c', 'd', 'f', 'g']
>>> print t3.keys()  # t3 saw only it's change
['a', 'b', 'd', 'e', 'f', 'g']
## Python API

TODO: Write this

Various details

## Transactions ## Storage details TODO: Write this
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