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ajgu 1.99.2

GraphDB based on pybsddb3

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Ajgu is a simple graph database power by BerkleyDB key-value store.

from ajgu import Graph

This is the only import you need to do any other object, is created by the graph object you will create.

Sensible default are used so that you only have to do the following to instantiate a graph:

graph = Graph('my-super-dupper-project')

When you are finished you have to close the database:

graph.close()

Every operation must happen in a transaction. For that matter there is context manager Graph.transaction() that handle does what must be done in most cases. If you want to do weird things, you can use Graph.txn() to create a transaction and manage it yourself.

The transaction object works as a shadow copy of the graph. You perform operation against it, until the transaction is commited, then changes if any are persisted.

To create a vertex use the following:

with graph.transaction() as txn:
    vertex = txn.create_vertex()

A vertex works kind of a dict but has not all its methods (yet). You can use subscript aka. [] to change or retrieve properties of the vertex. The handy Vertex.get(key, default) is also available. Once you are finished you have to call Vertex.save(). You can also delete a vertex with Vertex.delete(). You can use Vertex.id() to retrieve the unique identifier of the vertex. Those methods are the base methods of any graph element (vertex or edge). Remember that everything must happen in a transaction.

For instance to create a vertex and give it a name you can do the following:

with graph.transaction() as txn:
    vertex = txn.create_vertex()
    vertex['name'] = "super-dupper-vertex"
    vertex.save()  # don't forget to save before leaving the transaction

Creating and mutating the vertex in two transaction is possible, only if you retrieve again the vertex for the current transaction:

with graph.transaction() as txn:
    vertex = txn.create_vertex()
    # in this case saving is useless

with graph.transaction() as txn:
    vertex = txn.vertex(vertex.id())
    vertex['name'] = "super-dupper-vertex"
    vertex.save()  # don't forget to save before leaving the transaction

In the above we introduced another method of Transaction, Transaction.vertex(id) method. There is also a similar method for edge Transaction.edge(id) which allows to retrieve an element with its identifier.

Vertex other methods Vertex.outgoings(label=None) and Vertex.incomings(label=None). They allow to retrieve edges. The label argument allows to filter based on the label of the edge. They returns a generator. They will be explained further below.

Creating a edge is also very simple:

with graph.transaction() as txn:
    start = txn.create_vertex()
    end = txn.create_vertex()
    edge = txt.create_edge(start, 'super-dupper-label', end)

The edge is persisted, connected to both start and end. For start it's an outgoing edge, because it starts there. For end it's an incoming edge, because it ends there. This is translated in terms of code as follow:

with graph.transaction() as txn:
    start = txn.vertex(start.id())
    unique_outgoing = next(start.outgoings())
    assert unique_outgoing.id() == edge.id()

Similarly:

with graph.transaction() as txn:
    end = txn.vertex(end.id())
    unique_incoming = next(start.incomings())
    assert unique_incoming.id() == edge.id()

Edge as also a Edge.start() and Edge.end() methods which allows to retrieve the start and end vertex of the edge:

with graph.transaction() as txn:
    edge = txn.edge(edge.txn())
    assert start.id() == edge.start()

Similarly:

with graph.transaction() as txn:
    edge = txn.edge(edge.txn())
    assert end.id() == edge.end()

There is also a Edge.label() to retrieve the edge label.

The graph works also as a key/value store via Transaction.get(key) and Transaction.set(key, value). A common pratice is to store a vertex identifier in an index to retrieve it later:

with graph.transaction() as txn:
     txn.set('my-super-dupper-vertex', vertex.id())

Later...:

with graph.transaction() as txn:
     identifier = txn.get('my-super-dupper-vertex')
     again = txn.vertex(identifier)
     assert again.id() == vertex.id()

That is all! Don't forget to read the code, it's not big.

FAQ

  1. What ajgu means?

    ajgu is a beam used in house construction in amazigh language.

  2. Do you use ajgu in real product?

    Yes, for an internal app at $WORK.

  3. Where can I find a graph server with multithread and python querying?

    GraphitiDB does just that.

  4. Are GraphDBs useful beyond prototyping?

    You tell me!

  5. GraphDBs are awesome, where can I learn more?

    There is not much information on the web on how to architecture programs using graphdbs. Patterns exists but they are very simple and you can guess them on your own. The API is not as complex as RDBMS and in this case worse is better holds True ;)

  6. Why do ajgu use the Transaction object to mutate the Graph?

    Because it makes it possible to have several transctions in the same thread.

 
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