Prerequisites¶
The tutorial examples can be run on any system which supports Python 2.6, 2.7 and 3.4+. The tutorial assumes that AllegroGraph and the Python client have been installed and configured using the procedure described in the Installation document.
To make the code in this document compatible with Python 2 we need the following import statement (not required on Python 3):
from __future__ import print_function
Setting the environment for the tutorial¶
Before running any of the tutorial examples it is necessary to set a few variables that describe the location of the AllegroGraph server and credentials used to access it. By default the Python client looks for these parameters in environment variables.
The variables that must be set are:
AGRAPH_HOST: Specifies the IP address or hostname of the agraph server. Defaults to'127.0.0.1'.AGRAPH_PORT: Specifies the port of the agraph server atAGRAPH_HOST. The port used by the server is specified in the agraph.cfg file. The default is10035.AGRAPH_USER: Specifies the username for authentication. For best results, this should name an agraph user with superuser privileges. If a non-superuser is specified, that user must have Start Session privileges to run example 6 and examples dependent on that example, and Evaluate Arbitrary Code privileges to run example 17. There is no default.AGRAPH_PASSWORD: Specifies the password for authentication ofAGRAPH_USER. There is no default..
The agraph.cfg file (see the Server Installation document) specifies values like the port
used by the server and the catalogs defined. Refer to that file if
necessary (or ask your system administrator if you do not have access
to the file).
Troubleshooting¶
If the tutorial examples throw a connection error in Example 6: Importing triples, see the discussion Session Port Setup in the Server Installation document.
If you see an error similar to the following
ImportError: pycurl: libcurl link-time ssl backend (nss) is
different from compile-time ssl backend (none/other)
Perform this procedure (replacing {agraph-version} with the actual version)
# Uninstall pycurl
pip uninstall pycurl
# Set the required compile-time option for pycurl
export PYCURL_SSL_LIBRARY=nss
# Reinstall, but ignore cached packages (force recompile)
pip install --no-cache-dir agraph-{agraph-version}-client-python.tar.gz
Terminology¶
We need to clarify some terminology before proceeding.
“RDF” is the Resource Description Framework defined by the World Wide Web Consortium (W3C). It provides an elegantly simple means for describing multi-faceted resource objects and for linking them into complex relationship graphs. AllegroGraph Server creates, searches, and manages such RDF graphs.
A “URI” is a Uniform Resource Identifier. It is label used to uniquely
identify various types of entities in an RDF graph. A typical URI
looks a lot like a web address:
<http://www.company.com/project/class#number>. In spite of the
resemblance, a URI is not a web address. It is simply a unique label.
A “triple” is a data statement, a “fact”, stored in RDF format. It states that a resource has an attribute with a value. It consists of three fields:
- Subject: The first field contains the URI that uniquely identifies the resource that this triple describes.
- Predicate: The second field contains the URI identifying a property of this resource, such as its color or size, or a relationship between this resource and another one, such as parentage or ownership.
- Object: The third field is the value of the property. It could be a literal value, such as “red”, or the URI of a linked resource.
A “quad” is a triple with an added “context” field, which is used to divide the repository into “subgraphs.” This context or subgraph is just a URI label that appears in the fourth field of related triples.
A “resource description” is defined as a collection of triples that all have the same URI in the subject field. In other words, the triples all describe attributes of the same thing.
A “statement” is a client-side Python object that describes a triple.
In the context of AllegroGraph Server:
- A “catalog” is a list of repositories owned by an AllegroGraph server.
- A “repository” is a collection of triples within a catalog.
- A “context” is a subgraph of the triples in a repository.
- If contexts are not in use, the triples are stored in the default graph.
Creating Users with WebView¶
Each connection to an AllegroGraph server runs under the credentials of a registered AllegroGraph user account.
Initial Superuser Account¶
The installation instructions for AllegroGraph advise you to create a
superuser called “test”. This is the user which is used by the
Python tutorial if the environment variables AGRAPH_USER and
AGRAPH_PASSWORD are not set.
Users, Permissions, Access Rules, and Roles¶
AllegroGraph user accounts may be given any combination of the following three permissions:
- Superuser
- Start Session
- Evaluate Arbritrary Code
In addition, a user account may be given read, write or read/write access to individual repositories.
It is easiest to run the Python tutorial as a Superuser. That way you do not have to worry about permissions.
If you run as a non-Superuser, you need Start Session permission for Example 6: Importing triples and other examples that require a session. You also need read/write access on the appropriate catalogs and repositories.
You can also define a role (such as “librarian”) and give the role a set of permissions and access rules. Then you can assign this shared role to several users. This lets you manage their permissions and access by editing the role instead of the individual user accounts.
A superuser automatically has all possible permissions and unlimited access. A superuser can also create, manage and delete other user accounts. Non-superusers cannot view or edit account settings.
A user with the Start Sessions permission can use the AllegroGraph features that require spawning a dedicated session, such as transactions and Social Network Analysis (SNA). If you try to use these features without the appropriate permission, you’ll encounter errors.
A user with permission to Evaluate Arbitrary Code can run Prolog Rule Queries. This user can also do anything else that allows executing Lisp code, such as defining select-style generators, doing eval-in-server, or loading server-side files.
WebView¶
WebView is AllegroGraph’s browser-based graphical user interface for user and repository management. It allows you to create, query, and maintain repositories interactively.
To connect to WebView, simply direct your web browser to the AllegroGraph port of your server. If you have installed AllegroGraph locally (and used the default port number), use:
http://localhost:10035
You will be asked to log in. Use the superuser credentials described in the previous section.
The first page of WebView is a summary of your catalogs and repositories. Select from the navigation menu at the top of the page.
This exposes the Users and Roles page. This is the page for
creating and managing user accounts.
To create a new user, click the [add a user] link.
This exposes a small form where you can enter the username and password. Click OK to save the new account.
The new user will appear in the list of users. Click the [edit] link to open a control panel for the new user account:
Use the checkboxes to apply permissions to this account (start session is needed by Example 6: Importing triples).
It is important that you set up access permissions for the new user. Use the form to create an access rule by selecting read, write or read/write access, naming a catalog (or * for all), and naming a repository within that catalog (or * for all). Click the [add] link.
This creates an access rule for your new user. The access rule will appear in the permissions display:
This new user can log in and perform transactions on any repository in the system.
To repeat, the “test” superuser is all you need to run all of the tutorial examples. This section is for the day when you want to issue more modest credentials to some of your users.
