16.8.1.Simple Mapping Example -- Northwind Linked Data View

Here is example of the basic Northwind Linked Data Views deployment. The sequence of operations is very common for adding SPARQL access to existing application.

There exist few important questions to answer. Who should have access to data behind Linked Data View? Should someone have access to other sorts of RDF data but not to the new View? What are applications that should be interoperable with the new RDF data source? Are there any applications that produce similar data but that data sould be kept apart from data made by view? How to ensure that deployment the view will not cause problems for other applications?

First of all, we decide whether the default web service endpoint should have access to the data in question. If it should then we have to grant SELECT privileges to the account "SPARQL" that is used for the default endpoint; if it should not but some custom edpoint should then grant to the owner account of that account. Granting access is less trivial that it is usual. On one hand, those who can make SQL SELECT statements on application's tables can also make SPARQL queries on Linked Data View over that tables, because it makes SQL inside. On the other hand, those who do not intend to query that data at all may get unexpected "permission denied" errors on queries that worked fine before adding an Linked Data View. If SPARQL compiler can not prove that the query can not access data from the view then it will generate SQL code that will access tables behind the view. In some cases permission problems should be resolved by creating Linked Data View in a separate RDF storage . In this example, data are public:

use DB;

GRANT SELECT ON "Demo"."demo"."Products" TO "SPARQL";
GRANT SELECT ON "Demo"."demo"."Suppliers" TO "SPARQL";
GRANT SELECT ON "Demo"."demo"."Shippers" TO "SPARQL";
GRANT SELECT ON "Demo"."demo"."Categories" TO "SPARQL";
GRANT SELECT ON "Demo"."demo"."Customers" TO "SPARQL";
GRANT SELECT ON "Demo"."demo"."Employees" TO "SPARQL";
GRANT SELECT ON "Demo"."demo"."Orders" TO "SPARQL";
GRANT SELECT ON "Demo"."demo"."Order_Details" TO "SPARQL";
GRANT SELECT ON "Demo"."demo"."Countries" TO "SPARQL";
GRANT SELECT ON "Demo"."demo"."Provinces" TO "SPARQL";

Interoperability is the next question. The example is not interoperable with anything so in can provide data of any form, a real application will probably use some ontology from external source. Sometimes data should be converted from internal application's representation to something different (such as metric to imperial or ATT country code to two-character country id); sometimes composed IRIs should follow special rules; function-based IRI classes may help in that cases. As this is the first example, only plain format-string-based IRI classes are used.

We should also ensure that data generated by the new view will not be accidentally mixed with other data of the database. For that purpose the example will use a unique graph name that includes both application name and host name. In addition, the script will drop declarations that might remain from a previous run of the same script. The script is executed many times during the development so erasing previous version is worth writing. It will report an error if there's nothing to erase but it's better than unpredictable errors due to writing new declarations over existing ones.

[Note] Note

Making graph name unique for every host is not needed if the application is supposed to be "local" and nobody will access more than one installation of the application. If this is the case, use some fixed graph IRI, not necessarily starting with hostname at all; this is much more convenient for querying because you don't have to calculate the graph name in each query. With fixed graph in use, it is still possible to clone the Linked Data View to map to a unique graph as soon as the application become "public" and requires merging data from many installations.

SPARQL drop quad map graph iri("http://^{URIQADefaultHost}^/Northwind")
SPARQL drop quad map virtrdf:NorthwindDemo

The {URIQADefaultHost} macro is replaced with the value of DefaultHost parameter of [URIQA] section of configuration file. The IRI is written as iri("http://^{URIQADefaultHost}^/Northwind") , not as <http://^{URIQADefaultHost}^/Northwind> because macro of this sort works only inside SPARQL string values.

Now it's safe to create IRI classes needed for the view. If these classes are used only in the view we define then it is safe to create all of them in a single statement. If some of them are used across multiple declarations then errors may occur. it is impossible to redefine an IRI class that is in use; the compiler will try to avoid reporting errors by checking whether the new declaration is identical to the existing one and by trying garbage collection in hope that the IRI class is used only in garbage, but errors may occur anyway. Thus it is better to declare "shared" IRI classes by individual statements and group together only "private" IRI classes of a view. If a "class redefinition" error occurs in the middle of a group then "undefined class" errors may occur after because the processing of the group was interrupted before rest of group was not executed. When in trouble, try DB.DBA.RDF_AUDIT_METADATA procedure.

create iri class northwind:Category "http://^{URIQADefaultHost}^/Northwind/Category/%d#this" (in category_id integer not null) .
create iri class northwind:Shipper "http://^{URIQADefaultHost}^/Northwind/Shipper/%d#this" (in shipper_id integer not null) .
create iri class northwind:Supplier "http://^{URIQADefaultHost}^/Northwind/Supplier/%d#this" (in supplier_id integer not null) .
create iri class northwind:Product   "http://^{URIQADefaultHost}^/Northwind/Product/%d#this" (in product_id integer not null) .
create iri class northwind:Customer "http://^{URIQADefaultHost}^/Northwind/Customer/%U#this" (in customer_id varchar not null) .
create iri class northwind:Employee "http://^{URIQADefaultHost}^/Northwind/Employee/%U%U%d#this" (in employee_firstname varchar not null, in employee_lastname varchar not null, in employee_id integer not null) .
create iri class northwind:Order "http://^{URIQADefaultHost}^/Northwind/Order/%d#this" (in order_id integer not null) .
create iri class northwind:CustomerContact "http://^{URIQADefaultHost}^/Northwind/CustomerContact/%U#this" (in customer_id varchar not null) .
create iri class northwind:OrderLine "http://^{URIQADefaultHost}^/Northwind/OrderLine/%d/%d#this" (in order_id integer not null, in product_id integer not null) .
create iri class northwind:Province "http://^{URIQADefaultHost}^/Northwind/Province/%U/%U#this" (in country_name varchar not null, in province_name varchar not null) .
create iri class northwind:Country "http://^{URIQADefaultHost}^/Northwind/Country/%U#this" (in country_name varchar not null) .
create iri class northwind:Flag "http://^{URIQADefaultHost}^%U#this" (in flag_path varchar not null) .
create iri class northwind:dbpedia_iri "http://dbpedia.org/resource/%U" (in uname varchar not null) .
create iri class northwind:EmployeePhoto "http://^{URIQADefaultHost}^/DAV/VAD/demo/sql/EMP%d#this" (in emp_id varchar not null) .
create iri class northwind:CategoryPhoto "http://^{URIQADefaultHost}^/DAV/VAD/demo/sql/CAT%d#this" (in category_id varchar not null) .

One IRI class per subject type; format strings begin with same host but different directory names so this will let the compiler to guess the type of subject by the text of IRI. Most of declarations are bijections and may get option (bijection) hint but these format strings are so simple that the compiler may understand it by itself. (northwind:Employee is not a bijection because sprintf_inverse will be unable to split the tail of IRI string and find the boundary between first and last name.)

The final operation is extending the default quad storage with new tree of quad map patterns.

prefix northwind: <http://demo.openlinksw.com/schemas/northwind#>
prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
prefix sioc: <http://rdfs.org/sioc/ns#>
prefix foaf: <http://xmlns.com/foaf/0.1/>
prefix owl: <http://www.w3.org/2002/07/owl#>
prefix wgs: <http://www.w3.org/2003/01/geo/wgs84_pos#>

alter quad storage virtrdf:DefaultQuadStorage
from Demo.demo.Products as products
from Demo.demo.Suppliers as suppliers
from Demo.demo.Shippers as shippers
from Demo.demo.Categories as categories
from Demo.demo.Customers as customers
from Demo.demo.Employees as employees
from Demo.demo.Orders as orders
from Demo.demo.Order_Details as order_lines
from Demo.demo.Countries as countries
from Demo.demo.Provinces as provinces
where (^{suppliers.}^.Country = ^{countries.}^.Name)
where (^{customers.}^.Country = ^{countries.}^.Name)
where (^{employees.}^.Country = ^{countries.}^.Name)
where (^{orders.}^.ShipCountry = ^{countries.}^.Name)
        create virtrdf:NorthwindDemo as graph iri ("http://^{URIQADefaultHost}^/Northwind") option (exclusive)
                northwind:CustomerContact (customers.CustomerID)
                        a foaf:Person
                                as virtrdf:CustomerContact-foaf_Person .

                northwind:CustomerContact (customers.CustomerID)
                        a northwind:CustomerContact
                                as virtrdf:CustomerContact-CustomerContact;
                        foaf:name customers.ContactName
                                as virtrdf:CustomerContact-contact_name ;
                        foaf:phone customers.Phone
                                as virtrdf:CustomerContact-foaf_phone ;
                        northwind:is_contact_at northwind:Customer (customers.CustomerID)
                                as virtrdf:CustomerContact-is_contact_at ;
                        northwind:country northwind:Country (customers.Country)
                                as virtrdf:CustomerContact-country ;
                        rdfs:isDefinedBy northwind:customercontact_iri (customers.CustomerID) ;
                        rdfs:isDefinedBy northwind:CustomerContact (customers.CustomerID) .

                northwind:Country (customers.Country)
                northwind:CustomerContact (customers.CustomerID) as virtrdf:CustomerContact-is_country_of .

                northwind:Product (products.ProductID)
                        a northwind:Product
                                as virtrdf:Product-ProductID ;
                        northwind:has_category northwind:Category (products.CategoryID)
                                as virtrdf:Product-product_has_category ;
                        northwind:has_supplier northwind:Supplier (products.SupplierID)
                                as virtrdf:Product-product_has_supplier ;
                        northwind:productName products.ProductName
                                as virtrdf:Product-name_of_product ;
                        northwind:quantityPerUnit products.QuantityPerUnit
                                as virtrdf:Product-quantity_per_unit ;
                        northwind:unitPrice products.UnitPrice
                                as virtrdf:Product-unit_price ;
                        northwind:unitsInStock products.UnitsInStock
                                as virtrdf:Product-units_in_stock ;
                        northwind:unitsOnOrder products.UnitsOnOrder
                                as virtrdf:Product-units_on_order ;
                        northwind:reorderLevel products.ReorderLevel
                                as virtrdf:Product-reorder_level ;
                        northwind:discontinued products.Discontinued
                                as virtrdf:Product-discontinued ;
                        rdfs:isDefinedBy northwind:product_iri (products.ProductID) ;
                        rdfs:isDefinedBy northwind:Product (products.ProductID) .

                northwind:Category (products.CategoryID)
                        northwind:category_of northwind:Product (products.ProductID) as virtrdf:Product-category_of .

                northwind:Supplier (products.SupplierID)
                        northwind:supplier_of northwind:Product (products.ProductID) as virtrdf:Product-supplier_of .

                northwind:Supplier (suppliers.SupplierID)
                        a northwind:Supplier
                                as virtrdf:Supplier-SupplierID ;
                        northwind:companyName suppliers.CompanyName
                                as virtrdf:Supplier-company_name ;
                        northwind:contactName suppliers.ContactName
                                as virtrdf:Supplier-contact_name ;
                        northwind:contactTitle suppliers.ContactTitle
                                as virtrdf:Supplier-contact_title ;
                        northwind:address suppliers.Address
                                as virtrdf:Supplier-address ;
                        northwind:city suppliers.City
                                as virtrdf:Supplier-city ;
                        northwind:dbpedia_city northwind:dbpedia_iri(suppliers.City)
                                as virtrdf:Supplier-dbpediacity ;
                        northwind:region suppliers.Region
                                as virtrdf:Supplier-region ;
                        northwind:postalCode suppliers.PostalCode
                                as virtrdf:Supplier-postal_code ;
                        northwind:country northwind:Country(suppliers.Country)
                                as virtrdf:Supplier-country ;
                        northwind:phone suppliers.Phone
                                as virtrdf:Supplier-phone ;
                        northwind:fax suppliers.Fax
                                as virtrdf:Supplier-fax ;
                        northwind:homePage suppliers.HomePage
                                as virtrdf:Supplier-home_page ;
                        rdfs:isDefinedBy northwind:supplier_iri (suppliers.SupplierID) ;
                        rdfs:isDefinedBy northwind:Supplier (suppliers.SupplierID) .

                northwind:Country (suppliers.Country)
                northwind:Supplier (suppliers.SupplierID) as virtrdf:Supplier-is_country_of .

                northwind:Category (categories.CategoryID)
                        a northwind:Category
                                as virtrdf:Category-CategoryID ;
                        northwind:categoryName categories.CategoryName
                                as virtrdf:Category-home_page ;
                        northwind:description categories.Description
                                as virtrdf:Category-description ;
                        foaf:img northwind:CategoryPhoto(categories.CategoryID)
                                as virtrdf:Category-categories.CategoryPhoto ;
                        rdfs:isDefinedBy northwind:category_iri (categories.CategoryID) ;
                        rdfs:isDefinedBy northwind:Category (categories.CategoryID) .

                        a northwind:CategoryPhoto
                                as virtrdf:Category-categories.CategoryPhotoID ;
                        rdfs:isDefinedBy northwind:categoryphoto_iri (categories.CategoryID) ;
                        rdfs:isDefinedBy northwind:CategoryPhoto(categories.CategoryID) .

                northwind:Shipper (shippers.ShipperID)
                        a northwind:Shipper
                                as virtrdf:Shipper-ShipperID ;
                        northwind:companyName shippers.CompanyName
                                as virtrdf:Shipper-company_name ;
                        northwind:phone shippers.Phone
                                as virtrdf:Shipper-phone ;
                        rdfs:isDefinedBy northwind:shipper_iri (shippers.ShipperID) ;
                        rdfs:isDefinedBy northwind:Shipper (shippers.ShipperID) .

                northwind:Customer (customers.CustomerID)
                        a  northwind:Customer
                                as virtrdf:Customer-CustomerID2 ;
                        a  foaf:Organization
                                as virtrdf:Customer-CustomerID ;
                        foaf:name customers.CompanyName
                                as virtrdf:Customer-foaf_name ;
                        northwind:companyName customers.CompanyName
                                as virtrdf:Customer-company_name ;
                        northwind:has_contact northwind:CustomerContact (customers.CustomerID)
                                as virtrdf:Customer-contact ;
                        northwind:country northwind:Country (customers.Country)
                                as virtrdf:Customer-country ;
                        northwind:contactName customers.ContactName
                                as virtrdf:Customer-contact_name ;
                        northwind:contactTitle customers.ContactTitle
                                as virtrdf:Customer-contact_title ;
                        northwind:address customers.Address
                                as virtrdf:Customer-address ;
                        northwind:city customers.City
                                as virtrdf:Customer-city ;
                        northwind:dbpedia_city northwind:dbpedia_iri(customers.City)
                                as virtrdf:Customer-dbpediacity ;
                        northwind:region customers.Region
                                as virtrdf:Customer-region ;
                        northwind:PostalCode customers.PostalCode
                                as virtrdf:Customer-postal_code ;
                        foaf:phone customers.Phone
                                as virtrdf:Customer-foaf_phone ;
                        northwind:phone customers.Phone
                                as virtrdf:Customer-phone ;
                        northwind:fax customers.Fax
                                as virtrdf:Customer-fax ;
                        rdfs:isDefinedBy northwind:customer_iri (customers.CustomerID) ;
                        rdfs:isDefinedBy northwind:Customer (customers.CustomerID) .

                northwind:Country (customers.Country)
                northwind:Customer (customers.CustomerID) as virtrdf:Customer-is_country_of .

                northwind:Employee (employees.FirstName, employees.LastName, employees.EmployeeID)
                        a northwind:Employee
                                as virtrdf:Employee-EmployeeID2 ;
                        a foaf:Person
                                as virtrdf:Employee-EmployeeID ;
                        foaf:surname employees.LastName
                                as virtrdf:Employee-foaf_last_name ;
                        northwind:lastName employees.LastName
                                as virtrdf:Employee-last_name ;
                        foaf:firstName employees.FirstName
                                as virtrdf:Employee-foaf_first_name ;
                        northwind:firstName employees.FirstName
                                as virtrdf:Employee-first_name ;
                        foaf:title employees.Title
                                as virtrdf:Employee-title ;
                        northwind:titleOfCourtesy employees.TitleOfCourtesy
                                as virtrdf:Employee-title_of_courtesy ;
                        foaf:birthday employees.BirthDate
                                as virtrdf:Employee-foaf_birth_date ;
                        northwind:birthday employees.BirthDate
                                as virtrdf:Employee-birth_date ;
                        northwind:hireDate employees.HireDate
                                as virtrdf:Employee-hire_date ;
                        northwind:address employees.Address
                                as virtrdf:Employee-address ;
                        northwind:city employees.City
                                as virtrdf:Employee-city ;
                        northwind:dbpedia_city northwind:dbpedia_iri(employees.City)
                                as virtrdf:Employee-dbpediacity ;
                        northwind:region employees.Region
                                as virtrdf:Employee-region ;
                        northwind:postalCode employees.PostalCode
                                as virtrdf:Employee-postal_code ;
                        northwind:country northwind:Country(employees.Country)
                                as virtrdf:Employee-country ;
                        foaf:phone employees.HomePhone
                                as virtrdf:Employee-home_phone ;
                        northwind:extension employees.Extension
                                as virtrdf:Employee-extension ;
                        northwind:notes employees.Notes
                                as virtrdf:Employee-notes ;
                        northwind:reportsTo northwind:Employee(employees.FirstName, employees.LastName, employees.ReportsTo) where (^{employees.}^.ReportsTo = ^{employees.}^.EmployeeID)
                                as virtrdf:Employee-reports_to ;
                        foaf:img northwind:EmployeePhoto(employees.EmployeeID)
                                as virtrdf:Employee-employees.EmployeePhoto ;
                        rdfs:isDefinedBy northwind:employee_iri (employees.EmployeeID) ;
                        rdfs:isDefinedBy northwind:Employee (employees.FirstName, employees.LastName, employees.EmployeeID) .

                        a northwind:EmployeePhoto
                                as virtrdf:Employee-employees.EmployeePhotoId ;
                        rdfs:isDefinedBy northwind:employeephoto_iri (employees.EmployeeID) ;
                        rdfs:isDefinedBy northwind:EmployeePhoto (employees.EmployeeID) .

                northwind:Employee (employees.FirstName, employees.LastName, orders.EmployeeID)
                northwind:Order (orders.OrderID) where (^{orders.}^.EmployeeID = ^{employees.}^.EmployeeID) as virtrdf:Order-is_salesrep_of .

                northwind:Country (employees.Country)
                northwind:Employee (employees.FirstName, employees.LastName, employees.EmployeeID) as virtrdf:Employee-is_country_of .

                northwind:Order (orders.OrderID)
                        a northwind:Order
                                as virtrdf:Order-Order ;
                        northwind:has_customer northwind:Customer (orders.CustomerID)
                                as virtrdf:Order-order_has_customer ;
                        northwind:has_salesrep northwind:Employee (employees.FirstName, employees.LastName, orders.EmployeeID) where (^{orders.}^.EmployeeID = ^{employees.}^.EmployeeID)
                                as virtrdf:Customer-has_salesrep ;
                        northwind:has_employee northwind:Employee (employees.FirstName, employees.LastName, orders.EmployeeID) where (^{orders.}^.EmployeeID = ^{employees.}^.EmployeeID)
                                as virtrdf:Order-order_has_employee ;
                        northwind:orderDate orders.OrderDate
                                as virtrdf:Order-order_date ;
                        northwind:requiredDate orders.RequiredDate
                                as virtrdf:Order-required_date ;
                        northwind:shippedDate orders.ShippedDate
                                as virtrdf:Order-shipped_date ;
                        northwind:order_ship_via northwind:Shipper (orders.ShipVia)
                                as virtrdf:Order-order_ship_via ;
                        northwind:freight orders.Freight
                                as virtrdf:Order-freight ;
                        northwind:shipName orders.ShipName
                                as virtrdf:Order-ship_name ;
                        northwind:shipAddress orders.ShipAddress
                                as virtrdf:Order-ship_address ;
                        northwind:shipCity orders.ShipCity
                                as virtrdf:Order-ship_city ;
                        northwind:dbpedia_shipCity northwind:dbpedia_iri(orders.ShipCity)
                                as virtrdf:Order-dbpediaship_city ;
                        northwind:shipRegion orders.ShipRegion
                                as virtrdf:Order-ship_region ;
                        northwind:shipPostal_code orders.ShipPostalCode
                                as virtrdf:Order-ship_postal_code ;
                        northwind:shipCountry northwind:Country(orders.ShipCountry)
                                as virtrdf:ship_country ;
                        rdfs:isDefinedBy northwind:order_iri (orders.OrderID) ;
                        rdfs:isDefinedBy northwind:Order (orders.OrderID) .

                northwind:Country (orders.ShipCountry)
                northwind:Order (orders.OrderID) as virtrdf:Order-is_country_of .

                northwind:Customer (orders.CustomerID)
                        northwind:has_order northwind:Order (orders.OrderID) as virtrdf:Order-has_order .

                northwind:Shipper (orders.ShipVia)
                        northwind:ship_order northwind:Order (orders.OrderID) as virtrdf:Order-ship_order .

                northwind:OrderLine (order_lines.OrderID, order_lines.ProductID)
                        a northwind:OrderLine
                                as virtrdf:OrderLine-OrderLines ;
                        northwind:has_order_id northwind:Order (order_lines.OrderID)
                                as virtrdf:order_lines_has_order_id ;
                        northwind:has_product_id northwind:Product (order_lines.ProductID)
                                as virtrdf:order_lines_has_product_id ;
                        northwind:unitPrice order_lines.UnitPrice
                                as virtrdf:OrderLine-unit_price ;
                        northwind:quantity order_lines.Quantity
                                as virtrdf:OrderLine-quantity ;
                        northwind:discount order_lines.Discount
                                as virtrdf:OrderLine-discount ;
                        rdfs:isDefinedBy northwind:orderline_iri (order_lines.OrderID, order_lines.ProductID) ;
                        rdfs:isDefinedBy northwind:OrderLine (order_lines.OrderID, order_lines.ProductID) .

                northwind:Order (orders.OrderID)
                northwind:OrderLine (order_lines.OrderID, order_lines.ProductID) where (^{orders.}^.OrderID = ^{order_lines.}^.OrderID) as virtrdf:Order-is_order_of .

                northwind:Product (products.ProductID)
                northwind:OrderLine (order_lines.OrderID, order_lines.ProductID) where (^{products.}^.ProductID = ^{order_lines.}^.ProductID) as virtrdf:Product-is_product_of .

                northwind:Country (countries.Name)
                        a northwind:Country
                                as virtrdf:Country-Type2 ;
                        a wgs:SpatialThing
                                as virtrdf:Country-Type ;
                        owl:sameAs northwind:dbpedia_iri (countries.Name) ;
                        northwind:name countries.Name
                                as virtrdf:Country-Name ;
                        northwind:code countries.Code
                                as virtrdf:Country-Code ;
                        northwind:smallFlagDAVResourceName countries.SmallFlagDAVResourceName
                                as virtrdf:Country-SmallFlagDAVResourceName ;
                        northwind:largeFlagDAVResourceName countries.LargeFlagDAVResourceName
                                as virtrdf:Country-LargeFlagDAVResourceName ;
                        northwind:smallFlagDAVResourceURI northwind:Flag(countries.SmallFlagDAVResourceURI)
                                as virtrdf:Country-SmallFlagDAVResourceURI ;
                        northwind:largeFlagDAVResourceURI northwind:Flag(countries.LargeFlagDAVResourceURI)
                                as virtrdf:Country-LargeFlagDAVResourceURI ;
                        wgs:lat countries.Lat
                                as virtrdf:Country-Lat ;
                        wgs:long countries.Lng
                                as virtrdf:Country-Lng ;
                        rdfs:isDefinedBy northwind:country_iri (countries.Name) ;
                        rdfs:isDefinedBy northwind:Country (countries.Name) .

                northwind:Country (countries.Name)
                northwind:Province (provinces.CountryCode, provinces.Province) where (^{provinces.}^.CountryCode = ^{countries.}^.Code) as virtrdf:Country-has_province .

                northwind:Province (provinces.CountryCode, provinces.Province)
                        a northwind:Province
                                as virtrdf:Province-Provinces ;
                        northwind:has_country_code provinces.CountryCode
                                as virtrdf:has_country_code ;
                        northwind:provinceName provinces.Province
                                as virtrdf:Province-ProvinceName ;
                        rdfs:isDefinedBy northwind:province_iri (provinces.CountryCode, provinces.Province) ;
                        rdfs:isDefinedBy northwind:Province (provinces.CountryCode, provinces.Province) .

                northwind:Province (provinces.CountryCode, provinces.Province)
                northwind:Country (countries.Name) where  (^{countries.}^.Code = ^{provinces.}^.CountryCode) as virtrdf:Province-country_of .

The created Linked Data View is sufficient for querying relational data via SPARQL but not for accessing data by dereferencing IRIs of subjects. Making IRIs dereferenceable requires configuring HTTP server; that is explained in second part of the example .