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| <h1 class="title">ARQ - Custom Service Executors</h1> |
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| <main class="d-flex flex-xl-row flex-column"> |
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| <h2 class="h6 sticky-top m-0 p-2 bg-body-tertiary">On this page</h2> |
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| <li><a href="#examples">Examples</a> |
| <ul> |
| <li><a href="#global-vs-local-service-executor-registration">Global vs Local Service Executor Registration</a></li> |
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| <article class="flex-column me-lg-4"> |
| <p>Since Jena 4.2.0, ARQ features a plugin system for custom service executors. |
| The relevant classes are located in the package <code>org.apache.jena.sparql.service</code> and are summarized as follows:</p> |
| <ul> |
| <li> |
| <p><code>ServiceExecutorRegistry</code>: A registry that holds a list of service executors. When Jena starts up, it configures a default registry to handle <code>SERVICE</code> requests against HTTP SPARQL endpoints and registers it with the global ARQ context accessible under <code>ARQ.getContext()</code>.</p> |
| </li> |
| <li> |
| <p><code>ServiceExecutorFactory</code>: This is the main interface for custom SERVICE handler implementations:</p> |
| </li> |
| </ul> |
| <div class="highlight"><pre tabindex="0" style="background-color:#f8f8f8;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-java" data-lang="java"><span style="display:flex;"><span><span style="color:#a2f;font-weight:bold">public</span> <span style="color:#a2f;font-weight:bold">interface</span> <span style="color:#00f">ServiceExecutorFactory</span> <span style="color:#666">{</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#a2f;font-weight:bold">public</span> ServiceExecution <span style="color:#00a000">createExecutor</span><span style="color:#666">(</span>OpService substituted<span style="color:#666">,</span> OpService original<span style="color:#666">,</span> Binding binding<span style="color:#666">,</span> ExecutionContext execCxt<span style="color:#666">);</span> |
| </span></span><span style="display:flex;"><span><span style="color:#666">}</span> |
| </span></span></code></pre></div><p>The second OpService parameter represents the original <code>SERVICE</code> clause as it occurs in the query, whereas the first parameter is the OpService obtained after substitution of all mentioned variables w.r.t. the current binding. |
| A <code>ServiceExecutorFactory</code> can indicate its non-applicability for handling a request simply by returning <code>null</code>. In that case, Jena will ask the next service executor factory in the registry. If a request remains unhandled then the QueryExecException <code>No SERVICE handler</code> is raised.</p> |
| <ul> |
| <li><code>ServiceExecution</code>: If a <code>ServiceExectorFactory</code> can handle a request then it needs to returns a <code>ServiceExecution</code> instance:</li> |
| </ul> |
| <div class="highlight"><pre tabindex="0" style="background-color:#f8f8f8;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-java" data-lang="java"><span style="display:flex;"><span><span style="color:#a2f;font-weight:bold">public</span> <span style="color:#a2f;font-weight:bold">interface</span> <span style="color:#00f">ServiceExecution</span> <span style="color:#666">{</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#a2f;font-weight:bold">public</span> QueryIterator <span style="color:#00a000">exec</span><span style="color:#666">();</span> |
| </span></span><span style="display:flex;"><span><span style="color:#666">}</span> |
| </span></span></code></pre></div><p>The actual execution is started by calling the <code>exec()</code> method which returns a <code>QueryIterator</code>. |
| Note, that there are uses cases where ServiceExecution instances may not have to be executed. For example, one may analyze which service executor factories among a set of them claim to be capable of handling a request. This can be useful for debugging or display in a dashboard of applicable service executors.</p> |
| <h2 id="examples">Examples</h2> |
| <p>A runnable example suite is located in the jena-examples module at <a href="https://github.com/apache/jena/blob/main/jena-examples/src/main/java/arq/examples/service/CustomServiceExecutor.java">CustomServiceExecutor.java</a>.</p> |
| <p>In the remainder we summarize the essentials of setting up a custom service executor. |
| The following snippet sets up a simple service executor factory that relays queries targeted at Wikidata to DBpedia:</p> |
| <div class="highlight"><pre tabindex="0" style="background-color:#f8f8f8;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-java" data-lang="java"><span style="display:flex;"><span>Node WIKIDATA <span style="color:#666">=</span> NodeFactory<span style="color:#666">.</span><span style="color:#b44">createURI</span><span style="color:#666">(</span><span style="color:#b44">"http://query.wikidata.org/sparql"</span><span style="color:#666">);</span> |
| </span></span><span style="display:flex;"><span>Node DBPEDIA <span style="color:#666">=</span> NodeFactory<span style="color:#666">.</span><span style="color:#b44">createURI</span><span style="color:#666">(</span><span style="color:#b44">"http://dbpedia.org/sparql"</span><span style="color:#666">);</span> |
| </span></span><span style="display:flex;"><span> |
| </span></span><span style="display:flex;"><span>ServiceExecutorFactory myExecutorFactory <span style="color:#666">=</span> <span style="color:#666">(</span>opExecute<span style="color:#666">,</span> original<span style="color:#666">,</span> binding<span style="color:#666">,</span> execCxt<span style="color:#666">)</span> <span style="color:#666">-></span> <span style="color:#666">{</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#a2f;font-weight:bold">if</span> <span style="color:#666">(</span>opExecute<span style="color:#666">.</span><span style="color:#b44">getService</span><span style="color:#666">().</span><span style="color:#b44">equals</span><span style="color:#666">(</span>WIKIDATA<span style="color:#666">))</span> <span style="color:#666">{</span> |
| </span></span><span style="display:flex;"><span> opExecute <span style="color:#666">=</span> <span style="color:#a2f;font-weight:bold">new</span> OpService<span style="color:#666">(</span>DBPEDIA<span style="color:#666">,</span> opExecute<span style="color:#666">.</span><span style="color:#b44">getSubOp</span><span style="color:#666">(),</span> opExecute<span style="color:#666">.</span><span style="color:#b44">getSilent</span><span style="color:#666">());</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#a2f;font-weight:bold">return</span> ServiceExecutorRegistry<span style="color:#666">.</span><span style="color:#b44">httpService</span><span style="color:#666">.</span><span style="color:#b44">createExecutor</span><span style="color:#666">(</span>opExecute<span style="color:#666">,</span> original<span style="color:#666">,</span> binding<span style="color:#666">,</span> execCxt<span style="color:#666">);</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#666">}</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#a2f;font-weight:bold">return</span> <span style="color:#a2f;font-weight:bold">null</span><span style="color:#666">;</span> |
| </span></span><span style="display:flex;"><span><span style="color:#666">};</span> |
| </span></span></code></pre></div><h3 id="global-vs-local-service-executor-registration">Global vs Local Service Executor Registration</h3> |
| <p>The global registry can be accessed and modified as shown below:</p> |
| <div class="highlight"><pre tabindex="0" style="background-color:#f8f8f8;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-java" data-lang="java"><span style="display:flex;"><span>ServiceExecutorRegistry globalRegistry <span style="color:#666">=</span> ServiceExecutorRegistry<span style="color:#666">.</span><span style="color:#b44">get</span><span style="color:#666">();</span> |
| </span></span><span style="display:flex;"><span> |
| </span></span><span style="display:flex;"><span><span style="color:#080;font-style:italic">// Note: registry.add() prepends executor factories to the internal list such |
| </span></span></span><span style="display:flex;"><span><span style="color:#080;font-style:italic">// that they are consulted first! |
| </span></span></span><span style="display:flex;"><span><span style="color:#080;font-style:italic"></span>globalRegistry<span style="color:#666">.</span><span style="color:#b44">add</span><span style="color:#666">(</span>myExecutorFactory<span style="color:#666">);</span> |
| </span></span></code></pre></div><p>The following snippet shows how a custom service executor can be configured locally for an individual query execution:</p> |
| <div class="highlight"><pre tabindex="0" style="background-color:#f8f8f8;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-java" data-lang="java"><span style="display:flex;"><span>Context cxt <span style="color:#666">=</span> ARQ<span style="color:#666">.</span><span style="color:#b44">getContext</span><span style="color:#666">().</span><span style="color:#b44">copy</span><span style="color:#666">();</span> |
| </span></span><span style="display:flex;"><span>ServiceExecutorRegistry localRegistry <span style="color:#666">=</span> ServiceExecutorRegistry<span style="color:#666">().</span><span style="color:#b44">get</span><span style="color:#666">().</span><span style="color:#b44">copy</span><span style="color:#666">();</span> |
| </span></span><span style="display:flex;"><span>localRegistry<span style="color:#666">.</span><span style="color:#b44">add</span><span style="color:#666">(</span>myExecutorFactory<span style="color:#666">);</span> |
| </span></span><span style="display:flex;"><span> |
| </span></span><span style="display:flex;"><span>String queryStr <span style="color:#666">=</span> <span style="color:#b44">"SELECT * { SERVICE <http://query.wikidata.org/sparql> { ?s ?p "</span>Apache Jena<span style="color:#b44">"@en } }"</span><span style="color:#666">;</span> |
| </span></span><span style="display:flex;"><span><span style="color:#a2f;font-weight:bold">try</span> <span style="color:#666">(</span>QueryExecution qe <span style="color:#666">=</span> QueryExecutionFactory<span style="color:#666">.</span><span style="color:#b44">create</span><span style="color:#666">(</span>queryStr<span style="color:#666">))</span> <span style="color:#666">{</span> |
| </span></span><span style="display:flex;"><span> ServiceExecutorRegistry<span style="color:#666">.</span><span style="color:#b44">set</span><span style="color:#666">(</span>qe<span style="color:#666">.</span><span style="color:#b44">getContext</span><span style="color:#666">(),</span> registry<span style="color:#666">);</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#080;font-style:italic">// ... |
| </span></span></span><span style="display:flex;"><span><span style="color:#080;font-style:italic"></span><span style="color:#666">}</span> |
| </span></span></code></pre></div> |
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