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| <h1 class="title">The DatasetGraph hierarchy.</h1> |
| |
| <p><em>These notes were written February 2016.</em></p> |
| <p><code>DatasetGraph</code> forms the basic of storage as |
| <a href="https://www.w3.org/TR/rdf11-concepts/#section-dataset">RDFDataset</a>. There |
| is a class hierarchy to make implementation a matter of choosing the style |
| of implementation and adding specific functionality.</p> |
| <p>The hierarchy of the significant classes is: |
| (there are others adding special features)</p> |
| <pre><code>DatasetGraph - the interface |
| DatasetGraphBase |
| DatasetGraphBaseFind |
| DatasetGraphCollection |
| DatasetGraphMapLink - ad hoc collection of graphs |
| DatasetGraphOne |
| DatasetGraphTriplesQuads |
| DatasetGraphInMemory - fully transactional in-memory. |
| DatasetGraphMap |
| DatasetGraphQuads |
| DatasetGraphTrackActive - transaction support |
| DatasetGraphTransaction - This is the main TDB dataset. |
| DatasetGraphWithLock - MRSW support |
| DatasetGraphWrapper |
| DatasetGraphTxn - TDB usage |
| DatasetGraphViewGraphs |
| </code></pre> |
| <p>Other important classes:</p> |
| <pre><code>GraphView |
| </code></pre> |
| <h3 id="datasetgraph">DatasetGraph</h3> |
| <p>This is the interface. Includes <code>Transactional</code> operations.</p> |
| <p>There are two markers for transaction features supported.</p> |
| <p>If <code>begin</code>, <code>commit</code> and <code>end</code> are supported (which is normally the case) |
| <code>supportsTransactions</code> returns true.</p> |
| <p>If, further, <code>abort</code> is supported, then <code>supportsTransactionAbort</code> is true.</p> |
| <h3 id="general-hierarchy">General hierarchy</h3> |
| <p><strong>DatasetGraphBase</strong></p> |
| <p>This provides some basic machinery and provides implementations of |
| operations that have alternative styles. It converts <code>add(G,S,P,O)</code> to |
| <code>add(quad)</code> and <code>delete(G,S,P,O)</code> to <code>delete(quad)</code> and converts |
| <code>find(quad)</code> to <code>find(G,S,P,O)</code>.</p> |
| <p>It provides basic implementations of <code>deleteAny(?,?,?,?)</code> and <code>clear()</code>.</p> |
| <p>It provides a Lock (LockMRSW) and the Context.</p> |
| <p>From here on down, the storage aspect of the hierarchy splits depending on |
| implementation style.</p> |
| <p><strong>DatasetGraphBaseFind</strong></p> |
| <p>This is the beginning of the hierarchy for DSGs that store using different units for default graph and named graphs. |
| This class splits find/4 into the following variants:</p> |
| <pre><code> findInDftGraph |
| findInUnionGraph |
| findQuadsInUnionGraph |
| findUnionGraphTriples |
| findInSpecificNamedGraph |
| findInAnyNamedGraphs |
| </code></pre><p><strong>DatasetGraphTriplesQuads</strong></p> |
| <p>This is the beginning of the hierarchy for DSGs implemented as a set of triples for the default graph and a set of quads for all the named graphs.</p> |
| <p>It splits add(Quad) and delete(Quad) into:</p> |
| <pre><code>addToDftGraph |
| addToNamedGraph |
| deleteFromDftGraph |
| deleteFromNamedGraph |
| </code></pre> |
| <p>and makes</p> |
| <pre><code>setDefaultGraph |
| addGraph |
| removeGraph |
| </code></pre> |
| <p>copy-in operations - triples are copied into the graph or removed from the |
| graph, rather than the graph being shared.</p> |
| <p>** DatasetGraphInMemory**</p> |
| <p>The main in-memory implementation, providing full transactions (serializable isolation, abort).</p> |
| <p>Use this one!</p> |
| <p>This class backs <code>DatasetFactory.createTxnMem()</code>.</p> |
| <p><strong>DatasetGraphMap</strong></p> |
| <p>The in-memory implementation using in-memory Graphs as the storage for Triples. |
| It provides MRSW-transactions (serializable isolation, no real abort). |
| Use this if a single threaded application.</p> |
| <p>This class backs <code>DatasetFactory.create()</code>.</p> |
| <p><strong>DatasetGraphCollection</strong></p> |
| <p>Operations split into operations on a collection of Graphs, one for the default graph, and a map of (Node,Graph) for the named graphs. |
| It provides MRSW-transactions (serializable isolation, no real abort).</p> |
| <p><strong>DatasetGraphMapLink</strong></p> |
| <p>This implementation is manages Graphs provided by the application.</p> |
| <p>It provides MRSW-transactions (serializable isolation, no real abort). |
| Applications need to be careful when modifying the Graphs directly and also |
| modifying them via the DatasetGraph interface.</p> |
| <p>This class backs <code>DatasetFactory.createGeneral()</code>.</p> |
| <p><strong>DatasetGraphWrapper</strong></p> |
| <p>Indirection to another <code>DatasetGraph</code>.</p> |
| <p>Surprisingly useful.</p> |
| <p><strong>DatasetGraphViewGraphs</strong></p> |
| <p>A small class that provides the “get a graph” operations over a |
| <code>DatasetGraph</code> using <code>GraphView</code>.</p> |
| <p>Not used because subclasses usually want to inherit from a different part |
| fo the hierarchy but the idea of implementing <code>getDefaultGraph()</code> and |
| <code>getGraph(Node)</code> as calls to <code>GraphView</code> is used elsewhere.</p> |
| <p>Do not use with an implementations that store using graph |
| (e.g. <code>DatasetGraphMap</code>, <code>DatasetGraphMapLink</code>) because it goes into an |
| infinite recursion if they use GraphView internally.</p> |
| <p><strong>GraphView</strong></p> |
| <p>Implementation of the Graph interface as a view of a DatasetGraph including |
| providing a basic implementation of the union graph. Subclasses can, and do, |
| provide a better mechanisms for the union graph based on their internal |
| indexes.</p> |
| <p><strong>DatasetGraphOne</strong></p> |
| <p>An implement that only provides a default graph, given at creation time. |
| This is a fixed - the app can’t add named graphs.</p> |
| <p>Cuts through all the machinery to be a simple, direct implementation.</p> |
| <p>Backs up <code>DatasetGraphFactory.createOneGraph</code> but not |
| <code>DatasetFactory.create(Model)</code> which provided are adding named graphs.</p> |
| <p><strong>DatasetGraphQuads</strong></p> |
| <p>Root for implementations based on just quad storage, no special triples in |
| the default graph (e.g. the default graph is always the calculated union of |
| named graphs).</p> |
| <p>Not used currently.</p> |
| <p><strong>DatasetGraphTrackActive</strong></p> |
| <p>Framework for implementing transactions. Provides checking.</p> |
| <p><strong>DatasetGraphWithLock</strong></p> |
| <p>Provides transactions, without abort by default, using a lock. If the lock |
| is LockMRSW, we get multiple-readers or a single writer at any given moment |
| in time. As most datastructures are multi-thread reader safe, this style |
| works over systems that do not themselves provide transactions.</p> |
| <p>Abort requires work to be undone. Jena may in the future provide reverse |
| replay abort (do the adds and deletes in reverse operation, reverse order) |
| but this is partial. It does not protect against the DatasetGraph |
| implementation throwing exceptions nor JVM or machine crash (if any |
| persistence). It still needs MRSW to archive isolation.</p> |
| <p>Read-committed needs synchronization safe datastructures -including |
| co-ordinated changes to several places at once (ConcurrentHashMap isn’t |
| enough - need to update 2 or more ConcurrentHashMaps together).</p> |
| <h3 id="tdb">TDB</h3> |
| <p><strong>DatasetGraphTDB</strong></p> |
| <p><code>DatasetGraphTDB</code> is concerned with the storage |
| historical and not used directly by applications.</p> |
| <p><strong>DatasetGraphTransaction</strong></p> |
| <p>This is the class returned by <code>TDBFactory</code>, wrapped in <code>DatasetImpl</code>.</p> |
| <p>Different in TDB2 - DatasetGraphTransaction not used, DatasetGraphTDB is transactional.</p> |
| <p><strong>DatasetGraphTxn</strong></p> |
| <p>This is the TDB per-transaction <code>DatasetGraph</code> using the transaction view |
| of indexes. For the application, it is held in the transactions |
| <code>ThreadLocal</code> in <code>DatasetGraphTransaction</code>.</p> |
| <p>Internally, each read transaction for the same generation of the data uses |
| the same <code>DatasetGraphTransaction</code>.</p> |
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