Add example for optimistic concurrency support

1 files changed, 159 insertions, 0 deletions

diff --git a/optimistic/driver.cxx b/optimistic/driver.cxx
new file mode 100644
index 0000000..78cb14d
--- /dev/null
+++ b/optimistic/driver.cxx
@@ -0,0 +1,159 @@
+// file      : optimistic/driver.cxx
+// author    : Boris Kolpackov <boris@codesynthesis.com>
+// copyright : not copyrighted - public domain
+
+#include <memory>   // std::auto_ptr
+#include <iostream>
+
+#include <odb/database.hxx>
+#include <odb/transaction.hxx>
+
+#include "database.hxx" // create_database
+
+#include "person.hxx"
+#include "person-odb.hxx"
+
+using namespace std;
+using namespace odb::core;
+
+int
+main (int argc, char* argv[])
+{
+  try
+  {
+    auto_ptr<database> db (create_database (argc, argv));
+
+    // Persist the object.
+    //
+    // At this point the initial version (1) is assigned.
+    //
+    unsigned long id;
+    {
+      person p ("John", "Doe", 21);
+
+      transaction t (db->begin ());
+      id = db->persist (p);
+      t.commit ();
+
+      cout << "version after persist: " << p.version () << endl;
+    }
+
+    // Process 1: load the object.
+    //
+    person p1;
+    {
+      transaction t (db->begin ());
+      db->load (id, p1);
+      t.commit ();
+
+      cout << "process 1 version after load: " << p1.version () << endl;
+    }
+
+    // Process 2: load the object.
+    //
+    person p2;
+    {
+      transaction t (db->begin ());
+      db->load (id, p2);
+      t.commit ();
+
+      cout << "process 2 version after load: " << p2.version () << endl;
+    }
+
+    // Process 1: update the object.
+    //
+    // At this point the version is incremented and becomes 2.
+    //
+    {
+      p1.age (20); // Correct the wrong age.
+
+      transaction t (db->begin ());
+      db->update (p1);
+      t.commit ();
+
+      cout << "process 1 version after update: " << p1.version () << endl;
+    }
+
+    // Process 2: update the object.
+    //
+    // Since the object version in this process is 1 while in the database
+    // it is 2, this operation will fail.
+    //
+    {
+      p2.age (p2.age () + 1); // Increment the age.
+
+      transaction t (db->begin ());
+
+      try
+      {
+        db->update (p2);
+      }
+      catch (const object_changed&)
+      {
+        cout << "process 2 version is out of date: " << p2.version () << endl;
+
+        // Reload the object and retry the operation. Note that the second
+        // update call cannot throw object_changed since we reloaded the
+        // object and are trying to update it in a single transaction.
+        //
+        db->reload (p2);
+
+        cout << "process 2 version after reload: " << p2.version () << endl;
+
+        p2.age (p2.age () + 1);
+        db->update (p2);
+      }
+
+      t.commit ();
+
+      cout << "process 2 version after update: " << p2.version () << endl;
+      cout << "final age value: " << p2.age () << endl;
+    }
+
+    // Process 1: delete the object if the person is younger than 21.
+    //
+    // Since the object version in this process is 2 while in the database
+    // it is 3, this operation will fail. Note that this will only hold
+    // true if we are deleting the object by passing an object instance
+    // to the erase() function. If instead we pass object id, then the
+    // object will be deleted regardless of the version.
+    //
+    if (p1.age () < 21)
+    {
+      transaction t (db->begin ());
+
+      try
+      {
+        db->erase (p1);
+        // db->erase (id); // Never throws object_changed.
+      }
+      catch (const object_changed&)
+      {
+        cout << "process 1 version is out of date: " << p1.version () << endl;
+
+        // Reload the object and retry the operation. Similar to update, the
+        // second erase call cannot throw object_changed since we reloaded
+        // the object and are trying to erase it in a single transaction.
+        //
+        db->reload (p1);
+
+        cout << "process 1 version after reload: " << p2.version () << endl;
+
+        if (p1.age () < 21)
+        {
+          db->erase (p1);
+          cout << "object deleted" << endl;
+        }
+        else
+          cout << "object not deleted" << endl;
+      }
+
+      t.commit ();
+    }
+  }
+  catch (const odb::exception& e)
+  {
+    cerr << e.what () << endl;
+    return 1;
+  }
+}