diff --git a/application/clicommands/ImportCommand.php b/application/clicommands/ImportCommand.php
index fdd319d2..9da9a297 100644
--- a/application/clicommands/ImportCommand.php
+++ b/application/clicommands/ImportCommand.php
@@ -13,7 +13,9 @@ class ImportCommand extends Command
 
     public function runAction()
     {
-        if ($runId = Import::run($id = ImportSource::load($this->params->shift(), $this->db()))) {
+        $id = $this->params->shift();
+        $import = new Import(ImportSource::load($id, $this->db()));
+        if ($runId = $import->run()) {
             echo "Triggered new import\n";
         } else {
             echo "Nothing changed\n";
diff --git a/application/controllers/ImportsourceController.php b/application/controllers/ImportsourceController.php
index ebf1efba..c9ae7dde 100644
--- a/application/controllers/ImportsourceController.php
+++ b/application/controllers/ImportsourceController.php
@@ -22,7 +22,9 @@ class ImportsourceController extends ActionController
 
     public function runAction()
     {
-        if ($runId = Import::run($id = ImportSource::load($this->params->get('id'), $this->db()))) {
+        $id = $this->params->get('id');
+        $import = new Import(ImportSource::load($id, $this->db()));
+        if ($runId = $import->run()) {
             Notification::success('Import succeeded');
             $this->redirectNow(Url::fromPath('director/importrun', array('id' => $runId)));
         } else {
diff --git a/library/Director/Import/Import.php b/library/Director/Import/Import.php
index 6cd27db5..03dbbb65 100644
--- a/library/Director/Import/Import.php
+++ b/library/Director/Import/Import.php
@@ -25,22 +25,135 @@ class Import
      */
     protected $db;
 
+    /**
+     * Raw data that should be imported, array of stdClass objects
+     *
+     * @var array
+     */
     protected $data;
 
-    protected function __construct(ImportSource $source)
+    /**
+     * Checksum of the rowset that should be imported
+     *
+     * @var string
+     */
+    private $rowsetChecksum;
+
+    /**
+     * Checksum-indexed rows
+     *
+     * @var array
+     */
+    private $rows;
+
+    /**
+     * Checksum-indexed row -> property
+     *
+     * @var array
+     */
+    private $rowProperties;
+
+    /**
+     * Whether this rowset exists, for caching purposes
+     *
+     * @var boolean
+     */
+    private $rowsetExists;
+
+    protected $properties = array();
+
+    /**
+     * Checksums of all rows
+     */
+    private $rowChecksums;
+
+    public function __construct(ImportSource $source)
     {
         $this->source = $source;
         $this->connection = $source->getConnection();
-        $this->db = $connection->getDbAdapter();
+        $this->db = $this->connection->getDbAdapter();
     }
 
-    public static function run(ImportSource $source)
+    /**
+     * Whether this import provides modified data
+     *
+     * @return boolean
+     */
+    public function providesChanges()
     {
-        $import = new self($source);
-        return $import->importFromSource($source);
+        return ! $this->rowsetExists()
+            || ! $this->lastRowsetIs($this->rowsetChecksum());
     }
 
-    protected function rawData()
+    /**
+     * Trigger an import run
+     *
+     * @return int Last import run ID
+     */
+    public function run()
+    {
+        if ($this->providesChanges() && ! $this->rowsetExists()) {
+            $this->storeRowset();
+        }
+
+        $this->db->insert(
+            'import_run',
+            array(
+                'source_id'       => $this->source->id,
+                'rowset_checksum' => $this->rowsetChecksum(),
+                'start_time'      => date('Y-m-d H:i:s'),
+                'succeeded'       => 'y'
+            )
+        );
+
+        return $this->db->lastInsertId();
+    }
+
+    /**
+     * Whether there are no rows to be fetched from import source
+     *
+     * @return boolean
+     */
+    public function isEmpty()
+    {
+        $rows = $this->checksummedRows();
+        return empty($rows);
+    }
+
+    /**
+     * Checksum of all available rows
+     *
+     * @return string
+     */
+    protected function & rowsetChecksum()
+    {
+        if ($this->rowsetChecksum === null) {
+            $this->prepareChecksummedRows();
+        }
+
+        return $this->rowsetChecksum;
+    }
+
+    /**
+     * Checksum of all available rows
+     *
+     * @return string
+     */
+    protected function & checksummedRows()
+    {
+        if ($this->rows === null) {
+            $this->prepareChecksummedRows();
+        }
+
+        return $this->rows;
+    }
+
+    /**
+     * Checksum of all available rows
+     *
+     * @return string
+     */
+    protected function & rawData()
     {
         if ($this->data === null) {
             $this->data = ImportSourceHook::loadByName(
@@ -49,145 +162,159 @@ class Import
             )->fetchData();
         }
 
-        return & $this->data;
+        return $this->data;
     }
 
-    protected function importFromSource()
+    /**
+     * Prepare and remember an ImportedProperty
+     *
+     * @return array
+     */
+    protected function prepareImportedProperty($key, $rawValue)
     {
-        $source = $this->source;
-        $connection = $source->getConnection();
-        $this->db = $db = $connection->getDbAdapter();
+        if (is_array($rawValue)) {
+            $value = json_encode($rawValue);
+            $format = 'json';
+        } elseif ($rawValue instanceof stdClass) {
+            $value = json_encode($this->sortObject($rawValue));
+            $format = 'json';
+        } else {
+            $value = $rawValue;
+            $format = 'string';
+        }
 
-        $keyColumn = $source->key_column;
-        $rows = array();
-        $props = array();
-        $rowsProps = array();
+        $checksum = sha1(sprintf('%s=(%s)%s', $key, $format, $value), true);
+
+        if (! array_key_exists($checksum, $this->properties)) {
+            $this->properties[$checksum] = array(
+                'checksum'       => $checksum,
+                'property_name'  => $key,
+                'property_value' => $value,
+                'format'         => $format
+            );
+        }
+
+        return $this->properties[$checksum];
+    }
+
+    /**
+     * Walk through each row, prepare properties and calculate checksums
+     */
+    protected function prepareChecksummedRows()
+    {
+        $keyColumn = $this->source->key_column;
+        $this->rows = array();
+        $this->rowProperties = array();
+        $objects = array();
+        $rowCount = 0;
 
         foreach ($this->rawData() as $row) {
-            // TODO: Check for name collision
+            $rowCount++;
+
+            // Key column must be set
             if (! isset($row->$keyColumn)) {
-                // TODO: re-enable errors
-                continue;
                 throw new IcingaException(
-                    'No key column "%s" in row: %s',
+                    'No key column "%s" in row %d: %s',
                     $keyColumn,
+                    $rowCount,
                     json_encode($row)
                 );
             }
 
             $object_name = $row->$keyColumn;
+
+            // Check for name collision
+            if (array_key_exists($object_name, $objects)) {
+                throw new IcingaException(
+                    'Duplicate entry: %s',
+                    $object_name
+                );
+            }
+
             $rowChecksums = array();
             $keys = array_keys((array) $row);
             sort($keys);
 
             foreach ($keys as $key) {
-
                 // TODO: Specify how to treat NULL values. Ignoring for now.
+                //       One option might be to import null (checksum '(null)')
+                //       and to provide a flag at sync time
                 if ($row->$key === null) {
                     continue;
                 }
 
-                $pval = $row->$key;
-                if (is_array($pval)) {
-                    $pval = json_encode($pval);
-                    $format = 'json';
-                } elseif ($pval instanceof stdClass) {
-                    $pval = json_encode($this->sortObject($pval));
-                    $format = 'json';
-                } else {
-                    $format = 'string';
-                }
-
-                $checksum = sha1(sprintf('%s=(%s)%s', $key, $format, $pval), true);
-
-                if (! array_key_exists($checksum, $props)) {
-                    $props[$checksum] = array(
-                        'checksum'       => $checksum,
-                        'property_name'  => $key,
-                        'property_value' => $pval,
-                        'format'         => $format
-                    );
-                }
-
-                $rowChecksums[] = $checksum;
+                $property = $this->prepareImportedProperty($key, $row->$key);
+                $rowChecksums[] = $property['checksum'];
             }
 
             $checksum = sha1($object_name . ';' . implode(';', $rowChecksums), true);
-            if (array_key_exists($checksum, $rows)) {
+            if (array_key_exists($checksum, $this->rows)) {
                 die('WTF, collision?');
             }
 
-            $rows[$checksum] = array(
+            $this->rows[$checksum] = array(
                 'checksum'    => $checksum,
                 'object_name' => $object_name
             );
 
-            $rowsProps[$checksum] = $rowChecksums;
+            $this->rowProperties[$checksum] = $rowChecksums;
+
+            $objects[$object_name] = $checksum;
         }
 
-        $rowSums = array_keys($rows);
-        $rowset = sha1(implode(';', $rowSums), true);
+        $this->rowChecksums = array_keys($this->rows);
+        $this->rowsetChecksum = sha1(implode(';', $this->rowChecksums), true);
+        return $this;
+    }
 
-        if ($this->rowSetExists($rowset)) {
-            if ($this->lastRowsetIs($rowset)) {
-                return false;
-            }
-        }
+    /**
+     * Store our new rowset
+     */
+    protected function storeRowset()
+    {
+        $db = $this->db;
+        $rowset = $this->rowsetChecksum();
+        $rows = $this->checksummedRows();
 
         $db->beginTransaction();
-        if (! $this->rowSetExists($rowset)) {
 
-            if (empty($rowSums)) {
-                $newRows = array();
-            } else {
-                $newRows = $this->newChecksums('imported_row', $rowSums);
-            }
+        if ($this->isEmpty()) {
+            $newRows = array();
+            $newProperties = array();
+        } else {
+            $newRows = $this->newChecksums('imported_row', $this->rowChecksums);
+            $newProperties = $this->newChecksums('imported_property', array_keys($this->properties));
+        }
 
-            if (empty($rowSums)) {
-                $newProperties = array();
-            } else {
-                $newProperties = $this->newChecksums('imported_property', array_keys($props));
-            }
+        $db->insert('imported_rowset', array('checksum' => $rowset));
 
-            $db->insert('imported_rowset', array('checksum' => $rowset));
+        foreach ($newProperties as $checksum) {
+            $db->insert('imported_property', $this->properties[$checksum]);
+        }
 
-            foreach ($newProperties as $checksum) {
-                $db->insert('imported_property', $props[$checksum]);
-            }
-
-            foreach ($newRows as $checksum) {
-                $db->insert('imported_row', $rows[$checksum]);
-                foreach ($rowsProps[$checksum] as $propChecksum) {
-                    $db->insert('imported_row_property', array(
-                        'row_checksum'      => $checksum,
-                        'property_checksum' => $propChecksum
-                    ));
-                }
-            }
-
-            foreach (array_keys($rows) as $checksum) {
-                $db->insert(
-                    'imported_rowset_row',
-                    array(
-                        'rowset_checksum' => $rowset,
-                        'row_checksum'    => $checksum
-                    )
-                );
+        foreach ($newRows as $row) {
+            $db->insert('imported_row', $rows[$row]);
+            foreach ($this->rowProperties[$row] as $property) {
+                $db->insert('imported_row_property', array(
+                    'row_checksum'      => $row,
+                    'property_checksum' => $property
+                ));
             }
         }
-        $db->insert(
-            'import_run',
-            array(
-                'source_id' => $source->id,
-                'rowset_checksum' => $rowset,
-                'start_time' => date('Y-m-d H:i:s'),
-                'succeeded' => 'y'
-            )
-        );
-        $id =  $db->lastInsertId();
+
+        foreach (array_keys($rows) as $row) {
+            $db->insert(
+                'imported_rowset_row',
+                array(
+                    'rowset_checksum' => $rowset,
+                    'row_checksum'    => $row
+                )
+            );
+        }
+
         $db->commit();
 
-        return $id;
+        $this->rowsetExists = true;
     }
 
     /**
@@ -199,11 +326,36 @@ class Import
             === Util::binary2hex($checksum);
     }
 
-    protected function rowSetExists($checksum)
+    /**
+     * Whether our rowset already exists in the database
+     *
+     * @return boolean
+     */
+    protected function rowsetExists()
     {
-        return count($this->newChecksums('imported_rowset', array($checksum))) === 0;
+        if (null === $this->rowsetExists) {
+            $this->rowsetExists = 0 === count(
+                $this->newChecksums(
+                    'imported_rowset',
+                    array($this->rowsetChecksum())
+                )
+            );
+        }
+
+        return $this->rowsetExists;
     }
 
+    /**
+     * Finde new checksums for a specific table
+     *
+     * Accepts an array of checksums and gives you an array with those checksums
+     * that are missing in the given table
+     *
+     * @param string $table Database table name
+     * @param array  $checksums Array with the checksums that should be verified
+     *
+     * @return array
+     */
     protected function newChecksums($table, $checksums)
     {
         $db = $this->db;