diff --git a/src/Entity/TravelSnapshot.php b/src/Entity/TravelSnapshot.php
index 57b7071..683a40b 100644
--- a/src/Entity/TravelSnapshot.php
+++ b/src/Entity/TravelSnapshot.php
@@ -13,6 +13,18 @@ use Doctrine\ORM\Mapping as ORM;
 #[ORM\Index(name: 'idx_snapshot_product_id', columns: ['product_id'])]
 #[ORM\Index(name: 'idx_snapshot_date_to', columns: ['date_to'])]
 #[ORM\Index(name: 'idx_snapshot_extended_refreshed_at', columns: ['extended_refreshed_at'])]
+/**
+ * Persisted copy of a Travel aggregate for travels whose source XML may no longer be available.
+ *
+ * Each row stores the full serialized Travel graph as a JSON payload alongside a set of
+ * denormalized metadata columns (code, label, dates, hotel) that allow filtering and
+ * mapping lookups without deserializing the payload.
+ *
+ * Three timestamps track the lifecycle of the snapshot:
+ *   - capturedAt:          when the snapshot was first written
+ *   - updatedAt:           when the payload or metadata was last modified
+ *   - extendedRefreshedAt: when extended availability (VERFUEGBARKEIT2) was last applied
+ */
 class TravelSnapshot
 {
     #[ORM\Id]
@@ -50,9 +62,11 @@ class TravelSnapshot
     #[ORM\Column(type: 'date_immutable', nullable: true)]
     private ?\DateTimeImmutable $dateTo = null;
 
+    /** Full JSON-serialized Travel aggregate. */
     #[ORM\Column(type: 'text')]
     private string $payload;
 
+    /** SHA-256 of payload; compared before every write to skip no-op flushes. */
     #[ORM\Column(type: 'string', length: 64)]
     private string $payloadHash;
 
diff --git a/src/Repository/TravelSnapshotRepository.php b/src/Repository/TravelSnapshotRepository.php
index 9ffe846..86cbfa5 100644
--- a/src/Repository/TravelSnapshotRepository.php
+++ b/src/Repository/TravelSnapshotRepository.php
@@ -98,7 +98,9 @@ class TravelSnapshotRepository extends ServiceEntityRepository
         int $limit,
     ): array {
         return $this->createQueryBuilder('s')
+            // dateTo IS NULL: snapshots without a known end date are always considered active.
             ->where('s.dateTo IS NULL OR s.dateTo >= :dateToThreshold')
+            // extendedRefreshedAt IS NULL: never-refreshed snapshots rank as oldest.
             ->andWhere('s.extendedRefreshedAt IS NULL OR s.extendedRefreshedAt < :refreshBefore')
             ->setParameter('dateToThreshold', $dateToThreshold)
             ->setParameter('refreshBefore', $refreshBefore)
@@ -116,6 +118,8 @@ class TravelSnapshotRepository extends ServiceEntityRepository
     {
         return (int) $this->createQueryBuilder('s')
             ->delete()
+            // Snapshots with no known end date are excluded from purging as a safety net;
+            // they must be cleaned up manually if they become stale.
             ->where('s.dateTo IS NOT NULL')
             ->andWhere('s.dateTo < :beforeDate')
             ->setParameter('beforeDate', $beforeDate)
diff --git a/src/Service/TravelSnapshotService.php b/src/Service/TravelSnapshotService.php
index f8e3b05..7a81b34 100644
--- a/src/Service/TravelSnapshotService.php
+++ b/src/Service/TravelSnapshotService.php
@@ -97,6 +97,7 @@ class TravelSnapshotService
      */
     public function upsertFromTravel(Travel $travel): void
     {
+        // Both fields form the composite unique key; nothing to persist without them.
         if (null === $travel->id || null === $travel->hotelId) {
             return;
         }
@@ -191,6 +192,10 @@ class TravelSnapshotService
         $extendedResponseByDateId = [];
 
         foreach ($candidates as $snapshot) {
+            // Skip travels whose XML is still live; their snapshot is kept authoritative
+            // by the XML load path (getTravelDataFromXml → upsertFromTravel). An empty
+            // $xmlAvailableDateIds means the file map could not be loaded, so fall back
+            // to refreshing everything rather than skipping all candidates.
             if ([] !== $xmlAvailableDateIds && in_array($snapshot->getDateId(), $xmlAvailableDateIds, true)) {
                 continue;
             }
@@ -203,6 +208,8 @@ class TravelSnapshotService
                 continue;
             }
 
+            // Cache the API response by dateId: multiple hotel snapshots share one travel
+            // date, so a single API call covers all of them within the same batch.
             if (false === array_key_exists($dateId, $extendedResponseByDateId)) {
                 $response = null;
                 try {
@@ -215,6 +222,8 @@ class TravelSnapshotService
                     ]);
                 }
 
+                // A Notification response signals a business-level "no data" answer
+                // from the API rather than an error; treat it the same as a null response.
                 if (null !== $response && false === $response instanceof Notification) {
                     $extendedResponseByDateId[$dateId] = $response;
                 } elseif (false === isset($extendedResponseByDateId[$dateId])) {
@@ -236,6 +245,8 @@ class TravelSnapshotService
                 $snapshot->setPayload($payload)->setPayloadHash($payloadHash);
                 $this->applyTravelMetadata($snapshot, $travel);
             }
+            // Always stamp the refresh timestamp even when the payload was unchanged,
+            // so the candidate query does not re-select this snapshot on the next run.
             $snapshot->setExtendedRefreshedAt(new \DateTimeImmutable())->touchUpdatedAt();
             $this->entityManager->flush();