diff --git a/src/db/Capacitor.php b/src/db/Capacitor.php
index 396fa17..0c2f161 100644
--- a/src/db/Capacitor.php
+++ b/src/db/Capacitor.php
@@ -1,11 +1,14 @@
 <?php
 namespace nur\sery\db;
 
+use nur\sery\php\func;
+use nur\sery\ValueException;
+
 /**
  * Class Capacitor: un objet permettant d'attaquer un canal spécifique d'une
  * instance de {@link CapacitorStorage}
  */
-class Capacitor {
+class Capacitor implements ITransactor {
   function __construct(CapacitorStorage $storage, CapacitorChannel $channel, bool $ensureExists=true) {
     $this->storage = $storage;
     $this->channel = $channel;
@@ -34,6 +37,80 @@ class Capacitor {
     return $this->getChannel()->getTableName();
   }
 
+  /** @var CapacitorChannel[]  */
+  protected ?array $subChannels = null;
+
+  protected ?array $subManageTransactions = null;
+
+  function willUpdate(...$channels): self {
+    if ($this->subChannels === null) {
+      # désactiver la gestion des transaction sur le channel local aussi
+      $this->subChannels[] = $this->channel;
+    }
+    if ($channels) {
+      foreach ($channels as $channel) {
+        if ($channel instanceof Capacitor) $channel = $channel->getChannel();
+        if ($channel instanceof CapacitorChannel) {
+          $this->subChannels[] = $channel;
+        } else {
+          throw ValueException::invalid_type($channel, CapacitorChannel::class);
+        }
+      }
+    }
+    return $this;
+  }
+
+  function inTransaction(): bool {
+    return $this->db()->inTransaction();
+  }
+
+  function beginTransaction(?callable $func=null): void {
+    if ($this->subManageTransactions === null && $this->subChannels !== null) {
+      foreach ($this->subChannels as $channel) {
+        $name = $channel->getName();
+        $this->subManageTransactions ??= [];
+        if (!array_key_exists($name, $this->subManageTransactions)) {
+          $this->subManageTransactions[$name] = $channel->isManageTransactions();
+        }
+        $channel->setManageTransactions(false);
+      }
+      $db = $this->db();
+      if (!$db->inTransaction()) $db->beginTransaction();
+    }
+    if ($func !== null) {
+      $commited = false;
+      try {
+        func::call($func, $this);
+        $this->commit();
+        $commited = true;
+      } finally {
+        if (!$commited) $this->rollback();
+      }
+    }
+  }
+
+  protected function beforeEndTransaction(): void {
+    if ($this->subManageTransactions !== null) {
+      foreach ($this->subChannels as $channel) {
+        $name = $channel->getName();
+        $channel->setManageTransactions($this->subManageTransactions[$name]);
+      }
+      $this->subManageTransactions = null;
+    }
+  }
+
+  function commit(): void {
+    $this->beforeEndTransaction();
+    $db = $this->db();
+    if ($db->inTransaction()) $this->db()->commit();
+  }
+
+  function rollback(): void {
+    $this->beforeEndTransaction();
+    $db = $this->db();
+    if ($db->inTransaction()) $this->db()->rollback();
+  }
+
   function getCreateSql(): string {
     return $this->storage->_getCreateSql($this->channel);
   }
@@ -51,6 +128,7 @@ class Capacitor {
   }
 
   function charge($item, $func=null, ?array $args=null, ?array &$values=null): int {
+    $this->beginTransaction();
     return $this->storage->_charge($this->channel, $item, $func, $args, $values);
   }
 
@@ -71,10 +149,12 @@ class Capacitor {
   }
 
   function each($filter, $func=null, ?array $args=null): int {
+    $this->beginTransaction();
     return $this->storage->_each($this->channel, $filter, $func, $args);
   }
 
   function delete($filter, $func=null, ?array $args=null): int {
+    $this->beginTransaction();
     return $this->storage->_delete($this->channel, $filter, $func, $args);
   }
 
diff --git a/src/db/CapacitorStorage.php b/src/db/CapacitorStorage.php
index 862f1e1..cb65f49 100644
--- a/src/db/CapacitorStorage.php
+++ b/src/db/CapacitorStorage.php
@@ -204,6 +204,7 @@ EOT;
   function _charge(CapacitorChannel $channel, $item, $func, ?array $args, ?array &$values=null): int {
     $this->_create($channel);
     $tableName = $channel->getTableName();
+    $db = $this->db();
 
     $initFunc = [$channel, "getItemValues"];
     $initArgs = $args;
@@ -217,7 +218,7 @@ EOT;
     $rowIds = $this->getRowIds($channel, $row, $primaryKeys);
     if ($rowIds !== null) {
       # modification
-      $prow = $this->db()->one([
+      $prow = $db->one([
         "select",
         "from" => $tableName,
         "where" => $rowIds,
@@ -276,28 +277,41 @@ EOT;
       }
     }
 
-    if ($insert === null) {
-      # aucune modification
-      return 0;
-    } elseif ($insert) {
-      $id = $this->db()->exec([
-        "insert",
-        "into" => $tableName,
-        "values" => $row,
-      ]);
-      if (count($primaryKeys) == 1 && $rowIds === null) {
-        # mettre à jour avec l'id généré
-        $values[$primaryKeys[0]] = $id;
-      }
-    } else {
-      $this->db()->exec([
-        "update",
-        "table" => $tableName,
-        "values" => $row,
-        "where" => $rowIds,
-      ]);
+    # aucune modification
+    if ($insert === null) return 0;
+
+    $manageTransactions = $channel->isManageTransactions();
+    if ($manageTransactions) {
+      $commited = false;
+      $db->beginTransaction();
+    }
+    try {
+      if ($insert) {
+        $id = $db->exec([
+          "insert",
+          "into" => $tableName,
+          "values" => $row,
+        ]);
+        if (count($primaryKeys) == 1 && $rowIds === null) {
+          # mettre à jour avec l'id généré
+          $values[$primaryKeys[0]] = $id;
+        }
+      } else {
+        $db->exec([
+          "update",
+          "table" => $tableName,
+          "values" => $row,
+          "where" => $rowIds,
+        ]);
+      }
+      if ($manageTransactions) {
+        $db->commit();
+        $commited = true;
+      }
+      return 1;
+    } finally {
+      if ($manageTransactions && !$commited) $db->rollback();
     }
-    return 1;
   }
 
   function charge(?string $channel, $item, $func=null, ?array $args=null, ?array &$values=null): int {
diff --git a/src/db/IDatabase.php b/src/db/IDatabase.php
index b5a9047..64b0b0c 100644
--- a/src/db/IDatabase.php
+++ b/src/db/IDatabase.php
@@ -1,13 +1,7 @@
 <?php
 namespace nur\sery\db;
 
-interface IDatabase {
-  function beginTransaction(): void;
-
-  function commit(): void;
-
-  function rollback(): void;
-
+interface IDatabase extends ITransactor {
   /**
    * - si c'est un insert, retourner l'identifiant autogénéré de la ligne
    * - sinon retourner le nombre de lignes modifiées en cas de succès, ou false
diff --git a/src/db/ITransactor.php b/src/db/ITransactor.php
new file mode 100644
index 0000000..a485a7b
--- /dev/null
+++ b/src/db/ITransactor.php
@@ -0,0 +1,22 @@
+<?php
+namespace nur\sery\db;
+
+/**
+ * Class ITransactor: un objet qui peut faire des opérations dans une
+ * transaction
+ */
+interface ITransactor {
+  /**
+   * Indiquer qu'une transaction va être étendue à tous les objets mentionnés
+   */
+  function willUpdate(...$transactors): self;
+
+  function inTransaction(): bool;
+
+  /** si $func!==null, la lancer puis commiter la transaction */
+  function beginTransaction(?callable $func=null): void;
+
+  function commit(): void;
+
+  function rollback(): void;
+}
diff --git a/src/db/pdo/Pdo.php b/src/db/pdo/Pdo.php
index b51d4fc..89c2ee1 100644
--- a/src/db/pdo/Pdo.php
+++ b/src/db/pdo/Pdo.php
@@ -4,9 +4,11 @@ namespace nur\sery\db\pdo;
 use Generator;
 use nur\sery\cl;
 use nur\sery\db\IDatabase;
+use nur\sery\db\ITransactor;
 use nur\sery\php\func;
 use nur\sery\php\time\Date;
 use nur\sery\php\time\DateTime;
+use nur\sery\ValueException;
 
 class Pdo implements IDatabase {
   static function with($pdo, ?array $params=null): self {
@@ -144,16 +146,60 @@ class Pdo implements IDatabase {
     }
   }
 
-  function beginTransaction(): void {
+  /** @var ITransactor[] */
+  protected ?array $transactors = null;
+
+  function willUpdate(...$transactors): self {
+    foreach ($transactors as $transactor) {
+      if ($transactor instanceof ITransactor) {
+        $this->transactors[] = $transactor;
+        $transactor->willUpdate();
+      } else {
+        throw ValueException::invalid_type($transactor, ITransactor::class);
+      }
+    }
+    return $this;
+  }
+
+  function inTransaction(): bool {
+    return $this->db()->inTransaction();
+  }
+
+  function beginTransaction(?callable $func=null): void {
     $this->db()->beginTransaction();
+    if ($this->transactors !== null) {
+      foreach ($this->transactors as $transactor) {
+        $transactor->beginTransaction();
+      }
+    }
+    if ($func !== null) {
+      $commited = false;
+      try {
+        func::call($func, $this);
+        $this->commit();
+        $commited = true;
+      } finally {
+        if (!$commited) $this->rollback();
+      }
+    }
   }
 
   function commit(): void {
     $this->db()->commit();
+    if ($this->transactors !== null) {
+      foreach ($this->transactors as $transactor) {
+        $transactor->commit();
+      }
+    }
   }
 
   function rollback(): void {
     $this->db()->rollBack();
+    if ($this->transactors !== null) {
+      foreach ($this->transactors as $transactor) {
+        $transactor->rollback();
+      }
+    }
   }
 
   /**
diff --git a/src/db/pdo/_query_base.php b/src/db/pdo/_query_base.php
index f339f11..ad04447 100644
--- a/src/db/pdo/_query_base.php
+++ b/src/db/pdo/_query_base.php
@@ -82,7 +82,10 @@ class _query_base extends _base {
     }
   }
 
+  const DEBUG_QUERIES = false;
+
   function useStmt(\PDO $db, ?\PDOStatement &$stmt=null, ?string &$sql=null): bool {
+    if (static::DEBUG_QUERIES) error_log($this->sql); #XXX
     if ($this->bindings !== null) {
       $stmt = $db->prepare($this->sql);
       foreach ($this->bindings as $name => $value) {
diff --git a/src/db/sqlite/Sqlite.php b/src/db/sqlite/Sqlite.php
index 99577f4..2c30763 100644
--- a/src/db/sqlite/Sqlite.php
+++ b/src/db/sqlite/Sqlite.php
@@ -4,6 +4,9 @@ namespace nur\sery\db\sqlite;
 use Generator;
 use nur\sery\cl;
 use nur\sery\db\IDatabase;
+use nur\sery\db\ITransactor;
+use nur\sery\php\func;
+use nur\sery\ValueException;
 use SQLite3;
 use SQLite3Result;
 use SQLite3Stmt;
@@ -82,6 +85,7 @@ class Sqlite implements IDatabase {
     $this->migration = $params["migrate"] ?? static::MIGRATE;
     #
     $defaultAutoOpen = self::params_SCHEMA["auto_open"][1];
+    $this->inTransaction = false;
     if ($params["auto_open"] ?? $defaultAutoOpen) {
       $this->open();
     }
@@ -113,11 +117,14 @@ class Sqlite implements IDatabase {
   /** @var SQLite3 */
   protected $db;
 
+  protected bool $inTransaction;
+
   function open(): self {
     if ($this->db === null) {
       $this->db = new SQLite3($this->file, $this->flags, $this->encryptionKey);
       _config::with($this->config)->configure($this);
       _migration::with($this->migration)->migrate($this);
+      $this->inTransaction = false;
     }
     return $this;
   }
@@ -126,6 +133,7 @@ class Sqlite implements IDatabase {
     if ($this->db !== null) {
       $this->db->close();
       $this->db = null;
+      $this->inTransaction = false;
     }
   }
 
@@ -172,16 +180,64 @@ class Sqlite implements IDatabase {
     }
   }
 
-  function beginTransaction(): void {
+  /** @var ITransactor[] */
+  protected ?array $transactors = null;
+
+  function willUpdate(...$transactors): self {
+    foreach ($transactors as $transactor) {
+      if ($transactor instanceof ITransactor) {
+        $this->transactors[] = $transactor;
+        $transactor->willUpdate();
+      } else {
+        throw ValueException::invalid_type($transactor, ITransactor::class);
+      }
+    }
+    return $this;
+  }
+
+  function inTransaction(): bool {
+    #XXX très imparfait, mais y'a rien de mieux pour le moment :-(
+    return $this->inTransaction;
+  }
+
+  function beginTransaction(?callable $func=null): void {
     $this->db()->exec("begin");
+    $this->inTransaction = true;
+    if ($this->transactors !== null) {
+      foreach ($this->transactors as $transactor) {
+        $transactor->beginTransaction();
+      }
+    }
+    if ($func !== null) {
+      $commited = false;
+      try {
+        func::call($func, $this);
+        $this->commit();
+        $commited = true;
+      } finally {
+        if (!$commited) $this->rollback();
+      }
+    }
   }
 
   function commit(): void {
+    $this->inTransaction = false;
     $this->db()->exec("commit");
+    if ($this->transactors !== null) {
+      foreach ($this->transactors as $transactor) {
+        $transactor->commit();
+      }
+    }
   }
 
   function rollback(): void {
-    $this->db()->exec("commit");
+    $this->inTransaction = false;
+    $this->db()->exec("rollback");
+    if ($this->transactors !== null) {
+      foreach ($this->transactors as $transactor) {
+        $transactor->rollback();
+      }
+    }
   }
 
   function _get(string $query, bool $entireRow=false) {
diff --git a/src/db/sqlite/_query_base.php b/src/db/sqlite/_query_base.php
index 58605fd..d2e3ceb 100644
--- a/src/db/sqlite/_query_base.php
+++ b/src/db/sqlite/_query_base.php
@@ -32,7 +32,10 @@ class _query_base extends _base {
     }
   }
 
+  const DEBUG_QUERIES = false;
+
   function useStmt(SQLite3 $db, ?SQLite3Stmt &$stmt=null, ?string &$sql=null): bool {
+    if (static::DEBUG_QUERIES) error_log($this->sql); #XXX
     if ($this->bindings !== null) {
       /** @var SQLite3Stmt $stmt */
       $stmt = SqliteException::check($db, $db->prepare($this->sql));
diff --git a/src/php/time/DateTime.php b/src/php/time/DateTime.php
index 24310b8..c978663 100644
--- a/src/php/time/DateTime.php
+++ b/src/php/time/DateTime.php
@@ -233,6 +233,18 @@ class DateTime extends \DateTime {
     return static::with($this->add(new \DateInterval("P${nbDays}D")));
   }
 
+  function getElapsedAt(?DateTime $now=null, ?int $resolution=null): string {
+    return Elapsed::format_at($this, $now, $resolution);
+  }
+
+  function getElapsedSince(?DateTime $now=null, ?int $resolution=null): string {
+    return Elapsed::format_since($this, $now, $resolution);
+  }
+
+  function getElapsedDelay(?DateTime $now=null, ?int $resolution=null): string {
+    return Elapsed::format_delay($this, $now, $resolution);
+  }
+
   function __toString(): string {
     return $this->format();
   }
diff --git a/src/php/time/Elapsed.php b/src/php/time/Elapsed.php
index 9d7ed6f..62d5d9e 100644
--- a/src/php/time/Elapsed.php
+++ b/src/php/time/Elapsed.php
@@ -10,14 +10,16 @@ class Elapsed {
   const DAY = 60 * 60 * 24;
 
   /** @var int résolution */
-  const RES_SECONDS = 0, RES_MINUTES = 1, RES_HOURS = 2, RES_DAYS = 3;
+  const RESOLUTION_SECONDS = 0, RESOLUTION_MINUTES = 1, RESOLUTION_HOURS = 2, RESOLUTION_DAYS = 3;
+
+  const DEFAULT_RESOLUTION = self::RESOLUTION_SECONDS;
 
   private static function format(int $seconds, int $resolution, ?string $prefix=null, ?string $zero=null): string {
     if ($prefix === null) $prefix = "depuis";
     switch ($resolution) {
-    case self::RES_DAYS: return self::format_days($seconds, $prefix, $zero);
-    case self::RES_HOURS: return self::format_hours($seconds, $prefix, $zero);
-    case self::RES_MINUTES: return self::format_minutes($seconds, $prefix, $zero);
+    case self::RESOLUTION_DAYS: return self::format_days($seconds, $prefix, $zero);
+    case self::RESOLUTION_HOURS: return self::format_hours($seconds, $prefix, $zero);
+    case self::RESOLUTION_MINUTES: return self::format_minutes($seconds, $prefix, $zero);
     default: return self::format_seconds($seconds, $prefix, $zero);
     }
   }
@@ -122,27 +124,28 @@ class Elapsed {
     return self::format_generic($prefix, $d, 0, 0);
   }
   
-  static function format_at(DateTime $start, ?DateTime $now=null): string {
+  static function format_at(DateTime $start, ?DateTime $now=null, ?int $resolution=null): string {
     $now ??= new DateTime();
     $seconds = $now->getTimestamp() - $start->getTimestamp();
-    return (new self($seconds))->formatAt();
+    return (new self($seconds, $resolution))->formatAt();
   }
   
-  static function format_since(DateTime $start, ?DateTime $now=null): string {
+  static function format_since(DateTime $start, ?DateTime $now=null, ?int $resolution=null): string {
     $now ??= new DateTime();
     $seconds = $now->getTimestamp() - $start->getTimestamp();
-    return (new self($seconds))->formatSince();
+    return (new self($seconds, $resolution))->formatSince();
   }
   
-  static function format_delay(DateTime $start, ?DateTime $now=null): string {
+  static function format_delay(DateTime $start, ?DateTime $now=null, ?int $resolution=null): string {
     $now ??= new DateTime();
     $seconds = $now->getTimestamp() - $start->getTimestamp();
-    return (new self($seconds))->formatDelay();
+    return (new self($seconds, $resolution))->formatDelay();
   }
 
-  function __construct(int $seconds, int $resolution=self::RES_SECONDS) {
-    if ($resolution < self::RES_SECONDS) $resolution = self::RES_SECONDS;
-    elseif ($resolution > self::RES_DAYS) $resolution = self::RES_DAYS;
+  function __construct(int $seconds, ?int $resolution=null) {
+    $resolution ??= static::DEFAULT_RESOLUTION;
+    if ($resolution < self::RESOLUTION_SECONDS) $resolution = self::RESOLUTION_SECONDS;
+    elseif ($resolution > self::RESOLUTION_DAYS) $resolution = self::RESOLUTION_DAYS;
     $this->seconds = $seconds;
     $this->resolution = $resolution;
   }