diff --git a/src/Exceptions/AlreadyProcessingException.php b/src/Exceptions/AlreadyProcessingException.php
deleted file mode 100644
index 2a07996..0000000
--- a/src/Exceptions/AlreadyProcessingException.php
+++ /dev/null
@@ -1,15 +0,0 @@
-<?php
-
-declare(strict_types=1);
-
-namespace Wwwision\DCBLibrary\Exceptions;
-
-use RuntimeException;
-
-final class AlreadyProcessingException extends RuntimeException
-{
-    public function __construct()
-    {
-        parent::__construct('Subscription engine is already processing');
-    }
-}
diff --git a/src/Subscription/Engine/DefaultSubscriptionEngine.php b/src/Subscription/Engine/DefaultSubscriptionEngine.php
index bfc9cbb..2c14dfa 100644
--- a/src/Subscription/Engine/DefaultSubscriptionEngine.php
+++ b/src/Subscription/Engine/DefaultSubscriptionEngine.php
@@ -11,27 +11,25 @@
 use Wwwision\DCBEventStore\Types\SequenceNumber;
 use Wwwision\DCBLibrary\DomainEvent;
 use Wwwision\DCBLibrary\EventSerializer;
-use Wwwision\DCBLibrary\Exceptions\AlreadyProcessingException;
 use Wwwision\DCBLibrary\ProvidesSetup;
+use Wwwision\DCBLibrary\Subscription\RetryStrategy\RetryStrategy;
 use Wwwision\DCBLibrary\Subscription\RunMode;
 use Wwwision\DCBLibrary\Subscription\Status;
 use Wwwision\DCBLibrary\Subscription\Store\SubscriptionCriteria;
 use Wwwision\DCBLibrary\Subscription\Store\SubscriptionStore;
 use Wwwision\DCBLibrary\Subscription\Subscriber\Subscribers;
 use Wwwision\DCBLibrary\Subscription\Subscription;
-use Wwwision\DCBLibrary\Subscription\SubscriptionId;
 use Wwwision\DCBLibrary\Subscription\Subscriptions;
 
 final class DefaultSubscriptionEngine implements SubscriptionEngine
 {
 
-    private bool $processing = false;
-
     public function __construct(
         private readonly EventStore $eventStore,
         private readonly SubscriptionStore $subscriptionStore,
         private readonly Subscribers $subscribers,
         private readonly EventSerializer $eventSerializer,
+        private readonly RetryStrategy $retryStrategy,
         private readonly LoggerInterface|null $logger = null,
     ) {
     }
@@ -40,25 +38,13 @@ public function setup(
         SubscriptionEngineCriteria $criteria = null,
         int $limit = null,
     ): void {
-        if ($this->processing) {
-            throw new AlreadyProcessingException();
-        }
         $criteria ??= SubscriptionEngineCriteria::noConstraints();
-        // TODO acquire lock
-        $this->processing = true;
-
         $subscriptionCriteria = SubscriptionCriteria::create(
             ids: $criteria->ids,
             groups: $criteria->groups,
-            status: [Status::NEW]
+            status: [Status::NEW],
         );
-
-        try {
-            $this->runInternal($subscriptionCriteria, 'setup', $limit);
-        } finally {
-            $this->processing = false;
-            // TODO release lock
-        }
+        $this->runInternal($subscriptionCriteria, 'setup', $limit);
     }
 
 
@@ -66,24 +52,33 @@ public function run(
         SubscriptionEngineCriteria $criteria = null,
         int $limit = null,
     ): void {
-        if ($this->processing) {
-            throw new AlreadyProcessingException();
-        }
         $criteria ??= SubscriptionEngineCriteria::noConstraints();
-        // TODO acquire lock
-        $this->processing = true;
 
         $subscriptionCriteria = SubscriptionCriteria::create(
             ids: $criteria->ids,
             groups: $criteria->groups,
-            status: [Status::ACTIVE]
+            status: [Status::ACTIVE],
         );
+        $this->runInternal($subscriptionCriteria, 'run', $limit);
+    }
 
-        try {
-            $this->runInternal($subscriptionCriteria, 'run', $limit);
-        } finally {
-            $this->processing = false;
-            // TODO release lock
+    private function lockSubscriptions(Subscriptions $subscriptions): void
+    {
+        foreach ($subscriptions as $subscription) {
+            $sT = microtime(true);
+            while (!$this->subscriptionStore->acquireLock($subscription->id)) {
+                if (microtime(true) - $sT > 5) {
+                    // TODO better exception handling
+                    throw new \RuntimeException(sprintf('Failed to acquire lock for subscription "%s"', $subscription->id->value), 1721895494);
+                }
+            }
+        }
+    }
+
+    private function releaseSubscriptions(Subscriptions $subscriptions): void
+    {
+        foreach ($subscriptions as $subscription) {
+            $this->subscriptionStore->releaseLock($subscription->id);
         }
     }
 
@@ -91,13 +86,16 @@ private function runInternal(SubscriptionCriteria $criteria, string $process, in
     {
         $this->logger?->info(sprintf('Subscription Engine: %s: Start.', $process));
         $this->discoverNewSubscriptions();
-        // TODO $this->discoverDetachedSubscriptions($criteria);
-        // TODO $this->retrySubscriptions($criteria);
+        //$this->discoverDetachedSubscriptions($criteria);
+        $this->retrySubscriptions($criteria);
         $subscriptions = $this->subscriptionStore->findByCriteria($criteria);
         if ($subscriptions->isEmpty()) {
             $this->logger?->info(sprintf('Subscription Engine: %s: No subscriptions to process, finishing', $process));
             return;// new ProcessedResult(0, true);
         }
+
+        $this->lockSubscriptions($subscriptions);
+
         $startSequenceNumber = $this->lowestSubscriptionPosition($subscriptions)->next();
         $this->logger?->debug(
             sprintf(
@@ -151,6 +149,7 @@ private function runInternal(SubscriptionCriteria $criteria, string $process, in
                         $limit,
                     ),
                 );
+                $this->releaseSubscriptions($subscriptions);
 
                 return;// new ProcessedResult($messageCounter, false, $errors);
             }
@@ -179,6 +178,7 @@ private function runInternal(SubscriptionCriteria $criteria, string $process, in
                 $lastSequenceNumber?->value ?: $startSequenceNumber->value,
             ),
         );
+        $this->releaseSubscriptions($subscriptions);
 
         return;// new ProcessedResult($messageCounter, true, $errors);
     }
@@ -253,6 +253,47 @@ private function discoverNewSubscriptions(): void
         }
     }
 
+    private function retrySubscriptions(SubscriptionCriteria $criteria): void
+    {
+        $failedSubscriptions = $this->subscriptionStore->findByCriteria(
+            SubscriptionCriteria::create(
+                ids: $criteria->ids,
+                groups: $criteria->groups,
+                status: [Status::ERROR],
+            )
+        );
+        foreach ($failedSubscriptions as $subscription) {
+            if ($subscription->error === null) {
+                continue;
+            }
+            $retryable = in_array(
+                $subscription->error->previousStatus,
+                [Status::NEW, Status::BOOTING, Status::ACTIVE],
+                true,
+            );
+            if (!$retryable) {
+                continue;
+            }
+            if (!$this->retryStrategy->shouldRetry($subscription)) {
+                continue;
+            }
+            $this->subscriptionStore->update($subscription->id, static fn(Subscription $subscription) => $subscription->with(
+                status: $subscription->error->previousStatus,
+                retryAttempt: $subscription->retryAttempt + 1,
+            )->withoutError());
+
+            $this->logger?->info(
+                sprintf(
+                    'Subscription Engine: Retry subscription "%s" (%d) and set back to %s.',
+                    $subscription->id->value,
+                    $subscription->retryAttempt + 1,
+                    $subscription->error->previousStatus->name,
+                ),
+            );
+        }
+    }
+
+
     private function lastSequenceNumber(): SequenceNumber
     {
         $events = $this->eventStore->readAll(ReadOptions::create(backwards: true));
diff --git a/src/Subscription/RetryStrategy/ClockBasedRetryStrategy.php b/src/Subscription/RetryStrategy/ClockBasedRetryStrategy.php
new file mode 100644
index 0000000..f025cf6
--- /dev/null
+++ b/src/Subscription/RetryStrategy/ClockBasedRetryStrategy.php
@@ -0,0 +1,61 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Wwwision\DCBLibrary\Subscription\RetryStrategy;
+
+use DateTimeImmutable;
+use Psr\Clock\ClockInterface;
+use Wwwision\DCBLibrary\Subscription\Subscription;
+
+final class ClockBasedRetryStrategy implements RetryStrategy
+{
+    public const DEFAULT_BASE_DELAY = 5;
+    public const DEFAULT_DELAY_FACTOR = 2;
+    public const DEFAULT_MAX_ATTEMPTS = 5;
+
+    /**
+     * @param int $baseDelay in seconds
+     * @param positive-int $maxAttempts
+     */
+    public function __construct(
+        private readonly ClockInterface $clock,
+        private readonly int $baseDelay = self::DEFAULT_BASE_DELAY,
+        private readonly float $delayFactor = self::DEFAULT_DELAY_FACTOR,
+        private readonly int $maxAttempts = self::DEFAULT_MAX_ATTEMPTS,
+    ) {
+    }
+
+    public function shouldRetry(Subscription $subscription): bool
+    {
+        if ($subscription->retryAttempt >= $this->maxAttempts) {
+            return false;
+        }
+
+        $lastSavedAt = $subscription->lastSavedAt;
+
+        if ($lastSavedAt === null) {
+            return false;
+        }
+
+        $nextRetryDate = $this->calculateNextRetryDate($lastSavedAt, $subscription->retryAttempt);
+
+        return $nextRetryDate <= $this->clock->now();
+    }
+
+    private function calculateNextRetryDate(DateTimeImmutable $lastDate, int $attempt): DateTimeImmutable
+    {
+        $nextDate = $lastDate->modify(sprintf('+%d seconds', $this->calculateDelay($attempt)));
+
+        if ($nextDate === false) {
+            throw new \RuntimeException('Could not calculate next retry date.', 1721897113);
+        }
+
+        return $nextDate;
+    }
+
+    private function calculateDelay(int $attempt): int
+    {
+        return (int)round($this->baseDelay * ($this->delayFactor ** $attempt));
+    }
+}
\ No newline at end of file
diff --git a/src/Subscription/RetryStrategy/NoRetryStrategy.php b/src/Subscription/RetryStrategy/NoRetryStrategy.php
new file mode 100644
index 0000000..5f746ef
--- /dev/null
+++ b/src/Subscription/RetryStrategy/NoRetryStrategy.php
@@ -0,0 +1,15 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Wwwision\DCBLibrary\Subscription\RetryStrategy;
+
+use Wwwision\DCBLibrary\Subscription\Subscription;
+
+final class NoRetryStrategy implements RetryStrategy
+{
+    public function shouldRetry(Subscription $subscription): bool
+    {
+        return false;
+    }
+}
\ No newline at end of file
diff --git a/src/Subscription/RetryStrategy/RetryStrategy.php b/src/Subscription/RetryStrategy/RetryStrategy.php
new file mode 100644
index 0000000..587eaa2
--- /dev/null
+++ b/src/Subscription/RetryStrategy/RetryStrategy.php
@@ -0,0 +1,12 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Wwwision\DCBLibrary\Subscription\RetryStrategy;
+
+use Wwwision\DCBLibrary\Subscription\Subscription;
+
+interface RetryStrategy
+{
+    public function shouldRetry(Subscription $subscription): bool;
+}
\ No newline at end of file
diff --git a/src/Subscription/Store/SubscriptionStore.php b/src/Subscription/Store/SubscriptionStore.php
index da8e494..6ddeba9 100644
--- a/src/Subscription/Store/SubscriptionStore.php
+++ b/src/Subscription/Store/SubscriptionStore.php
@@ -16,6 +16,10 @@ public function findOneById(SubscriptionId $subscriptionId): ?Subscription;
 
     public function findByCriteria(SubscriptionCriteria $criteria): Subscriptions;
 
+    public function acquireLock(SubscriptionId $subscriptionId): bool;
+
+    public function releaseLock(SubscriptionId $subscriptionId): void;
+
     public function add(Subscription $subscription): void;
 
     /**
diff --git a/src/Subscription/Subscription.php b/src/Subscription/Subscription.php
index b44fb42..db6dae9 100644
--- a/src/Subscription/Subscription.php
+++ b/src/Subscription/Subscription.php
@@ -16,6 +16,7 @@ public function __construct(
         public readonly RunMode $runMode,
         public readonly Status $status,
         public readonly SequenceNumber $position,
+        public readonly bool $locked = false,
         public readonly SubscriptionError|null $error = null,
         public readonly int $retryAttempt = 0,
         public readonly DateTimeImmutable|null $lastSavedAt = null,
@@ -47,6 +48,7 @@ public function with(
             $this->runMode,
             $status ?? $this->status,
             $position ?? $this->position,
+            $this->locked,
             $this->error,
             $retryAttempt ?? $this->retryAttempt,
             $this->lastSavedAt,
@@ -66,25 +68,12 @@ public function withError(Throwable|string $throwableOrMessage): self
             $this->runMode,
             Status::ERROR,
             $this->position,
+            $this->locked,
             $error,
             $this->retryAttempt,
             $this->lastSavedAt,
         );
     }
-
-    public function withLastSavedAt(DateTimeImmutable $lastSavedAt): self
-    {
-        return new self(
-            $this->id,
-            $this->group,
-            $this->runMode,
-            $this->status,
-            $this->position,
-            $this->error,
-            $this->retryAttempt,
-            $lastSavedAt,
-        );
-    }
 //
 //    public function doRetry(): void
 //    {
@@ -104,6 +93,7 @@ public function withoutError(): self
             $this->runMode,
             $this->status,
             $this->position,
+            $this->locked,
             null,
             $this->retryAttempt,
             $this->lastSavedAt,