diff --git a/src/FuzeWorks/Async/Constraint.php b/src/FuzeWorks/Async/Constraint.php
index 9b3f100..32c87a7 100644
--- a/src/FuzeWorks/Async/Constraint.php
+++ b/src/FuzeWorks/Async/Constraint.php
@@ -44,9 +44,10 @@ interface Constraint
      * Return true ONLY when this constraint changes the execution of the Task. Otherwise return false.
      *
      * @param Task $task
+     * @param Task[] $tasks
      * @return bool
      */
-    public function intervene(Task $task): bool;
+    public function intervene(Task $task, array $tasks): bool;
 
     /**
      * When intervene() returns true, this method should return the new status of the task.
diff --git a/src/FuzeWorks/Async/Constraint/DependencyConstraint.php b/src/FuzeWorks/Async/Constraint/DependencyConstraint.php
index ff78586..3938833 100644
--- a/src/FuzeWorks/Async/Constraint/DependencyConstraint.php
+++ b/src/FuzeWorks/Async/Constraint/DependencyConstraint.php
@@ -84,12 +84,12 @@ class DependencyConstraint implements Constraint
     /**
      * @inheritDoc
      */
-    public function intervene(Task $task): bool
+    public function intervene(Task $task, array $tasks): bool
     {
         // Fetch taskStorage
         try {
-            $tasks = $this->loadTasksLib();
-            $taskStorage = $tasks->getTaskStorage();
+            $tasksLib = $this->loadTasksLib();
+            $taskStorage = $tasksLib->getTaskStorage();
 
             // Is any dependency unresolved?
             $hasUnresolved = false;
diff --git a/src/FuzeWorks/Async/Constraint/FixedTimeConstraint.php b/src/FuzeWorks/Async/Constraint/FixedTimeConstraint.php
index eb17a0c..3ecd1b0 100644
--- a/src/FuzeWorks/Async/Constraint/FixedTimeConstraint.php
+++ b/src/FuzeWorks/Async/Constraint/FixedTimeConstraint.php
@@ -70,7 +70,7 @@ class FixedTimeConstraint implements Constraint
     /**
      * @inheritDoc
      */
-    public function intervene(Task $task): bool
+    public function intervene(Task $task, array $tasks): bool
     {
         if ($task->getStatus() === Task::PENDING && time() < $this->timestamp)
             return true;
diff --git a/src/FuzeWorks/Async/Constraint/GroupConstraint.php b/src/FuzeWorks/Async/Constraint/GroupConstraint.php
new file mode 100644
index 0000000..86e781e
--- /dev/null
+++ b/src/FuzeWorks/Async/Constraint/GroupConstraint.php
@@ -0,0 +1,119 @@
+<?php
+/**
+ * FuzeWorks Component.
+ *
+ * The FuzeWorks PHP FrameWork
+ *
+ * Copyright (C) 2013-2019 TechFuze
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ *
+ * @author    TechFuze
+ * @copyright Copyright (c) 2013 - 2019, TechFuze. (http://techfuze.net)
+ * @license   https://opensource.org/licenses/MIT MIT License
+ *
+ * @link  http://techfuze.net/fuzeworks
+ * @since Version 1.2.0
+ *
+ * @version Version 1.2.0
+ */
+
+namespace FuzeWorks\Async\Constraint;
+use FuzeWorks\Async\Constraint;
+use FuzeWorks\Async\Task;
+
+/**
+ * Class GroupConstraint
+ *
+ * Use this constraint to maximize the amount of concurrent tasks within a certain group.
+ * If the amount of running tasks equals the maximum concurrent tasks, this constraint will issue a delay.
+ * Useful when multiple tasks are not dependent on each other, but should be run separately.
+ *
+ * @package FuzeWorks\Async\Constraint
+ */
+class GroupConstraint implements Constraint
+{
+
+    /**
+     * @var string
+     */
+    protected $groupName;
+
+    /**
+     * @var int
+     */
+    protected $maxConcurrent = 3;
+
+    protected $returnStatus = Task::DELAYED;
+
+    /**
+     * GroupConstraint constructor.
+     *
+     * To add a group, simply provide the $groupName and the maximum of concurrent tasks within this group.
+     *
+     * @param string $groupName
+     * @param int $maxConcurrent
+     */
+    public function __construct(string $groupName, int $maxConcurrent = 3)
+    {
+        $this->groupName = $groupName;
+        $this->maxConcurrent = $maxConcurrent;
+    }
+
+    public function getGroupName(): string
+    {
+        return $this->groupName;
+    }
+
+    public function intervene(Task $task, array $tasks): bool
+    {
+        // Find all tasks within this group and that are running
+        $runningTasks = 0;
+        foreach ($tasks as $t)
+        {
+            foreach ($t->getConstraints() as $constraint)
+            {
+                if ($constraint instanceof GroupConstraint && $this->groupName === $constraint->getGroupName())
+                {
+                    if ($t->getStatus() === Task::RUNNING || $t->getStatus() === Task::POST)
+                        $runningTasks++;
+                }
+            }
+
+            // If the running tasks exceeds the maximum, block this task from running
+            if ($runningTasks >= $this->maxConcurrent)
+            {
+                $this->returnStatus = Task::DELAYED;
+                return true;
+            }
+        }
+
+        return false;
+    }
+
+    public function blockCode(): int
+    {
+        return $this->returnStatus;
+    }
+
+    public function delayTime(): int
+    {
+        return time() + 1;
+    }
+}
\ No newline at end of file
diff --git a/src/FuzeWorks/Async/Constraint/GroupDependencyConstraint.php b/src/FuzeWorks/Async/Constraint/GroupDependencyConstraint.php
new file mode 100644
index 0000000..fe8ce55
--- /dev/null
+++ b/src/FuzeWorks/Async/Constraint/GroupDependencyConstraint.php
@@ -0,0 +1,118 @@
+<?php
+/**
+ * FuzeWorks Component.
+ *
+ * The FuzeWorks PHP FrameWork
+ *
+ * Copyright (C) 2013-2019 TechFuze
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ *
+ * @author    TechFuze
+ * @copyright Copyright (c) 2013 - 2019, TechFuze. (http://techfuze.net)
+ * @license   https://opensource.org/licenses/MIT MIT License
+ *
+ * @link  http://techfuze.net/fuzeworks
+ * @since Version 1.2.0
+ *
+ * @version Version 1.2.0
+ */
+
+namespace FuzeWorks\Async\Constraint;
+use FuzeWorks\Async\Constraint;
+use FuzeWorks\Async\Task;
+use FuzeWorks\Factory;
+
+class GroupDependencyConstraint implements Constraint
+{
+
+    /**
+     * @var string
+     */
+    protected $groupName;
+
+    protected $returnStatus = Task::DELAYED;
+
+    public function __construct(string $groupName)
+    {
+        $this->groupName = $groupName;
+    }
+
+    /**
+     * @inheritDoc
+     */
+    public function intervene(Task $task, array $tasks): bool
+    {
+        // Find whether any task is not completed
+        $hasFailed = false;
+        $hasUnresolved = false;
+        foreach ($tasks as $t)
+        {
+            foreach ($t->getConstraints() as $constraint)
+            {
+                // Check whether the constraint is a GroupConstraint
+                if ($constraint instanceof GroupConstraint && $this->groupName === $constraint->getGroupName())
+                {
+                    // Check whether GroupConstraint has failed
+                    if ($t->getStatus() === Task::CANCELLED)
+                        $hasFailed = true;
+                    // If the task is not cancelled and not completed, there are unresolved tasks
+                    elseif ($t->getStatus() !== Task::COMPLETED)
+                        $hasUnresolved = true;
+                }
+            }
+
+            // If there are failed tasks, cancel this dependent
+            if ($hasFailed)
+            {
+                $this->returnStatus = Task::CANCELLED;
+                $task->setOutput('', 'Task cancelled due to failed group dependency.');
+                return true;
+            }
+            elseif ($hasUnresolved)
+            {
+                $this->returnStatus = Task::DELAYED;
+                return true;
+            }
+        }
+
+        // Return the taskList
+        //$task->setOutput(serialize($tasks), '');
+        //$this->returnStatus = Task::CANCELLED;
+        //return true;
+
+        return false;
+    }
+
+    /**
+     * @inheritDoc
+     */
+    public function blockCode(): int
+    {
+        return $this->returnStatus;
+    }
+
+    /**
+     * @inheritDoc
+     */
+    public function delayTime(): int
+    {
+        return time() + 3;
+    }
+}
\ No newline at end of file
diff --git a/src/FuzeWorks/Async/Executors/ShellExecutor.php b/src/FuzeWorks/Async/Executors/ShellExecutor.php
index 247adda..8b3eaa0 100644
--- a/src/FuzeWorks/Async/Executors/ShellExecutor.php
+++ b/src/FuzeWorks/Async/Executors/ShellExecutor.php
@@ -59,13 +59,23 @@ class ShellExecutor implements Executor
         if (!isset($parameters['workerFile']) || !isset($parameters['bootstrapFile']))
             throw new TasksException("Could not construct ShellExecutor. Parameter failure.");
 
+        // Determine alternative worker file
+        $alternative = dirname(__DIR__, 4) . DIRECTORY_SEPARATOR . 'bin' . DIRECTORY_SEPARATOR . 'worker';
+
         // Fetch workerFile
-        $this->worker = $parameters['workerFile'];
-        if (!file_exists($this->worker))
+        $workerFile = null;
+        if (file_exists($parameters['workerFile']))
+            $workerFile = $parameters['workerFile'];
+        elseif (file_exists($alternative))
+            $workerFile = $alternative;
+
+        // If no workerFile is found, throw an exception
+        if (is_null($workerFile))
             throw new TasksException("Could not construct ShellExecutor. ShellWorker script does not exist.");
 
         // First determine the PHP binary
         $this->binary = PHP_BINDIR . DS . 'php';
+        $this->worker = $workerFile;
         $this->bootstrapFile = $parameters['bootstrapFile'];
         if (!file_exists($this->bootstrapFile))
             throw new TasksException("Could not construct ShellExecutor. No bootstrap file found.");
diff --git a/src/FuzeWorks/Async/Handler/ControllerHandler.php b/src/FuzeWorks/Async/Handler/ControllerHandler.php
index 15f88d8..f6966cd 100644
--- a/src/FuzeWorks/Async/Handler/ControllerHandler.php
+++ b/src/FuzeWorks/Async/Handler/ControllerHandler.php
@@ -149,6 +149,9 @@ class ControllerHandler implements Handler
      */
     public function getOutput(): string
     {
+        if (!is_string($this->output))
+            return '';
+
         return $this->output;
     }
 
@@ -189,6 +192,9 @@ class ControllerHandler implements Handler
      */
     public function getPostOutput(): string
     {
+        if (!is_string($this->postOutput))
+            return '';
+
         return $this->postOutput;
     }
 
diff --git a/src/FuzeWorks/Async/Supervisors/ParallelSuperVisor.php b/src/FuzeWorks/Async/Supervisors/ParallelSuperVisor.php
index ee6277a..5d6d85b 100644
--- a/src/FuzeWorks/Async/Supervisors/ParallelSuperVisor.php
+++ b/src/FuzeWorks/Async/Supervisors/ParallelSuperVisor.php
@@ -62,6 +62,7 @@ class ParallelSuperVisor implements SuperVisor
     {
         $this->taskStorage = $taskStorage;
         $this->executor = $executor;
+        gc_enable();
     }
 
     /**
@@ -88,7 +89,10 @@ class ParallelSuperVisor implements SuperVisor
 
                 // If the task changed status, task is no longer pending and should be processed by another statement
                 if ($task->getStatus() !== Task::PENDING)
+                {
+                    fwrite(STDOUT, "\nChanged status of task '".$task->getId()."' to status " . Task::getStatusType($task->getStatus()));
                     continue;
+                }
 
                 // Start the process using the executor service
                 $task = $this->executor->startTask($task);
@@ -101,6 +105,7 @@ class ParallelSuperVisor implements SuperVisor
             }
 
             // DELAYED: If task is delayed, and enough time has passed, change the status back to pending
+            // @todo Don't set to pending if still constrained
             elseif ($task->getStatus() === Task::DELAYED && time() > $task->getDelayTime())
             {
                 $task->setStatus(Task::PENDING);
@@ -262,12 +267,19 @@ class ParallelSuperVisor implements SuperVisor
             return SuperVisor::RUNNING;
     }
 
+    /**
+     * @todo Here it saves the constraint code regardless of whether there is a change. Fix!
+     *
+     * @param Task $task
+     * @return Task
+     * @throws TasksException
+     */
     private function testConstraints(Task $task): Task
     {
         $constraints = $task->getConstraints();
         foreach ($constraints as $constraint)
         {
-            if ($constraint->intervene($task) && $constraint->blockCode() != 0)
+            if ($constraint->intervene($task, $this->tasks) && $constraint->blockCode() != 0)
             {
                 $task->setStatus($constraint->blockCode());
                 if ($constraint->blockCode() === Task::DELAYED)
@@ -275,7 +287,6 @@ class ParallelSuperVisor implements SuperVisor
 
                 // Save changes to TaskStorage
                 $this->taskStorage->modifyTask($task);
-                fwrite(STDOUT, "\nChanged status of task '".$task->getId()."' to status " . Task::getStatusType($task->getStatus()));
             }
         }
 
diff --git a/src/FuzeWorks/Async/Task.php b/src/FuzeWorks/Async/Task.php
index 2da3945..08c95a5 100644
--- a/src/FuzeWorks/Async/Task.php
+++ b/src/FuzeWorks/Async/Task.php
@@ -262,6 +262,9 @@ class Task
      */
     public function addConstraint(Constraint $constraint)
     {
+        if (method_exists($constraint, 'init'))
+            $constraint->init($this);
+
         $this->constraints[] = $constraint;
     }
 
@@ -319,6 +322,16 @@ class Task
 
     /* ---------------------------------- Attributes setters and getters ------------------ */
 
+    /**
+     * Retrieve every attribute of this Task
+     *
+     * @return array
+     */
+    public function getAttributes(): array
+    {
+        return $this->attributes;
+    }
+
     /**
      * Fetch an attribute of this task
      *
diff --git a/src/FuzeWorks/Async/TaskStorage.php b/src/FuzeWorks/Async/TaskStorage.php
index f0ffafe..ddf5ef8 100644
--- a/src/FuzeWorks/Async/TaskStorage.php
+++ b/src/FuzeWorks/Async/TaskStorage.php
@@ -74,6 +74,15 @@ interface TaskStorage
      */
     public function getTaskById(string $identifier): Task;
 
+    /**
+     * Retrieve a list of all tasks that have a certain attribute
+     *
+     * @param string $attributeKey
+     * @param string $attributeValue
+     * @return array
+     */
+    public function getTasksByAttribute(string $attributeKey, string $attributeValue): array;
+
     /**
      * Modifies a task
      *
diff --git a/src/FuzeWorks/Async/TaskStorage/RedisTaskStorage.php b/src/FuzeWorks/Async/TaskStorage/RedisTaskStorage.php
index 1f6d3b3..352e597 100644
--- a/src/FuzeWorks/Async/TaskStorage/RedisTaskStorage.php
+++ b/src/FuzeWorks/Async/TaskStorage/RedisTaskStorage.php
@@ -54,6 +54,8 @@ class RedisTaskStorage implements TaskStorage
 
     protected $indexSet = 'async_index';
     protected $unfinishedSet = 'async_index_unfinished';
+    protected $attributesSet = 'async_attributes';
+    protected $attributesSetPrefix = 'async_attributes_';
     protected $key_prefix = 'async_task_';
 
     /**
@@ -115,6 +117,22 @@ class RedisTaskStorage implements TaskStorage
         if ($this->conn->hSet($this->key_prefix . $taskId, 'data', $taskData) === FALSE)
             return false;
 
+        // And create the attributes sets
+        foreach ($task->getAttributes() as $key => $val)
+        {
+            // If the attribute value is not a string it can't be used as a searchable attribute
+            if (!is_string($val))
+                continue;
+
+            // And finally add it to the set
+            $setKey = $this->attributesSetPrefix . $key . '_' . $val;
+            $this->conn->sAdd($setKey, $taskId);
+
+            // And add the attribute combo to the attributesSet
+            if (!$this->conn->sIsMember($this->attributesSet, $key . '_' . $val))
+                $this->conn->sAdd($this->attributesSet, $key . '_' . $val);
+        }
+
         return true;
     }
 
@@ -155,6 +173,22 @@ class RedisTaskStorage implements TaskStorage
         return $task;
     }
 
+    /**
+     * @inheritDoc
+     */
+    public function getTasksByAttribute(string $attributeKey, string $attributeValue): array
+    {
+        // Fetch the taskList by the attribute
+        $setKey = $this->attributesSetPrefix . $attributeKey . '_' . $attributeValue;
+        $taskList = $this->conn->sMembers($setKey);
+
+        $tasks = [];
+        foreach ($taskList as $taskId)
+            $tasks[] = unserialize($this->conn->hGet($this->key_prefix . $taskId, 'data'));
+
+        return $tasks;
+    }
+
     /**
      * @inheritDoc
      */
@@ -182,6 +216,29 @@ class RedisTaskStorage implements TaskStorage
         if ($this->conn->hSet($this->key_prefix . $taskId, 'data', $taskData) === FALSE)
             return false;
 
+        // Delete all the attributes
+        $allLoggedAttributes = $this->conn->sMembers($this->attributesSet);
+        foreach ($allLoggedAttributes as $attribute)
+        {
+            if ($this->conn->sIsMember($this->attributesSetPrefix . $attribute, $taskId))
+                $this->conn->sRem($this->attributesSetPrefix . $attribute, $taskId);
+        }
+
+        // And write all the attributes
+        foreach ($task->getAttributes() as $key => $val)
+        {
+            // If the attribute value is not a string it can't be used as a searchable attribute
+            if (!is_string($val))
+                continue;
+
+            // And finally add it to the set
+            $setKey = $this->attributesSetPrefix . $key . '_' . $val;
+            $this->conn->sAdd($setKey, $taskId);
+
+            // And add the attribute combo to the attributesSet
+            $this->conn->sAdd($this->attributesSet, $key . '_' . $val);
+        }
+
         // Modify the unfinished set
         if ($this->conn->sIsMember($this->unfinishedSet, $taskId) && ($task->getStatus() === Task::COMPLETED || $task->getStatus() === Task::CANCELLED ))
             $this->conn->sRem($this->unfinishedSet, $taskId);
@@ -212,6 +269,14 @@ class RedisTaskStorage implements TaskStorage
         if ($this->conn->sIsMember($this->unfinishedSet, $taskId))
             $this->conn->sRem($this->unfinishedSet, $taskId);
 
+        // Delete all the attributes
+        $allLoggedAttributes = $this->conn->sMembers($this->attributesSet);
+        foreach ($allLoggedAttributes as $attribute)
+        {
+            if ($this->conn->sIsMember($this->attributesSetPrefix . $attribute, $taskId))
+                $this->conn->sRem($this->attributesSetPrefix . $attribute, $taskId);
+        }
+
         // Delete the task itself
         if ($this->conn->del($this->key_prefix . $taskId) > 0)
             return true;