diff --git a/state/task_log.h b/state/task_log.h
new file mode 100644
index 0000000..d5aa4f0
--- /dev/null
+++ b/state/task_log.h
@@ -0,0 +1,33 @@
+#ifndef TASK_LOG_H
+#define TASK_LOG_H
+
+#include "db.h"
+#include "task.h"
+
+/* The task log is a message bus that is used for all communication between
+ * local and global schedulers (and also persisted to the state database).
+ * Here are examples of events that are recorded by the task log:
+ *
+ * 1) local scheduler writes it when submits a task to the global scheduler;
+ * 2) global scheduler reads it to get the task submitted by local schedulers;
+ * 3) global scheduler writes it when assigning the task to a local scheduler;
+ * 4) local scheduler reads it to get its tasks assigned by global scheduler;
+ * 5) local scheduler writes it when a task finishes execution;
+ * 6) global scheduler reads it to get the tasks that have finished; */
+
+/* Callback for subscribing to the task log. */
+typedef void (*task_log_callback)(task_instance* task_instance, void *userdata);
+
+/* Initially add a task instance to the task log. */
+void task_log_add_task(db_handle *db, task_iid task_iid, task_instance* task_instance);
+
+/* Update task instance in the task log. */
+void task_log_update_task(db_handle *db, task_iid task_iid, int32_t state, node_id node);
+
+/* Register callback for a certain event. The node specifies the node whose
+ * events we want to listen to. If you want to listen to all events for this node,
+ * use state_filter = TASK_WAITING | TASK_SCHEDULED | TASK_RUNNING | TASK_DONE.
+ * If you want to register to updates from all nodes, set node = NIL_ID. */
+void task_log_register_callback(db_handle *db, task_log_callback callback, node_id node, int32_t state_filter, void *userdata);
+
+#endif /* TASK_LOG_H */
diff --git a/state/task_queue.h b/state/task_queue.h
deleted file mode 100644
index 9296870..0000000
--- a/state/task_queue.h
+++ /dev/null
@@ -1,33 +0,0 @@
-#ifndef TASK_QUEUE_H
-#define TASK_QUEUE_H
-
-#include "db.h"
-#include "task.h"
-
-/* The task ID is a deterministic hash of the function ID that
- * the task executes and the argument IDs or argument values */
-typedef unique_id task_id;
-
-/* The task instance ID is a globally unique ID generated which
- * identifies this particular execution of the task */
-typedef unique_id task_iid;
-
-/* The node id is an identifier for the node the task is
- * scheduled on */
-typedef unique_id node_id;
-
-/* Callback for subscribing to the task queue. The only argument this
- * callback gets is the task_id of the. */
-typedef void (*task_queue_callback)(task_iid *task_iid, task_spec *task);
-
-/* Submit task to the global scheduler. */
-void task_queue_submit_task(db_handle *db, task_iid task_iid, task_spec *task);
-
-/* Submit task to a local scheduler based on the decision made by the global
- * scheduler. */
-void task_queue_schedule_task(db_handle *db, task_iid task_iid, node_id node);
-
-/* Subscribe to task queue. */
-void task_queue_register_callback(db_handle *db, task_queue_callback callback);
-
-#endif
diff --git a/task.h b/task.h
index 96c97b8..6603f1a 100644
--- a/task.h
+++ b/task.h
@@ -1,7 +1,8 @@
+
 #ifndef TASK_H
 #define TASK_H
 
-/* This API specifies the task data structure. It is in C so we can
+/* This API specifies the task data structures. It is in C so we can
  * easily construct tasks from other languages like Python. The datastructures
  * are also defined in such a way that memory is contiguous and all pointers
  * are relative, so that we can memcpy the datastructure and ship it over the
@@ -15,6 +16,24 @@
 typedef unique_id function_id;
 typedef unique_id object_id;
 
+/* The task ID is a deterministic hash of the function ID that
+ * the task executes and the argument IDs or argument values */
+typedef unique_id task_id;
+
+/* The task instance ID is a globally unique ID generated which
+ * identifies this particular execution of the task */
+typedef unique_id task_iid;
+
+/* The node id is an identifier for the node the task is
+ * scheduled on */
+typedef unique_id node_id;
+
+/*
+ * TASK SPECIFICATIONS: Contain all the information neccessary
+ * to execute the task (function id, arguments, return object ids).
+ * 
+ */
+
 typedef struct task_spec_impl task_spec;
 
 /* If argument is passed by value or reference. */
@@ -68,4 +87,45 @@ void print_task(task_spec *spec, UT_string *output);
 /* Parse task as printed by print_task. */
 task_spec *parse_task(char *task_string, int64_t task_length);
 
+/*
+ * SCHEDULED TASK: Contains information about a scheduled task:
+ * the task iid, the task specification and the task status
+ * (WAITING, SCHEDULED, RUNNING, DONE) and which node the
+ * task is scheduled on.
+ *
+ */
+
+/* The scheduling_state can be used as a flag when we are listening for an event,
+ * for example TASK_WAITING | TASK_SCHEDULED. */
+enum scheduling_state {
+  TASK_WAITING = 1,
+  TASK_SCHEDULED = 2,
+  TASK_RUNNING = 4,
+  TASK_DONE = 8
+};
+
+/* A task instance is one execution of a task specification.
+ * It has a unique instance id, a state of execution (see scheduling_state)
+ * and a node it is scheduled on or running on. */
+typedef struct task_instance_impl task_instance;
+
+/* Allocate and initialize a new task instance. Must be freed with
+ * scheduled_task_free after use. */
+task_instance *make_task_instance(task_iid task_iid, task_spec *task, int32_t state, node_id node);
+
+/* Size of task instance structure in bytes. */
+int64_t task_instance_size(task_instance *instance);
+
+/* Instance ID of the task instance. */
+task_iid *task_instance_id(task_instance *instance);
+
+/* Node this task instance has been assigned to or is running on. */
+node_id *task_instance_node(task_instance *instance);
+
+/* Task specification of this task instance. */
+task_spec *task_instance_task_spec(task_instance *instance);
+
+/* Free this task instance datastructure. */
+void task_instance_free(task_instance *instance);
+
 #endif