diff --git a/common.c b/common.c
index 53e32fe13..d9fe1e951 100644
--- a/common.c
+++ b/common.c
@@ -6,6 +6,9 @@
 #include <sys/stat.h>
 #include <fcntl.h>
 
+const unique_id NIL_ID = {{255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
+                           255, 255, 255, 255, 255, 255, 255, 255, 255, 255}};
+
 unique_id globally_unique_id(void) {
   /* Use /dev/urandom for "real" randomness. */
   int fd;
@@ -31,63 +34,3 @@ char *sha1_to_hex(const unsigned char *sha1, char *buffer) {
 
   return buffer;
 }
-
-const signed char hexval_table[256] = {
-    -1, -1, -1, -1, -1, -1, -1, -1, /* 00-07 */
-    -1, -1, -1, -1, -1, -1, -1, -1, /* 08-0f */
-    -1, -1, -1, -1, -1, -1, -1, -1, /* 10-17 */
-    -1, -1, -1, -1, -1, -1, -1, -1, /* 18-1f */
-    -1, -1, -1, -1, -1, -1, -1, -1, /* 20-27 */
-    -1, -1, -1, -1, -1, -1, -1, -1, /* 28-2f */
-    +0, +1, +2, +3, +4, +5, +6, +7, /* 30-37 */
-    +8, +9, -1, -1, -1, -1, -1, -1, /* 38-3f */
-    -1, 10, 11, 12, 13, 14, 15, -1, /* 40-47 */
-    -1, -1, -1, -1, -1, -1, -1, -1, /* 48-4f */
-    -1, -1, -1, -1, -1, -1, -1, -1, /* 50-57 */
-    -1, -1, -1, -1, -1, -1, -1, -1, /* 58-5f */
-    -1, 10, 11, 12, 13, 14, 15, -1, /* 60-67 */
-    -1, -1, -1, -1, -1, -1, -1, -1, /* 68-67 */
-    -1, -1, -1, -1, -1, -1, -1, -1, /* 70-77 */
-    -1, -1, -1, -1, -1, -1, -1, -1, /* 78-7f */
-    -1, -1, -1, -1, -1, -1, -1, -1, /* 80-87 */
-    -1, -1, -1, -1, -1, -1, -1, -1, /* 88-8f */
-    -1, -1, -1, -1, -1, -1, -1, -1, /* 90-97 */
-    -1, -1, -1, -1, -1, -1, -1, -1, /* 98-9f */
-    -1, -1, -1, -1, -1, -1, -1, -1, /* a0-a7 */
-    -1, -1, -1, -1, -1, -1, -1, -1, /* a8-af */
-    -1, -1, -1, -1, -1, -1, -1, -1, /* b0-b7 */
-    -1, -1, -1, -1, -1, -1, -1, -1, /* b8-bf */
-    -1, -1, -1, -1, -1, -1, -1, -1, /* c0-c7 */
-    -1, -1, -1, -1, -1, -1, -1, -1, /* c8-cf */
-    -1, -1, -1, -1, -1, -1, -1, -1, /* d0-d7 */
-    -1, -1, -1, -1, -1, -1, -1, -1, /* d8-df */
-    -1, -1, -1, -1, -1, -1, -1, -1, /* e0-e7 */
-    -1, -1, -1, -1, -1, -1, -1, -1, /* e8-ef */
-    -1, -1, -1, -1, -1, -1, -1, -1, /* f0-f7 */
-    -1, -1, -1, -1, -1, -1, -1, -1, /* f8-ff */
-};
-
-static inline unsigned int hexval(unsigned char c) {
-  return hexval_table[c];
-}
-
-/*
- * Convert two consecutive hexadecimal digits into a char.  Return a
- * negative value on error.  Don't run over the end of short strings.
- */
-static inline int hex2chr(const char *s) {
-  int val = hexval(s[0]);
-  return (val < 0) ? val : (val << 4) | hexval(s[1]);
-}
-
-int hex_to_sha1(const char *hex, unsigned char *sha1) {
-  int i;
-  for (i = 0; i < UNIQUE_ID_SIZE; i++) {
-    int val = hex2chr(hex);
-    if (val < 0)
-      return -1;
-    *sha1++ = val;
-    hex += 2;
-  }
-  return 0;
-}
diff --git a/common.h b/common.h
index 5444739bc..047644cc8 100644
--- a/common.h
+++ b/common.h
@@ -38,6 +38,8 @@
 
 typedef struct { unsigned char id[UNIQUE_ID_SIZE]; } unique_id;
 
+extern const unique_id NIL_ID;
+
 /* Generate a globally unique ID. */
 unique_id globally_unique_id(void);
 
@@ -46,11 +48,6 @@ unique_id globally_unique_id(void);
  * UNIQUE_ID_SIZE + 1 */
 char *sha1_to_hex(const unsigned char *sha1, char *buffer);
 
-/* Convert a hexdecimal string of length 40 to a 20 byte sha1 hash. This
- * function assumes that sha1 points to an already allocated char array of size
- * UNIQUE_ID_SIZE. */
-int hex_to_sha1(const char *hex, unsigned char *sha1);
-
 typedef unique_id object_id;
 
 #endif
diff --git a/state/redis.c b/state/redis.c
index aa2011d9a..db008cb5d 100644
--- a/state/redis.c
+++ b/state/redis.c
@@ -9,7 +9,7 @@
 #include "common.h"
 #include "db.h"
 #include "object_table.h"
-#include "task_queue.h"
+#include "task_log.h"
 #include "event_loop.h"
 #include "redis.h"
 #include "io.h"
@@ -65,22 +65,28 @@ db_handle *db_connect(const char *address,
 
   db->client_type = strdup(client_type);
   db->client_id = num_clients;
-  db->reading = 0;
-  db->writing = 0;
   db->service_cache = NULL;
   db->sync_context = context;
+  utarray_new(db->callback_freelist, &ut_ptr_icd);
 
   /* Establish async connection */
   db->context = redisAsyncConnect(address, port);
   CHECK_REDIS_CONNECT(redisAsyncContext, db->context,
                       "could not connect to redis %s:%d", address, port);
   db->context->data = (void *) db;
+  /* Establish async connection for subscription */
+  db->sub_context = redisAsyncConnect(address, port);
+  CHECK_REDIS_CONNECT(redisAsyncContext, db->sub_context,
+                      "could not connect to redis %s:%d", address, port);
+  db->sub_context->data = (void *) db;
+
   return db;
 }
 
 void db_disconnect(db_handle *db) {
   redisFree(db->sync_context);
   redisAsyncFree(db->context);
+  redisAsyncFree(db->sub_context);
   service_cache_entry *e, *tmp;
   HASH_ITER(hh, db->service_cache, e, tmp) {
     free(e->addr);
@@ -88,18 +94,22 @@ void db_disconnect(db_handle *db) {
     free(e);
   }
   free(db->client_type);
+  void **p = NULL;
+  while ((p = (void **) utarray_next(db->callback_freelist, p))) {
+    free(*p);
+  }
+  utarray_free(db->callback_freelist);
   free(db);
 }
 
 void db_attach(db_handle *db, event_loop *loop) {
   redisAeAttach(loop, db->context);
+  redisAeAttach(loop, db->sub_context);
 }
 
 void object_table_add(db_handle *db, unique_id object_id) {
-  static char hex_object_id[2 * UNIQUE_ID_SIZE + 1];
-  sha1_to_hex(&object_id.id[0], &hex_object_id[0]);
-  redisAsyncCommand(db->context, NULL, NULL, "SADD obj:%s %d",
-                    &hex_object_id[0], db->client_id);
+  redisAsyncCommand(db->context, NULL, NULL, "SADD obj:%b %d", &object_id.id[0],
+                    UNIQUE_ID_SIZE, db->client_id);
   if (db->context->err) {
     LOG_REDIS_ERR(db->context, "could not add object_table entry");
   }
@@ -148,30 +158,75 @@ void object_table_get_entry(redisAsyncContext *c, void *r, void *privdata) {
 void object_table_lookup(db_handle *db,
                          object_id object_id,
                          lookup_callback callback) {
-  static char hex_object_id[2 * UNIQUE_ID_SIZE + 1];
-  sha1_to_hex(&object_id.id[0], &hex_object_id[0]);
   lookup_callback_data *cb_data = malloc(sizeof(lookup_callback_data));
   cb_data->callback = callback;
   cb_data->object_id = object_id;
   redisAsyncCommand(db->context, object_table_get_entry, cb_data,
-                    "SMEMBERS obj:%s", &hex_object_id[0]);
+                    "SMEMBERS obj:%b", &object_id.id[0], UNIQUE_ID_SIZE);
   if (db->context->err) {
     LOG_REDIS_ERR(db->context, "error in object_table lookup");
   }
 }
 
-void task_queue_submit_task(db_handle *db, task_iid task_iid, task_spec *task) {
-  /* For converting an id to hex, which has double the number
-   * of bytes compared to the id (+ 1 byte for '\0'). */
-  static char hex[2 * UNIQUE_ID_SIZE + 1];
-  UT_string *command;
-  utstring_new(command);
-  sha1_to_hex(&task_iid.id[0], &hex[0]);
-  utstring_printf(command, "HMSET queue:%s ", &hex[0]);
-  print_task(task, command);
-  redisAsyncCommand(db->context, NULL, NULL, utstring_body(command));
+void task_log_add_task(db_handle *db, task_instance *task_instance) {
+  task_iid task_iid = *task_instance_id(task_instance);
+  redisAsyncCommand(db->context, NULL, NULL, "HMSET tasklog:%b 0 %b",
+                    (char *) &task_iid.id[0], UNIQUE_ID_SIZE,
+                    (char *) task_instance, task_instance_size(task_instance));
   if (db->context->err) {
-    LOG_REDIS_ERR(db->context, "error in task_queue submit_task");
+    LOG_REDIS_ERR(db->context, "error setting task in task_log_add_task");
+  }
+  node_id node = *task_instance_node(task_instance);
+  int32_t state = *task_instance_state(task_instance);
+  redisAsyncCommand(db->context, NULL, NULL, "PUBLISH task_log:%b:%d %b",
+                    (char *) &node.id[0], UNIQUE_ID_SIZE, state,
+                    (char *) task_instance, task_instance_size(task_instance));
+  if (db->context->err) {
+    LOG_REDIS_ERR(db->context, "error publishing task in task_log_add_task");
+  }
+}
+
+void task_log_redis_callback(redisAsyncContext *c,
+                             void *reply,
+                             void *privdata) {
+  redisReply *r = reply;
+  if (reply == NULL)
+    return;
+  CHECK(r->type == REDIS_REPLY_ARRAY);
+  /* First entry is message type, second is topic, third is payload. */
+  CHECK(r->elements > 2);
+  /* If this condition is true, we got the initial message that acknowledged the
+   * subscription. */
+  if (r->element[2]->str == NULL) {
+    return;
+  }
+  /* Otherwise, parse the task and call the callback. */
+  CHECK(privdata);
+  task_log_callback_data *callback_data = privdata;
+  task_instance *instance = malloc(r->element[2]->len);
+  memcpy(instance, r->element[2]->str, r->element[2]->len);
+  callback_data->callback(instance, callback_data->userdata);
+  task_instance_free(instance);
+}
+void task_log_register_callback(db_handle *db,
+                                task_log_callback callback,
+                                node_id node,
+                                int32_t state,
+                                void *userdata) {
+  task_log_callback_data *callback_data =
+      malloc(sizeof(task_log_callback_data));
+  utarray_push_back(db->callback_freelist, &callback_data);
+  callback_data->callback = callback;
+  callback_data->userdata = userdata;
+  if (memcmp(&node.id[0], &NIL_ID.id[0], UNIQUE_ID_SIZE) == 0) {
+    redisAsyncCommand(db->sub_context, task_log_redis_callback, callback_data,
+                      "PSUBSCRIBE task_log:*:%d", state);
+  } else {
+    redisAsyncCommand(db->sub_context, task_log_redis_callback, callback_data,
+                      "SUBSCRIBE task_log:%b:%d", (char *) &node.id[0],
+                      UNIQUE_ID_SIZE, state);
+  }
+  if (db->sub_context->err) {
+    LOG_REDIS_ERR(db->sub_context, "error in task_log_register_callback");
   }
-  utstring_free(command);
 }
diff --git a/state/redis.h b/state/redis.h
index 23ebb2ba6..51479f8f0 100644
--- a/state/redis.h
+++ b/state/redis.h
@@ -3,10 +3,12 @@
 
 #include "db.h"
 #include "object_table.h"
+#include "task_log.h"
 
 #include "hiredis/hiredis.h"
 #include "hiredis/async.h"
 #include "uthash.h"
+#include "utarray.h"
 
 typedef struct {
   /* Unique ID for this service. */
@@ -17,6 +19,13 @@ typedef struct {
   UT_hash_handle hh;
 } service_cache_entry;
 
+typedef struct {
+  /* The callback that will be called. */
+  task_log_callback callback;
+  /* Userdata associated with the callback. */
+  void *userdata;
+} task_log_callback_data;
+
 struct db_handle_impl {
   /* String that identifies this client type. */
   char *client_type;
@@ -24,8 +33,10 @@ struct db_handle_impl {
   int64_t client_id;
   /* Redis context for this global state store connection. */
   redisAsyncContext *context;
-  /* Which events are we processing (read, write)? */
-  int reading, writing;
+  /* Redis context for "subscribe" communication.
+   * Yes, we need a separate one for that, see
+   * https://github.com/redis/hiredis/issues/55 */
+  redisAsyncContext *sub_context;
   /* The event loop this global state store connection is part of. */
   event_loop *loop;
   /* Index of the database connection in the event loop */
@@ -35,6 +46,8 @@ struct db_handle_impl {
   /* Redis context for synchronous connections.
    * Should only be used very rarely, it is not asynchronous. */
   redisContext *sync_context;
+  /* Data structure for callbacks that needs to be freed. */
+  UT_array *callback_freelist;
 };
 
 typedef struct {
diff --git a/state/task_log.h b/state/task_log.h
new file mode 100644
index 000000000..acf5dbcd0
--- /dev/null
+++ b/state/task_log.h
@@ -0,0 +1,41 @@
+#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_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 92968707e..000000000
--- 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/state/task_table.h b/state/task_table.h
index 71e879c2c..3a1852522 100644
--- a/state/task_table.h
+++ b/state/task_table.h
@@ -5,9 +5,16 @@
 #include "task.h"
 
 /* Add task to the task table, handle errors here. */
-status task_table_add_task(db_handle *db, task_iid task_iid, task_spec *task);
+status task_table_add_task(db_handle *db, task_spec *task);
+
+/* Callback for getting an entry from the task table. Task spec will be freed
+ * by the system after the callback */
+typedef void (*task_table_callback)(task_spec *task, void *context);
 
 /* Get specific task from the task table. */
-status task_table_get_task(db_handle *db, task_iid task_iid, task_spec *task);
+status task_table_get_task(db_handle *db,
+                           task_id task_id,
+                           task_table_callback callback,
+                           void *context);
 
 #endif /* TASK_TABLE_H */
diff --git a/task.c b/task.c
index 30fde8d31..0086eb337 100644
--- a/task.c
+++ b/task.c
@@ -8,6 +8,8 @@
 #include "common.h"
 #include "io.h"
 
+/* TASK SPECIFICATIONS */
+
 /* Tasks are stored in a consecutive chunk of memory, the first
  * sizeof(task_spec) bytes are arranged according to the struct
  * task_spec. Then there is an array of task_args of length
@@ -168,65 +170,49 @@ void print_task(task_spec *spec, UT_string *output) {
   }
 }
 
-UT_icd unique_id_icd = {sizeof(unique_id), NULL, NULL, NULL};
+/* TASK INSTANCES */
 
-task_spec *parse_task(char *task_string, int64_t task_length) {
-  /* We make one pass through task_string to store all the argument ids
-   * in "args" and all the return ids in "returns". */
-  UT_array *args;
-  utarray_new(args, &unique_id_icd);
-  UT_array *returns;
-  utarray_new(returns, &unique_id_icd);
-  function_id function_id;
-  char *cursor = strtok(task_string, " ");
-  int index = 0;
-  while (cursor != NULL) {
-    /* This will be equal to "args" or "returns" depending on whether we
-     * are processing an argument id or a return id. */
-    UT_array *target = NULL;
-    if (strncmp("fun", cursor, 3) == 0) {
-      /* Parse function id. */
-      CHECK(cursor + 2 * UNIQUE_ID_SIZE + 1 <= task_string + task_length);
-      cursor = strtok(NULL, " ");
-      hex_to_sha1(cursor, &function_id.id[0]);
-      cursor = strtok(NULL, " ");
-      CHECK(cursor);
-      continue;
-    } else if (strncmp("id:", cursor, 3) == 0) {
-      /* Parse pass by reference argument. */
-      sscanf(cursor, "id:%d", &index);
-      target = args;
-    } else if (strncmp("val:", cursor, 4) == 0) {
-      /* Parse pass by value argument. */
-      sscanf(cursor, "val:%d", &index);
-      CHECK(0); /* Not implemented yet */
-    } else if (strncmp("ret:", cursor, 4) == 0) {
-      /* Parse return object reference. */
-      sscanf(cursor, "ret:%d", &index);
-      target = returns;
-    }
-    cursor = strtok(NULL, " ");
-    CHECK(cursor);
-    if (index >= utarray_len(target)) {
-      utarray_resize(target, index + 1);
-    }
-    object_id *id = (object_id *) utarray_eltptr(target, index);
-    hex_to_sha1(cursor, &id->id[0]);
-    cursor = strtok(NULL, " ");
-  }
-  /* TODO(pcm): Implement pass by value. */
-  /* Now assemble the task specification. */
-  task_spec *spec =
-      alloc_task_spec(function_id, utarray_len(args), utarray_len(returns), 0);
-  for (int i = 0; i < utarray_len(args); ++i) {
-    object_id *id = (object_id *) utarray_eltptr(args, i);
-    task_args_add_ref(spec, *id);
-  }
-  for (int i = 0; i < utarray_len(returns); ++i) {
-    object_id *id = (object_id *) utarray_eltptr(returns, i);
-    *task_return(spec, i) = *id;
-  }
-  utarray_free(args);
-  utarray_free(returns);
-  return spec;
+struct task_instance_impl {
+  task_iid iid;
+  int32_t state;
+  node_id node;
+  task_spec spec;
+};
+
+task_instance *make_task_instance(task_iid task_iid,
+                                  task_spec *spec,
+                                  int32_t state,
+                                  node_id node) {
+  int64_t size = sizeof(task_instance) - sizeof(task_spec) + task_size(spec);
+  task_instance *result = malloc(size);
+  memset(result, 0, size);
+  result->iid = task_iid;
+  result->state = state;
+  result->node = node;
+  memcpy(&result->spec, spec, task_size(spec));
+  return result;
+}
+
+int64_t task_instance_size(task_instance *instance) {
+  return sizeof(task_instance) - sizeof(task_spec) + task_size(&instance->spec);
+}
+
+task_iid *task_instance_id(task_instance *instance) {
+  return &instance->iid;
+}
+
+int32_t *task_instance_state(task_instance *instance) {
+  return &instance->state;
+}
+
+node_id *task_instance_node(task_instance *instance) {
+  return &instance->node;
+}
+
+task_spec *task_instance_task_spec(task_instance *instance) {
+  return &instance->spec;
+}
+
+void task_instance_free(task_instance *instance) {
+  free(instance);
 }
diff --git a/task.h b/task.h
index 96c97b80d..9267edb65 100644
--- a/task.h
+++ b/task.h
@@ -1,7 +1,7 @@
 #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 +15,24 @@
 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. */
@@ -65,7 +83,51 @@ task_spec *read_task(int fd);
 /* Print task as a humanly readable string. */
 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);
+
+/* The scheduling state of the task instance. */
+int32_t *task_instance_state(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
diff --git a/test/common_tests.c b/test/common_tests.c
index 3673c335d..47b643039 100644
--- a/test/common_tests.c
+++ b/test/common_tests.c
@@ -6,11 +6,8 @@ SUITE(common_tests);
 
 TEST sha1_test(void) {
   static char hex[2 * UNIQUE_ID_SIZE + 1];
-  static unsigned char id[UNIQUE_ID_SIZE];
   unique_id uid = globally_unique_id();
   sha1_to_hex(&uid.id[0], &hex[0]);
-  hex_to_sha1(&hex[0], &id[0]);
-  ASSERT(memcmp(&uid.id[0], &id[0], 20) == 0);
   PASS();
 }
 
diff --git a/test/db_tests.c b/test/db_tests.c
index 99fad7e18..96f16b528 100644
--- a/test/db_tests.c
+++ b/test/db_tests.c
@@ -6,7 +6,7 @@
 #include "test/example_task.h"
 #include "state/db.h"
 #include "state/object_table.h"
-#include "state/task_queue.h"
+#include "state/task_log.h"
 #include "state/redis.h"
 #include "task.h"
 
@@ -72,27 +72,70 @@ TEST object_table_lookup_test(void) {
   PASS();
 }
 
-TEST task_queue_test(void) {
+void task_log_test_callback(task_instance *instance, void *userdata) {
+  task_instance *other = userdata;
+  CHECK(*task_instance_state(instance) == TASK_SCHEDULED);
+  CHECK(task_instance_size(instance) == task_instance_size(other));
+  CHECK(memcmp(instance, other, task_instance_size(instance)) == 0);
+}
+
+TEST task_log_test(void) {
   event_loop *loop = event_loop_create();
   db_handle *db = db_connect("127.0.0.1", 6379, "local_scheduler", "", -1);
   db_attach(db, loop);
-
+  node_id node = globally_unique_id();
   task_spec *task = example_task();
-  task_queue_submit_task(db, globally_unique_id(), task);
+  task_instance *instance =
+      make_task_instance(globally_unique_id(), task, TASK_SCHEDULED, node);
+  task_log_register_callback(db, task_log_test_callback, node, TASK_SCHEDULED,
+                             instance);
+  task_log_add_task(db, instance);
   event_loop_add_timer(loop, 100, timeout_handler, NULL);
   event_loop_run(loop);
-
+  task_instance_free(instance);
   free_task_spec(task);
   db_disconnect(db);
   event_loop_destroy(loop);
   PASS();
 }
 
+int num_test_callback_called = 0;
+
+void task_log_all_test_callback(task_instance *instance, void *userdata) {
+  num_test_callback_called += 1;
+}
+
+TEST task_log_all_test(void) {
+  event_loop *loop = event_loop_create();
+  db_handle *db = db_connect("127.0.0.1", 6379, "local_scheduler", "", -1);
+  db_attach(db, loop);
+  task_spec *task = example_task();
+  /* Schedule two tasks on different nodes. */
+  task_instance *instance1 = make_task_instance(
+      globally_unique_id(), task, TASK_SCHEDULED, globally_unique_id());
+  task_instance *instance2 = make_task_instance(
+      globally_unique_id(), task, TASK_SCHEDULED, globally_unique_id());
+  task_log_register_callback(db, task_log_all_test_callback, NIL_ID,
+                             TASK_SCHEDULED, NULL);
+  task_log_add_task(db, instance1);
+  task_log_add_task(db, instance2);
+  event_loop_add_timer(loop, 100, timeout_handler, NULL);
+  event_loop_run(loop);
+  task_instance_free(instance2);
+  task_instance_free(instance1);
+  free_task_spec(task);
+  db_disconnect(db);
+  event_loop_destroy(loop);
+  ASSERT(num_test_callback_called == 2);
+  PASS();
+}
+
 SUITE(db_tests) {
   redisContext *context = redisConnect("127.0.0.1", 6379);
   freeReplyObject(redisCommand(context, "FLUSHALL"));
   RUN_REDIS_TEST(context, object_table_lookup_test);
-  RUN_REDIS_TEST(context, task_queue_test);
+  RUN_REDIS_TEST(context, task_log_test);
+  RUN_REDIS_TEST(context, task_log_all_test);
   redisFree(context);
 }
 
diff --git a/test/task_tests.c b/test/task_tests.c
index fcb714737..f72a0e2c2 100644
--- a/test/task_tests.c
+++ b/test/task_tests.c
@@ -63,28 +63,9 @@ TEST send_task(void) {
   PASS();
 }
 
-TEST print_and_parse_task(void) {
-  task_spec *task = example_task();
-
-  UT_string *output;
-  utstring_new(output);
-  print_task(task, output);
-  task_spec *result = parse_task(utstring_body(output), utstring_len(output));
-  utstring_free(output);
-
-  ASSERT_EQ(task_size(task), task_size(result));
-  ASSERT(memcmp(task, result, task_size(task)) == 0);
-
-  free_task_spec(task);
-  free_task_spec(result);
-
-  PASS();
-}
-
 SUITE(task_tests) {
   RUN_TEST(task_test);
   RUN_TEST(send_task);
-  RUN_TEST(print_and_parse_task);
 }
 
 GREATEST_MAIN_DEFS();