Introduce local scheduler heartbeats which carry load information. (#155)

* Introduce local scheduler heartbeats which carry load information.

src/photon/photon_algorithm.c

@@ -5,6 +5,7 @@
 #include "utlist.h"
 
 #include "state/task_table.h"
+#include "state/local_scheduler_table.h"
 #include "state/object_table.h"
 #include "photon.h"
 #include "photon_scheduler.h"
@@ -90,6 +91,14 @@ void free_scheduling_algorithm_state(
   free(algorithm_state);
 }
 
+void provide_scheduler_info(local_scheduler_state *state,
+                            scheduling_algorithm_state *algorithm_state,
+                            local_scheduler_info *info) {
+  task_queue_entry *elt;
+  DL_COUNT(algorithm_state->task_queue, elt, info->task_queue_length);
+  info->available_workers = utarray_len(algorithm_state->available_workers);
+}
+
 /**
  * Check if all of the remote object arguments for a task are available in the
  * local object store.

src/photon/photon_algorithm.h

@@ -3,6 +3,7 @@
 
 #include "photon.h"
 #include "common/task.h"
+#include "state/local_scheduler_table.h"
 
 /* The duration that the local scheduler will wait before reinitiating a fetch
  * request for a missing task dependency. TODO(rkn): We may want this to be
@@ -33,6 +34,13 @@ scheduling_algorithm_state *make_scheduling_algorithm_state(void);
 void free_scheduling_algorithm_state(
     scheduling_algorithm_state *algorithm_state);
 
+/**
+ *
+ */
+void provide_scheduler_info(local_scheduler_state *state,
+                            scheduling_algorithm_state *algorithm_state,
+                            local_scheduler_info *info);
+
 /**
  * This function will be called when a new task is submitted by a worker for
  * execution.

src/photon/photon_scheduler.c

@@ -291,6 +291,18 @@ void handle_task_scheduled_callback(task *original_task, void *user_context) {
                         task_task_spec(original_task));
 }
 
+int heartbeat_handler(event_loop *loop, timer_id id, void *context) {
+  local_scheduler_state *state = context;
+  scheduling_algorithm_state *algorithm_state = state->algorithm_state;
+  local_scheduler_info info;
+  /* Ask the scheduling algorithm to fill out the scheduler info struct. */
+  provide_scheduler_info(state, algorithm_state, &info);
+  /* Publish the heartbeat to all subscribers of the local scheduler table. */
+  local_scheduler_table_send_info(state->db, &info, NULL);
+  /* Reset the timer. */
+  return LOCAL_SCHEDULER_HEARTBEAT_TIMEOUT_MILLISECONDS;
+}
+
 void start_server(const char *node_ip_address,
                   const char *socket_name,
                   const char *redis_addr,
@@ -323,6 +335,13 @@ void start_server(const char *node_ip_address,
                          TASK_STATUS_SCHEDULED, handle_task_scheduled_callback,
                          NULL, &retry, NULL, NULL);
   }
+  /* Create a timer for publishing information about the load on the local
+   * scheduler to the local scheduler table. This message also serves as a
+   * heartbeat. */
+  if (g_state->db != NULL) {
+    event_loop_add_timer(loop, LOCAL_SCHEDULER_HEARTBEAT_TIMEOUT_MILLISECONDS,
+                         heartbeat_handler, g_state);
+  }
   /* Run event loop. */
   event_loop_run(loop);
 }

src/photon/photon_scheduler.h

@@ -4,6 +4,9 @@
 #include "task.h"
 #include "event_loop.h"
 
+/* The duration between local scheduler heartbeats. */
+#define LOCAL_SCHEDULER_HEARTBEAT_TIMEOUT_MILLISECONDS 1000
+
 /**
  * Establish a connection to a new client.
  *