Function select_task

apps/backend/services/task_service.js:113–164

* @brief Select next available task for processing * * @return Task record or null if no task available * * Selection Logic: * 1. No running tasks (STATUS=1) in system * 2. Task status is NEW(0) or RETRY(-2,-3) * 3. Task not finished and has content * 4. Prioritize NEW tasks over RETRY *

()

 * @note Ensures single task execution across processes
 */
async function select_task() {
  const query_select_task = TASK_QUERIES.SELECT_TASK;
  // IMPORTANT!!!!  The NOT EXISTS line ensures that even if there are multiple processes, 
  // only one process is executing at any given time, meaning if a task is in status=1, 
  // other processes will not proceed to process other tasks with status=0.  
  // SQL statment:
  // WITH T AS (
  //   SELECT * 
  //     FROM TASK
  //    WHERE (STATUS=0 AND PID = ${process.pid})
  //       OR STATUS=-2
  //       OR STATUS=-3
  // )
  // SELECT TOP 1 *
  //   FROM T
  //  WHERE NOT EXISTS (SELECT 1 FROM TASK WHERE STATUS=1)
  //    AND FINISH_AT IS NULL
  //    AND FILE_SIZE > 0
  //    AND IS_DELETE IS NULL
  //  ORDER BY 
  //   CASE WHEN STATUS=0 THEN 0 ELSE 1 END,
  //   CASE WHEN STATUS=0 THEN CREATE_AT END,
  //   CASE WHEN STATUS=-2 OR STATUS=-3 THEN RETRY END;
  // 

  try {
    const pool = await pool_promise;
    const result = await pool.request()
      .input('pid', sql.Int, process.pid)
      .query(query_select_task);

    // Currently no unexecuted tasks
    if (!result.recordset.length) {
      return null;
    }

    const task = result.recordset[0];
    const memo = operation_memo({ ref: task.OBJID });
    
    task.CREATE_AT = new Date(task.CREATE_AT).toISOString();
    
    logger(LOG_LEVEL.INFO, `Task ${task.OBJID} selected for execution`, memo);
    logger(LOG_LEVEL.INFO_TABLE, task);

    return task;
  } catch (err) {
    logger(LOG_LEVEL.ERROR, `select_task ${err}.`, operation_memo({}));

    // No need to throw err, just return null or an empty array if preferred
    return null; 
  }
}    

/**
 * @brief Cancel a transcription task by ID