In this section you bring together your skills to write a basic monitoring engine in PHP. Because you never know how your needs will change, you should make it as flexible as possible.

The logger should be able to support arbitrary service checks (for example, HTTP and FTP services) and be able to log events in arbitrary ways (via email, to a logfile, and so on). You, of course, want it to run as a daemon, so you should be able to request it to give its complete current state.

A service needs to implement the following abstract class:

abstract class ServiceCheck {

 const FAILURE = 0;
 const SUCCESS = 1;
 
 protected $timeout = 30;
 protected $next_attempt;
 protected $current_status = ServiceCheck::SUCCESS;
 protected $previous_status = ServiceCheck::SUCCESS;
 protected $frequency = 30;
 protected $description;
 protected $consecutive_failures = 0;
 protected $status_time;
 protected $failure_time;
 protected $loggers = array();
 
 abstract public function _ _construct($params);

 public function _ _call($name, $args)
 {
  if(isset($this->$name)) {
   return $this->$name;
  }
 }
 
 public function set_next_attempt()
 {
  $this->next_attempt = time() + $this->frequency;
 }
 
 public abstract function run();

 public function post_run($status)
 {
  if($status !== $this->current_status) {
   $this->previous_status = $this->current_status;
  }
  if($status === self::FAILURE) {
   if( $this->current_status === self::FAILURE ) {
    $this->consecutive_failures++;
   }
   else {
    $this->failure_time = time();
   }
  }
  else {
   $this->consecutive_failures = 0;
  }
  $this->status_time = time();
  $this->current_status = $status;
  $this->log_service_event();
 }  

 public function log_current_status()
 {
  foreach($this->loggers as $logger) {
   $logger->log_current_status($this);
  }
 }

 private function log_service_event()
 {
  foreach($this->loggers as $logger) {
   $logger->log_service_event($this);
  }
 }

 public function register_logger(ServiceLogger

$logger)
 {
  $this->loggers[] = $logger;
 }
}

The _ _call() overload method provides read-only access to the parameters of a ServiceCheck object:

• timeout—How long the check can hang before it is to be terminated by the engine.

• next_attempt—When the next attempt to contact this server should be made.

• current_status—The current state of the service: SUCCESS or FAILURE.

• previous_status—The status before the current one.

• frequency—How often the service should be checked.

• description—A description of the service.

• consecutive_failures—The number of consecutive times the service check has failed because it was last successful.

• status_time—The last time the service was checked.

• failure_time—If the status is FAILED, the time that failure occurred.

The class also implements the observer pattern, allowing objects of type ServiceLogger to register themselves and then be called whenever log_current_status() or log_service_event() is called.

The critical function to implement is run(), which defines how the check should be run. It should return SUCCESS if the check succeeded and FAILURE if not.

The post_run() method is called after the service check defined in run() returns. It handles setting the status of the object and performing logging.

The ServiceLogger interface: specifies that a logging class need only implement two methods, log_service_event() and log_current_status(), which are called when a run() check returns and when a generic status request is made, respectively.

The interface is as follows:

interface ServiceLogger {
 public function log_service_event(ServiceCheck

$service);
 public function log_current_status(ServiceCheck

$service);
}

Finally, you need to write the engine itself. The idea is similar to the ideas behind the simple programs in the "Writing Daemons" section earlier in this chapter: The server should fork off a new process to handle each check and use a SIGCHLD handler to check the return value of checks when they complete. The maximum number of checks that will be performed simultaneously should be configurable to prevent overutilization of system resources. All the services and logging will be defined in an XML file.

The following is the ServiceCheckRunner class that defines the engine:

class ServiceCheckRunner {

 private $num_children;
 private $services = array();
 private $children = array();

 public function _ _construct($conf, $num_children)
 {
  $loggers = array();
  $this->num_children = $num_children;
  $conf = simplexml_load_file($conf);
  foreach($conf->loggers->logger as $logger) {
   $class = new Reflection_Class("$logger->class");
   if($class->isInstantiable()) {
    $loggers["$logger->id"] = $class->newInstance();
   }
   else {
    fputs(STDERR, "{$logger->class} cannot be

instantiated.\n");
    exit;
   }
  }
  foreach($conf->services->service as $service) {
   $class = new Reflection_Class("$service->class");
   if($class->isInstantiable()) {
    $item = $class->newInstance($service->params);
    foreach($service->loggers->logger as $logger) {
     $item->register_logger($loggers["$logger"]);
    }
    $this->services[] = $item;
   }
   else {
    fputs(STDERR, "{$service->class} is not

instantiable.\n");
    exit;
   }
  }
 }

 private function next_attempt_sort($a, $b)
 {
  if($a->next_attempt() == $b->next_attempt()) {
   return 0;
  }
  return ($a->next_attempt() < $b->next_attempt())

? -1 : 1;
 }

 private function next()
 {
  usort($this->services,

array($this,'next_attempt_sort'));
  return $this->services[0];
 }

 public function loop()
 {
  declare(ticks=1);
  pcntl_signal(SIGCHLD, array($this, "sig_child"));
  pcntl_signal(SIGUSR1, array($this, "sig_usr1"));
  while(1) {
   $now = time();
   if(count($this->children)

< $this->num_children) {
    $service = $this->next();
    if($now < $service->next_attempt()) {
     sleep(1);
     continue;
    }
    $service->set_next_attempt();
    if($pid = pcntl_fork()) {
     $this->children[$pid] = $service;
    } 
    else {
     pcntl_alarm($service->timeout());
     exit($service->run());
    }
   }  
  } 
 }

 public function log_current_status()
 {
  foreach($this->services as $service) {
   $service->log_current_status();
  }
 }
 
 private function sig_child($signal)
 {
  $status = ServiceCheck::FAILURE;
  pcntl_signal(SIGCHLD, array($this, "sig_child"));
  while(($pid = pcntl_wait($status, WNOHANG)) > 0)

{
   $service = $this->children[$pid];
   unset($this->children[$pid]);
   if(pcntl_wifexited($status) && 
     pcntl_wexitstatus($status) ==

ServiceCheck::SUCCESS) 
   {
    $status = ServiceCheck::SUCCESS;
   }
   $service->post_run($status);
  }
 }
 
 private function sig_usr1($signal)
 {
  pcntl_signal(SIGUSR1, array($this, "sig_usr1"));
  $this->log_current_status();
 }
}

This is an elaborate class. The constructor reads in and parses an XML file, creating all the services to be monitored and the loggers to record them. You'll learn more details on this in a moment.

The loop() method is the main method in the class. It sets the required signal handlers and checks whether a new child process can be created. If the next event (sorted by next_attempt timestamp) is okay to run now, a new process is forked off. Inside the child process, an alarm is set to keep the test from lasting longer than its timeout, and then the test defined by run() is executed.

There are also two signal handlers. The SIGCHLD handler sig_child() collects on the terminated child processes and executes their service's post_run() method. The SIGUSR1 handler sig_usr1() simply calls the log_current_status() methods of all registered loggers, which can be used to get the current status of the entire system.

As it stands, of course, the monitoring architecture doesn't do anything. First, you need a service to check. The following is a class that checks whether you get back a 200 Server OK response from an HTTP server:

class HTTP_ServiceCheck extends ServiceCheck
{
 public $url;
 public function _ _construct($params)
 {
  foreach($params as $k => $v) {
   $k = "$k";
   $this->$k = "$v";
  }
 }

 public function run()
 {
  if(is_resource(@fopen($this->url, "r"))) {
   return ServiceCheck::SUCCESS;
  }
  else {
   return ServiceCheck::FAILURE;
  }
 }
}

Compared to the framework you built earlier, this service is extremely simple—and that's the point: the effort goes into building the framework, and the extensions are very simple.