Submitting jobs with AMQP to a Condor based Grid

I recently took MRG Grid’s low-latency feature for a spin. It’s basically a way to use AMQP to submit jobs onto Condor managed nodes, but without going through normal scheduling, called matchmaking. The jobs are AMQP messages, and they don’t go to a Condor scheduler (a Schedd) to be matched with an execute node (a Startd). They go directly to the execute nodes, and they can go there really quickly.

This all works because Condor nodes can operate in a very autonomous fashion. Even when traditional jobs get matched with a node by the global scheduler (the Negotiator), the node always has the final say about if it will run the matched job or not. This has all sorts of benefits in a distributed system, but for low-latency it means that an execute node can take jobs from the Condor scheduler or from some other source. That other source here is an AMQP message queue.

To get going I setup the low-latency feature as described in MRG Grid User Guide: Low Latency Scheduling with the help of condor_configure_node. Since I was doing this using MRG I also used the Qpid implementation of AMQP. Qpid comes in the qpidc package, and in qpidc-devel I found some really helpful examples – the direct example and request-response client were all I needed.

The code’s flow is pretty natural…

Setup the AMQP queues

From the low-latency config (carod.conf) I know that jobs are read from a queue named grid. So the first thing I need to do is setup a way to send messages to the grid queue.

   session.queueDeclare(arg::queue="grid");
   session.exchangeBind(arg::exchange="amq.direct",
                        arg::queue="grid",
                        arg::bindingKey="grid_key");

Next, I want to make sure there’s a way for result to get back to me, so I setup my own unique queue where I can receive results.

   session.queueDeclare(arg::queue="my_grid_results",
				 // Automatically delete the queue
				 // when finished
			 arg::exclusive=true, arg::autoDelete=true);
   session.exchangeBind(arg::exchange="amq.direct",
                        arg::queue="my_grid_results",
                        arg::bindingKey="my_grid_results_key");

Construct the Condor job

With the queues all setup, I need to construct a job. In Condor all jobs are represented as ClassAds, which is basically a bunch of (attribute,value) pairs. The low-latency code needs a job in a similar form, and does so with the message’s application headers.

   message.getHeaders().setString("Cmd", "\"/bin/echo\"");
   message.getHeaders().setString("Arguments", "\"Hello Grid!\"");
   message.getHeaders().setString("Out", "\"job.out\""); // stdout
   message.getHeaders().setString("Err", "\"job.err\""); // stderr
   message.getHeaders().setString("Requirements", "TRUE");

Immediately, it looks like crazy quoting going on here, and there is. In ClassAds, the values of attributes are typed in expected ways, e.g. ints floats strings, but there is also an expression type that can be evaluated to a boolean. An expression is something like FALSE, ClusterId > 15, ((KeyboardIdle 2 * 60) && (CurrentTime - JobStart) > 90)) or regexp(".*mf.*", Name). So, to distinguish between a string and an expression, strings are quoted and expressions are not, e.g. "/bin/echo" is a string and TRUE is an expression.

By the way, at first I thought I’d need to specify the MyType, TargetType, Owner, User, In, JobUniverse, … attributes found on traditional jobs, but I discovered MyType, TargetType, and In were superfluous, and JobUniverse has a sane default that can run /bin/echo, or essentially anything. I also didn’t care about the identify of the job owner for accessing resources, so I skipped the Owner and User attributes. The job just ran as the nobody user on the execute node.

One thing that I did on purpose was set the Requirements to TRUE, which means that job is happy to run anywhere. I can do that because I know all the nodes I setup to run jobs have /bin/echo.

Send and receive messages

There are a few final steps to do to setup the job message before it can be sent. It needs a unique message id, required by the low-latency feature.

   message.getMessageProperties().setMessageId(Uuid(true));

And, it needs to pass along information about where to send results.

   message.getMessageProperties().setReplyTo(ReplyTo("amq.direct",
                                                     "my_grid_results_key"));

Finally, the message can be sent.

   message.getDeliveryProperties().setRoutingKey("grid_key");
   session.messageTransfer(arg::content=message,
                           arg::destination="amq.direct");

This got the job out onto the network, but didn’t actually give me a way to get the results back. For that I setup a function to receive messages on the my_grid_results queue.

void
Listener::received(Message &message) {
   const MessageProperties properties = message.getMessageProperties();
   const FieldTable headers = properties.getApplicationHeaders();

   const string state = headers.getAsString("JobState");
   const int status = headers.getAsInt("JobStatus");
   const int exitCode = headers.getAsInt("ExitCode");
   const string exitBySignal = headers.getAsString("ExitBySigbal");

   std::cout
      << "Response: " << properties.getMessageId() << std::endl
      << "JobState: " << state << std::endl
      << "JobStatus: " << status << std::endl
      << "ExitCode: " << exitCode << std::endl
      << "ExitBySignal: " << exitBySignal << std::endl
      << "Is Body Empty? " << (message.getData().empty() ? "Yes" : "No") << std::endl;
//    << headers << std::endl;

   if ("\"Exited\"" == state && 4 == status) {
         // There were some results returned, they're stored in the
         // message body as a zip archive
      if (!message.getData().empty()) {
         std::ofstream out;
         out.open("job.results");
         out << message.getData();
         out.close();
      }
      subscriptions.cancel(message.getDestination());
   }
}

The function is pretty simple. It prints out information about the messages on the my_grid_results queue, and when it sees a message that represents the completion of my job it writes out the results and stops listening.

To get the receive function called when messages come in, it needs to be setup and run. I started it running after I sent the job message.

   SubscriptionManager subscriptions(session);
   Listener listener(subscriptions);
   subscriptions.subscribe(listener, "my_grid_results");
   subscriptions.run();

See it all work

That was basically it. I compiled the program, ran it, and in short order found the job.results file created.

$ ./a.out
Response: 00000000-0000-0000-0000-000000000000
JobState: 
JobStatus: 2
ExitCode: 0
ExitBySignal: 
Is Body Empty? Yes
Response: 00000000-0000-0000-0000-000000000000
JobState: "Exited"
JobStatus: 4
ExitCode: 0
ExitBySignal: 
Is Body Empty? No
$ unzip -l job.results   
Archive:  job.results
  Length     Date   Time    Name
 --------    ----   ----    ----
        0  03-06-09 13:33   job.err
       12  03-06-09 13:33   job.out
 --------                   -------
       12                   2 files

What happens when my job completes is all the files in its working directory are wrapped up in a zip archive and sent back in the body of a message that has a JobState header with a value of "Exited" and a JobStatus of 4. JobStatus 4 means Completed in the Condor world.

That’s pretty much it. The full example is in LL.cpp.

Tags: AMQP, Condor, Grid, MRG, Qpid