In the previous post, I covered the printer. In this post, I focus on the API.

The Architecture

The user sends quotes to a queue. The printer polls for available new quotes every 10 seconds. If a quote is available in the queue, it prints it. If no quote is available, it just sleeps for another 10 seconds.

Adding quotes to the printer queue

Since I’m hooking this to my existing quote app, I just added a new command:

{
  "name": "Print",
  "help": "p. Sends a quote to the configured subscriber printer.",
  "verbs": ["p", "prn", "print"],
  "requiresSubscription": true,
  "admin": false,
  "handler": (done) => {
      print(locals.subscriber, done);
  },
}

The actual handler:

/*
  Puts a quote in the printing queue for the subscriber (if they have a printer).
*/
function print(subscriber, done){
  if(!subscriber) { 
    return done(null, "Print. Please subscribe before running this command."); 
  }

  domain.putQuoteInPrinterQueue(subscriber, subscriber.lastQuoteId, (error, msg)=>{
    if(error) { 
      return done("Print. Error while sending quote to printer. Please retry."); 
    }
    done(null, "Your quote will print soon!");
  });
}

/*
  Print sends quote to a queue for the user.
*/
domain.putQuoteInPrinterQueue = (phone, quoteId, done) => {
  connectDb((err, client) => {
    if(err) return done(err, "System error. Please try sometime else");
      const queue_name = "Printer-" + phone.replace('+');
      const queue = mongoDbQueue(client.db(), queue_name);
      queue.add(quoteId, (q_err, msg_id) =>{
        if(q_err){
          return done(q_err, "Couldn't add quote to printing queue.");
        }
        return done(null,"Quote added to subscriber printing queue.");
      });
  });  
}

I am using the mongodbqueue module, which is straightforward to use. Some notes on the implementation:

  1. I am merely storing the quote id as opposed to the entire quote.
  2. There’s a queue per subscriber, with a naming convention Printer-{phone}.

A MongoDb collection behind the scenes provides storage for every queue. mongodbqueue takes care of the details.

Printing quotes

The printer polls the API for a new quote on a single protected endpoint every 10 seconds:

GET /printer
Authorization: Bearer {ACCESS_TOKEN}

This request can return:

  1. 200 and a quote in the body
  2. 404 if no quote is available
  3. 409 if there are too many requests
  4. 500 if something wrong happens
/*
  Retrieves messages from the printer queue
  returns 404 if no quotes are available
*/
server.get('/printer', [limiter, read], (req, res, next) => {
  
  var phone = getPhone();

  domain.getQuoteFromPrinterQueue(phone, (error, quote) => {
    if(error){
      return next(boom.serverUnavailable("Error retrieving quote from queue", err)); 
    }
    if(!quote){
      return next(boom.notFound());
    }
    return res.json({
      quote: quote.quote,
      author: quote.author
    });
  });
});

There are two middlewares on this route:

var read = jwtAuthz(["read:quotes"],{ failWithError: true });

const rateLimit = require("express-rate-limit");
  
const limiter = rateLimit({
  windowMs: 1 * 30 * 1000,
  max: 20 // limit each IP to 100 requests per windowMs
});

Rate limiter is meant to protect against runaway printers polling the endpoint like crazy. The read middleware checks that the there’s the appropriate scope.

The access_token also contains the phone claim, used to identify the user’s print queue.

Finally, getQuoteFromPrinterQueue, the function that does all the work:

domain.getQuoteFromPrinterQueue = (phone, done) => {
  connectDb((err, client) => {
    if(err) return done(err, "System error. Please try sometime else");
      const queue_name = "Printer-" + phone.replace('+');
      const queue = mongoDbQueue(client.db(), queue_name);

      queue.get((q_err, msg) =>{
        if(q_err){
          return done(q_err, "Couldn't get quote from printing queue.");
        }

        //No message
        if(!msg){
          return done(null, null);
        }
        
        domain.getQuotes({_id: new ObjectID(msg.payload)}, null, 0, (e, quotes) =>{
          if(e || !quotes || quotes.length === 0){ 
            return done(e, 'Get Print Queue. Quote not found'); 
          }

          //Before returning, we delete message from queue
          //If there's an error, we just ignore.
          queue.ack(msg.ack, (err, id) => {
            if(err){
              console.log("Error ACK'ing msg from queue");
            }
            return done(null, quotes[0]);
          });
        });
      });
  });  
}

As you might have noticed, there’re not many provisions for retries or anything.

Sequence of Events