Reading 24: Message-Passing

In our introduction to concurrency, we saw two models for concurrent programming: shared memory and message passing.

• 

  In the shared memory model, concurrent modules interact by reading and writing shared mutable objects. Creating multiple threads inside a single process is our primary example of shared-memory concurrency. Because TypeScript has no API for creating threads, many of our code examples have used the filesystem as the shared mutable state.

• 

  In the message passing model, concurrent modules interact by sending immutable messages to one another over a communication channel. That communication channel might connect different computers over a network, as in some of our initial examples: web browsing, instant messaging, etc.

The message passing model has several advantages over the shared memory model, which boil down to greater safety from bugs. In message-passing, concurrent modules interact explicitly, by passing messages through the communication channel, rather than implicitly through mutation of shared data. The implicit interaction of shared memory can too easily lead to inadvertent interaction, sharing and manipulating data in parts of the program that don’t know they’re concurrent and aren’t cooperating properly in the concurrency safety strategy. Message passing also shares only immutable objects (the messages) between modules, whereas shared memory requires sharing mutable objects, which even in non-concurrency programming can be a source of bugs.

We’ll discuss in this reading how to implement message passing in two contexts:

• between Python threads
• between TypeScript workers

We will start with Python, because it makes the mechanism of message passing more explicit. We’ll use blocking queues to implement message passing between Python threads. Some of the operations of a blocking queue are blocking in the sense that calling the operation blocks the progress of the thread until the operation can return a result. Blocking makes writing code easier, but it also means we must continue to contend with bugs that cause deadlock.

For message passing between TypeScript workers, we’ll use a channel abstraction for sending and receiving messages. Instead of using blocking operations, we will provide a callback function to receive messages.

In the next concurrency reading we’ll see how to implement message passing between client/server processes over the network.

We saw in Mutual Exclusion that a thread blocks trying to acquire a lock until the lock has been released by its current owner. Blocking means that a thread waits (without doing further work) until an event occurs. We can use this term to describe methods and method calls: if a method is a blocking method, then a call to that method can block, waiting until some event occurs before it returns to the caller.

We can use a queue with blocking operations for message passing between threads — and the buffered network communication channel in client/server message passing will work the same way. Python provides the queue.Queue class for queues with blocking operations.

In an ordinary queue ADT, we might have operations:

• add(e) adds element e to the end of the queue.
• remove() removes and returns the element at the head of the queue, or throws an exception if the queue is empty.

If we use an ordinary TypeScript array as a queue, we could use e.g. push(..) for add and shift() for remove. (Using push with pop() instead gives us a last-in-first-out queue, instead of a first-in-first-out queue.)

The blocking Queue provides similar operations that block:

• put(e) blocks until it can add element e to the end of the queue (if the queue does not have a size bound, put will not block).
• get() blocks until it can remove and return the element at the head of the queue, waiting until the queue is non-empty.

We will implement the producer-consumer design pattern for message passing between threads. Producer threads and consumer threads share a synchronized queue. Producers put data or requests onto the queue, and consumers remove and process them. One or more producers and one or more consumers might all be adding and removing items from the same queue. This queue must be safe for concurrency.

We must also choose or design a type for messages in the queue: we will choose an immutable type because our goal in using message passing is to avoid the problems of shared memory. Producers and consumers will communicate only by sending and receiving messages, and there will be no opportunity for (mis)communication by mutating an aliased message object.

And just as we designed the operations on a threadsafe ADT to prevent race conditions and enable clients to perform the atomic operations they need, we will design our message objects with those same requirements.

Message passing example

class DrinksFridge:
    '''
    A mutable type representing a refrigerator containing drinks, using
    message passing to communicate with clients.
    '''

    # Abstraction function:
    #   AF(drinksInFridge, incoming, outgoing) = a refrigerator containing
    #                                            `drinksInFridge` drinks that takes
    #                                            requests from `incoming` and sends
    #                                            replies to `outgoing`
    # Rep invariant:
    #   drinksInFridge >= 0

    def __init__(self, requests, replies):
        '''
        Make a DrinksFridge that will listen for requests and generate replies.
        requests: Queue to receive requests from
        replies: Queue to send replies to
        '''
        self.drinksInFridge = 0
        self.incoming = requests
        self.outgoing = replies
        self.checkRep()

    ...

def start(self):
    '''
    Start handling drink requests.
    '''
    def runFridge():
        while True:
            n = self.incoming.get()
            result = self.handleDrinkRequest(n)
            self.outgoing.put(result)
    Thread(target=runFridge).start()

def handleDrinkRequest(self, n):
    '''
    Take (or add) drinks from the fridge.
    n: if >= 0, removes up to n drinks from the fridge;
    if < 0, adds -n drinks to the fridge.
    Returns: FridgeResult reporting how many drinks were actually added or removed,
            and how many drinks are left in the fridge.
    '''
    change = min(n, self.drinksInFridge)
    self.drinksInFridge -= change
    self.checkRep()
    return FridgeResult(change, self.drinksInFridge)

class FridgeResult:
    '''
    A threadsafe immutable message describing the result of taking or adding
    drinks to a DrinksFridge.
    '''

    # Rep invariant: TODO

    def __init__(self, drinksTakenOrAdded, drinksLeftInFridge):
        '''
        Make a new result message.
        drinksTakenOrAdded: (precondition? TODO)
        drinksLeftInFridge: (precondition? TODO)
        '''
        self.drinksTakenOrAdded = drinksTakenOrAdded
        self.drinksLeftInFridge = drinksLeftInFridge

    def __str__(self):
        return ('you took ' if self.drinksTakenOrAdded >= 0 else 'you put in ') + \
               f'{abs(self.drinksTakenOrAdded)} drinks, fridge has {self.drinksLeftInFridge} left'

requests = Queue()
replies = Queue()

# start an empty fridge
fridge = DrinksFridge(requests, replies)
fridge.start()

# deliver some drinks to the fridge
requests.put(-42)

# maybe do something concurrently...

# read the reply
print(replies.get())

print('done')
os._exit(0) # abruptly ends the program, including DrinksFridge, generally a bad idea

def checkRep(self):
    assert REP_INVARIANT

Now that we’ve seen how message passing works in Python, using a blocking queue of message we create and share, let’s turn our attention to TypeScript workers. We’ll focus on the Node interface for Worker (found in the package worker_threads). The web browser version of Worker (called Web Workers) differs slightly in detail, but the basic operations of message-passing exist in that context too.

When a worker is created, it comes with a two-way communication channel, the ends of which are called message ports. One end of the channel is accessible to the code that created the worker, using the postMessage operation on the Worker object:

import { Worker } from 'worker_threads';

const worker = new Worker('./hello.js');
worker.postMessage('hello!'); // sends a message to the worker

The other end of the channel is accessible to the worker, as a global object parentPort from the worker_threads module:

// hello.ts
import { parentPort } from 'worker_threads';

parentPort.postMessage('bonjour!'); // sends a message back to parent

These examples are sending simple strings as messages, and neither side is actually receiving the message yet. Let’s address each of those problems in turn.

Message types

type FridgeResult = {
  drinksTakenOrAdded: number,
  drinksLeftInFridge: number
};

type DepositRequest =  { name: 'deposit', amount: number };
type WithdrawRequest = { name: 'withdrawal', amount: number };
type BalanceRequest =  { name: 'balance' };
type BankRequest = DepositRequest | WithdrawRequest | BalanceRequest;

Receiving messages

import { parentPort } from 'worker_threads';

parentPort.addListener('message', (greeting: string) => {
  console.log('received a message', greeting);
});

TypeScript message passing example

/**
 * A mutable type representing a refrigerator containing drinks.
 */
class DrinksFridge {

    private drinksInFridge: number = 0;

    // Abstraction function:
    //   AF(drinksInFridge, port) = a refrigerator containing `drinksInFridge` drinks
    //                              that takes requests from and sends replies to `port`
    // Rep invariant:
    //   drinksInFridge >= 0

    /**
     * Make a DrinksFridge that will listen for requests and generate replies.
     * 
     * @param port port to receive requests from and send replies to
     */
    public constructor(
        private readonly port: MessagePort
    ) {
        this.checkRep();
    }

    ...
}

/** Start handling drink requests. */
public start(): void {
    this.port.addListener('message', (n: number) => {
        const reply: FridgeResult = this.handleDrinkRequest(n);
        this.postMessage(reply);
    });
}

private handleDrinkRequest(n: number): FridgeResult {
    const change = Math.min(n, this.drinksInFridge);
    this.drinksInFridge -= change;
    this.checkRep();
    return { drinksTakenOrAdded: change, drinksLeftInFridge: this.drinksInFridge };
}

/** Record type for DrinksFridge's reply messages. */
type FridgeResult = {
    drinksTakenOrAdded: number,
    drinksLeftInFridge: number 
};

What if we want to shut down the DrinksFridge so it is no longer waiting for new inputs? One strategy is a poison pill: a special message that signals the consumer of that message to end its work.

To shut down the fridge, since its input messages are merely integers, we would have to choose a magic poison integer (maybe nobody will ever ask for 0 drinks…? bad idea, don’t use magic numbers) or use null (bad idea, don’t use null). Instead, we might change the type of input messages to a discriminated union:

type FridgeRequest = DrinkRequest | StopRequest;
type DrinkRequest = { name: 'drink', drinksRequested: number };
type StopRequest =  { name: 'stop' };

When we want to stop the fridge, we send a StopRequest, i.e. fridge.postMessage({ name:'stop' }).

And when the DrinksFridge receives this stop message, it will need to stop listening for incoming messages. That requires us to keep track of the listener callback so it can be removed later.

For example, fill in the blanks in DrinksFridge in the exercise below:

class DrinksFridge {

    ...

    // callback function is now stored as a private field
    private readonly messageCallback = (req: FridgeRequest) => {
        // see if we should stop
        if (req.name === 'stop') {
            // TypeScript infers that req must be a StopRequest here
            this.stop();
            ▶▶A◀◀;
        }
        // with the correct code in blank A,
        //   TypeScript infers that req must be a DrinkRequest here
        // compute the answer and send it back
        const n = ▶▶B◀◀;
        const reply:FridgeResult = this.handleDrinkRequest(n);
        this.port.postMessage(reply);
    };

    /** Start handling drink requests. */
    public start(): void {
        this.port.addListener('message', this.messageCallback);
    }

    private stop(): void {
        this.port.removeListener('message', ▶▶C◀◀);
        // note that once this thread has no more code to run, and no more
        //   listeners attached, it terminates
    }

    ...
}

(If you’re surprised that the messageCallback arrow function can reference this, good eye! Remember that arrow functions do not define their own this. Instead, this will be looked up in the surrounding scope. It turns out that field value expressions in a TypeScript class body are evaluated in the constructor, which is why this is defined as we need it.)

It is also possible to stop a Worker by calling its terminate() method. But like our initial Python example that used the inadvisable os._exit(0), this method summarily ends execution in that Worker, dropping whatever unfinished work it had on the floor, and potentially leaving shared state in the filesystem, or database, or communication channels broken for the rest of the program. Using the message passing channel to shut down cleanly is the way to go.

In a previous reading, we saw in the bank account example that message-passing doesn’t eliminate the possibility of race conditions. It’s still possible for concurrent message-passing processes to interleave their work in bad ways.

This particularly happens when a client must send multiple messages to the module to do what it needs, because those messages (and the client’s processing of their responses) may interleave with messages sent by other clients. The message protocol for DrinksFridge has been carefully designed to manage some of this interleaving, but there are still situations where a race condition can arise. The next exercises explore this problem.

The blocking behavior of blocking queues is very convenient for programming, but blocking also introduces the possibility of deadlock. In a deadlock, two (or more) concurrent modules are both blocked waiting for each other to do something. Since they’re blocked, no module will be able to make anything happen, and none of them will break the deadlock.

In general, in a system of multiple concurrent modules communicating with each other, we can imagine drawing a graph in which the nodes of the graph are modules, and there’s an edge from A to B if module A is blocked waiting for module B to do something. The system is deadlocked if at some point in time, there is a cycle in this graph. The simplest case is the two-node deadlock, A → B and B → A, but more complex systems can encounter larger deadlocks.

Deadlock is much more common with locks than with message-passing — but when the message-passing queues have limited capacity, and become filled up to that capacity with messages, then even a message-passing system can experience deadlock. A message passing system in deadlock appears to simply get stuck.

Let’s see an example of message-passing deadlock, using the same Python DrinksFridge we’ve been discussing. This time, instead of using Queue instances that can grow arbitrarily (limited only by the size of memory), we will limit them to a fixed capacity by passing a maxsize argument. Many message-passing systems use fixed-capacity queues for performance reasons, so this is a common situation.

Finally, to create the conditions needed for deadlock, the client code will make N requests, each asking for a drink, before checking for any of the replies from DrinksFridge. Here is the full code:

QUEUE_SIZE = 100
N = 100

requests = Queue(maxsize=QUEUE_SIZE)
replies = Queue(maxsize=QUEUE_SIZE)

fridge = DrinksFridge(requests, replies)
fridge.start()

# put enough drinks in the fridge to start
requests.put(-N)
print(replies.get())

# send the requests
for x in range(N):
    requests.put(1)
    print(f'person #{x} is looking for a drink')
# collect the replies
for x in range(N):
    print(f'person #{x}: {replies.get()}')

print('done')
os._exit(0) # abruptly ends the program, generally a bad idea

It turns out with QUEUE_SIZE=100 and N=100, the code above works and doesn’t reach a deadlock. But notice that our client is making N requests before reading any replies. If N is larger than QUEUE_SIZE, the replies queue fills up with unread replies. Then DrinksFridge blocks trying to put one more reply into that queue, and it stops calling get on the requests queue. The client can continue putting more requests into the requests queue, but only up to the size of that queue. If there are more additional requests than can fit in that queue – i.e., when N is greater than 2×QUEUE_SIZE – then the client’s call to requests.put() will block too. And now we have our deadly embrace. DrinksFridge is waiting for the client to read some replies and free up space on the replies queue, but the client is waiting for DrinksFridge to accept some requests and free up space on the requests queue. Deadlock.

Final suggestions for preventing deadlock

• Rather than synchronize with locks, message passing systems synchronize on a shared communication channel, e.g. a stream or a queue.

• Threads communicating with blocking queues is a useful pattern for message passing within a single process.