Seastar

Introduction

SeaStar is an event-driven framework allowing you to write non-blocking, asynchronous code in a relatively straightforward manner (once understood). It is based on futures.

Futures and promises

A future is a result of a computation that may not be available yet. Examples include:

• a data buffer that we are reading from the network
• the expiration of a timer
• the completion of a disk write
• the result computation that requires the values from one or more other futures.

a promise is an object or function that provides you with a future, with the expectation that it will fulfill the future.

Promises and futures simplify asynchronous programming since they decouple the event producer (the promise) and the event consumer (whoever uses the future). Whether the promise is fulfilled before the future is consumed, or vice versa, does not change the outcome of the code.

Consuming a future

You consume a future by using its then() method, providing it with a callback (typically a lambda). For example, consider the following operation:

future<int> get();   // promises an int will be produced eventually
future<> put(int)    // promises to store an int

void f() {
    get().then([] (int value) {
        put(value + 1).then([] {
            std::cout << "value stored successfully\n";
        });
    });
}

Here, we initate a get() operation, requesting that when it completes, a put() operation will be scheduled with an incremented value. We also request that when the put() completes, some text will be printed out.

Chaining futures

If a then() lambda returns a future (call it x), then that then() will return a future (call it y) that will receive the same value. This removes the need for nesting lambda blocks; for example the code above could be rewritten as:

future<int> get();   // promises an int will be produced eventually
future<> put(int)    // promises to store an int

void f() {
    get().then([] (int value) {
        return put(value + 1);
    }).then([] {
        std::cout << "value stored successfully\n";
    });
}

Loops

Loops are achieved with a tail call; for example:

future<int> get();   // promises an int will be produced eventually
future<> put(int)    // promises to store an int

future<> loop_to(int end) {
    get().then([end] (int value) {
        if (value == end) {
            return make_ready_future<>();
        }
        return put(value + 1);
    }).then([end] {
        return loop_to(end);
    });
}

The make_ready_future() function returns a future that is already available --- corresponding to the loop termination condition, where no further I/O needs to take place.