A Future for R: Future API Backend Specification

Future API Backend Specification

Version 0.1.9-9000

WARNING: Starting with future 1.40.0 (2025-04-10), we are migrating to a new way to write future backends. This is work in progress, so some of the below is subject to change for the next few release cycles.

Introduction

This document is written to serve as a reference for developers who are developing a future backend to the future framework as implemented in the future package for R that available on CRAN. The Future Application Programming Interface (API) has three fundamental functions at its core:

f <- future(expr) - create a future from an R expression (non-blocking but may be blocking)
r <- resolved(f) - check whether a future is resolved or not (non-blocking)
v <- value(f) - retrieve the value of a future (blocking)

With these three functions alone, it is possible to evaluate one or more R expressions synchronously and asynchronously. How and where these expressions are resolved depends on which “future backend” is in use. For example, one backend may evaluated the expressions sequentially (synchronously) while another may evaluated them in parallel (asynchronously). Regardless of backend, the value of a future expression is always the same.

It is fundamental to the future ecosystem that all future backends conform to the Future API specification. Conformance serves as a guarantor of correctness and behavior for both the developer who use futures in their software as well as the end-user of their software. A future backend that meets the requirements can be used in any software that use futures internally.

For example, the above three functions serve as building blocks in several higher-level map-reduce APIs. One example is the [future.apply] package on CRAN that provides future_lapply(), which is a futurized version of lapply() available in the base package. This function can be used to perform the lapply-like processing in parallel using a parallel backend. The implementation of the future.apply package is 100% invariant to the parallel backend used. This is possible because all future backends conform to a set of rules. Rules that are documented below.

A supplement to the specification herein is the ‘Test Suite for Future API Backends’, which consists of a set of tests that can be used to validated that a future backend meets the minimal requirements of the Future API. These tests run from the command-line, from the R prompt, or as part of the package tests of a backend package. This test suite is documented and implemented in the future.tests package available on CRAN.

Feedback

If you find that something in this document to be missing, unclear, or faulty, please report your feedback using the official issue tracker for the future package at https://github.com/futureverse/future. If you have feedback that is specific to the test suite, please use the official issue tracker for the future.tests package at https://github.com/futureverse/future.tests.

Overview of the Future API

The Future API has three fundamental functions at its core:

f <- future(expr) - create a future from an R expression (non-blocking but may be blocking)
r <- resolved(f) - check whether a future is resolved or not (non-blocking)
v <- value(f) - retrieve the value of a future (blocking)

The implementation of a future backend for these involves several steps. For simplicity, lets say we call our future backend ‘myparallel’. As a broad summary, a future backend needs to implement the following components:

A myparallel function that inherits class future. This function must never be called - it is used as a no-op placeholder for setting the future backend via plan().
A MyParallelFutureBackend function that returns and an object of class MyParallelFutureBackend inheriting the FutureBackend. This function should be set as attribute factory for the above myparallel function.
A launchFuture() method for the MyParallelFutureBackend class taking arguments backend and future. This method is responsible for starting the concurrent evaluation of the Future object and returning it as an instance of class MyParallelFuture inheriting the Future class. This method is often non-blocking for parallel backends, but may be blocking if all compute resources are exhausted. It is typically blocking for sequential backends.
An S3 method of resolved() for MyParallelFuture that, in a non-blocking fashion, returns TRUE if the future is resolved and FALSE if not.
An S3 method of result() for MyParallelFuture that returns a FutureResult object (as defined by the future package) when the future is resolved or otherwise fails to resolve. If the future is not yet resolved, this method should block until the future is resolved.

With this in place, the selection of using this backend as the future plan, will be done as plan(myparallel) with the option of specifying certain arguments to be passed to myparallel(). With the plan set, a call to f <- future(expr) will then launch the evaluation of the future via the launchFuture() method for the current set future backend and return then launch the future now inheriting MyParallelFuture. When calling resolved(f) to query whether the future expression is resolved or not, the underlying S3 method for this class will then check in with the parallel worker whether the expression is resolved or not. When calling value(f), the S3 method for the Future class calls result(f), which will return the FutureResult object for this future. If the future is not yet resolved, this call will block until it is. If no errors occurred while resolving the future expression, then value(f) will return the value of the expression, which is recorded by the backend in the FutureResult object. If there was an evaluation error, then value(f) will resignal (“relayed”) that error. Any captured conditions or standard output will also be relayed at this point.

Requirements for the backend Future API

This section describes in detail what the requirements of the above four components are. The requirements are given as a continuation of the above ‘myparallel’ example. If otherwise not specified, all functions mentioned below are from the future package.

Constructor function creating a Future

The place-holder function myparallel() that is used by plan() must inherits from class future such that inherits(myparallel, "future") is true. It must also have attribute factory set to the corresponding FutureBackend function, i.e. MyParallelFutureBackend.

launchFuture() method of the FutureBackend class

An S3 method launchFuture() for MyParallelFutureBackend that takes a FutureBackend object as its first argument and a Future object as the second is required. It should accept additional arguments via ..., which are currently not used.

The launchFuture() method should invisibly return the Future object of desired class, e.g. MyParallelFuture.

The launchFuture() method is responsible for evaluation the Future object. The evaluation of the future expression should respect any global variables in the FutureGlobals object returned by globals() with the Future object as the first argument. The evaluation should also respect any package names returned by packages() with the Future object as the first argument.

If the backend provides parallel processing, then launchFuture() should return the future as soon as possible and without waiting for it to be resolved. If all workers are occupied, then launchFuture() is responsible for waiting until a worker becomes available and then launch the future on that worker and immediatedly return the future.

The launchFuture() method may produce an error of class FutureError in case it fails to launch the future on the worker or the worker has terminated unexpectedly.

The launchFuture() method must not update the RNG state.

resolved() method for the Future class

An S3 method resolved() for MyParallelFuture that takes a Future object as its first argument and return either TRUE or FALSE is required. It should accept additional arguments via ..., which are currently not used.

The method may be called zero or more times.

The method should return FALSE as long as the future is unresolved. It may also return FALSE if it fail to establish the state of the future within a reasonable time period (“timeout”). It should return TRUE as soon as it can be established that the future is resolved. After it has returned TRUE once, any succeeding calls should return TRUE.

If resolved() is called on a future that yet has not been launched, it should launch the future by calling run(). This is the only occasion when resolved() may block. In all other cases, it should return promptly.

The resolved() method may produce FutureError error as created by FutureError() in case communication with the worker has broken down or the worker has terminated unexpectedly.

The resolved() method must not update the RNG state.

result() method for the Future class

An S3 method result() for MyParallelFuture that takes a Future object as its first argument and return a FutureResult object is required. It should accept additional arguments via ..., which are currently not used.

The method may be called zero or more times.

If result() is called on a future that yet has not been launched, it should launch the future by calling run().

If result() is called on a future that is not yet resolved, it should block until the future is resolved.

The value of result() should be the value from evaluating the getExpression() expression that run() launched.

The result() method may produce FutureError error as created by FutureError() in case communication with the worker has broken down or the worker has terminated unexpectedly.

The result() method must not update the RNG state.

Henrik Bengtsson