Concurrency is a property of a system representing the fact that multiple activities are executed at the same time. According to Van Roy [Roy04], a program having "several independent activities, each of which executes at its own pace". In addition, the activities may perform some kind of interaction among them. The concurrent execution of activities can take place in different environments, such as single-core processors, multi-core processors, multi-processors or even on multiple machines as part of a distributed system. Yet, they all share the same underlying challenges: providing mechanisms to control the different flows of execution via coordination and synchronization, while ensuring consistency.
Apart from recent hardware trends towards multi-core and multiprocessor systems, the use of concurrency in applications is generally motivated by performance gains. Cantrill et al. [Can08] describe three fundamental ways how the concurrent execution of an application can improve its performance:
The presence of concurrency is an intrinsic property for any kind of distributed system. Processes running on different machines form a common system that executes code on multiple machines at the same time.
Conceptually, all web applications can be used by various users at the same time. Thus, a web application is also inherently concurrent. This is not limited to the web server that must handle multiple client connections in parallel. Also the application that executes the associated business logic of a request and the backend data storage are confronted with concurrency.
In the literature, there are varying definitions of the terms concurrency and parallelism, and their relation. Sometimes both terms are even used synonymously. We will now introduce a distinction of both terms and of their implications on programming.
Concurrency is a conceptual property of a program, while parallelism is a runtime state. Concurrency of a program depends on the programming language and the way it is coded, while parallelism depends on the actual runtime environment. Given two tasks to be executed concurrently, there are several possible execution orders. They may be performed sequentially (in any order), alternately, or even simultaneously. Only executing two different tasks simultaneously yields true parallelism. In terms of scheduling, parallelism can only be achieved if the hardware architecture supports parallel execution, like multi-core or multi-processor systems do. A single-core machine will also be able to execute multiple threads concurrently, however it can never provide true parallelism.
Differentiating concurrent and parallel programming is more tedious, as both are targeting different goals on different conceptual levels. Concurrent programming tackles concurrent and interleaving tasks and the resulting complexity due to a nondeterministic control flow. Reducing and hiding latency is equally important to improving throughput. Instead, parallel programming favors a deterministic control flow and mainly reaches for optimized throughput. The internals of a web server are the typical outcome of concurrent programming, while the parallel abstractions such as Google's MapReduce [Dea08] or Java's fork/join [Lea00] provide a good example of what parallel programming is about. Parallel programming is also essential for several specialized tasks. For instance, a graphics processing unit is designed for massive floating-point computational power and usually runs a certain numerical computation in parallel on all of its units at the same time. High-performance computing is another important area of parallel programming. It takes advantage of computer clusters and distributes sub-tasks to cluster nodes, thus speeding up complex computations.
|Single Instruction||Multiple Instruction|
The previous differentiation can also be backed by a closer look on the architectures where physical concurrency is actually possible. We will therefore refer to Flynn's taxonomy [Fly72], which is shown in table 2.3. This classification derives four different types of computer architectures, based on the instruction concurrency and the available data streams. The SISD class is represented by traditional single processor machines. We do not consider them further due to their lack of physical concurrency. MISD is a rather exotic architecture class, sometimes used for fault-tolerant computing where the same instructions are executed redundantly. It is also not relevant for our considerations. Real parallelism can only be exploited on architectures that support multiple data streams--SIMD and MIMD. SIMD represents the aforementioned architecture class targeting dedicated parallel executions of computations such as graphics processing unit and vector processors. SIMD exploits data-level parallelism which is not suitable for handling web requests. Accordingly, this type of architecture is not relevant for our consideration. We will focus on the last remaining class, MIMD. It represents architectures that rely on a shared or distributed memory and thus includes architectures possessing multiple cores, multiple CPUs or even multiple machines. In the following, we will primarily focus on concurrent programming (parallel execution of subtasks is partially relevent in chapter 5) and only on the MIMD class of architectures.
Van Roy [Roy04] introduces four main approaches for programming concurrency that we will examine briefly (see figure :autorefModels for Programming Concurrency (Van Roy [Roy04])2.0). The more important models will be studied later as potential solutions for programming concurrency inside web architectures, especially in chapter 5.
In this deterministic programming model, no concurrency is used at all. In its strongest form, there is a total order of all operations of the program. Weaker forms still keep the deterministic behaviour. However, they either make no guarantees on the exact execution order to the programmer a priori. Or they provide mechanisms for explicit preemption of the task currently active, as co-routines do, for instance.
Declarative programming is a programming model that favors implicit control flow of computations. Control flow is not described directly, it is rather a result of computational logic of the program. The declarative concurrency model extends the declarative programming model by allowing multiple flows of executions. It adds implicit concurrency that is based on a data-driven or a demand-driven approach. While this introduces some form of nondeterminism at runtime, the nondeterminism is generally not observable from the outside.
This model is a programming style that allows concurrent activities to communicate via messages. Generally, this is the only allowed form of interaction between activities which are otherwise completely isolated. Message passing can be either synchronous or asynchronous resulting in different mechanisms and patterns for synchronization and coordination.
Shared-state concurrency is an extended programming model where multiple activities are allowed to access contended resources and states. Sharing the exact same resources and states among different activities requires dedicated mechanisms for synchronization of access and coordination between activities. The general nondeterminism and missing invariants of this model would otherwise directly cause problems regarding consistency and state validity.
Regardless of the actual programming model, there must be an implicit or explicit control over concurrency (at least within the runtime environment). It is both hazardous and unsafe when multiple flows of executions simultaneously operate in the same address space without any kind of agreement on ordered access. Two or more activities might access the same data and thus induce data corruption as well as inconsistent or invalid application state. Furthermore, multiple activities that work jointly on a problem need an agreement on their common progress. Both issues represent fundamental challenges of concurrency and concurrent programming.
Synchronization and coordination are two mechanisms attempting to tackle this. Synchronization, or more precisely competition synchronization as labeled by Sebesta [Seb05], is a mechanism that controls access on shared resources between multiple activities. This is especially important when multiple activities require access to resources that cannot be accessed simultaneously. A proper synchronization mechanism enforces exclusiveness and ordered access on the resource by different activities. Coordination, sometimes also named cooperation synchronization (Sebesta [Seb05]), aims at the orchestration of collaborating activities.
Synchronization and coordination are sometimes conflated in practice. Both mechanisms can be either implicit or explicit (Van Roy [Roy04]). Implicit synchronization hides the synchronization as part of the operational semantics of the programming language. Thus, it is not part of the visible program code. On the contrary, explicit synchronization requires the programmer to add explicit synchronization operations to the code.
So far, our considerations of concurrency were based on computer architectures and programming models. We will now show how they interrelate by introducing the actual activity entities used and provided by operating systems and how they are mapped to the hardware. Note that we will use the term task as a general abstraction for a unit of execution from now on.
The ability to execute multiple tasks concurrently has been a crucial requirement for operating systems. It is addressed by multitasking. This mechanism manages an interleaved and alternating execution of tasks. In case of multiple CPU cores or CPUs, multitasking can be complemented by multiprocessing, which allocates different tasks on available cores/CPUs. The key concept for both mechanisms is scheduling, which organizes the assignment of processing time to tasks using scheduling strategies. Appropriate strategies can have different aims, such as fair scheduling between tasks, fixed latencies for task executions or maximum utilization. Another distinction of scheduling strategies is their preemption model. In the preemptive model, the scheduler assigns processing time to a task and eventually revokes it. The task has no control over the preemption. In a cooperative model, the task itself has the responsibility of yielding after some time to allow another task to run. Scheduling is an important duty of any operating system. However, it is also noteworthy that applications themselves can provide some kind of scheduling of their own internal tasks, as we will see in chapter 4 and chapter 5.
Operating systems generally provide different types of tasks--processes and threads. Essentially, they represent different task granularities. A process is a heavyweight task unit and owns system resources such as memory and file descriptors that are allocated from the operating system. Threads are leightweight task units that belong to a certain process. All threads allocated within the same process share memory, file descriptors and other related resources. Creating threads is a relatively cheap operation compared to the creation of new processes.
Most concurrent applications make heavy use of multiple threads. However, this does not imply the availability of threads as explicit entities of the programming language itself. Instead, the execution environment might map other concurrency constructs of a programming language to actual threads at runtime. Similarly, the shared-state property of multithreading might be idiomatically hidden by the language.
Next, we consider the strong relationship between concurrent programming, programming languages and distributed systems when building large architectures. Programming distributed systems introduces a set of additional challenges compared to regular programming. The "Fallacies of Distributed Computing" [RGO06] provide a good overview on some of the important pitfalls that must be addressed. For instance, not anticipating errors in a network or ignoring the different semantics of local and remote operation are common misconceptions that are described.
From a software engineering perspective, the major challenges are fault tolerance, integration of distribution aspects and reliability. As we have already seen before, distributed systems are inherently concurrent and parallel, thus concurrency control is also essential.
Programming languages to be used for distributed systems must either incorporate appropriate language idioms and features to meet these requirements. Otherwise, frameworks are necessary to provide additional features on top of the core language.
Ghosh et al. [Gho11] have considered the impact of programming languages on distributed systems. They pointed out that mainstream languages like Java and C++ are still the most popular choice of developing distributed systems. They are combined with middleware frameworks most of the time, providing additional features. However, the strengths of general purpose languages do not cover the main requirements of distributed systems to a great extent. The experiences with RPC-based systems (see Kendall et al. [Ken94]) and their object-based descendents (see Vinoski [Vin08]) have raised some questions to this approach. Middleware systems providing distributability compensate for features missing at the core of a language. Thus, the systems actually meet the necessary requirements, but they are often also cumbersome to use and introduce superfluous complexity.
Recently, there has been an increasing interest in various alternative programming languages embracing high-level concurrency and distributed computing. Being less general, these languages focus on important concepts and idioms for distributed systems, such as component abstractions, fault tolerance and distribution mechanisms. It is interesting for our considerations that most of these languages oriented towards distributed computing also incorporate alternative concurrency approaches. We will have a brief look on some of these languages as part of chapter 5.