Real-Time Systems: High level introduction

I've been working with real-time distributed systems for quite some time now. This one of the topics I like the most. In this post I'd like to spend some time explaining at a high level what real-time systems are, where they're used, some of their requirements and I'd also like to conclude the post with a small section explaining why I think the term real-time is neither accurate nor correct to describe these systems.

Real Time Systems

A system is said to be real-time , when it's subjected to execution time constraints. What this means is that whatever it takes to complete the execution of a specific task will determine whether the task ended successfully or not. There are different groups of real-time systems. The first group is composed by systems that are fully dependent of a specific deadline. Any deviation from the time constraint will be considered a total failure. This systems are said to be hard real time systems. The second group is composed by systems whose quality may be affected by missed deadlines but those misses won't be considered failures. Nonetheless, missed deadlines will invalidate the usefulness of the result provided. This group is usually called firm and it's not so common. The third group, though, is said to be soft real time because the deadlines misses are not considered failures and the usefulness of their results will decrease when a deadline is missed but it won't be invalidated.

The different levels of constraints exist to satisfy multiple scenarios. For instance, hard real-time systems are commonly used for stock quotas transactions, airplane systems, car systems etc, whereas soft real-time systems are used when the availability of the result is important but not as much to make it mission critical. In other words, hard real-time systems main goal is to meet all deadlines, whereas soft real-time systems just need to meet a subset.

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Real time systems implications

The hardest thing about real-time systems are not the systems themselves but the things they imply. A real-time system, for instance, implies some level of determinism, scale, fault-tolerance etc. This all depends on the level of strictness the system has. A system that needs to meet all the deadlines will need to have fault-tolerance as well. In the event of a node failure, the system has to send the result back before the deadline, otherwise it will be considered a total failure.

Lets dive into some of those implications.

Time Constraint

At this point it is clear that real-time systems are not the same thing as really-fast systems. A system is said to be real-time when its results are tight to an execution time constraint. Therefore, it is necessary to establish what that time constraint is and how strict it is. There's not just 1 time constraint that has to be established, though. If your real-time system is composed by more than 1 component, you'll need to establish a time constraint for the inter-communications of your system. Each one of the inner constraints have to be smaller than the constraint applied to the whole system. What that time constraint is, depends on the system itself and its purpose.

Besides defining the time constraint, it's also necessary to establish how it will be enforced, measured and what actions will be taken in case of failure. Furthermore, it is necessary to determine where this enforcement will happen. More about this later.

Integration Requirements

This pretty much falls into what systems integration is. However, since a real-time system is not necessarily distributed, this will also apply to non-distributed systems.

Integration requirements refer to the semantics, technology and methods used by the system to enforce both the execution and the distribution of the task. Things like whether the execution needs to be synchronous or asynchronous need to be sorted out in this step. Therefore, it is also necessary to establish what the components of the system are, what those components do and how they interact with the rest of the environment.

This section has different applications. Depending on the system, it may be implemented differently. An example of this are non distributed real-time system - perhaps applications would be more accurate here - where the integration with other systems through a non-deterministic environment is not required. However, it is necessary to integrate the different components of that single, most likely multi-threaded, system. Although this may seem obvious, and perhaps implicit in every system aiming to run concurrently.

Determinism

I won't go deep into Determinism, the concepts and rules behind this topic are big and out of the scope of this post. Visit wikipedia for more information.

Determinism is not a strict requirement for every real-time system. It is possible to have real-time systems that don't behave deterministically. Although this is certainly the least common case for real-time systems, the previous statement could be argued as a whole given the fact that these systems would benefit from a deterministic behavior.

Determinism brings predictability to the system, which allows it to be more reliable and lower the difficulties of meeting the goals of the tasks being executed.

Fault Tolerance

Just as with Determinism I won't go deep in the concepts behind fault-tolerance, for more information check wikipedia out.

Unlike determinism, fault-tolerance is applicable just to multi-component systems. That is, in most of the cases, a distributed system.

Fault tolerance is perhaps a stronger requirement for real-time systems that what determinism itself is. A system willing to respect the imposed time constraint has to survive possible failures and complete the task.

It is also worth mentioning that deterministic systems have to be fault-tolerant, which is not necessarily true the other way around. Failing to survive failures will introduce non-deterministic behaviors throughout the system, therefore making the whole system behave non-deterministically.

Requirements Enforcement

We've made it clear that every real-time system has intrinsic requirements that should be met in order for it to meet its goals. The list of requirements is far from being complete but it introduces some of the most relevant ones.

Some of the requirements described above need to be enforced at some point in time during the execution of every task and the life of the system. In order to do that, it is necessary to determine where this enforcement will happen and when.

This enforcement is usually implemented along side with the system itself. That means, if the system is distributed, the enforcement of the time constraint, support for determinism and support for fault-tolerance will be distributed as well.

This step adds more complexity to the system. For instance, determining whether the system is behaving deterministically, whether the system nodes' health is fine or even whether the goals are being met is often the most critical task of a successful real-time system.

Real-Time applicability

This post has been mostly focused on distributed systems. However, the term real-time is not bounded to those systems. Here's a list of other type of applications for this term:

Real time systems misconception

At this point, I don't expect you to be an expert on this field. In fact, I think some of the topics explained above could certainly have been explained more in detail. However, I do expect you to know that real-time does not mean fast nor it means immediately. A real-time system is a system tight to time constraints, which in most cases are very low. Therefore, I believe the term is wrong and doesn't describe the real goal of the system.

Given the fact that there's no such thing as real-time and that computers are governed by the laws of physics, it'd be accurate to say that unless the point of reference for real-time systems is explicitly defined, the measurement could be relative to any point of the system since the task was executed. Common sense leads us to use the time when the task started as a reference point to measure the success of the execution. This, though, implies that no matter how fast the system is, the result won't ever be immediate and the execution time is not actually real.

In my humble opinion, a more accurate term for this kind of systems would be one that explicitly specifies the time constraints of the system itself. For example: time-bound, time-constrained, etc.

Unfortunately, the misconception around the term has led people to use it erroneously to describe things that are supposed to be fast as real-time.

Some References