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Designing for Scalability with Erlang/OTP by Steve Vinoski, Francesco Cesarini

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Chapter 1. Introduction

You need to implement a fault-tolerant, scalable, soft real-time system with requirements for high availability. It has to be event-driven and react to external stimuli, load, and failure. It must always be responsive. You have heard, rightfully so, of many success stories telling you Erlang is the right tool for the job. And indeed it is—but while Erlang is a powerful programming language, it’s not enough on its own to group these features all together and build complex reactive systems. To get the job done correctly, quickly, and efficiently, you also need middleware, reusable libraries, tools, design principles, and a programming model that tells you how to architect and distribute your system.

Our goal with this book is to explore multiple facets of availability and scalability, as well as related topics such as concurrency, distribution, and fault tolerance, in the context of the Erlang programming language and its OTP framework. Erlang/OTP was created when the team at the Ericsson Computer Science Laboratory (CS Lab) set out to investigate how they could efficiently develop the next generation of telecommunications systems in an industry where time to market was becoming critical. This was before the Web, before tablets and smartphones, massively multiuser online gaming, messaging, and the Internet of Things.

At that time, the only systems that required the levels of scalability and fault tolerance we take for granted today were boring phone switches. They had to handle massive traffic spikes on New Year’s Eve, fulfill regulatory obligations for the availability of calls to emergency services, and avoid the painfully expensive contractual penalties forced on infrastructure suppliers whose equipment caused outages. In layman’s terms, if you picked up the phone and did not hear the dial tone on the other end, you could be sure of two things: top-level management would get into serious trouble and the outage would make the front page news in the papers. No matter what, those switches were not allowed to fail. Even when components and infrastructure around them were failing, requests had to be handled. Today, regulators and fines have been replaced with impatient users with no loyalty who will not hesitate to switch suppliers, and front-page newspaper articles have been replaced by mass hysteria on social media. But the core problems of availability and scalability remain.

As a result, telecoms switches and modern systems alike have to react to failure as much as they have to react to load and internal events. So while the folks at the Ericsson Computer Science Lab did not set out to invent a programming language, the solution to the problem they were out to solve happened to be one. It’s a great example of inventing a language and programming model that facilitates the task of solving a specific, well-defined problem.

Defining the Problem

As we show throughout this book, Erlang/OTP is unique among programming languages and frameworks in the breadth, depth, and consistency of the features it provides for scalable, fault-tolerant systems with requirements for high availability. Designing, implementing, operating, and maintaining these systems is challenging. Teams that succeed in building and running them do so by continuously iterating through those four phases, constantly using feedback from production metrics and monitoring to help find areas they can improve not only in their code, but also in their development and operating processes. Successful teams also learn how to improve scalability through other means, such as testing, experimentation, and benchmarking, and they keep up on research and development relevant to their system characteristics. Nontechnical issues such as organizational values and culture can also play a significant part in determining whether teams can meet or exceed their system requirements.

We used the terms distributed, fault-tolerant, scalable, soft real-time, and highly available to describe the systems we plan on building with OTP. But what do these words actually mean?

Scalable refers to how well a computing system can adapt to changes in load or available resources. Scalable websites, for example, are able to smoothly handle traffic spikes without dropping any client requests, even when hardware fails. A scalable chat system might be able to accommodate thousands of new users per day without disruption of the service it provides to its current users.

Distributed refers to how systems are clustered together and interact with each other. Clusters can be architected to scale horizontally by adding commodity (or regular) hardware, or on a single machine, where additional instances of standalone nodes are deployed to better utilize the available cores. Single machines can also be virtualized, so that instances of an operating system run on other operating systems or share the bare-metal resources. Adding more processing power to a database cluster could enable it to scale in terms of the amount of data it can store or how many requests per second it can handle. Scaling downward is often equally as important; for example, a web application built on cloud services might want to deploy extra capacity at peak times and release unused computing instances as soon as usage drops.

Systems that are fault tolerant continue to operate predictably when things in their environment are failing. Fault tolerance has to be designed into a system from the start; don’t even consider adding it as an afterthought. What if there is a bug in your code or your state gets corrupted? Or what if you experience a network outage or hardware failure? If a user sending a message causes a process to crash, the user is notified of whether the message was delivered or not and can be assured that the notification received is correct.

By soft real-time, we mean the predictability of response and latency, handling a constant throughput, and guaranteeing a response within an acceptable time frame. This throughput has to remain constant regardless of traffic spikes and number of concurrent requests. No matter how many simultaneous requests are going through the system, throughput must not degrade under heavy loads. Response time, also known as latency, has to be relative to the number of simultaneous requests, avoiding large variances in requests caused by “stop the world” garbage collectors or other sequential bottlenecks. If your system throughput is a million messages per second and a million simultaneous requests happen to be processed, it should take 1 second to process and deliver a request to its recipient. But if during a spike, two million requests are sent, there should be no degradation in the throughput; not some, but all of the requests should be handled within 2 seconds.

High availability minimizes or completely eliminates downtime as a result of bugs, outages, upgrades, or other operational activities. What if a process crashes? What if the power supply to your data center is cut off? Do you have a redundant supply or battery backup that gives you enough time to migrate your cluster and cleanly shut down the affected servers? Or network and hardware redundancy? Have you dimensioned your system ensuring that, even after losing part of your cluster, the remaining hardware has enough CPU capacity to handle peak loads? It does not matter if you lose part of your infrastructure, if your cloud provider is experiencing an embarrassing outage, or if you are doing maintenance work; a user sending a chat message wants to be reassured that it reaches its intended recipient. The system’s users expect it to just work. This is in contrast to fault tolerance, where the user is told it did not work, but the system itself is unaffected and continues to run. Erlang’s ability to do software upgrades during runtime helps. But if you start thinking of what is involved when dealing with database schema changes, or upgrades to non–backward-compatible protocols in potentially distributed environments handling requests during the upgrade, simplicity fades very quickly. When doing your online banking on weekends or at night, you want to be sure you will not be met with an embarrassing “closed for routine maintenance” sign posted on the website.

Erlang indeed facilitates solving many of these problems. But at the end of the day, it is still just a programming language. For the complex systems you are going to implement, you need ready-built applications and libraries you can use out of the box. You also need design principles and patterns that inform the architecture of your system with an aim to create distributed, reliable clusters. You need guidelines on how to design your system, together with tools to implement, deploy, monitor, operate, and maintain it. In this book we cover libraries and tools that allow you to isolate failure on a node level, and create and distribute multiple nodes for scalability and availability.

You need to think hard about your requirements and properties, making certain you pick the right libraries and design patterns that ensure the final system behaves the way you want it to and does what you originally intended. In your quest, you will have to make tradeoffs that are mutually dependent—tradeoffs on time, resources, and features and tradeoffs on availability, scalability, and reliability. No ready-made library can help you if you do not know what you want to get out of your system. In this book, we guide you through the steps in understanding these requirements, and walk you through the steps involved in making design choices and the tradeoffs needed to achieve them.

OTP

OTP is a domain-independent set of frameworks, principles, and patterns that guide and support the structure, design, implementation, and deployment of Erlang systems. Using OTP in your projects will help you avoid accidental complexity: things that are difficult because you picked inadequate tools. But other problems remain difficult, irrespective of the programming tools and middleware you choose.

Ericsson realized this very early on. In 1993, alongside the development of the first Erlang product, Ericsson started a project to tackle tools, middleware, and design principles. The developers wanted to avoid accidental difficulties that had already been solved, and instead focus their energy on the hard problems. The result was BOS, the Basic Operating System. In 1995, BOS merged with the development of Erlang, bringing everything under one roof to form Erlang/OTP as we know it today. You might have heard the dream team that supports Erlang being referred to as the OTP team. This group was a spinoff of this merge, when Erlang was moved out of a research organization and a product group was formed to further develop and maintain it.

Spreading knowledge of OTP can promote Erlang adoption in more “tried and true” corporate IT environments. Just knowing there is a stable and mature platform available for application development helps technologists sell Erlang to management, a crucial step in making its industrial adoption more widespread. Startups, on the other hand, just get on with it, with Erlang/OTP allowing them to achieve speed to market and reduce their development and operations costs.

OTP is said to consist of three building blocks (Figure 1-1) that, when used together, provide a solid approach to designing and developing systems in the problem domain we’ve just described. They are Erlang itself, tools and libraries, and a set of design principles. We’ll look at each in turn.

Figure 1-1. OTP components

Erlang

The first building block is Erlang itself, which includes the semantics of the language and its underlying virtual machine. Key language features such as lightweight processes, lack of shared memory, and asynchronous message passing will bring you a step closer to your goal. Just as important are links and monitors between processes, and dedicated channels for the propagation of the error signals. The monitors and error reporting allow you to build, with relative ease, complex supervision hierarchies with built-in fault recovery. Because message passing and error propagation are asynchronous, the semantics and logic of a system that was developed to run in a single Erlang node can be easily distributed without having to change any of the code base.

One significant difference between running on a single node and running in a distributed environment is the latency with which messages and errors are delivered. But in soft real-time systems, you have to consider latency regardless of whether the system is distributed or under heavy load. So if you have solved one facet of the problem, you have solved both.

Erlang lets you run all your code on top of a virtual machine highly optimized for concurrency, with a per-process garbage collector, yielding predictable and simple system behavior. Other programming environments do not have this luxury because they need an extra layer to emulate Erlang’s concurrency model and error semantics. To quote Joe Armstrong, coinventor of Erlang, “You can emulate the logic of Erlang, but if it is not running on the Erlang virtual machine, you cannot emulate the semantics.” The only languages that today get away with this are built on the BEAM emulator, the prevailing Erlang virtual machine. There is a whole ecosystem of them, with the Elixir and Lisp Flavored Erlang languages being the ones gaining most traction at the time of writing. What we write in this book about Erlang also applies to them.

Tools and Libraries

The second building block, which came about before open source became the widespread norm for software projects, includes applications that ship as part of the standard Erlang/OTP distribution. You can view each application as a way of packaging resources in OTP, where applications may have dependencies on other applications. The applications include tools, libraries, interfaces toward other languages and programming environments, databases and database drivers, standard components, and protocol stacks. The OTP documentation does a fine job of separating them into the following subsets:

  • The basic applications include the following:

    • The Erlang runtime system (erts)

    • The kernel

    • The standard libraries (stdlib)

    • The system architecture support libraries (sasl)

    They provide the tools and basic building blocks needed to architect, create, start, and upgrade your system. We cover the basic applications in detail throughout this book. Together with the compiler, these are the minimal subset of applications necessary in any system written in Erlang/OTP to do anything meaningful.

  • The database applications include mnesia, Erlang’s distributed database, and odbc, an interface used to communicate with relational SQL databases. Mnesia is a popular choice because it is fast, runs and stores its data in the same memory space as your applications, and is easy to use, as it is accessed through an Erlang API.

  • The operations and maintenance applications include os_mon, an application that allows you to monitor the underlying operating system; snmp, a Simple Network Management Protocol agent and client; and otp_mibs, management information bases that allow you to manage Erlang systems using SNMP.

  • The collection of interface and communication applications provide protocol stacks and interfaces to work with other programming languages, including an ASN.1 (asn1) compiler and runtime support, direct hooks into C (ei and erl_interface) and Java (jinterface) programs, along with an XML parser (xmerl). There are security applications for SSL/TLS, SSH, cryptography, and public key infrastructure. Graphics packages include a port of wxWidgets (wx), together with an easy-to-use interface. The eldap application provides a client interface toward the Lightweight Directory Access Protocol (LDAP). And for telecom aficionados, there is a Diameter stack (as defined in RFC 6733), used for policy control and authorization, alongside authentication and accounting. Dig even deeper and you will find the Megaco stack. Megaco/H.248 is a protocol for controlling elements of a physically decomposed multimedia gateway, separating the media conversion from the call control. If you have ever used a smartphone, you have very likely indirectly taken the Erlang diameter and megaco applications for a spin.

  • The collection of tools applications facilitate the development, deployment, and management of your Erlang system. We cover only the most relevant ones in this book, but outline them all here so you are aware of their existence:

    • The debugger is a graphical tool that allows you to step through your code while influencing the state of the functions.

    • The observer integrates the application monitor and the process manager, alongside basic tools to monitor your Erlang systems as they are being developed and in production.

    • The dialyzer is a static analysis tool that finds type discrepancies, dead code, and other issues.

    • The event tracer (et) uses ports to collect trace events in distributed environments, and percept allows you to locate bottlenecks in your system by tracing and visualizing concurrency-related activities.

    • Erlang Syntax Tools (syntax_tools) contains modules for handling Erlang syntax trees in a way that is compatible with other language-related tools. It also includes a module merger allowing you to merge Erlang modules, together with a renamer, solving the issue of clashes in a nonhierarchical module space.

    • The parsetools application contains the parse generator (yecc) and a lexical analyzer generator for Erlang (leex).

    • Reltool is a release management tool that provides a graphical front end together with back-end hooks that can be used by more generic build systems.

    • Runtime_tools is a collection of utilities including DTrace and SystemTap probes, and dbg, a user-friendly wrapper around the trace built-in functions (BIFs).

    • Finally, the tools application is a collection of profilers, code coverage tools, and module cross-reference analysis tools, as well as the Erlang mode for the emacs editor.

  • The test applications provide tools for unit testing (eunit), system testing, and black-box testing. The Test Server (packaged in the test_server application) is a framework that can be used as the engine of a higher-level test tool application. Chances are that you will not be using it, because OTP provides one of these higher-level test tools in the form of common_test, an application suited for black-box testing. Common_test supports automated execution of Erlang-based test cases toward most target systems irrespective of programming language.

  • We need to mention the Object Request Brokers (ORBs) and interface definition language (IDL) applications for nostalgic reasons, reminding one of the coauthors of his past sins. They include a broker called orber, an IDL compiler called ic, and a few other CORBA Common Object Services no longer used by anyone.

We cover and refer to some of these applications and tools in this book. Some of the tools we do not cover are described in Erlang Programming (O’Reilly), and those that aren’t are covered by the set of reference manual pages and the user’s guide that comes as part of the standard Erlang/OTP documentation.

These applications are not the full extent of tool support for Erlang; they are enhanced by thousands of other applications implemented and supported by the community and available as open source. We cover some of the prevailing applications in the latter half of the book, where we focus on distributed architectures, availability, scalability, and monitoring. They include the Riak Core and Scalable Distributed (SD) Erlang frameworks; load regulation applications such as jobs and safetyvalve; and monitoring and logging applications such as elarm, folsom, exometer, and lager. Once you’ve read this book and before starting your project, review the standard and open source Erlang/OTP reference manuals and user’s guides, because you never know when they will come in handy.

System Design Principles

The third building block of OTP consists of a set of abstract principles, design rules, and generic behaviors. The abstract principles describe the software architecture of an Erlang system, using processes in the form of generic behaviors as basic ingredients. Design rules keep the tools you use compatible with the system you are developing. Using this approach provides a standard way of solving problems, making code easier to understand and maintain, as well as providing a common language and vocabulary among the teams.

OTP generic behaviors can be seen as formalizations of concurrent design patterns. Behaviors are packaged into library modules containing generic code that solves a common problem. They have built-in support for debugging, software upgrade, generic error handling, and built-in functionality for upgrades.

Behaviors can be worker processes, which do all of the hard work, and supervisor processes, whose only tasks are to start, stop, and monitor workers or other supervisors. Because supervisors can monitor other supervisors, the functionality within an application can be chained so that it can be more easily developed in a modular fashion. The processes monitored by a supervisor are called its children.

OTP provides predefined libraries for workers and supervisors, allowing you to focus on the business logic of the system. We structure processes into hierarchical supervision trees, yielding fault-tolerant structures that isolate failure and facilitate recovery. OTP allows you to package a supervision tree into an application, as seen in Figure 1-2, where circles with double rings are supervisors and the other processes are workers.

Figure 1-2. OTP application

Generic behaviors that come as part of the OTP middleware include:

  • Generic servers, providing a client-server design pattern

  • Generic finite state machines, allowing you to implement FSMs

  • Event handlers and managers, allowing you to generically deal with event streams

  • Supervisors, monitoring other worker and supervision processes

  • Applications, allowing you to package resources, including supervision trees

We cover them all in detail in this book, as well as explaining how to implement your own. We use behaviors to create supervision trees, which are packaged into applications. We then group applications together to form a release. A release describes what runs in a node.

Erlang Nodes

An Erlang node consists of several loosely coupled applications, which might be comprised of some of the applications described in “Tools and Libraries” combined with other third-party applications and applications you write specifically for the system you are trying to implement. These applications could be independent of each other or rely on the services and APIs of other applications. Figure 1-3 illustrates a typical release of an Erlang node with the virtual machine (VM) dependent on the hardware and operating system, and Erlang applications running on top of the VM interfacing with non-Erlang components that are OS and hardware dependent.

Figure 1-3. An Erlang node

Group together a cluster of Erlang nodes—potentially pairing them up with nodes written in other programming languages—and you have a distributed system. You can now scale your system by adding nodes until you hit certain physical limits. These may be dictated by how you shared your data, by hardware or network constraints, or by external dependencies that act as bottlenecks.

Distribution, Infrastructure, and Multicore

Fault tolerance—one of Erlang’s fundamental requirements from its telecom roots—has distribution as its mainspring. Without distribution, the reliability and availability of an application running on just a single host would depend heavily on the reliability of the hardware and software comprising that host. Any problems with the host’s CPU, memory, persistent storage, peripherals, power supply, or backplane could easily take down the entire machine and the application along with it. Similarly, problems in the host’s operating system or support libraries could bring down the application or otherwise render it unavailable. Achieving fault tolerance requires multiple computers with some degree of coordination between them, and distribution provides the avenue for that coordination.

For decades, the computing industry has explored how programming languages can support distribution. Designing general-purpose languages is difficult enough; designing them to support distribution significantly adds to that difficulty. Because of this, a common approach is to add distribution support to nondistributed programming languages through optional libraries. This approach has the benefit of allowing distribution support to evolve separately from the language itself, but it often suffers from an impedance mismatch with the language, feeling to developers as if it were “bolted on.” Since most languages use function calls as the primary means of transferring control and data from one part of an application to another, add-on distribution libraries often model exchanges between distributed parts of an application as function calls as well. While convenient, this approach is fundamentally broken because the semantics of local and remote function calls, especially their failure modes, are markedly different.

In Erlang, processes communicate via asynchronous message passing. This works even if a process is on a remote node because the Erlang virtual machine supports passing messages from one node to another. When one node joins another, it also becomes aware of any nodes already known to the other. In this manner, all the nodes in a cluster form a mesh, enabling any process to send a message to another process on any other node in the cluster. Each node in the cluster also automatically tracks liveness of other nodes in order to become aware of nonresponsive nodes. The advantages of asynchronous message passing in systems running on a node is extended to systems running in clusters, as replies can be received alongside errors and timeouts.

Erlang’s message passing and clustering primitives can serve as the basis for a wide variety of distributed system architectures. For example, service-oriented architecture (SOA), especially in its more modern variant, microservices, is a natural fit for Erlang given the ease of developing and deploying server-like processes. Clients treat such processes as services, communicating with them by exchanging messages. As another example, consider that Erlang clusters do not require master or leader nodes, which means that using them for peer-to-peer systems of replicas works well. Clients can send service request messages to any peer node in the cluster, and the peer can either handle the request itself or route it to another peer. The concept of standalone clusters, known as groups that communicate with each other through gateway nodes that can go up and down or lose connectivity exists in a framework called SD Erlang. Another popular distributed framework, inspired by the Amazon Dynamo paper published in 2007, is Riak Core, offering consistent hashing to schedule jobs, recovery from partitioned networks and failed nodes through consistent hashing, eventual consistency, and virtual nodes dividing state and the data into small, manageable entities that can be replicated and moved across nodes.

With distributed systems, you can also achieve scalability. In fact, availability, consistency, and scalability go hand in hand, each affecting the others. It starts with the concurrency model and the concept of message passing within the node, which we extend across the network to use for clustering nodes. Erlang’s virtual machine takes advantage of today’s multicore systems by allowing processes to execute with true concurrency, running simultaneously on different cores. Because of the symmetric multiprocessing (SMP) capabilities of the Erlang virtual machine, Erlang is already prepared to help applications scale vertically as the number of cores per CPU continues to increase. And because adding new nodes to a cluster is easy—all it takes is to have that node contact just one other node to join the mesh—horizontal scaling is also well within easy reach. This, in turn, allows you to focus on the real challenge when dealing with distributed systems: namely, distributing your data and state across hosts and networks that are unreliable.

Summing Up

To make design, implementation, operation, and maintainability easier and more robust, your programming language and middleware have to be compact, their behavior in runtime predictable, and the resulting code base maintainable. We keep talking about fault-tolerant, scalable, soft real-time systems with requirements for high availability. The problems you have to solve do not have to be complicated in order to benefit from the advantages Erlang/OTP brings to the table. Advantages will be evident if you are developing solutions targeted for embedded hardware platforms such as the Parallela board, the BeagleBoard, or the Raspberry Pi. You will find Erlang/OTP ideal for the orchestration code in embedded devices, for server-side development where concurrency comes in naturally, and all the way up to scalable and distributed multicore architectures and supercomputers. It eases the development of the harder software problems while making simpler programs even easier to implement.

What You’ll Learn in This Book

This book is divided into two sections. The first part, from Chapter 3 to Chapter 10, deals with the design and implementation of a single node. You should read these chapters sequentially, because their examples and explanations build on prior ones. The second half of the book, from Chapter 11 to Chapter 16, focuses on tools, techniques, and architectures used for deployment, monitoring, and operations, while explaining the theoretical approaches needed to tackle issues such as reliability, scalability, and high availability. The second half builds in part on the examples covered in the first half of the book, but can be read independently of it.

We begin with an overview of Erlang in Chapter 2, intended not to teach you the language but rather as a refresher course. If you do not yet know Erlang, we recommend that you first consult one or more of the excellent books designed to help you learn the language, such as Simon St. Laurent’s Introducing Erlang, Erlang Programming by Francesco Cesarini and Simon Thompson, or any of the other books we mention in Chapter 2. Our overview touches on the major elements of the language, such as lists, functions, processes and messages, and the Erlang shell, as well as those features that make Erlang unique among languages, such as process linking and monitoring, live upgrades, and distribution.

Following the Erlang overview, Chapter 3 dives into process structures. Erlang processes can handle a wide variety of tasks, yet regardless of the particular tasks or their problem domains, similar code structures and process lifecycles surface, akin to the common design patterns that have been observed and documented for popular object-oriented languages like Java and C++. OTP captures and formalizes these common process-oriented structures and lifecycles into behaviors, which serve as the base elements of OTP’s reusable frameworks.

In Chapter 4 we explore in detail our first worker process. It is the most popular and frequently used OTP behavior, the gen_server. As its name implies, it supports generic client-server structures, with the server governing particular computing resources—perhaps just a simple Erlang Term Storage (ETS) instance, or a pool of network connections to a remote non-Erlang server—and granting clients access to them. Clients communicate with generic servers synchronously in a call-response fashion, asynchronously via a one-way message called a cast, or via regular Erlang messaging primitives. Full consideration of these modes of communication requires us to scrutinize various aspects of the processes involved, such as what happens if the client or server dies in the middle of a message exchange, how timeouts apply, and what might happen if a server receives a message it does not understand. By addressing these and other common issues, the gen_server handles a lot of details independently of the problem domain, allowing developers to focus more of their time and energy on their applications. The gen_server behavior is so useful that it not only appears in most nontrivial Erlang applications but is used throughout OTP itself as well.

Prior to examining more OTP behaviors, we follow our discussion of gen_server with a look at some of the control and observation points the OTP behaviors provide (Chapter 5). These features reflect another aspect of Erlang/OTP that sets it apart from other languages and frameworks: built-in observability. If you want to know what your gen_server process is doing, you can simply enable debug tracing for that process, either at compile time or at runtime from an Erlang shell. Enabling traces causes it to emit information that indicates what messages it is receiving and what actions it is taking to handle them. Erlang/OTP also provides functions for peering into running processes to see their backtraces, process dictionaries, parent processes, linked processes, and other details. There are also OTP functions for examining status and internal state specifically for behaviors and other system processes. Because of these debug-oriented features, Erlang programmers often forego the use of traditional debuggers and instead rely on tracing to help them diagnose errant programs, as it is typically both faster to set up and more informative.

We then examine another OTP behavior, gen_fsm (Chapter 6), which supports a generic FSM pattern. As you may already know, an FSM is a system that has a finite number of states, and incoming messages can advance the system from one state to another, with side effects potentially occurring as part of the transitions. For example, you might consider your television set-top box as being an FSM where the current state represents the selected channel and whether any on-screen display is shown. Pressing buttons on your remote causes the set-top box to change state, perhaps selecting a different channel, or changing its on-screen display to show the channel guide or list any on-demand shows that might be available for purchase. FSMs are applicable to a wide variety of problem domains because they allow developers to more easily reason about and implement the potential states and state transitions of their applications. Knowing when and how to use gen_fsm can save you from trying to implement your own ad hoc state machines, which often quickly devolve into spaghetti code that is hard to maintain and extend.

Logging and monitoring are critical parts of any scalability success story, since they allow you to glean important information about your running systems that can help pinpoint bottlenecks and problematic areas that require further investigation. The Erlang/OTP gen_event behavior (Chapter 7) provides support for subsystems that emit and manage event streams reflecting changes in system state that can impact operational characteristics, such as sustained increases in CPU load, queues that appear to grow without bound, or the inability of one node in a distributed cluster to reach another. These streams do not have to stop with your system events. They could handle your application-specific events originating from user interaction, sensor networks, or third-party applications. In addition to exploring the gen_event behavior, we also take a look at the OTP system architecture support libraries (SASL) error-logging event handlers, which provide flexibility for managing supervisor reports, crash reports, and progress reports.

Event handlers and error handlers are staples of numerous programming languages, and they are incredibly useful in Erlang/OTP as well, but do not let their presence here fool you: dealing with errors in Erlang/OTP is strikingly different from the approaches to which most programmers are accustomed.

After gen_event, the next behavior we study is the supervisor (Chapter 8), which manages worker processes. In Erlang/OTP, supervisor processes start workers and then keep an eye on them while they carry out application tasks. Should one or more workers die unexpectedly, the supervisor can deal with the problem in one of several ways that we explain later in the book. This form of handling errors, known as “let it crash,” differs significantly from the defensive programming tactics that most programmers employ. “Let it crash” and supervision, together a critical cornerstone of Erlang/OTP, are highly effective in practice.

We then look into the final fundamental OTP behavior, the application (Chapter 9), which serves as the primary point of integration between the Erlang/OTP runtime and your code. OTP applications have configuration files that specify their names, versions, modules, the applications upon which they depend, and other details. When started by the Erlang/OTP runtime, your application instance in turn starts a top-level supervisor that brings up the rest of the application. Structuring modules of code into applications also lets you perform code upgrades on live systems. A release of an Erlang/OTP package typically comprises a number of applications, some of which are part of the Erlang/OTP open source distribution and others that you provide.

Having examined the standard behaviors, we next turn our attention to explaining how to write your own behaviors and special processes (Chapter 10). Special processes are processes that follow certain design rules, allowing them to be added to OTP supervision trees. Knowing these design rules can not only help you understand implementation details of the standard behaviors, but also inform you of their tradeoffs and allow you to better decide when to use them and when to write your own instead.

Chapter 11 describes how OTP applications in a single node are coupled together and started as a whole. You will have to create your own release files, referred to in the Erlang world as rel files. The rel file lists the versions of the applications and the runtime system that are used by the systools module to bundle up the software into a standalone release directory that includes the virtual machine. This release directory, once configured and packaged, is ready to be deployed and run on target hosts. We cover the community-contributed tools rebar3 and relx, the best way to build your code and your releases.

The Erlang virtual machine has configurable system limits and settings you need to be aware of when deploying your systems. There are many, ranging from limits regulating the maximum number of ETS tables or processes to included code search paths and modes used for loading modules. Modules in Erlang can be loaded at startup, or when they are first called. In systems with strict revision control, you will have to run them in embedded mode, loading modules at startup and crashing if modules do not exist, or in interactive mode, where if a module is not available, an attempt to load it is made before terminating the process. An external monitoring heart process monitors the Erlang virtual machine by sending heartbeats and invoking a script that allows you to react when these heartbeats are not acknowledged. You implement the script yourself, allowing you to decide whether restarting the node is enough or whether—based on a history of previous restarts—you want to escalate the crash and terminate the virtual instance or reboot the whole machine.

Although Erlang’s dynamic typing allows you to upgrade your module at runtime while retaining the process state, it does not coordinate dependencies among modules, changes in process state, or non–backward-compatible protocols. OTP has the tools to support system upgrades on a system level, including not only the applications, but also the runtime system. The principles and supporting libraries are presented in Chapter 12, from defining your own application-upgrade scripts to writing scripts that support release upgrades. Approaches and strategies for handling changes to your database schema are provided, as are guidelines for upgrades in distributed environments and non–backward-compatible protocols. For major upgrades in distributed environments where bugs are fixed, protocols improved, and database schema changed, runtime upgrades are not for the faint of heart. But they are incredibly powerful, allowing automated upgrades and nonstop operations. Finding your online banking is unavailable because of maintenance should now be a thing of the past. If it isn’t, send a copy of this book to your bank’s IT department.

Operating and maintaining any system requires visibility into what is going on. Scaling clusters require strategies for how you share your data and state. And fault tolerance requires an approach to how you replicate and persist it. In doing so, you have to deal with unreliable networks, failure, and recovery strategies. While each of these subjects merits a book of its own, the final chapters of this book will provide you with the theoretical background needed when distributing your systems and making them reliable and scalable. We provide this theory by describing the steps needed to design a scalable, highly available architecture in Erlang/OTP.

Chapter 13 will give you an overview of the approaches needed when designing your distributed architecture, breaking up your functionality into standalone nodes. In doing so, each standalone node type will be assigned a specific purpose, such as acting as a client gateway managing TCP/IP connection pools or providing a service such as authentication or payments. For each node type, we define an approach to specifying interfaces and defining the state and data each node needs. We conclude the chapter by describing the most common distributed architectural patterns and the different network protocols that can be used to connect them.

When you have your distributed architecture in place, you need to make design choices that will impact fault tolerance, resilience, reliability, and availability. You know what data and state you need in your node types, but how are you going to distribute it and keep it consistent? Are you going for the share-everything, share-something, or share-nothing approach, and what are the tradeoffs you need to make when choosing strong, causal, or eventual consistency? In Chapter 14, we describe the different approaches you can take, introducing the retry strategies you need to be aware of in case a request times out as the result of process, node, or network failure or the mere fact that the network or your servers are running over capacity.

It is easy to say that you are going to add hardware to make your system scale horizontally, but alas, the design choices introduced in Chapter 14 will have an impact on your system’s scalability. In Chapter 15, we describe the impacts resulting from your data-sharing strategy, consistency model, and retry strategy. We cover capacity planning, including the load, peak, and stress tests you need to subject your system to to guarantee it behaves in a predictable way under heavy load even when the hardware, software, and infrastructure around it are failing.

Once you’ve designed your scalability and availability strategies, you need to tackle monitoring. If you want to achieve five-nines uptime, you need to not only know what is going on, but also be able to quickly determine what happened, and why. We conclude the book with Chapter 16, looking at how monitoring is used for preemptive support and postmortem debugging.

Monitoring focuses on metrics, alarms, and logs. This chapter discusses the importance of system and business metrics. Examples of system metrics include the amount of memory your node is using, process message queue length, and hard-disk utilization. Combining these with business metrics, such as the number of failed and successful login attempts, message throughput per second, and session duration, yields full visibility of how your business logic is affecting your system resources.

Complementing metrics is alarming, where you detect and report anomalies, allowing the system to take action to try to resolve them or to alert an operator when human intervention is required. Alarms could include a system running out of disk space (resulting in the automatic invocation of scripts for compressing or deleting logs) or a large number of failed message submissions (requiring human intervention to troubleshoot connectivity problems). Preemptive support at its best, detecting and resolving issues before they escalate, is a must when dealing with high availability. If you do not have a real-time view of what is going on, resolving issues before they escalate becomes extremely difficult and cumbersome.

And finally, logging of major events in the system helps you troubleshoot your system after a crash where you lost its state, so you can retrieve the call flow of a particular request among millions of others to handle a customer services query, or just provide data records for billing purposes.

With your monitoring in place, you will be ready to architect systems that are not only scalable, but also resilient and highly available. Happy reading! We hope you enjoy the book as much as we enjoyed writing it.

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