A microservice architecture – a variant of the

service-oriented architecture In software engineering, service-oriented architecture (SOA) is an architectural style that focuses on discrete services instead of a monolithic design. By consequence, it is also applied in the field of software design where services are provide ...

structural style – is an architectural pattern that arranges an application as a collection of loosely-coupled,

fine-grained Granularity (also called graininess), the condition of existing in granules or grains, refers to the extent to which a material or system is composed of distinguishable pieces. It can either refer to the extent to which a larger entity is sub ...

services, communicating through lightweight protocols. One of its goals is that teams can develop and deploy their services independently of others. This is achieved by the reduction of several dependencies in the code base, allowing for developers to evolve their services with limited restrictions from users, and for additional complexity to be hidden from users. As a consequence, organizations are able to develop software with fast growth and size, as well as use off-the-shelf services more easily. Communication requirements are reduced. These benefits come at a cost to maintaining the decoupling. Interfaces need to be designed carefully and treated as a public

API An application programming interface (API) is a way for two or more computer programs to communicate with each other. It is a type of software interface, offering a service to other pieces of software. A document or standard that describes how ...

. One technique that is used is having multiple interfaces on the same service, or multiple versions of the same service, so as to not disrupt existing users of the code.

Introduction

There is no single definition for microservices. A consensus view has evolved over time in the industry. Some of the defining characteristics that are frequently cited include: * Services in a microservice architecture are often processes that communicate over a

network Network, networking and networked may refer to: Science and technology * Network theory, the study of graphs as a representation of relations between discrete objects * Network science, an academic field that studies complex networks Mathematics ...

to fulfill a goal using technology-agnostic

protocols Protocol may refer to: Sociology and politics * Protocol (politics), a formal agreement between nation states * Protocol (diplomacy), the etiquette of diplomacy and affairs of state * Etiquette, a code of personal behavior Science and technology ...

such as HTTP. * Services are organized around business capabilities. * Services can be implemented using different

programming language A programming language is a system of notation for writing computer programs. Most programming languages are text-based formal languages, but they may also be graphical. They are a kind of computer language. The description of a programming ...

database In computing, a database is an organized collection of data stored and accessed electronically. Small databases can be stored on a file system, while large databases are hosted on computer clusters or cloud storage. The design of databases sp ...

s, hardware and software environments, depending on what fits best. * Services are small in size, messaging-enabled, bounded by contexts, autonomously developed, independently deployable,Nadareishvili, I., Mitra, R., McLarty, M., Amundsen, M., Microservice Architecture: Aligning Principles, Practices, and Culture, O'Reilly 2016 decentralized and built and released with automated processes. A microservice is not a layer within a monolithic application (example, the web controller, or the backend-for-frontend). Rather, it is a self-contained piece of business functionality with clear interfaces, and may, through its own internal components, implement a layered architecture. From a strategical perspective, microservice architecture essentially follows the

Unix philosophy The Unix philosophy, originated by Ken Thompson, is a set of cultural norms and philosophical approaches to minimalist, modular software development. It is based on the experience of leading developers of the Unix operating system. Early Unix de ...

of "Do one thing and do it well". Martin Fowler describes a microservices-based architecture as having the following properties: * Lends itself to a

continuous delivery Continuous delivery (CD) is a software engineering approach in which teams produce software in short cycles, ensuring that the software can be reliably released at any time and, following a pipeline through a "production-like environment", withou ...

software development process. A change to a small part of the application only requires rebuilding and redeploying only one or a small number of services. * Adheres to principles such as

interface Interface or interfacing may refer to: Academic journals * ''Interface'' (journal), by the Electrochemical Society * '' Interface, Journal of Applied Linguistics'', now merged with ''ITL International Journal of Applied Linguistics'' * '' Int ...

s (to independently deployable services), business-driven development (e.g.

domain-driven design Domain-driven design (DDD) is a major software design approach, focusing on modeling software to match a domain according to input from that domain's experts. Under domain-driven design, the structure and language of software code (class name ...

). It is common for microservices architectures to be adopted for cloud-native applications,

serverless computing Serverless computing is a cloud computing execution model in which the cloud provider allocates machine resources on demand, taking care of the servers on behalf of their customers. "Serverless" is a misnomer in the sense that servers are still ...

, and applications using lightweight

container A container is any receptacle or enclosure for holding a product used in storage, packaging, and transportation, including shipping. Things kept inside of a container are protected on several sides by being inside of its structure. The term ...

deployment. According to Fowler, because of the large number (when compared to monolithic application implementations) of services, decentralized continuous delivery and

DevOps DevOps is a set of practices that combines software development (''Dev'') and IT operations (''Ops''). It aims to shorten the systems development life cycle and provide continuous delivery with high software quality. DevOps is complementary to ...

with holistic service monitoring are necessary to effectively develop, maintain, and operate such applications. A consequence of (and rationale for) following this approach is that the individual microservices can be individually scaled. In the monolithic approach, an application supporting three functions would have to be scaled in its entirety even if only one of these functions had a resource constraint. With microservices, only the microservice supporting the function with resource constraints needs to be scaled out, thus providing resource and cost optimization benefits.

History

There are numerous claims as to the origin of the term microservices. Whilst vice president of ThoughtWorks in 2004, Fred George began working on prototype architectures based on what he called the "Baysean Principles" named after Jeff Bay. As early as 2005, Peter Rodgers introduced the term "Micro- Web-Services" during a presentation at the Web Services Edge conference. Against conventional thinking and at the height of the

SOAP Soap is a salt of a fatty acid used in a variety of cleansing and lubricating products. In a domestic setting, soaps are surfactants usually used for washing, bathing, and other types of housekeeping. In industrial settings, soaps are use ...

SOA architecture hype curve he argued for "

REST Rest or REST may refer to: Relief from activity * Sleep ** Bed rest * Kneeling * Lying (position) * Sitting * Squatting position Structural support * Structural support ** Rest (cue sports) ** Armrest ** Headrest ** Footrest Arts and enter ...

-services" and on slide #4 of the conference presentation, he discusses "

Software component Component-based software engineering (CBSE), also called component-based development (CBD), is a branch of software engineering that emphasizes the separation of concerns with respect to the wide-ranging functionality available throughout a give ...

s are Micro-Web-Services". He goes on to say "Micro-Services are composed using Unix-like pipelines (the Web meets Unix = true loose-coupling). Services can call services (+multiple language run-times). Complex service assemblies are abstracted behind simple

URI Uri may refer to: Places * Canton of Uri, a canton in Switzerland * Úri, a village and commune in Hungary * Uri, Iran, a village in East Azerbaijan Province * Uri, Jammu and Kashmir, a town in India * Uri (island), an island off Malakula Islan ...

interfaces. Any service, at any granularity, can be exposed." He described how a well-designed microservices platform "applies the underlying architectural principles of the Web and REST services together with Unix-like scheduling and pipelines to provide radical flexibility and improved simplicity in service-oriented architectures. Rodgers' work originated in 1999 with the Dexter research project at

Hewlett Packard Labs Hewlett Packard Labs is the exploratory and advanced research group for Hewlett Packard Enterprise and its businesses. It was formed in November, 2015 when HP Labs spun off Hewlett Packard Labs to reflect the spin off of Hewlett Packard Enterpris ...

, whose aim was to make code less brittle and to make large-scale, complex software systems

robust Robustness is the property of being strong and healthy in constitution. When it is transposed into a system, it refers to the ability of tolerating perturbations that might affect the system’s functional body. In the same line ''robustness'' ca ...

to change. Ultimately this path of research led to the development of

resource-oriented computing Resource Oriented Computing (ROC) is a simple abstract computing model used for describing, designing, and implementing software and software systems. The fundamental idea behind ROC is derived from the World Wide Web, Unix, and other sources as we ...

(ROC), a generalized computation abstraction in which REST is a special subset. In 2007, Juval Löwy in his writing and speaking called for building systems in which every class was a service. Löwy realized this required the use of a technology that can support such granular use of services, and he extended Windows Communication Foundation (WCF) to do just that, taking every class and treating it as a service while maintaining the conventional programming model of classes. In 2005

Alistair Cockburn Alistair Cockburn ( ) is an American computer scientist, known as one of the initiators of the agile movement in software development. He cosigned (with 17 others) the Manifesto for Agile Software Development. Life and career Cockburn starte ...

wrote about

Hexagonal architecture (software) The hexagonal architecture, or ports and adapters architecture, is an architectural pattern used in software design. It aims at creating loosely coupled application components that can be easily connected to their software environment by means of ...

which is a software design pattern that is used along with the microservices. This pattern makes the design of the microservice possible since it isolates in layers the business logic from the auxiliary services needed in order to deploy and run the microservice completely independent from others. A workshop of software architects held near Venice in May 2011 used the term "microservice" to describe what the participants saw as a common architectural style that many of them had been recently exploring. In May 2012, the same group decided on "microservices" as the most appropriate name. James Lewis presented some of those ideas as a

case study A case study is an in-depth, detailed examination of a particular case (or cases) within a real-world context. For example, case studies in medicine may focus on an individual patient or ailment; case studies in business might cover a particular fi ...

in March 2012 at 33rd Degree in Kraków in Microservices - Java, the Unix Way, as did Fred George about the same time. Adrian Cockcroft, former director for the Cloud Systems at Netflix, described this approach as "fine-grained SOA", pioneered the style at web-scale, as did many of the others mentioned in this article - Joe Walnes, Dan North, Evan Bottcher, and Graham Tackley. Microservices is a specialization of an implementation approach for

s (SOA) used to build flexible, independently deployable

software systems A software system is a system of intercommunicating components based on software forming part of a computer system (a combination of hardware and software). It "consists of a number of separate programs, configuration files, which are used to ...

. The microservices approach is the first realisation of SOA that followed the introduction of

and is becoming more popular for building continuously deployed systems. In February 2020, the Cloud Microservices Market Research Report predicted that the global microservice architecture market size will increase at a

CAGR Compound annual growth rate (CAGR) is a business and investing specific term for the geometric progression ratio that provides a constant rate of return over the time period. CAGR is not an accounting term, but it is often used to describe some ele ...

of 21.37% from 2019 to 2026 and reach $3.1 billion by 2026.

Service granularity

A key step in defining a microservice architecture is figuring out how big an individual microservice has to be. There is no consensus or litmus test for this, as the right answer depends on the business and organizational context. For instance,

Amazon Amazon most often refers to: * Amazons, a tribe of female warriors in Greek mythology * Amazon rainforest, a rainforest covering most of the Amazon basin * Amazon River, in South America * Amazon (company), an American multinational technology c ...

uses a

where service often maps 1:1 with a team of 3 to 10 engineers. Generally, the terminology goes as such: services that are dedicated to a single task, such as calling a particular backend system or making a particular type of calculation, are called ''atomic services''. Similarly, services that call such atomic services in order to consolidate an output, are called ''composite services''. It is considered bad practice to make the service too small, as then the runtime overhead and the operational complexity can overwhelm the benefits of the approach. When things get too fine-grained, alternative approaches must be considered - such as packaging the function as a library, moving the function into other microservices. If the

is being employed in modeling the domain for which the system is being built, then a microservice could be as small as an aggregate or as large as a bounded Context. In the granularity of microservices discussion, there is a spectrum, in one end there are the Anaemic Services, which do not have a large number of responsibilities, and on the other end, the Modular Monolith, which are large modules of a system.

Benefits

The benefit of decomposing an application into different smaller services are numerous: * Modularity: This makes the application easier to understand, develop, test, and become more resilient to architecture erosion. This benefit is often argued in comparison to the complexity of monolithic architectures. *

Scalability Scalability is the property of a system to handle a growing amount of work by adding resources to the system. In an economic context, a scalable business model implies that a company can increase sales given increased resources. For example, a ...

: Since microservices are implemented and deployed independently of each other, i.e. they run within independent processes, they can be monitored and scaled independently. *

Integration Integration may refer to: Biology *Multisensory integration *Path integration * Pre-integration complex, viral genetic material used to insert a viral genome into a host genome *DNA integration, by means of site-specific recombinase technology, ...

of heterogeneous and

legacy system In computing, a legacy system is an old method, technology, computer system, or application program, "of, relating to, or being a previous or outdated computer system", yet still in use. Often referencing a system as "legacy" means that it paved ...

s: microservices is considered a viable means for modernizing existing monolithic software application. There are experience reports of several companies who have successfully replaced (parts of) their existing software with microservices or are in the process of doing so. The process for

Software modernization Legacy modernization, also known as software modernization or platform modernization, refers to the conversion, rewriting or porting of a legacy system to modern computer programming languages, architectures (e.g. microservices), software libraries ...

of legacy applications is done using an incremental approach. * Distributed development: it parallelizes

development Development or developing may refer to: Arts *Development hell, when a project is stuck in development *Filmmaking, development phase, including finance and budgeting *Development (music), the process thematic material is reshaped * Photograph ...

by enabling small autonomous teams to develop, deploy and scale their respective services independently. It also allows the architecture of an individual service to emerge through continuous

refactoring In computer programming and software design, code refactoring is the process of restructuring existing computer code—changing the '' factoring''—without changing its external behavior. Refactoring is intended to improve the design, structu ...

. Microservice-based architectures facilitate continuous integration,

and deployment.

Criticism and concerns

The microservices approach is subject to criticism for a number of issues: * Services form information barriers. * Inter-service calls over a network have a higher cost in terms of network latency and message processing time than in-process calls within a

monolithic A monolith is a monument or natural feature consisting of a single massive stone or rock. Monolith or monolithic may also refer to: Architecture * Monolithic architecture, a style of construction in which a building is carved, cast or excavated ...

service process. *

Testing An examination (exam or evaluation) or test is an educational assessment intended to measure a test-taker's knowledge, skill, aptitude, physical fitness, or classification in many other topics (e.g., beliefs). A test may be administered verba ...

and

deployment Deployment may refer to: Engineering and software Concepts * Blue-green deployment, a method of installing changes to a web, app, or database server by swapping alternating production and staging servers * Continuous deployment, a software e ...

are more complicated. * Moving responsibilities between services is more difficult. It may involve communication between different teams, rewriting the functionality in another language or fitting it into a different infrastructure. However, microservices can be deployed independently from the rest of the application, while teams working on monoliths need to synchronize to deploy together. * Viewing the size of services as the primary structuring mechanism can lead to too many services when the alternative of internal modularization may lead to a simpler design. This requires understanding the overall architecture of the applications and interdependencies between components. * Two-phased commits are regarded as an anti-pattern in microservices-based architectures as this results in a tighter coupling of all the participants within the transaction. However, the lack of this technology causes awkward dances which have to be implemented by all the transaction participants in order to maintain data consistency. * Development and support of many services are more challenging if they are built with different tools and technologies - this is especially a problem if engineers move between projects frequently. * The protocol typically used with microservices (HTTP) was designed for public-facing services, and as such is unsuitable for working internal microservices that often must be impeccably reliable. * While not specific to microservices, the decomposition methodology often uses functional decomposition, which does not handle changes in the requirements while still adding the complexity of services. * The very concept of microservice is misleading since there are only services. There is no sound definition of when a service starts or stops being a microservice. * Data aggregation. In order to have a full view of a working system, it is required to extract data sets from the microservices repositories and aggregate them into a single schema. For example, to be able to create operational reports that are not possible using a single microservice repository.

Cognitive load

The architecture introduces additional complexity and new problems to deal with, such as

network latency Network delay is a design and performance characteristic of a telecommunications network. It specifies the Latency (engineering), latency for a bit of data to travel across the network from one communication endpoint to another. It is typically ...

, message format design, Backup/Availability/Consistency (BAC), load balancing and

fault tolerance Fault tolerance is the property that enables a system to continue operating properly in the event of the failure of one or more faults within some of its components. If its operating quality decreases at all, the decrease is proportional to the ...

. All of these problems have to be addressed at scale. The complexity of a

monolithic application In software engineering, a monolithic application describes a single-tiered software application in which the user interface and data access code are combined into a single program from a single platform. A monolithic application is self-contained ...

does not disappear if it is re-implemented as a set of microservices. Some of the complexity gets translated into operational complexity. Other places where the complexity manifests itself are increased network traffic and resulting in slower performance. Also, an application made up of any number of microservices has a larger number of interface points to access its respective

ecosystem An ecosystem (or ecological system) consists of all the organisms and the physical environment with which they interact. These biotic and abiotic components are linked together through nutrient cycles and energy flows. Energy enters the syste ...

, which increases the architectural complexity. Various organizing principles (such as

HATEOAS Hypermedia as the Engine of Application State (HATEOAS) is a constraint of the REST application architecture that distinguishes it from other network application architectures. With HATEOAS, a client interacts with a network application whose app ...

, interface and data model documentation captured via Swagger, etc.) have been applied to reduce the impact of such additional complexity.

Technologies

Computer microservices can be implemented in different programming languages and might use different infrastructures. Therefore, the most important technology choices are the way microservices communicate with each other (synchronous, asynchronous, UI integration) and the protocols used for the communication (RESTful HTTP, messaging,

GraphQL GraphQL is an open-source data query and manipulation language for APIs, and a runtime for fulfilling queries with existing data. GraphQL was developed internally by Facebook (now Meta) in 2012 before being publicly released in 2015. On 7 No ...

...). In a traditional system, most technology choices like the programming language impact the whole system. Therefore, the approach to choosing technologies is quite different. The

Eclipse Foundation The Eclipse Foundation AISBL is an independent, Europe-based not-for-profit corporation that acts as a steward of the Eclipse open source software development community, with legal jurisdiction in the European Union. It is an organization suppo ...

has published a specification for developing microservices, Eclipse MicroProfile.

Service mesh

In a service mesh, each service instance is paired with an instance of a reverse proxy server, called a service proxy, sidecar proxy, or sidecar. The service instance and sidecar proxy share a container, and the containers are managed by a container orchestration tool such as

Kubernetes Kubernetes (, commonly stylized as K8s) is an open-source container orchestration system for automating software deployment, scaling, and management. Google originally designed Kubernetes, but the Cloud Native Computing Foundation now maintains ...

Nomad A nomad is a member of a community without fixed habitation who regularly moves to and from the same areas. Such groups include hunter-gatherers, pastoral nomads (owning livestock), tinkers and trader nomads. In the twentieth century, the popu ...

, Docker Swarm, or DC/OS. The service proxies are responsible for communication with other service instances and can support capabilities such as service (instance) discovery, load balancing, authentication and authorization, secure communications, and others. In a service mesh, the service instances and their sidecar proxies are said to make up the data plane, which includes not only data management but also request processing and response. The service mesh also includes a control plane for managing the interaction between services, mediated by their sidecar proxies.

A comparison of platforms

Implementing a microservice architecture is very difficult. There are many concerns (see table below) that any microservice architecture needs to address.

Netflix Netflix, Inc. is an American subscription video on-demand over-the-top streaming service and production company based in Los Gatos, California. Founded in 1997 by Reed Hastings and Marc Randolph in Scotts Valley, California, it offers a fil ...

developed a microservice framework to support their internal applications, and then open-sourced many portions of that framework. Many of these tools have been popularized via the

Spring Framework The Spring Framework is an application framework and inversion of control container for the Java platform. The framework's core features can be used by any Java application, but there are extensions for building web applications on top of the Java ...

– they have been re-implemented as Spring-based tools under the umbrella of the Spring Cloud project. The table below shows a comparison of an implementing feature from the

ecosystem with an equivalent from the Spring Cloud world. One noteworthy aspect of the Spring Cloud ecosystem is that they are all Java-based technologies, whereas Kubernetes is a polyglot runtime platform.