There are a few competent API gateway solutions (open source as well as commercial grade).

Kong (https://konghq.com/) is an open source API gateway solution. The Kong server can run in front of any RESTful API. Kong fully supports popular REST APIs. Due to their lightweight and versatile nature, RESTful is the widely used API standard for all kinds of web, mobile, embedded, and cloud applications. The current capability of the Kong API gateway can be substantially extended through plug-ins, which bring in extra functionality to meet evolving needs. That is, the core features and facilities offered by Kong can be supplemented through additional functionalities provided by versatile plug-ins. That is, Kong natively supports plug and play architecture. Small, medium, and large-scale business enterprises across the globe are using this innovation-field product suite in production-grade IT environments. Kong can be deployed in on-demand, online, and off-premises clouds. Also, it can run on on-premises private clouds. As illustrated in the following diagram, OpenResty and Nginx are the core engines of Kong. OpenResty is a high-performance web platform that integrates the standard Nginx core, LuaJIT, and a host of libraries. Kong uses either Cassandra or PostgreSQL as the data store. The popular features accentuated by Kong include authentication, monitoring and analytics, and request/response translation and logging. 

To better understand how Kong works, here is a typical request workflow of an API that uses Kong:

Once Kong has started to run, every request made to the API will hit the Kong server first.  And the Kong server acts as the proxy to send the request to the API of the requested service. In short, everything in the microservice world gets initiated and implemented through an API gateway. When everything uses an API, the role of API gateway, management, and analytics solutions is bound to escalate in the days to unfold: 

Red Hat 3scale APIcast gateway (https://www.3scale.net/tag/open-source/)—the APIcast gateway is also Nginx-based and made available as an open source software solution. This API gateway is configured within 3scale's Admin Portal. The gateway is a crucial part of 3scale API management, which is a software as a service (SaaS) offering. The Admin Portal has a number of customization and configuration functionalities, such as allowing customers to define desired authentication methods, setting rate limits, getting analytics done on API usage data, and creating a developer portal for their API consumers. The 3scale APIcast gateway lets you deploy the API gateway service on a cloud environment with a few clicks. This deployment is much faster than others because there is no need for any kind of code modification at the backend. APIcast is emerging as the perfect solution for low or medium volume APIs. Enterprises widely use APIcast in staging environments and this speeds up the process of testing. 

Tyk (https://tyk.io/) is also an open source API gateway. This solution intrinsically takes care of API management activities. The Tyk package consists of an API gateway and an API management dashboard. It also has the API analytics feature. A developer portal is another interesting module of this package. Tyk can be installed in on-premises clouds. It is available in leading public clouds and can be purchased and used as a cloud service. Not only that, it integrates private and public clouds to contribute as the API gateway solution for hybrid clouds. Under load, this API gateway can do the full key validation, security verification, quota management, and data analytics, without any hitch or hurdle. The developer portal is made available to empower the development community.

Moesif (https://www.moesif.com/ ) is primarily an API analytics solution. This has the capability to understand how developers use APIs and to know why certain errors occur and repeat sometimes. Also, it helps to notify API providers of any hidden issues before the customers see them. Thus, this solution is a helping hand for service providers. As we tend toward the API economy, the API analytics solution plays an important role. Today, thousands of developers process billions of API calls through Moesif for debugging, monitoring, and understanding API usage. The first and foremost requirement to build  a great API (whether it is REST, GraphQL, or JSON-RPC API) is to precisely and concisely measure how developers use the APIs. Product teams use the API analytics capability offered by this solution to understand how their APIs are being used. By leveraging the proven and potential machine learning (ML) techniques, Moesif API insights enable data-driven teams to continuously improve their API and developer experience (DX).

Ambassador (https://www.getambassador.io/) is a popular open source and Kubernetes-native API gateway for the microservice world. This gateway solution can do several things for the container world. Ambassador can authenticate all kinds of incoming requests before intelligently routing them to backend services. It natively supports TLS termination. In addition to that, Ambassador supports rate limiting/throttling via an external third-party service. This rate limiting is facilitated through the Envoy proxy's rate limiting capabilities. A key feature of Envoy is the observability feature, which is enabled  by exposing a multitude of statistics about its own operations. Ambassador makes it easy to disseminate that knowledge to statistics and monitoring tools such as Prometheus, Datadog, and so on. Ambassador generally relies on the fast-evolving Kubernetes platform for ensuring service reliability, availability, and scalability.

Envoy (https://www.envoyproxy.io/) was originally built by Lyft. Envoy is a high performance C++ distributed proxy designed for single services and applications. Envoy turns out to be a competent solution as a communication bus. For microservice-centric applications, this contributes as the universal data plane. Finally, it is a core engine for service mesh solutions. Envoy runs alongside every application and abstracts the networking complexity by providing all the common network features in a platform-agnostic manner. By leveraging the Envoy mesh solution, it is possible to visualize and pinpoint problem areas because features such as consistent observability, tuning overall performance, and adding a substrate are made available in a single place.

Tree gateway (http://treegateway.org/) is a free and open source solution. This can create a complete and customizable pipeline to handle service requests. Tree gateway makes it easy to create and sustain big clusters. In addition to this, it supports the ready formation of Redis clusters to share configurations, circuit-breaker states, and cached content. Any API configuration can be changed at any point in time, and all configurations get propagated to other tree gateway cluster nodes, without any problems. This comes with an advanced circuit breaker module that can fast fail responses when any API fails, falls, and falters.  Furthermore, it innately supports real-time monitoring and analytics. A pluggable engine allows any kinds of transformations or verifications to any API requests.

Gravitee.io (https://gravitee.io/) is a flexible, lightweight, blazingly fast, and open source API management solution. This tool helps any business enterprise to control who, when, and how users access the APIs in a finely grained manner.

API Umbrella (https://apiumbrella.io/) is a proxy solution that sits in front of any API. It can seamlessly add API gateway and analytics functionalities, such as API keys, rate limiting, and so on. 

Express gateway (https://www.express-gateway.io/) is a microservice API gateway, which is built on Express.js. It is typically fast, flexible, and community driven.

API gateway solutions are very important for API-enabled microservices. RESTful APIs are the most prevalent and powerful. RESTful APIs are overwhelmingly used by web, mobile, cloud, and IoT applications and services. Not only software applications but also resource-constrained and intensive embedded devices are being linked with one another via RESTful APIs. With microservices quickly proliferating and as everything is stuffed with APIs, there is a mandate for advanced API gateway solutions to form integrated and orchestrated applications, which businesses increasingly prefer these days. API management and analytics capabilities are also attached with gateway solutions to make them comprehensive. A variety of plugins and utility services are being built and incorporated on demand. Furthermore, API gateway solutions are being integrated with third-party tools for monitoring, measurement, management, and visualization. The well-intended approach is to have a dynamic pool of modular (loosely and lightly coupled and highly cohesive) API gateway services instead of a monolithic API gateway solution, which is difficult to manage, inflexible, and closed.