MULE ESB provides an API layer that offers a decoupled interface for interacting with applications in a common, language-agnostic way. An API layer is commonly used to connect to legacy applications that lack REST APIs or to publish partner-facing APIs. The Anypoint Platform enables API-led connectivity by helping users build, manage, and secure APIs from a single platform. It includes an API gateway for connecting to backends, an API manager for administering users and traffic, an API contract manager for access control, and an API policy manager for security policies.

1. MULE ESB–API LayerMULE ESB–API Layer

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API Layer
A REST or Web services API layer offers a decoupled interface to data
and/or functionality of one or more applications. It provides a common,
language-agnostic way of interacting with an application

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Usecases
Here are some common examples of where an API layer would be used:
You need to connect to a legacy application that does not have a REST
API. For example you have a custom CRM or HR system that you want
employees to be able to access through a portal.
You want to publish an API that enables your partners to communicate with
your systems through a well-defined interface. Typically a Web services
API is created for this.

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Anypoint Platform for APIs
Make API-led connectivity a reality with Anypoint™ Platform.
Build new APIs, design new interfaces for existing APIs and more efficiently
manage all your APIs using a single platform to rapidly expose enterprise
data to mobile devices, web apps and connected things in a secure
controlled way.
Unleash the value of your information, eliminating the friction of integration
and enabling unprecedented speed, agility and rapid change.

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API Gateway
Creates a secure connection to any backend service or data source
Runs in the cloud or on-premises behind a firewall
Lets you enforce runtime policies and collect data for analytics

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API Manager
API management just got easier. With API Manager, you can manage
users, traffic and SLAs and secure APIs with just a few simple steps. The
built-in API Gateway allows you to easily connect to any backend service or
data source. Wherever your APIs are hosted and whatever technologies
they run on, you can keep tabs on all your APIs in one place.

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API contract manager
Manages access to your API by issuing contract keys
Monitors the API to confirm it is meeting all contract terms
Ensures compliance with service level agreements (SLAs)

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API policy manager
Applies policies to APIs or endpoints around security, throttling, rate
limiting, and more
Federate with PingFederate or OpenAM to provide identity management
and OAuth protection for your APIs
Lets you require or restrict certain behaviors in a few simple steps
Lets you add or remove policies at runtime with no API downtime