Mule Application Development | MuleSoft Documentation
# Mule Application Development
You create Mule applications to perform system integrations. Typically, the application reads data from internal and external sources, processes and transforms data to the required formats or structures, and writes that output to the systems and servers where you store or use the transformed data.
Mule applications are configured to run in Mule runtime engine (Mule). A request to a Mule application triggers Mule to encode the request and data in a Mule event and to pass it to either single or multiple threads.
## Getting Started with Mule Application Development
To get started with Mule application development, you can follow the steps in these tutorials:
  * Create a "Hello World" Mule app
Build a Mule application that interacts with a user in a simple HTTP request-response flow.
  * Create a Mule app that uses the Database Connector and DataWeave
Build a Mule application that retrieves data from a database and transforms it to a new structure.


## Building Blocks of a Mule Application
Mule applications use connectors, modules, and components to read, write, and process data.
  * Connectors and Modules
Anypoint connectors provide components, such as listeners, that interact with external API endpoints and act on data in the Mule application. Modules, such as the Validation, Java, Spring, and OAuth, provide modules that act on data in a Mule application without providing a direct connection to an endpoint.
  * Core components support flow control, error handling, logging, batch, and other programmatic operations on data that flows through your application.


## DataWeave Language
DataWeave is the primary language used for formulating expressions in Mule. Connectors, modules, and components support the use of DataWeave to access, manipulate, and transform data structures and output formats, and to extract data that is processed within the Mule application.
At runtime, Mule evaluates DataWeave expressions while executing a flow to:
  * Extract data needed to process the current message
  * Set or manipulate a value in the message


## Mule Flows
Understanding basic flow architecture is key to understanding a Mule application. Essentially, every Mule flow contains a series of Mule components that receive or process messages:
Figure 1. Simple Flow in Design Center
At the simplest level, flows are sequences of processors. A message that enters a flow can pass through a variety of processors. In a typical flow, a Mule application receives a message through a source (such as an HTTP Listener component), transforms that message into a new format, and processes any business logic before writing the processed message to an external system in a format that the system can read.
To separate processing into more manageable units, Mule applications often contain multiple, interrelated flows instead of just a single flow. One flow can call another flow as a direct reference.
For more information about this topic, see Flows and Subflows and Mule Components.
### Sources
A source component (or trigger) is the first component in a flow. It receives a triggering event, creates a corresponding Mule event, and forwards that event for processing by the next component in the flow.
External clients can trigger processing in a Mule flow through several communication protocols and methods, such as JMS, HTTP, FTP, JDBC, or File. Mule translates these communication protocols and methods into a standard message format, which passes through the flow’s processors.
Sources in Mule can connect to specific external sources, either through a standard protocol or a third-party API. It is also possible to set a Scheduler component. Some schedulers can poll for specific changes to external resources, such as new files or table rows in an external resource. Examples of listeners and connector operations that can trigger a flow include:
  * HTTP, JMS, and VM listeners in their associated connectors
  * On Table Row operation in the Database connector
  * On New or Updated File operation in the File and FTP connectors
  * Scheduler


Note that you can also create your own source using the Mule SDK.
### Processors
After a flow is triggered through the source component, subsequent components process the data as it travels through the flow. By default, each processor that receives a Mule event returns a new Mule message, typically with a set of attributes and the message payload that the processor returns. The processor forwards the new message as output to the next processor in the flow.
Processors available to Mule applications include:
  * Components from modules and connectors
Examples include operations that read from and write to an external resource and that validate data in the Mule application. Some operations can make client requests to external resources and services (including external databases and systems, such as Salesforce, Workday, ServiceNow, and many others) and to other Mule applications. Others can run your custom code, support OAuth configurations, and manage communication through asynchronous queues, for example. Many other operations are available.
  * Core components
Core components can route data, perform data transformations, handle errors that might occur when processing the event, and perform other tasks in a Mule application.
Transformers (such as the Transform Message, Set Variable, and others) are key to exchanging data between nodes. Transformers enable Mule to convert message data in the Mule event to a format that another application or service can read.
Mule also enables content enrichment of messages (through Target Variables) so that you can retrieve additional data and attach it to the message.


Note that you can also create your own event processors using the Mule SDK.
## Security
You can provide security to your Mule applications by encrypting properties, configuring secure communications over TLS, setting up authentication over OAuth 2.0, and providing cryptographic and other capabilities, such as FIPS compliance.
## Development Environments
You can develop a Mule application using Anypoint Studio (an Eclipse-based IDE), or, if you are an advanced developer, in your own IDE.
For example, in Studio, you build and design a Mule application in a project that contains one or more XML-based files. A Mule project supports all the dependencies required for development. The Package Explorer view in Studio provides access to the project folders and files that make up a Mule project. Studio provides a design-time environment in which you can also build, run, and test your Mule application.
## Mule Versioning
The Mule version you use determines what your Mule application, domain, or policy can do and what features and products are compatible with Mule. For example, Core components, which process the Mule event as it travels through flows in a Mule application, are part of a Core module that is bundled with and shares the same version as Mule. Modules, connectors, the DataWeave language, and several MuleSoft products have their own versioning system but are compatible with specific versions of Mule. For example, DataWeave 2.0 and Studio 7.x are compatible Mule 4.x runtime engines, while DataWeave 1.0 and Studio 6.x are compatible with Mule 3.x runtime engines. You need to make sure the connector or module you use in a Mule application is compatible with your Mule version.
## See Also
  *   * Variables in Mule Applications
  *