The Coyote Loader is a toolkit and a framework for loading components in a JRE and managing their life cycle. It supports multi-threading, but in a very simplistic manner to keep maintainability as high as possible.
This is an evolving project which provides a starting point for creating a loader in a variety of other projects. That is to say, this loader project will be used as a starting point for the loader of other projects and will, therefore, have rather unique adaptability requirements. For example, this loader framework must operate on embedded devices and there are currently several embedded Java projects underway to which this project will contribute.
By separating the Loader out into a separate project it is easier to focus on just the design and test of component loading without the distraction of the system as a whole. The hope is that a very flexible component loader can be developed and applied to several projects currently being developed. This framework will be tested and developed separately and merged into other projects when they are ready for implementing a command line loader.
Other container projects are far too complex for our needs as they try to be everything for everyone. This is a purposed built toolkit for a specific set of needs.
12 Factor Applications
The loader solves several of our problems for our scalable 12-factor applications. Everything is self-contained in our applications and reliance on an external container is eliminated. This means the loader can be used to stand up a complete running instance without external containers or other frameworks. Our Heroku slug sizes are significantly smaller than those with Jetty, Spring, Tomcat or other frameworks included. Because this was built to support deployment on single board computers (SBC) and embedded systems (e.g. field sensors), our cloud deployment footprints benefitted.
The encryption is completely pluggable, allowing any library to be used through a simple interface. The encryption algorithm and keys can be specified in environment variables, another tenant of a 12-factor application.
Environment variables are leveraged in the configuration and templating tools further reducing the reliance on file systems and assisting the developer ensure each environment is (development, test, quality assurance, certification, production, etc.) are configured correctly and project artifacts such as configuration files do not "point" to the wrong locations or backing services. Loggers tie into backing log streams allowing further independence from the ephemeral file systems used in many cloud infrastructures. While you can use a local file system by default, it is rather easy to send log events externally by simply using a different appender, configured (of course) through environment variables.
The Loader uses a set of JRE shutdown hooks to help ensure graceful shutdown when the SIGTERM event is caught. The Loader then calls the shutdown and terminate methods on all the components giving them a chance to gracefully terminate and will help the application handle life in the cloud as applications are terminated and moved to support scaling operations.
Coyote Loader allows us to create 12-factor applications which maximize automation, offer maximum portability, can be deployed on modern cloud platforms, allow for continuous deployment, and scale quickly.
This is a prototyping project which will be used to drive a loader for a set of IoT (Internet of Things) projects. It, therefore, must support traditional platforms (e.g. server installations) and the restricted resources of embedded systems. It, therefore, must not rely on classes or libraries which may not be available in JRE images with limited libraries.
- Small Footprint - Forego larger, general purpose libraries for simple, purpose-driven code. Resources spent on storing unused code are resources taken away from application data.
- Portability - Usable on as many publicly available embedded systems platforms as possible. If it runs Java, it should be able to run this loader.
- Simplicity over Elegance - Maintainability of the code is key to stable systems, this project uses simple concepts and plainly written code (and comments) so bugs are easier to spot and fix.
- Security Built-In, not Bolted-On, working in the utility industry has made it clear that security should be first on your requirements list and development plan.
- Micro Services - No need for containers and complex frameworks to expose your components through secure ReST APIs.
- 12-Factor Ready - Tools support all tenents of 12-factor applications while enabling you to use more traditional practices.
- Stay out of the developer's way; no surprises.
What this project is not:
- The best way to do
X - Everyone's needs will be different and this is just what has been found to solve many common problems in this area. YMMV
- Containerization - This is a JRE toolkit, not a self-contained environment to run traditional applications.
- Application Server - While it serves the same role as many App Servers, this is not intended to be the full-blown environments you find on the market today.
- Intended To Lock You In - This is a way to run your components your way, not ours. This project strives to let you load wrappers for POJOs and not specialized components (e.g. Servlets, EJBs).
This code was created sometime during Java 1.4 to support projects which needed to modularize their components and allow each of these components to be developed independently. The idea was to simply place the JAR of the component in the class path and let the loader load the component as directed from the configuration file.
The loader has been used in many different projects in the telecommunications and electric utility industries, allowing for the lifecycle control of components for applications which controlled and managed the provisioning of voice and data infrastructure and the monitoring and management for smart grid resources. It has evolved through many different incarnations until its latest form.
- Configuration File Driven - No coding, just specify a file to direct the operation.
- Component Life Cycle Management - Creation, monitoring and cleanup of components.
- HTTP Server - Lightweight and secure message exchange for component communications.
- Environment Variables - Environment variables override configuration for easier porting between environments.
- JDK 1.8 or later installed
- Ability to run bash (*nix) or batch (Windows) scripts
- Network connection to get the dependencies (there are ways around that)
- Assumes you do not have Gradle installed (if you do, you can replace gradlew with gradle)