# Backend ND4J works atop so-called backends, or linear-algebra libraries, such as Native `nd4j-native` and `nd4j-cuda-$SOME_CUDA_VERSION` (usually the 2 most recent cuda versions) (GPUs), which you can select by pasting the [right dependency into your project’s POM.xml file](https://deeplearning4j.konduit.ai/en-1.0.0-beta6/config/backends). A Java [ServiceLoader](https://docs.oracle.com/javase/6/docs/api/java/util/ServiceLoader.html), which is baked into the language itself, tells Java that the backend exists. It’s not necessary to concern yourself with how ND4J backends load and perform other basic functions, but you can explore [how ND4J loads and selects backends, according to your OS, here](https://github.com/eclipse/deeplearning4j/blob/master/nd4j/nd4j-backends/nd4j-api-parent/nd4j-api/src/main/java/org/nd4j/linalg/factory/Nd4jBackend.java). The core configurations for each backend are specified in a properties file. A few more points: * Regardless of the backend you choose, you use the same ND4J API for everything, including GPUs and distributed systems. * You can override all properties in the above file from the command line using *mvn -D$your\_parameter\_here*. * **Backend prioritization**: You can include multiple backends on the classpath. If you do, ND4J will run on as many as GPUs as you have available, exhaust them, and then start adding CPUs, allowing you to operate on mixed hardware. * C programmers engaged in numerical or scientific computing may ask (with a touch of disdain ;) why we built a Java API over several backends. This architecture allows us to largely abstract away the hardware, while optimizing for it under the hood. Software engineers writing in Java or Scala can build scalable numerical software once, and then deploy on multiple platforms, knowing that we’ve done the work of lower-level optimization, and that their algorithms will work on servers, desktops and Android phones. Another advantage is that you can build your own backends, test them in isolation, and benefit from a higher-level language. For enabling different backends at runtime, you set the priority with your environment via the environment variable: ``` BACKEND_PRIORITY_CPU=SOME_NUM BACKEND_PRIORITY_GPU=SOME_NUM ``` Relative to the priority, it will allow you to dynamically set the backend type. --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://deeplearning4j.konduit.ai/en-1.0.0-beta6/nd4j/backend.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.