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Persistent primitive arrays: PersistentBooleanArray, PersistentByteArray, PersistentCharArray, PersistentDoubleArray, PersistentFloatArray, PersistentIntArray, PersistentLongArray, PersistentShortArray.The following are the persistent collections supported in the API: There is even the possibility to create our own abstractions through a low-level accessor API (in the form of a MemoryRegion interface), but that is out of the scope of this article. In addition to built-in classes, programmers can define their own persistent classes (as we will see next). Instances of these classes persist (that is, remain reachable) beyond the life of the Java virtual machine (JVM) instance. The reason is that collection classes map well to the use cases observed for many persistent memory applications. The API provided for persistent memory programming in Java emphasizes persistent collections. Although the APIs for Java and Python are still early (experimental) solutions, work is in progress and evolving quickly. Although these libraries are implemented in C, there has been a significant effort to provide APIs for other popular languages, including C++, Java* (which is the topic of this article) and Python*. To avoid that, programmers need to design their data structures in such a way that temporary torn-writes are allowed, and make sure that the proper flushing instructions are issued at exactly the right time (too much flushing is not good either because it impacts performance).įortunately, Intel has developed the Persistent Memory Developer Kit (PMDK), an open source collection of libraries and tools that provide low-level primitives as well as useful high-level abstractions to help persistent memory programmers overcome these obstacles. If that were to happen while part of our writes are still sitting unflushed in the caches, we may end up with corrupted data structures. Unfortunately, these caches are not protected against a sudden loss of power. Even if we avoid block caching on dynamic random-access memory (DRAM), some of the most recent data writes may still reside-unflushed-in the CPU caches. Programming directly against memory-mapped files, however, is not trivial.
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Using a special FS, processes running in user space can (after opening and mapping a file) access this mapped memory directly without involving the FS itself, in turn avoiding expensive block caching/flushing and context switching to/from the operating system (OS). Thus, they are stable, well understood, and widely supported. In addition, memory-mapped files have been around for decades.
Cloud native java cat source code example how to#
This model was chosen primarily to avoid reinventing the wheel, given that the majority of problems they were trying to solve (such as how to collect, name, and find memory regions, or how to provide access control, permissions, and so on) were already solved by file systems (FS). Why Do We Need APIs?Īt the core of the NVM Programming Model (NPM) standard, developed by key players in the industry through the Storage and Networking Industry Association (SNIA), we have memory-mapped files. I finalize the article by showing how Java programs using PCJ can be compiled and run.
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Cloud native java cat source code example full#
A full example, comprised of a persistent array of employees (being an Employee a custom persistent class implemented from scratch) is described in detail (including source code). I show how it is possible to instantiate and store a persistent collection (without serialization), as well as fetch it later after a power cycle.
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This API emphasizes persistent collections, since collection classes map well to the use cases observed for many persistent memory applications. In this article, I present an introduction to the Persistent Collections for Java* (PCJ) API for persistent memory programming. (Programming Language, tool, IDE, Framework)Ĭ++ Compiler, JDK, Persistent Memory Developers Kit (PMDK) libraries and Persistent Collections for Java* (PCJ) Emulated: See How to Emulate Persistent Memory Using Dynamic Random-access Memory (DRAM)