Persistent Collections for Java*

OVERVIEW

This is a "pilot" project to develop a library for Java objects stored in persistent memory. Persistent collections are being emphasized because many applications for persistent memory seem to map well to the use of collections. One of this project's goals is to make programming with persistent objects feel natural to a Java developer, for example, by using familiar Java constructs when incorporating persistence elements such as data consistency and object lifetime.

The breadth of persistent types is currently limited and the code is not performance-optimized. We are making the code available because we believe it can be useful in experiments to retrofit existing Java code to use persistent memory and to explore persistent Java programming in general.

This library provides Java classes whose instances can persist (i.e. remain reachable) beyond the life of a Java VM instance. Persistent classes include:

1. Primitive arrays
2. Generic arrays
3. Tuples
4. ArrayList
5. HashMap
6. LinkedList
7. LinkedQueue
8. SkipListMap
9. FPTree
10. SIHashMap
11. ObjectDirectory
12. Boxed primitives
13. String
14. AtomicReference
15. ByteBuffer

This Java library uses the libpmemobj library from the Persistent Memory Development Kit (PMDK). For more information on PMDK, please visit http://pmem.io and https://github.com/pmem/pmdk.

For a brief introduction on use of the library, please see Introduction.txt.

HOW TO BUILD & RUN

PREREQUISITES TO BUILD

The following are the prerequisites for building this Java library:

1. Linux operating system (tested on CentOS 7.2 and Ubuntu 16.04)
2. Persistent Memory Development Kit (PMDK)
3. Java 8 or above
4. Build tools - g++ compiler and make

PREREQUISITES TO RUN

This library assumes the availability of hardware persistent memory or emulated persistent memory. Properties of this memory such as path and size can be specified in the config.properties file at the top level directory. Further information can be found inside the file.

Instructions for creating emulated persistent memory are shown below.

EMULATING PERSISTENT MEMORY

The preferred way is to create an in-memory DAX file system. This requires Linux kernel 4.2 or greater. Please follow the steps at:

http://pmem.io/2016/02/22/pm-emulation.html

Alternatively, for use with older kernels, create a tmpfs partition as follows (as root):

$ mount -t tmpfs -o size=4G tmpfs /mnt/mem  # creates a 4GB tmpfs partition
$ chmod -R a+rw /mnt/mem                    # enables read/write permissions to all users

STEPS TO BUILD AND RUN TESTS

Once all the prerequisites have been satisfied:

$ git clone https://github.com/pmem/pcj
$ cd pcj
$ make && make tests

Available Makefile targets include:

• sources - builds only sources
• examples - builds the sources and examples
• tests - builds and runs tests

USING THIS LIBRARY IN EXISTING JAVA APPLICATIONS

To import this library into an existing Java application, include the project's target/classes directory in your Java classpath and the project's target/cppbuild directory in your java.library.path. For example:

$ javac -cp .:<path>/pcj/target/classes <source>
$ java -cp .:<path>/pcj/target/classes -Djava.library.path=<path>/pcj/target/cppbuild <class>

NOTES ON FUNCTIONALITY

1. PersistentString objects are backed by a byte array and only supports ASCII characters.

CONTRIBUTING

Thanks for your interest! Right now, substantial architectural changes are still happening in the project. This makes it difficult to contribute code and difficult to effectively process pull requests. We expect these changes to settle out around December of this year and we look forward to code contributions once this happens. We will update this README then.

In the meantime, we would love to hear your comments and suggestions via the contacts listed below.

Contacts

For more information on this library, contact Olasoji Denloye ([email protected]) or Steve Dohrmann ([email protected]).

* Other names and brands may be claimed as the property of others.