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matt-welch authored Jul 16, 2021
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# Low Level Persistence Library #
# Low-Level Persistence Library #

## OVERVIEW ##
The Low Level Persistence Library offers access to blocks of persistent memory allocated on
a persistent heap.
The Low-Level Persistence Library (LLPL) is a Java library that provides access to off-heap persistent memory.
LLPL includes several kinds of components that can be allocated and used alone or together in building applications:
* <b>heaps</b>: a pool of memory and an allocator for it
* <b>memory blocks</b>: unstructured bytes that can be laid out for any purpose and linked to build data structures
* <b>pre-built data structures</b>: arrays, linked list, and radix trees
* <b>memory pools</b>: a process-shareable pool of memory

Data stored in the components above can persist beyond the life of a JVM instance, i.e. across application or system restarts.
LLPL provides APIs that help developers ensure consistency of stored data.

Memory allocated using LLPL is not garbage-collected and must be explicitly deallocated using LLPL APIs.

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

## HOW TO BUILD & RUN ##
Expand All @@ -14,12 +22,13 @@ For more information on PMDK, please visit http://pmem.io and https://github.com
The following are the prerequisites for building this Java library:

1. Linux operating system
2. Persistent Memory Development Kit (PMDK)
3. Java 8 or above
2. Persistent Memory Development Kit (PMDK) v1.5 or newer
3. Java 8 or newer
4. Build tools - `g++` compiler, `CMake` and `Maven`

### PREREQUISITES TO RUN ###
This library assumes the availability of hardware persistent memory or emulated persistent memory. Instructions for creating emulated persistent memory are shown below.
This library assumes the availability of hardware persistent memory or emulated persistent memory.
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
Expand All @@ -37,7 +46,6 @@ Once all the prerequisites have been satisfied:
```
$ git clone https://github.com/pmem/llpl.git
$ cd llpl
$ mvn compile
$ mvn test -Dtest.heap.path=<path to persistent memory mount point>
```
Available Maven commands include:
Expand All @@ -58,11 +66,20 @@ LLPL is available from the Maven central repository. Add the following dependenc
<type>jar</type>
</dependency>
```

#### WITH CLASSPATH ####
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:
To use this library in your Java application, build the LLPL jar and include
its location in your Java classpath. For example:
```
$ mvn package
$ javac -cp .:<path>/llpl/target/llpl-<version>.jar <source>
$ java -cp .:<path>/llpl/target/llpl-<version>.jar <class>
```

Alternatively, include LLPL's `target/classes` directory in your Java classpath and the
`target/cppbuild` directory in your `java.library.path`. For example:
```
$ mvn compile
$ javac -cp .:<path>/llpl/target/classes <source>
$ java -cp .:<path>/llpl/target/classes -Djava.library.path=<path>/llpl/target/cppbuild <class>
```
Expand All @@ -73,5 +90,4 @@ Thanks for your interest! Please see the CONTRIBUTING.md document for informatio
We would love to hear your comments and suggestions via https://github.com/pmem/llpl/issues.

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

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