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Chronicle-Core: An Advanced Low-Level Library

Table of Contents

About Chronicle-Core
System properties from file
Chronicle-Core Initialization
Off Heap Memory Access
JVM Access Methods
OS Calls
Deterministic Resource Management
Thread Safety Checks
- Thread safety patterns
Object Pooling
Class Local Caching
Maths Functions
Serializable Lambdas
Histogram
JLBH
Loop Block Monitor tool

About Chronicle-Core

Chronicle-Core is an advanced low-level library that equips developers with powerful tools to interact with the operating system, manage memory, handle resources, and more. However, it should be used with caution due to its low-level operations which, if misused, can lead to complex issues.

Here is a summary of the library’s key features:

Operating System Calls: Chronicle-Core provides access to various system calls such as retrieving the process ID, checking the operating system, and obtaining the hostname, among others.

int processId = OS.getProcessId();
boolean isWindows = OS.isWindows();
String hostname = OS.getHostName();

See the section on OS Calls

JVM Access Methods: To access platform specific features of the JVM.

See the section on JVM Access Methods

Memory Mapped Files: The library offers an interface for managing memory mapped files, which is useful for high-performance I/O operations.

FileChannel fc = new CleaningRandomAccessFile(fileName, "rw").getChannel();
long address = OS.map(fc, MapMode.READ_WRITE, 0, 64 << 10);
OS.memory().writeLong(1024L, 0x1234567890ABCDEFL);
OS.unmap(address, 64 << 10);

Deterministic Resource Management: Chronicle-Core features components that can be closed or reference-counted, and released deterministically without waiting for garbage collection.

Closeable Resources: Chronicle-Core provides an interface for managing closeable resources, which are open when created and can’t be used once closed. This helps in preventing resource leaks.

public class AbstractCloseableTest {
    @Test
    public void close() {
        MyCloseable mc = new MyCloseable();
        assertFalse(mc.isClosed());
        assertEquals(0, mc.performClose);
        mc.throwExceptionIfClosed();
        mc.close();
        assertTrue(mc.isClosed());
        assertEquals(1, mc.performClose);
    }
}

Resource Reference Counting: The library enables the use of reference counting for deterministic resource release. A reference-counted resource can add reservations until it’s closed.

public class AbstractReferenceCountedTest {
    @Test
    public void reserve() {
        assertTrue(Jvm.isResourceTracing());
        MyReferenceCounted rc = new MyReferenceCounted();
        assertEquals(1, rc.refCount());
        ReferenceOwner a = ReferenceOwner.temporary("a");
        rc.reserve(a);
        assertEquals(2, rc.refCount());
        //...
    }
}

See the section on Resource Reference Counting

Thread safety checks: The library enables the use of thread safety checks for single-threaded components. See the section on Thread Safety Checks

This library also wraps up low level access to

System properties from a file
Off Heap Memory Access
Object Pools
Class Local Caching
Maths Functions for casting types, rounding double, faster hashing.
Serializable Lambdas
Histogram A high performance wide range histogram.

System properties from file

Chronicle-Core’s Jvm class automatically loads system properties from a system.properties file if found in the current directory or parent directory. This feature aids in streamlining your command line. You can specify a different properties file with the -Dsystem.properties=my.properties command.

static {
    Jvm.init();
}

The choice of file to load can be overridden on the command line with -Dsystem.properties=my.properties

In Jvm.java it can be seen how to guarantee that JVM class is initialized before the system property is read. For example with Jvm.getInteger or Jvm.getLong.

A number of relevant system properties are listed in systemProperties.adoc.

Note	Command line-specified system properties override those in the `system.properties` file.

Chronicle-Core Initialization

Chronicle-Core offers an initialization class, ChronicleInit, that enables developers to run their own code at startup. This code can be executed before and/or after the execution of Chronicle’s static initializers, which perform tasks such as system property loading.

ChronicleInit allows the developer to hook in their own code to be run at startup before and/or after the Chronicle static initialisers are run. Chronicle static initialisers perform tasks such as loading system properties, so it is possible, for example, to override system properties using ChronicleInit. To this end, ChronicleInit introduces the following system properties:

"chronicle.init.runnable"

This system property specifies a fully qualified class name that will be run before any system property is read by Chronicle code, allowing the class to set them to the desired values. The class should contain an empty static init() method that is called to trigger class load.
"chronicle.postinit.runnable"

This system property specifies a fully qualified class name that will run only once after the Jvm initialisation static class. The class should contain an empty static postInit() method that is called to trigger class load.

The alternative way to using the above system properties is to implement the ChronicleInitRunnable interface whose implementing classes may be listed in the META-INF/services/net.openhft.chronicle.core.ChronicleInitRunnable file in any JAR in classpath to be discovered via ServiceLoader JVM facility. It can provide both init and post-init functionalities by implementing the ChronicleInitRunnableRunnable.run() and ChronicleInitRunnable.postInit() methods.

Off Heap Memory Access

This allows you to access native memory using primitives and some thread safe operations.

Memory memory = OS.memory();
long address = memory.allocate(1024);
try {
    memory.writeInt(address, 1);
    assert memory.readInt(address) == 1;
    final boolean swapped = memory.compareAndSwapInt(address, 1, 2);
    assert swapped;
    assert memory.readInt(address) == 2;
} finally {
    memory.freeMemory(address, 1024);
}

JVM Access Methods

Check the JVM is running in debug mode

if (Jvm.isDebug()) {
   // running in debug.

Rethrow a checked exception as an unchecked one.

try {
    // IO operation
} catch (IOException ioe) {
    throw Jvm.rethrow(ioe);
}

Get a Field for a Class by name

Field theUnsafe = Jvm.getField(Unsafe.class, "theUnsafe");
Unsafe unsafe = (Unsafe) theUnsafe.get(null);

OS Calls

Access to system calls

int processId = OS.getProcessId();
int maxProcessId = OS.getMaxProcessId();
int pageSize = OS.getPageSize();
boolean isWindows = OS.isWindows();
boolean is64bit = OS.is64Bit();
String hostname = OS.getHostName();
String username = OS.getUserName();
String targetDir = OS.getTarget(); // location the target directory during builds

Memory mapped files

FileChannel fc = new CleaningRandomAccessFile(fileName, "rw").getChannel();
// map in 64 KiB
long address = OS.map(fc, MapMode.READ_WRITE, 0, 64 << 10);
// use address
OS.memory().writeLong(1024L, 0x1234567890ABCDEFL);
// unmap memory region
OS.unmap(address, 64 << 10);

Deterministic Resource Management

Component which are closeable or reference counted can be released deterministically without waiting for a GC.

Closeable Resources

A Closeable resources has a simple lifecycle. It is open when created, and cannot be used once closed.

public class AbstractCloseableTest {

    @Test
    public void close() {
        MyCloseable mc = new MyCloseable();
        assertFalse(mc.isClosed());
        assertEquals(0, mc.performClose);

        mc.throwExceptionIfClosed();

        mc.close();
        assertTrue(mc.isClosed());
        assertEquals(1, mc.performClose);

        mc.close();
        assertTrue(mc.isClosed());
        assertEquals(1, mc.performClose);
    }

    @Test(expected = IllegalStateException.class)
    public void throwExceptionIfClosed() {
        MyCloseable mc = new MyCloseable();
        mc.close();
        mc.throwExceptionIfClosed();

 }

    @Test
    public void warnAndCloseIfNotClosed() {
        Map<ExceptionKey, Integer> map = Jvm.recordExceptions();
        MyCloseable mc = new MyCloseable();
        mc.warnAndCloseIfNotClosed();
        Jvm.resetExceptionHandlers();
        assertEquals("Discarded without closing\n" +
                        "java.lang.IllegalStateException: net.openhft.chronicle.core.StackTrace: Created Here",
                map.keySet().stream()
                        .map(e -> e.message + "\n" + e.throwable)
                        .collect(Collectors.joining(", ")));
    }

    static class MyCloseable extends AbstractCloseable {
        int performClose;

        @Override
        protected void performClose() {
            performClose++;
        }
    }
}

Resource Reference Counting

Use reference counting to deterministically release resources.

A reference counted resource can add reservations until closed.

public class AbstractReferenceCountedTest {

    @Test
    public void reserve() {
        assertTrue(Jvm.isResourceTracing());
        MyReferenceCounted rc = new MyReferenceCounted();
        assertEquals(1, rc.refCount());

        ReferenceOwner a = ReferenceOwner.temporary("a");
        rc.reserve(a);
        assertEquals(2, rc.refCount());

        ReferenceOwner b = ReferenceOwner.temporary("b");
        rc.reserve(b);
        assertEquals(3, rc.refCount());

        try {
            rc.reserve(a);
            fail();
        } catch (IllegalStateException ignored) {
        }
        assertEquals(3, rc.refCount());

        rc.release(b);
        assertEquals(2, rc.refCount());

        rc.release(a);
        assertEquals(1, rc.refCount());
        assertEquals(0, rc.performRelease);

        rc.releaseLast();
        assertEquals(0, rc.refCount());
        assertEquals(1, rc.performRelease);
    }

    @Test
    public void reserveWhenClosed() {
        MyReferenceCounted rc = new MyReferenceCounted();
        assertEquals(1, rc.refCount());

        ReferenceOwner a = ReferenceOwner.temporary("a");
        rc.reserve(a);
        assertEquals(2, rc.refCount());

        assertFalse(rc.isClosed());

        rc.closeable.close();

        assertEquals(2, rc.refCount());
        assertTrue(rc.isClosed());

        ReferenceOwner b = ReferenceOwner.temporary("b");
        try {
            rc.reserve(b);
            fail();
        } catch (IllegalStateException ignored) {
        }
        assertEquals(2, rc.refCount());

        assertFalse(rc.tryReserve(b));
        assertEquals(2, rc.refCount());

        rc.release(a);
        assertEquals(1, rc.refCount());
        assertEquals(0, rc.performRelease);

        rc.throwExceptionIfReleased();

        rc.releaseLast();
        assertEquals(0, rc.refCount());
        assertEquals(1, rc.performRelease);

        rc.throwExceptionBadResourceOwner();
        try {
            rc.throwExceptionIfClosed();
            fail();
        } catch (IllegalStateException ignored) {
        }
        try {
            rc.throwExceptionIfReleased();
            fail();
        } catch (IllegalStateException ignored) {
        }
    }

    @Test
    public void throwExceptionBadResourceOwner() {
        MyReferenceCounted rc = new MyReferenceCounted();
        MyReferenceCounted rc2 = new MyReferenceCounted();
        rc.reserve(rc2);
        rc.throwExceptionBadResourceOwner();

        rc2.closeable.close();
        try {
            rc.throwExceptionBadResourceOwner();
            fail();
        } catch (IllegalStateException ignored) {
        }
        rc.release(rc2);
        rc.releaseLast();
    }

    @Test
    public void throwExceptionIfClosed() {
        MyReferenceCounted rc = new MyReferenceCounted();
        rc.throwExceptionIfClosed();

        rc.closeable.close();
        try {
            rc.throwExceptionIfClosed();

           fail();
        } catch (IllegalStateException ignored) {

        }
    }

    static class MyReferenceCounted extends AbstractReferenceCounted {
        final AbstractCloseable closeable;
        int performRelease;

        public MyReferenceCounted() {
            this(new AbstractCloseableTest.MyCloseable());
        }

        public MyReferenceCounted(AbstractCloseable abstractCloseable) {
            super(abstractCloseable);
            closeable = abstractCloseable;
        }

        @Override
        protected void performRelease() {
            performRelease++;
        }
    }
}

MappedFile mf = MappedFile.mappedFile(tmp, chunkSize, 0);
MappedBytesStore bs = mf.acquireByteStore(chunkSize + (1 << 10));

assertEquals(2, mf.refCount());
assertEquals(3, bs.refCount());
assertEquals("refCount: 2, 0, 3", mf.referenceCounts());

mf.close();
assertEquals(2, bs.refCount());
assertEquals("refCount: 1, 0, 2", mf.referenceCounts());
bs2.releaseLast();
assertEquals(1, mf.refCount());
assertEquals(1, bs.refCount());
bs.releaseLast();
assertEquals(0, bs.refCount());
assertEquals(0, mf.refCount());
assertEquals("refCount: 0, 0, 0", mf.referenceCounts());

Releasing Resources

Releasing resources can be managed by starting the BACKGROUND_RESOURCE_RELEASER thread or alternatively it can be managed in a user defined thread. To start the BACKGROUND_RESOURCE_RELEASER thread, both system properties background.releaser and background.releaser.thread should be set to true. In this condition, the thread starts as a daemon thread and invokes BackgroundResourceReleaser.runReleaseResources().

If only background.releaser.thread is set to false, resources will still be queued for releasing, but they need to be released explicitly by calling BackgroundResourceReleaser.releasePendingResources().

If background.releaser is set to false regardless of background.releaser.thread, resources are not queued for release and release will be done synchronously (by calling the relevant close() function).

Calling BackgroundResourceReleaser.stop() releases pending resources and then stops the BACKGROUND_RESOURCE_RELEASER thread. To make sure the shutdown hook does not prevent classes from unloading, deregister the shutdown hook by calling PriorityHook.clear().

Table 1. Resource Release Configurations

`background.releaser.thread`	`background.releaser`	Release Behaviour
`true`	`true`	Resources are queued and then released in the `BACKGROUND_RESOURCE_RELEASER` thread.
`false`	`true`	Rresources are queued but should be released in a user thread by calling `BackgroundResourceReleaser.releasePendingResources()`.
X	`false`	Resources are not queued and are released synchronously.

Thread Safety Checks

Classes that are designed for single-threaded use can implement net.openhft.chronicle.core.io.SingleThreadedChecked. There are a number of implementations of this in Chronicle-Core, including net.openhft.chronicle.core.io.AbstractCloseable, and many Chronicle library classes extend this class. When the user calls a method which must be called single-threaded, these methods call a private method to check that Thread.currentThread() is the same as the first thread that used the object, and throws an exception if not. AbstractCloseable also provides very helpful functionality (if resource tracing is on) to keep track of the stack trace of where it was used first by the use-by thread - this provides invaluable when diagnosing unexpected sharing of an object between threads.

Thread safety checking can be turned off with the system property disable.single.threaded.check, once the application is thoroughly tested.

An object can be handed-off between threads by calling singleThreadedCheckReset.

Thread safety patterns

Some patterns which are commonly used in Chronicle’s libraries are below.

Initialise in one thread, use in another

If using an object which is single-threaded, a common pattern is to construct and initialise your object on the main thread before handing it off to an event loop or worker thread. Call singleThreadedCheckReset after initialisation.

// in the Main thread
ChronicleQueue q = ...;
ExcerptTailer tailer = q.createTailer().toEnd(); // toEnd() checks thread safety and thus initialises the used-by thread
tailer.singleThreadedCheckReset();
// now use the tailer in an event loop

Reset in constructor

If writing an object to use net.openhft.chronicle.core.io.SingleThreadedChecked then a common pattern (and a variant of the above) is to call singleThreadedCheckReset at the end of the constructor e.g. from Chronicle Queue:

public ExcerptTailer createTailer(String id) {
    ...
    final StoreTailer storeTailer = new StoreTailer(this, pool, indexUpdater); // initialises state and sets the used-by thread
    ...
    storeTailer.singleThreadedCheckReset();
    return storeTailer;
}

Delegate through SingleThreadedCheck methods

If implementing SingleThreadedCheck then any methods should also delegate to any contained SingleThreadedCheck objects i.e.

@Override
public void singleThreadedCheckReset() {
    // perform my reset
    ...
    // call singleThreadedCheckReset on any single-threaded fields
    this.bytes.singleThreadedCheckReset();
}

Object Pooling

Chronicle-Core provides object pooling for strings and enums, allowing you to convert a CharSequence into a String of a specific Enum type efficiently.

Bytes<?> b = Bytes.from("Hello World");
b.readSkip(6);

StringInterner si = new StringInterner(128);
String s = si.intern(b);
String s2 = si.intern(b);
assertEquals("World", s);
assertSame(s, s2);

Class Local Caching

Add caching of a data structure for each class using a lambda

public static final ClassLocal<EnumInterner> ENUM_INTERNER =
        ClassLocal.withInitial(c -> new EnumInterner<>(c));

E enumValue = ENUM_INTERNER.get(enumClass).intern(stringBuilder);

Maths Functions

Maths functions to support rounds

double a = 0.1;
double b = 0.3;
double c= Maths.round2(b - a); // 0.2 rounded to 2 decimal places

Checking type conversions

int i = Maths.toInt32(longValue);

Serializable Lambdas

There is a number of FunctionalInterfaces you can utilise as method arguments. This allows implicitly making a lambda Serializable.

// in KeyedVisitable
default <R> R applyToKey(K key, @NotNull SerializableFunction<E, R> function) {

// in code

String fullename = map.applyToKey("u:123223", u -> u.getFullName());

Histogram

A high dynamic range histogram with tunable accuracy.

Histogram h = new Histogram(32, 4);
long start = instance.ticks(), prev = start;
for (int i = 0; i <= 1000_000_000; i++) {
    long now = instance.ticks();
    long time = now - prev;
    h.sample(time);
    prev = now;
}
System.out.println(h.toLongMicrosFormat(instance::toMicros));

JLBH

JLBH has moved home and now lives in its own project, see JLBH.

Loop Block Monitor tool

The tool to summarise the thread stack traces is here.

net.openhft.chronicle.core.threads.MonitorProfileAnalyserMain

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