How to read last N records from Chronicle Queue?

Question

I have the following code to read from Chronicle queue (it's written in Kotlin but that does not matter):

val queue = ChronicleQueueBuilder.single(path).build()
val tailer = queue.createTailer()

tailer.toEnd()

// // This code is wrong
//    val lastIndex = tailer.index()
//
//    val shift = lastIndex - 10
//    if (shift > 0) {
//        tailer.moveToIndex(lastIndex)
//    }

while (true) {
    val text = await(tailer)

    if (prefix == null) {
        println(text)
    } else {
        if (text.startsWith(prefix)) {
            // Would be nice without additional allocation ...
            println(text.substring(prefix.length + 1))
        }
    }
}

How can I modify the commented code to read previous 10 records from the queue and continue on?

Rationale: It is useful in situations where the queue is used for displaying logs. You want to see a few previous logging statements and see new logging statements as they come.

Answer 1

I have written a test for you. Please run the same it should work.

public class ChronicleTest {

private String chroniclePath = "/tmp/chronicle-test";

private int msgCount = 10;

private int i = 0;

    @Test
    public void writeToQ() {
        ChronicleQueue queue = ChronicleQueueBuilder.single(chroniclePath).build();
        ExcerptAppender appender = queue.acquireAppender();
        for (i = 1; i <= msgCount; i++) {
            appender.writeBytes(b -> {
                b.writeInt(i);
            });
        }
        ExcerptTailer tailer = queue.createTailer();
        tailer.toEnd();
        long lastIndex = tailer.index();
        tailer.moveToIndex(lastIndex - 5);

        while (tailer.readBytes(b -> {
            int value = b.readInt();
            System.out.println("Received:" + value);
        }))
            System.out.println("Completed");
    }
}