Is there any Java blocking queue that can save data to hard drive when limit is reached

Question

I know that I can use JMS and ActiveMQ, but I really need something very simple and without a lot of overhead. I did some test with ActiveMQ and didn't really liked a performance of persistence queues.

What I'm looking for is basic implementation of any blocking queue with ability to store message on HDD (ideally) if some size limit is reached. Then it should be able to read stored message from HDD and if possible stop writing new to HDD (restore in memory use).

My scenario is very simple - messages (json) are coming from outside world. I do some processing and then send them to another REST service. Problem can occur when target REST service is down or network between us is bad. In this case ready to go events are stored in queue that can potentially fill up all available memory. I don't want/need to write every message to HDD/DB - only those that can't fit into memory.

Thank you!

Answer 1

This code should work for you - its an in memory persistent blocking queue - needs some file tuning but should work

    package test;

     import java.io.BufferedReader;
     import java.io.BufferedWriter;
     import java.io.File;
     import java.io.FileReader;
     import java.io.FileWriter;
     import java.io.IOException;
     import java.util.ArrayList;
     import java.util.Collections;
     import java.util.LinkedList;
     import java.util.List;

     public class BlockingQueue {

    //private static Long maxInMenorySize = 1L;
    private static Long minFlushSize = 3L;

    private static String baseDirectory = "/test/code/cache/";
    private static String fileNameFormat = "Table-";

    private static String  currentWriteFile = "";

    private static List<Object>  currentQueue = new LinkedList<Object>();
    private static List<Object>  lastQueue = new LinkedList<Object>();

    static{
        try {
            load();
        } catch (IOException e) {
            System.out.println("Unable To Load");
            e.printStackTrace();
        }
    }

    private static void load() throws IOException{
        File baseLocation = new File(baseDirectory);
        List<String> fileList = new ArrayList<String>();

        for(File entry : baseLocation.listFiles()){
            if(!entry.isDirectory() && entry.getName().contains(fileNameFormat)){
                fileList.add(entry.getAbsolutePath());
            }
        }

        Collections.sort(fileList);

        if(fileList.size()==0){
            //currentQueue = lastQueue = new ArrayList<Object>();
            currentWriteFile = baseDirectory + "Table-1";
            BufferedWriter writer = new BufferedWriter(new FileWriter(currentWriteFile));
            while (!lastQueue.isEmpty()){
                writer.write(lastQueue.get(0).toString()+ "\n");
                lastQueue.remove(0);
            }
            writer.close();
        }else{
            if(fileList.size()>0){
                    BufferedReader reader = new BufferedReader(new FileReader(fileList.get(0)));
                    String line=null;
                    while ((line=reader.readLine())!=null){
                        currentQueue.add(line);
                    }
                    reader.close();
                    File toDelete = new File(fileList.get(0));
                    toDelete.delete();
            }

            if(fileList.size()>0){
                BufferedReader reader = new BufferedReader(new FileReader(fileList.get(fileList.size()-1)));
                currentWriteFile = fileList.get(fileList.size()-1);
                String line=null;
                while ((line=reader.readLine())!=null){
                    lastQueue.add(line);
                }
                reader.close();
                //lastFileNameIndex=Long.parseLong(fileList.get(fileList.size()).substring(6, 9));
            }
        }

    }

    private void loadFirst() throws IOException{
        File baseLocation = new File(baseDirectory);
        List<String> fileList = new ArrayList<String>();

        for(File entry : baseLocation.listFiles()){
            if(!entry.isDirectory() && entry.getName().contains(fileNameFormat)){
                fileList.add(entry.getAbsolutePath());
            }
        }

        Collections.sort(fileList);

        if(fileList.size()>0){
                BufferedReader reader = new BufferedReader(new FileReader(fileList.get(0)));
                String line=null;
                while ((line=reader.readLine())!=null){
                    currentQueue.add(line);
                }
                reader.close();
                File toDelete = new File(fileList.get(0));
                toDelete.delete();
        }
    }

    public Object pop(){
        if(currentQueue.size()>0)
            return  currentQueue.remove(0);

        if(currentQueue.size()==0){
            try {
                loadFirst();
            } catch (IOException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }

        if(currentQueue.size()>0)
            return  currentQueue.remove(0);
        else
            return null;
    }

    public synchronized Object waitTillPop() throws InterruptedException{
        if(currentQueue.size()==0){
            try {
                loadFirst();
            } catch (IOException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
            if(currentQueue.size()==0)
                wait();
        }
        return currentQueue.remove(0);
    }

    public synchronized void push(Object data) throws IOException{
        lastQueue.add(data);
        this.notifyAll();
        if(lastQueue.size()>=minFlushSize){
            BufferedWriter writer = new BufferedWriter(new FileWriter(currentWriteFile));
            while (!lastQueue.isEmpty()){
                writer.write(lastQueue.get(0).toString() + "\n");
                lastQueue.remove(0);
            }
            writer.close();

            currentWriteFile  = currentWriteFile.substring(0,currentWriteFile.indexOf("-")+1) + 
                    (Integer.parseInt(currentWriteFile.substring(currentWriteFile.indexOf("-")+1,currentWriteFile.length())) + 1);
        }
    }

    public static void main(String[] args) {
        try {
            BlockingQueue bq = new BlockingQueue();

            for(int i =0 ; i<=8 ; i++){
                bq.push(""+i);
            }

            System.out.println(bq.pop());
            System.out.println(bq.pop());
            System.out.println(bq.pop());

            System.out.println(bq.waitTillPop());
            System.out.println(bq.waitTillPop());
            System.out.println(bq.waitTillPop());
            System.out.println(bq.waitTillPop());



        } catch (Exception e) {
            e.printStackTrace();
        }
    }


}

Answer 2

Okay, so having your queue persisted to disk would work if you back your queue with a RandomAccessFile, a MemoryMappedFile or a MappedByteBuffer.. or some other equivalent implementation. In the event of your JVM crashing or terminating prematurely, you can pretty much rely on your operating system to persist uncommitted buffers to disk. The caveat is that if your machine crashes beforehand, you can say goodbye to any updates in your queue, so make sure you understand this. You can sync your disk for a guaranteed persistence albeit with a heavy performance hit. From a more hardcore perspective, another option is to replicate to another machine for redundancy, which warrants a separate answer given it's complexity.