distributedlog-tutorials/distributedlog-basic/basic-1.md

Write Records using Core Library

This tutorial shows how to write records using core library.

Open a writer

Create distributedlog URI.

    String dlUriStr = ...;
    URI uri = URI.create(dlUriStr);

Create distributedlog configuration.

    DistributedLogConfiguration conf = new DistributedLogConfiguration();

Enable immediate flush.

    conf.setImmediateFlushEnabled(true);
    conf.setOutputBufferSize(0);
    conf.setPeriodicFlushFrequencyMilliSeconds(0);

Enable immediate locking. So if there is already a writer wring to the stream, opening another writer will fail because previous writer already held a lock.
```
    conf.setLockTimeout(DistributedLogConstants.LOCK_IMMEDIATE);
```

Build the distributedlog namespace.

    DistributedLogNamespace namespace = DistributedLogNamespaceBuilder.newBuilder()
            .conf(conf)
            .uri(uri)
            .regionId(DistributedLogConstants.LOCAL_REGION_ID)
            .clientId("console-writer")
            .build();

Open the writer.

    DistributedLogManager dlm = namespace.openLog("basic-stream-1");
    AsyncLogWriter writer = FutureUtils.result(dlm.openAsyncLogWriter());

Write Records

Construct a log record using System.currentTimeMillis() as the TransactionID.

    byte[] data = ...;
    LogRecord record = new LogRecord(System.currentTimeMillis(), data);

Write the log record.

    Future<DLSN> writeFuture = writer.write(record);

    writeFuture.addEventListener(new FutureEventListener<DLSN>() {
        @Override
        public void onFailure(Throwable cause) {
            // executed when write failed.
        }

        @Override
        public void onSuccess(DLSN value) {
            // executed when write completed.
        }
    });

Close the writer

Close the writer after usage.

    FutureUtils.result(writer.asyncClose());

Usage

Run the example in the following steps:

Start the local bookkeeper cluster. After the bookkeeper cluster is started, you could access it using distributedlog uri distributedlog://127.0.0.1:7000/messaging/distributedlog.
```
    // dlog local ${zk-port}
    ./distributedlog-core/bin/dlog local 7000
```

Create the stream under the distributedlog uri.

    // Create Stream `basic-stream-1`
    // dlog tool create -u ${distributedlog-uri} -r ${stream-prefix} -e ${stream-regex}
    ./distributedlog-core/bin/dlog tool create -u distributedlog://127.0.0.1:7000/messaging/distributedlog -r basic-stream- -e 1

Tailing the stream using TailReader to wait for new records.

    // Tailing Stream `basic-stream-1`
    // runner run org.apache.distributedlog.basic.TailReader ${distributedlog-uri} ${stream}
    ./distributedlog-tutorials/distributedlog-basic/bin/runner run org.apache.distributedlog.basic.TailReader distributedlog://127.0.0.1:7000/messaging/distributedlog basic-stream-1

Run the example to write records to the stream in a console.

    // Write Records into Stream `basic-stream-1`
    // runner run org.apache.distributedlog.basic.ConsoleWriter ${distributedlog-uri} ${stream}
    ./distributedlog-tutorials/distributedlog-basic/bin/runner run org.apache.distributedlog.basic.ConsoleWriter distributedlog://127.0.0.1:7000/messaging/distributedlog basic-stream-1

Example output from ConsoleWriter and TailReader.

    // Output of `ConsoleWriter`
    Opening log stream basic-stream-1
    [dlog] > test!
    [dlog] >


    // Output of `TailReader`
    Opening log stream basic-stream-1
    Log stream basic-stream-1 is empty.
    Wait for records starting from DLSN{logSegmentSequenceNo=1, entryId=0, slotId=0}
    Received record DLSN{logSegmentSequenceNo=1, entryId=0, slotId=0}
    """
    test!
    """

Open another terminal to run ConsoleWriter. It would fail with OwnershipAcquireFailedException as previous ConsoleWriter is still holding lock on writing to stream basic-stream-1.

    Opening log stream basic-stream-1
    Exception in thread "main" org.apache.distributedlog.exceptions.OwnershipAcquireFailedException: LockPath - /messaging/distributedlog/basic-stream-1/<default>/lock: Lock acquisition failed, the current owner is console-writer
        at org.apache.distributedlog.lock.ZKSessionLock$8.apply(ZKSessionLock.java:570)
        at org.apache.distributedlog.lock.ZKSessionLock$8.apply(ZKSessionLock.java:567)
        at com.twitter.util.Future$$anonfun$map$1$$anonfun$apply$8.apply(Future.scala:1041)
        at com.twitter.util.Try$.apply(Try.scala:13)
        at com.twitter.util.Future$.apply(Future.scala:132)
        at com.twitter.util.Future$$anonfun$map$1.apply(Future.scala:1041)
        at com.twitter.util.Future$$anonfun$map$1.apply(Future.scala:1040)
        at com.twitter.util.Promise$Transformer.liftedTree1$1(Promise.scala:112)
        at com.twitter.util.Promise$Transformer.k(Promise.scala:112)
        at com.twitter.util.Promise$Transformer.apply(Promise.scala:122)
        at com.twitter.util.Promise$Transformer.apply(Promise.scala:103)
        at com.twitter.util.Promise$$anon$1.run(Promise.scala:357)
        at com.twitter.concurrent.LocalScheduler$Activation.run(Scheduler.scala:178)
        at com.twitter.concurrent.LocalScheduler$Activation.submit(Scheduler.scala:136)
        at com.twitter.concurrent.LocalScheduler.submit(Scheduler.scala:207)
        at com.twitter.concurrent.Scheduler$.submit(Scheduler.scala:92)
        at com.twitter.util.Promise.runq(Promise.scala:350)
        at com.twitter.util.Promise.updateIfEmpty(Promise.scala:716)
        at com.twitter.util.Promise.update(Promise.scala:694)
        at com.twitter.util.Promise.setValue(Promise.scala:670)
        at org.apache.distributedlog.lock.ZKSessionLock$9.safeRun(ZKSessionLock.java:622)
        at org.apache.bookkeeper.util.SafeRunnable.run(SafeRunnable.java:31)
        at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:471)
        at java.util.concurrent.FutureTask.run(FutureTask.java:262)
        at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$201(ScheduledThreadPoolExecutor.java:178)
        at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:292)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
        at java.lang.Thread.run(Thread.java:745)