1   Tail reading records from a stream

This tutorial shows how to tail read records from a stream.

1.1   Open a distributedlog manager

1.1.1   Create distributedlog URI

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

1.1.2   Create distributedlog configuration

DistributedLogConfiguration conf = new DistributedLogConfiguration();

1.1.3   Build the distributedlog namespace

DistributedLogNamespace namespace = DistributedLogNamespaceBuilder.newBuilder()
        .conf(conf)
        .uri(uri)
        .build();

1.1.4   Open the distributedlog manager

DistributedLogManager dlm = namespace.openLog("basic-stream-9");

1.2   Get Last Record

Get the last record from the record. From the last record, we can use DLSN of last record to start tailing the stream.

LogRecordWithDLSN record = dlm.getLastLogRecord();
DLSN lastDLSN = record.getDlsn();

1.3   Read Records

1.3.1   Open the stream

Open the stream to start read the records.

AsyncLogReader reader = FutureUtils.result(dlm.openAsyncLogReader(lastDLSN));

1.3.2   Read the records

Read the next available record from the stream. The future is satisified when the record is available.

Future<LogRecordWithDLSN> readFuture = reader.readNext();

1.3.3   Register the read callback

Register a future listener on read completion. The reader will be notified once the record is ready for consuming.

final FutureEventListener<LogRecordWithDLSN> readListener = new FutureEventListener<LogRecordWithDLSN>() {
    @Override
    public void onFailure(Throwable cause) {
        // executed when read failed.
    }

    @Override
    public void onSuccess(LogRecordWithDLSN record) {
        // process the record
        ...
        // issue read next
        reader.readNext().addEventListener(this);
    }
};
reader.readNext().addEventListener(readListener);

1.4   Close the reader

Close the reader after usage.

FutureUtils.result(reader.asyncClose());

1.5   Run the tutorial

Run the example in the following steps:

1.5.1   Start the local bookkeeper cluster

You can use follow command to start the distributedlog stack locally. After the distributedlog 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

1.5.2   Start the write proxy

Start the write proxy, listening on port 8000.

// DistributedLogServerApp -p ${service-port} --shard-id ${shard-id} -sp ${stats-port} -u {distributedlog-uri} -mx -c ${conf-file}
./distributedlog-service/bin/dlog org.apache.distributedlog.service.DistributedLogServerApp -p 8000 --shard-id 1 -sp 8001 -u distributedlog://127.0.0.1:7000/messaging/distributedlog -mx -c ${distributedlog-repo}/distributedlog-service/conf/distributedlog_proxy.conf

1.5.3   Create the stream

Create the stream under the distributedlog uri.

// Create Stream `basic-stream-9`
// 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 9

1.5.4   Tail the stream

Tailing the stream using TailReader to wait for new records.

// Tailing Stream `basic-stream-9`
// 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-9

1.5.5   Write records

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

// Write Records into Stream `basic-stream-9`
// runner run org.apache.distributedlog.basic.ConsoleProxyWriter ${distributedlog-uri} ${stream}
./distributedlog-tutorials/distributedlog-basic/bin/runner run org.apache.distributedlog.basic.ConsoleProxyWriter 'inet!127.0.0.1:8000' basic-stream-9

1.5.6   Check the results

Example output from ConsoleProxyWriter and TailReader.

// Output of `ConsoleProxyWriter`
May 08, 2016 10:27:41 AM com.twitter.finagle.BaseResolver$$anonfun$resolvers$1 apply
INFO: Resolver[inet] = com.twitter.finagle.InetResolver(com.twitter.finagle.InetResolver@756d7bba)
May 08, 2016 10:27:41 AM com.twitter.finagle.BaseResolver$$anonfun$resolvers$1 apply
INFO: Resolver[fixedinet] = com.twitter.finagle.FixedInetResolver(com.twitter.finagle.FixedInetResolver@1d2e91f5)
May 08, 2016 10:27:41 AM com.twitter.finagle.BaseResolver$$anonfun$resolvers$1 apply
INFO: Resolver[neg] = com.twitter.finagle.NegResolver$(com.twitter.finagle.NegResolver$@5c707aca)
May 08, 2016 10:27:41 AM com.twitter.finagle.BaseResolver$$anonfun$resolvers$1 apply
INFO: Resolver[nil] = com.twitter.finagle.NilResolver$(com.twitter.finagle.NilResolver$@5c8d932f)
May 08, 2016 10:27:41 AM com.twitter.finagle.BaseResolver$$anonfun$resolvers$1 apply
INFO: Resolver[fail] = com.twitter.finagle.FailResolver$(com.twitter.finagle.FailResolver$@52ba2221)
May 08, 2016 10:27:41 AM com.twitter.finagle.Init$$anonfun$1 apply$mcV$sp
[dlog] > test-proxy-writer
[dlog] >


// Output of `TailReader`
Opening log stream basic-stream-9
Log stream basic-stream-9 is empty.
Wait for records starting from DLSN{logSegmentSequenceNo=1, entryId=0, slotId=0}
Received record DLSN{logSegmentSequenceNo=1, entryId=0, slotId=0}
"""
test-proxy-writer
"""