distributedlog-tutorials/distributedlog-basic/basic-5.md

### Tail reading records from a stream

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

#### Open a distributedlog manager

-   Create distributedlog URI.
    ```
        String dlUriStr = ...;
        URI uri = URI.create(dlUriStr);
    ```

-   Create distributedlog configuration.
    ```
        DistributedLogConfiguration conf = new DistributedLogConfiguration();
    ```

-   Build the distributedlog namespace.
    ```
        DistributedLogNamespace namespace = DistributedLogNamespaceBuilder.newBuilder()
                .conf(conf)
                .uri(uri)
                .build(); 
    ```

-   Open the distributedlog manager.
    ```
        DistributedLogManager dlm = namespace.openLog("basic-stream-9");
    ```

#### 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();
    ```

#### Read Records

-   Open the stream to start read the records.
    ```
        AsyncLogReader reader = FutureUtils.result(dlm.openAsyncLogReader(lastDLSN));
    ```

-   Read the next available record from the stream. The future is satisified when the record is available.
    ```
        Future<LogRecordWithDLSN> readFuture = reader.readNext();
    ```

-   Register a future listener on read completion.
    ```
        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);
    ```

#### Close the reader

-   Close the reader after usage.
    ```
        FutureUtils.result(reader.asyncClose());
    ```

#### Usage

Run the example in the following steps:

1.  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
    ```

2.  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-proxy-server/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-proxy-server/conf/distributedlog_proxy.conf
    ```

3.  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
    ```

4.  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
    ```

6.  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
    ```

7.  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
        """
    ```