[DISCUSS] BP-46: Pluggable journal file channel

[Jiabei Zhao <jb...@streamnative.io.INVALID>]

BP-46: Pluggable journal file channel

Motivation:

Currently, the Journal file channel implementation is based on the basic File API which can meet the vast majority of application scenarios. Persistent memory is a new generation technology which allows us to get lower latency and higher throughput and here is a blog https://www.verizonmedia.com/technology/blog/apache-pulsar-overview <https://www.verizonmedia.com/technology/blog/apache-pulsar-overview> comparing the performance for the different devices. If you want to know more about the pmem you can refer to https://software.intel.com/content/www/us/en/develop/articles/introduction-to-programming-with-persistent-memory-from-intel.html <https://software.intel.com/content/www/us/en/develop/articles/introduction-to-programming-with-persistent-memory-from-intel.html> and https://docs.pmem.io/persistent-memory/getting-started-guide/introduction <https://docs.pmem.io/persistent-memory/getting-started-guide/introduction>. 

The pmem providers also provide the SDK for the pmem device which can achieve lower latency than using the File API such as https://pmem.io/pmdk/ <https://pmem.io/pmdk/>. And the pmdk also provide libpmemlog lib which can easily to implement the Journal file. So the proposal is to allow users to use diversified Journal File implementations through plug-ins. 

And here is the in-progress PR https://github.com/apache/bookkeeper/pull/2742
Proposed Change: 
API Changes:

We need to abstract the `FileChannel` in the JournalChannel https://github.com/apache/bookkeeper/blob/d03b046c2526a3b89305da6460b601656ecd0700/bookkeeper-server/src/main/java/org/apache/bookkeeper/bookie/JournalChannel.java#L50 <https://github.com/apache/bookkeeper/blob/d03b046c2526a3b89305da6460b601656ecd0700/bookkeeper-server/src/main/java/org/apache/bookkeeper/bookie/JournalChannel.java#L50>
This proposal we will introduce a new configuration `journalChannelProvider` to choose which journal channel we are used for save journal files. By default it will keep the current way to write journal files.

```
protected static final String JOURNAL_CHANNEL_PROVIDER = "journalChannelProvider";
```

```
/**
 * A FileChannel for the JournalChannel read and write, we can use this interface to extend the FileChannel
 * which we use in the JournalChannel.
 */
interface JournalFileChannel {

    /**
     * An interface for get the FileChannel from the provider.
     * @return
     */
    FileChannel getFileChannel() throws FileNotFoundException;

    /**
     * Check the given file if exists.
     *
     * @param file
     * @return
     */
    boolean fileExists(File file);

    /**
     * Get the file descriptor of the opened file.
     *
     * @return
     * @throws IOException
     */
    FileDescriptor getFD() throws IOException;
}

```

```
/**
 * An interface of the FileChannelProvider.
 */
public interface FileChannelProvider {
    static FileChannelProvider newProvider(String providerClassName) throws IOException {
        try {
            Class<?> providerClass = Class.forName(providerClassName);
            Object obj = providerClass.newInstance();
            return (FileChannelProvider) obj;
        } catch (Exception e) {
            throw new IOException(e);
        }
    }

    /**
     * Get the journalFileChannel with the given file and configuration.
     *
     * @param file
     * @param configuration
     * @return
     * @throws IOException
     */
    JournalFileChannel open(File file, ServerConfiguration configuration) throws IOException;
}

```
Configuration Changes:
Users are expected to define `journalChannelProvider` in `conf/bk_server.conf`.
journalChannelProvider=org.apache.bookkeeper.bookie.DefaultFileChannelProvider

BookieAdmin:

Because we will support different way to read journal, we also need to change the `ReadJournal` command to make it can work with the new journal provider.
https://github.com/apache/bookkeeper/blob/d03b046c2526a3b89305da6460b601656ecd0700/bookkeeper-server/src/main/java/org/apache/bookkeeper/tools/cli/commands/bookie/ReadJournalCommand.java#L127


Migration Plan and Compatibility:

Because we abstract the write process, we can make the current implementation as the default provider. That will not introduce any compatibility issue.