apache
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<h1>Streaming with BookKeeper</h1>
<div id="front-matter">
<div id="minitoc-area">
<ul class="minitoc">
<li>
<a href="#bk_StreamSummary">Summary</a>
</li>
<li>
<a href="#bk_LedgerOutputStream">Writing a stream of bytes</a>
</li>
<li>
<a href="#bk_LedgerInputStream">Reading a stream of bytes</a>
</li>
</ul>
</div>
</div>
  

  
	
<a name="bk_StreamSummary"></a>
<h2 class="h3">Summary</h2>
<div class="section">
<p>
    When using the BookKeeper API, an application has to split the data to write into entries, each
    entry being a byte array. This is natural for many applications. For example, when using BookKeeper
    for write-ahead logging, an application typically wants to write the modifications corresponding
    to a command or a transaction. Some other applications, however, might not have a natural boundary
    for entries, and may prefer to write and read streams of bytes. This is exactly the purpose of the
    stream API we have implemented on top of BookKeeper.
    </p>
<p>
    The stream API is implemented in the package <span class="codefrag computeroutput">Streaming</span>, and it contains two main classes: <span class="codefrag computeroutput">LedgerOutputStream</span> and 
    <span class="codefrag computeroutput">LedgerInputStream</span>. The class names are indicative of what they do.
    </p>
</div>
    
    
<a name="bk_LedgerOutputStream"></a>
<h2 class="h3">Writing a stream of bytes</h2>
<div class="section">
<p>
    Class <span class="codefrag computeroutput">LedgerOutputStream</span> implements two constructors and five public methods:
    </p>
<p>
    
<span class="codefrag computeroutput">
	public LedgerOutputStream(LedgerHandle lh) 
    </span>
	
</p>
<p>
    where:
    </p>
<ul>
    	
<li>
    	
<p> 
        
<span class="codefrag computeroutput">lh</span> is a ledger handle for a previously created and open ledger.  
    	</p>
    	
</li>
    
</ul>
<p>
    
<span class="codefrag computeroutput">
	public LedgerOutputStream(LedgerHandle lh, int size) 
    </span>
	
</p>
<p>
    where:
    </p>
<ul>
    	
<li>
    	
<p> 
        
<span class="codefrag computeroutput">lh</span> is a ledger handle for a previously created and open ledger.  
    	</p>
    	
</li>

    	
<li>
    	
<p> 
        
<span class="codefrag computeroutput">size</span> is the size of the byte buffer to store written bytes before flushing.  
    	</p>
    	
</li>
    
</ul>
<p>
   	
<strong>Closing a stream.</strong> This call closes the stream by flushing the write buffer.
   	</p>
<p>
    
<span class="codefrag computeroutput">
	public void close() 
    </span>
	
</p>
<p>
    which has no parameters.
    </p>
<p>
   	
<strong>Flushing a stream.</strong> This call essentially flushes the write buffer.
   	</p>
<p>
    
<span class="codefrag computeroutput">
	public synchronized void flush() 
    </span>
	
</p>
<p>
    which has no parameters.
    </p>
<p>
   	
<strong>Writing bytes.</strong> There are three calls for writing bytes to a stream.
   	</p>
<p>
    
<span class="codefrag computeroutput">
	public synchronized void write(byte[] b) 
    </span>
	
</p>
<p>
    where:
    </p>
<ul>
    	
<li>
    	
<p> 
        
<span class="codefrag computeroutput">b</span> is an array of bytes to write.  
    	</p>
    	
</li>
    
</ul>
<p>
    
<span class="codefrag computeroutput">
	public synchronized void write(byte[] b, int off, int len) 
    </span>
	
</p>
<p>
    where:
    </p>
<ul>
    	
<li>
    	
<p> 
        
<span class="codefrag computeroutput">b</span> is an array of bytes to write.  
    	</p>
    	
</li>
    	
    	
<li>
    	
<p> 
        
<span class="codefrag computeroutput">off</span> is a buffer offset.  
    	</p>
    	
</li>
    	
    	
<li>
    	
<p> 
        
<span class="codefrag computeroutput">len</span> is the length to write.  
    	</p>
    	
</li>
    
</ul>
<p>
    
<span class="codefrag computeroutput">
	public synchronized void write(int b) 
    </span>
	
</p>
<p>
    where:
    </p>
<ul>
    	
<li>
    	
<p> 
        
<span class="codefrag computeroutput">b</span> contains a byte to write. The method writes the least significant byte of the integer four bytes.    
    	</p>
    	
</li>
    
</ul>
</div>
    
    
<a name="bk_LedgerInputStream"></a>
<h2 class="h3">Reading a stream of bytes</h2>
<div class="section">
<p>
    Class <span class="codefrag computeroutput">LedgerOutputStream</span> implements two constructors and four public methods:
    </p>
<p>
    
<span class="codefrag computeroutput">
	public LedgerInputStream(LedgerHandle lh)
	throws BKException, InterruptedException 
    </span>
	
</p>
<p>
    where:
    </p>
<ul>
    	
<li>
    	
<p> 
        
<span class="codefrag computeroutput">lh</span> is a ledger handle for a previously created and open ledger.  
    	</p>
    	
</li>
    
</ul>
<p>
    
<span class="codefrag computeroutput">
	public LedgerInputStream(LedgerHandle lh, int size) 
    throws BKException, InterruptedException
    </span>
	
</p>
<p>
    where:
    </p>
<ul>
    	
<li>
    	
<p> 
        
<span class="codefrag computeroutput">lh</span> is a ledger handle for a previously created and open ledger.  
    	</p>
    	
</li>

    	
<li>
    	
<p> 
        
<span class="codefrag computeroutput">size</span> is the size of the byte buffer to store bytes that the application
        will eventually read.  
    	</p>
    	
</li>
    
</ul>
<p>
   	
<strong>Closing.</strong> There is one call to close an input stream, but the call
   	is currently empty and the application is responsible for closing the ledger handle. 
   	</p>
<p>
    
<span class="codefrag computeroutput">
	public void close()
    </span>
	
</p>
<p>
    which has no parameters.
    </p>
<p>
   	
<strong>Reading.</strong> There are three calls to read from the stream.
   	</p>
<p>
    
<span class="codefrag computeroutput">
	public synchronized int read()
	throws IOException 
    </span>
	
</p>
<p>
    which has no parameters.
    </p>
<p>
    
<span class="codefrag computeroutput">
	public synchronized int read(byte[] b)
	throws IOException 
    </span>
	
</p>
<p>
    where:
    </p>
<ul>
    	
<li>
    	
<p> 
        
<span class="codefrag computeroutput">b</span> is a byte array to write to.  
    	</p>
    	
</li>
    
</ul>
<p>
    
<span class="codefrag computeroutput">
	public synchronized int read(byte[] b, int off, int len)
	throws IOException 
    </span>
	
</p>
<p>
    where:
    </p>
<ul>
    	
<li>
    	
<p> 
        
<span class="codefrag computeroutput">b</span> is a byte array to write to.  
    	</p>
    	
</li>
    	
    	
<li>
    	
<p> 
        
<span class="codefrag computeroutput">off</span> is an offset for byte array <span class="codefrag computeroutput">b</span>.  
    	</p>
    	
</li>
    	
    	
<li>
    	
<p> 
        
<span class="codefrag computeroutput">len</span> is the length in bytes to write to <span class="codefrag computeroutput">b</span>.  
    	</p>
    	
</li>
    
</ul>
</div>
  
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