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<h1>BookKeeper overview</h1>
<div id="minitoc-area">
<ul class="minitoc">
<li>
<a href="#bk_Overview">BookKeeper overview</a>
<ul class="minitoc">
<li>
<a href="#bk_basicComponents">Basic elements</a>
</li>
<li>
<a href="#bk_moreDetail">In slightly more detail...</a>
</li>
</ul>
</li>
</ul>
</div>
  

  
  
<a name="N10009"></a><a name="bk_Overview"></a>
<h2 class="h3">BookKeeper overview</h2>
<div class="section">
<p>This document explains basic concepts of BookKeeper. We start by discussing
    the basic elements of BookKeeper, and next we discuss how they work together. 
    </p>
<a name="N10012"></a><a name="bk_basicComponents"></a>
<h3 class="h4">Basic elements</h3>
<p> 
	BookKeeper uses four basic elements:
	</p>
<ul>
      
<li>
      
<p> 
		
<strong>Ledger</strong>: A ledger is a sequence of entries, and each entry is a sequence of bytes. Entries are
		written sequentially to a ledger and at most once. Consequently, ledgers have an append-only semantics;
	  </p>
	  
</li>
	
	  
<li>
	  
<p> 
		
<strong>BookKeeper client</strong>: A client runs along with a BookKeeper application, and it enables applications
		to execute operations on ledgers, such as creating a ledger and writing to it; 
	  </p>
	  
</li> 
	
	  
<li>
	  
<p>
		
<strong>Bookie</strong>: A bookie is a BookKeeper storage server. Bookies store the content of ledgers. For any given
		ledger L, we call an <em>ensemble</em> the group of bookies storing the content of L. For performance, we store on
		each bookie of an ensemble only a fragment of a ledger. That is, we stripe when writing entries to a ledger such that
		each entry is written to sub-group of bookies of the ensemble.
	  </p>
	  
</li>

	  
<li>
	  
<p> 	
		
<strong>Metadata storage service</strong>: BookKeeper requires a metadata storage service to store information related 
		to ledgers and available bookies. We currently use ZooKeeper for such a task.     
   	  </p>
   	  
</li>
    
</ul>
<a name="N10046"></a><a name="bk_moreDetail"></a>
<h3 class="h4">In slightly more detail...</h3>
<p> BookKeeper implements highly available logs, and it has been designed with write-ahead logging in mind. Besides high availability
    due to the replicated nature of the service, it provides high throughput due to striping. As we write entries in a subset of bookies of an
    ensemble and rotate writes across available quorums, we are able to increase throughput with the number of servers for both reads and writes. 
    Scalability is a property that is possible to achieve in this case due to the use of quorums. Other replication techniques, such as 
    state-machine replication, do not enable such a property. 
    </p>
<p> An application first creates a ledger before writing to bookies through a local BookKeeper client instance. To 
	create a ledger, an application has to specify which kind of ledger it wants to use: self-verifying or generic. Self-verifying
	includes a digest on every entry, which enables a reduction on the degree of replication. Generic ledgers do not store a digest
	along with entries at the cost of using more bookies.   
	</p>
<p> Upon creating a ledger, a BookKeeper clients writes metadata about the ledger to ZooKeeper. A given client first creates
	a znode named "L" as a child of "/ledger" with the SEQUENCE flag. ZooKeeper consequently assigns a unique sequence number to the 
	node, naming the node "/Lx", where x is the sequence number assigned. We use this sequence number as the identifier of the ledger. 
	This identifier is necessary when opening a ledger. We also store the ensemble composition so that readers know which set of bookies
	of access for a given ledger. 	
	</p>
<p>
	Each ledger currently has a single writer. This writer has to execute a close ledger operation before any other client can read
	from it. If the writer of a ledger does not close a ledger properly because, for example, it has crashed before having the 
	opportunity of closing the ledger, then the next client that tries to open a ledger executes an procedure to recover it. As closing a ledger
	consists essentially of writing the last entry written to a ledger to ZooKeeper, the recovery procedure simply finds the last entry
	written correctly and writes it to ZooKeeper in the form of a close znode as a child of "/Lx", where x is the identifier of the ledger.     
	</p>
<p>
	Note that currently this recovery procedure is executed automatically upon trying to open a ledger and no explicit action is necessary. 
	Although two clients may try to recover a ledger concurrently, only one will succeed, the first one that is able to create the close znode
	for the ledger.
	</p>
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