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# BookKeeper Getting Started Guide

- Programming with BookKeeper
  - Instantiating BookKeeper.
  - Creating a ledger.
  - Adding entries to a ledger.
  - Closing a ledger.
  - Opening a ledger.
  - Reading from ledger

## Programming with BookKeeper

- Instantiating BookKeeper.

- Creating a ledger.

- Adding entries to a ledger.

- Closing a ledger.

- Opening a ledger.

- Reading from ledger

### Instantiating BookKeeper.

The first step to use BookKeeper is to instantiate a BookKeeper object:

org.apache.bookkeeper.BookKeeper

There are three BookKeeper constructors:

public BookKeeper(String servers) throws KeeperException, IOException

where:

- servers is a comma-separated list of ZooKeeper servers.

public BookKeeper(ZooKeeper zk) throws InterruptedException, KeeperException

where:

- zk is a ZooKeeper object. This constructor is useful when the application also using ZooKeeper and wants to have a single instance of ZooKeeper.

public BookKeeper(ZooKeeper zk, ClientSocketChannelFactory channelFactory) throws InterruptedException, KeeperException

where:

- zk is a ZooKeeper object. This constructor is useful when the application also using ZooKeeper and wants to have a single instance of ZooKeeper.

- channelFactory is a netty channel object (org.jboss.netty.channel.socket).

### Creating a ledger.

Before writing entries to BookKeeper, it is necessary to create a ledger. With the current BookKeeper API, it is possible to create a ledger both synchronously or asynchronously. The following methods belong to org.apache.bookkeeper.client.BookKeeper.

Synchronous call:

public LedgerHandle createLedger(int ensSize, int qSize, DigestType type, byte passwd[]) throws KeeperException, InterruptedException, IOException, BKException

where:

- ensSize is the number of bookies (ensemble size);

- qSize is the write quorum size;

- type is the type of digest used with entries: either MAC or CRC32.

- passwd is a password that authorizes the client to write to the ledger being created.

All further operations on a ledger are invoked through the LedgerHandle object returned.

As a convenience, we provide a createLedger with default parameters (3,2,VERIFIABLE), and the only two input parameters it requires are a digest type and a password.

Asynchronous call:

public void asyncCreateLedger(int ensSize, int qSize, DigestType type, byte passwd[], CreateCallback cb, Object ctx )

The parameters are the same of the synchronous version, with the exception of cb and ctx. CreateCallback is an interface in org.apache.bookkeeper.client.AsyncCallback, and a class implementing it has to implement a method called createComplete that has the following signature:

void createComplete(int rc, LedgerHandle lh, Object ctx);

where:

- rc is a return code (please refer to org.apache.bookeeper.client.BKException for a list);

- lh is a LedgerHandle object to manipulate a ledger;

- ctx is a control object for accountability purposes. It can be essentially any object the application is happy with.

The ctx object passed as a parameter to the call to create a ledger is the one same returned in the callback.

### Adding entries to a ledger.

Once we have a ledger handle lh obtained through a call to create a ledger, we can start writing entries. As with creating ledgers, we can write both synchronously and asynchronously. The following methods belong to org.apache.bookkeeper.client.LedgerHandle.

Synchronous call:

public long addEntry(byte[] data) throws InterruptedException

where:

- data is a byte array;

A call to addEntry returns the status of the operation (please refer to org.apache.bookeeper.client.BKDefs for a list);

Asynchronous call:

public void asyncAddEntry(byte[] data, AddCallback cb, Object ctx)

It also takes a byte array as the sequence of bytes to be stored as an entry. Additionaly, it takes a callback object cb and a control object ctx. The callback object must implement the AddCallback interface in org.apache.bookkeeper.client.AsyncCallback, and a class implementing it has to implement a method called addComplete that has the following signature:

void addComplete(int rc, LedgerHandle lh, long entryId, Object ctx);

where:

- rc is a return code (please refer to org.apache.bookeeper.client.BKDefs for a list);

- lh is a LedgerHandle object to manipulate a ledger;

- entryId is the identifier of entry associated with this request;

- ctx is control object used for accountability purposes. It can be any object the application is happy with.

### Closing a ledger.

Once a client is done writing, it closes the ledger. The following methods belong to org.apache.bookkeeper.client.LedgerHandle.

Synchronous close:

public void close() throws InterruptedException

It takes no input parameters.

Asynchronous close:

public void asyncClose(CloseCallback cb, Object ctx) throws InterruptedException

It takes a callback object cb and a control object ctx. The callback object must implement the CloseCallback interface in org.apache.bookkeeper.client.AsyncCallback, and a class implementing it has to implement a method called closeComplete that has the following signature:

void closeComplete(int rc, LedgerHandle lh, Object ctx)

where:

- rc is a return code (please refer to org.apache.bookeeper.client.BKDefs for a list);

- lh is a LedgerHandle object to manipulate a ledger;

- ctx is control object used for accountability purposes.

### Opening a ledger.

To read from a ledger, a client must open it first. The following methods belong to org.apache.bookkeeper.client.BookKeeper.

Synchronous open:

public LedgerHandle openLedger(long lId, DigestType type, byte passwd[]) throws InterruptedException, BKException

- ledgerId is the ledger identifier;

- type is the type of digest used with entries: either MAC or CRC32.

- passwd is a password to access the ledger (used only in the case of VERIFIABLE ledgers);

Asynchronous open:

public void asyncOpenLedger(long lId, DigestType type, byte passwd[], OpenCallback cb, Object ctx)

It also takes a a ledger identifier and a password. Additionaly, it takes a callback object cb and a control object ctx. The callback object must implement the OpenCallback interface in org.apache.bookkeeper.client.AsyncCallback, and a class implementing it has to implement a method called openComplete that has the following signature:

public void openComplete(int rc, LedgerHandle lh, Object ctx)

where:

- rc is a return code (please refer to org.apache.bookeeper.client.BKDefs for a list);

- lh is a LedgerHandle object to manipulate a ledger;

- ctx is control object used for accountability purposes.

### Reading from ledger

Read calls may request one or more consecutive entries. The following methods belong to org.apache.bookkeeper.client.LedgerHandle.

Synchronous read:

public LedgerSequence readEntries(long firstEntry, long lastEntry) throws InterruptedException, BKException

- firstEntry is the identifier of the first entry in the sequence of entries to read;

- lastEntry is the identifier of the last entry in the sequence of entries to read.

Asynchronous read:

public void asyncReadEntries(long firstEntry, long lastEntry, ReadCallback cb, Object ctx) throws BKException, InterruptedException

It also takes a first and a last entry identifiers. Additionaly, it takes a callback object cb and a control object ctx. The callback object must implement the ReadCallback interface in org.apache.bookkeeper.client.AsyncCallback, and a class implementing it has to implement a method called readComplete that has the following signature:

void readComplete(int rc, LedgerHandle lh, Enumeration<LedgerEntry> seq, Object ctx)

where:

- rc is a return code (please refer to org.apache.bookeeper.client.BKDefs for a list);

- lh is a LedgerHandle object to manipulate a ledger;

- seq is a Enumeration<LedgerEntry> object to containing the list of entries requested;

- ctx is control object used for accountability purposes.