Page History

Motivation

Currently (in BookKeeper 4.5) we have several overloaded versions of the methods createLedger/createLedegerAdv/asyncCreateLeader/asyncCreateLeaderAdv. This methods are present because from version to version we added new configuration options for the Ledger.

In order to support new features in the future we need to have a more extensible API.

Proposed Change

We can use the Builder design pattern to define a new unified createLedger and asyncCreateLedger APISo we will introduce a new set of APIs to construct Ledgers, in the future this pattern will be extended to other operations, like deleteLedger/closeLedger/addEntry.

This is an example of the new APIhow the client code will look like:

        BookKeeper bookeeper = ....;
        LedgerConfiguration ledgerConfiguration CompletableFuture<WriteHandler> future = LedgerConfigurationbookkeeper.buildercreateLedger()
    .withEnsembleSize(3)
    .withWriteQuorumSize(2)
    .withAckQuorumSize(1)
    .withDigestType(DigestType.CRC32)                                       
    .withCustomMetadata(metadata)
    .withPassword(password)
    .apply();
WriteHandler lh = future.get();

 CompletableFuture<WriteAdvHandler> future = bookkeeper.createLedger()
    .ensembleSizewithEnsembleSize(3)
    .withWriteQuorumSize(2)
    .withAckQuorumSize(1)
    .withDigestType(DigestType.CRC32)                                       
    .withCustomMetadata(metadata)
    .withPassword(password)
    .withLedgerId(ledgerId)
    .makeAdv()
    .apply();
WriteAdvHandler lh =      .writeQuorumSize(2)
  future.get();
 

In order to achieve this goal we are going to introduce CreateBuilder and CreateBuilderAdv interfaces

interface CreateBuilder {
     CreateBuilder withEnsembleSize(...);
     CreateBuilder withWriteQuorumSize(...);
     ...
     CreateAdvBuilder makeAdv();

     // old style callbacks
     void execute(CreateCallback callback, Object ctx);
     // support java8 completable future
                           .ackQuorumSize(1)CompletableFuture<WriteHandler> apply();
     // sync method
     WriteHandler create() throws BKException, InterruptedException;
 }
interface CreateAdvBuilder {
     // old style callbacks
     void execute(CreateCallback callback, Object ctx);
     // support java8 completable future
     CompletableFuture<WriteAdvHandler> apply();
     // sync method
     WriteAdvHandler create() throws BKException, InterruptedException;

}
interface OpenBuilder {
     OpenBuilder    .digestType(DigestType.CRC32)withRecovery(boolean)
     OpenBuilder withPassword(password) 
     OpenBuilder withDigestType(digestType)
     CompletableFuture<ReadHandler> apply(long ledgerId)
     void open(long ledgerId, OpenCallback cb, Object ctx)
        
																			  .customMetadata(Map<String, byte[]> metadata)
     ReadHandler open(long ledgerId)
}

And we will make CrateLedgerOp implement such interfaces.

We are going to introduce a new BookKeeper interface

interface BookKeeper extends AutoCloseable { 
      CreateBuilder createLedger();
      OpenBuilder   openLedger();
}

We are going to introduce WriteHandler and WriteHandlerAdv interfaces which contains only the API related to writing to a Ledger, following the LedgerHandler/LedgerHandlerAdv semantics.

In a similar way we are going to introduce a ReadHandler which contains only the API related to reading to a Ledger and a OpenBuilder.

Please note that WriteHandler and WriteAdvHandler will extend ReadHandler as writer need to be able to read its own writes.

A common Handler interface will be a base interface for ReadHandler and it will contain basic operations like 'close'

interface Handler extends AutoCloseable {
     void close();
     void asyncClose(CloseCallback cb, Object ctx);
}

interface ReadHandler extends Handler {
     void readEntries();
                            .password(password)long readLastAddConfirmed();
     .....
     
}
interface WriteHandler extends ReadHandler {
     void addEntry(byte[] data)
     void sync(...)
}
interface WriteAdvHandler extends ReadHandler {
     void addEntry(long entryID, byte[] data)
     void sync(...)
}

Migration Plan and Compatibility

No issue about migration and compatibility.

Legacy methods will be retained. No @Deprecated annotation will be added.

Further removal of existing APIs will be evaluated in the future, maybe while starting a new major release, like 5.0.0

Further works

We can create similar API style for addEntry, readEntries and other operations which need an extensible API.

We would leverage CompletableFuture for operations like close, delete, addEntry, readEntries....

Rejected Alternatives

This is an example of an alternative API which has been rejected, as it will not be really extensible in the future.

The rejected idea is about using the builder pattern for creating LedgerConfiguration structures and not directly ledgers

        BookKeeper bookeeper                       .advanced(true|false)= ....;
        LedgerHandleLedgerConfiguration ledgerledgerConfiguration = bookeeperLedgerConfiguration.createLedgerbuilder(ledgerConfiguration);
        bookeeper.createLedgerAsync(ledgerConfiguration, createCallback,  ctx);
        CompletableFuture<LedgerHandle> future = bookeeper.asyncCreateLedger(ledgerConfiguration);

So we will introduce a new LedgerConfiguration class which defines the configuration options of the Ledger and we will support a new simple API which takes that structure.

Existing API will be supported, and maybe it will be deprecated and finally dropped in new major releases (5.0.0)

New or Changed Public Interfaces

We are going to introduce the LedgerConfiguration class which defines the specification for new ledgers.

public final LedgerConfiguration  {    
               private final int ensembleSize = 1;
               private final int writeQuorumSize = 1;
        .ensembleSize(3)
       private final int ackQuorumSize = 1;
               private final DigestType digestType = DigestType.CRC32;
               private final byte[] password = EMPTY_ARRAY;
               private final boolean advanced = false;
      .writeQuorumSize(2)
         private final Map<String, byte[]> customMetadata = null;   (will be an un-modifiable Map)
               /**
               * Create a new LedgerConfiguration with default values

               **/

    .ackQuorumSize(1)
           public static Builder builder() { return new Builder(); }
               /**
               * Create a new LedgerConfiguration using another configuration as starting values

               **/

    .digestType(DigestType.CRC32)          public static Builder cloning(LedgerConfiguration configuration) {......}

              public static final class Builder() {
     
																			  .customMetadata(Map<String, byte[]> metadata)
                ... builder methods...
                         public LedgerConfiguration build() throws BKException;

              } 
}

This new class will be immutable and it will provide only getters.

On BookKeeper class we are going to introduce this new methods

public LedgerHandle createLedger(LedgerConfiguration ledgerConfiguration);

public void asyncCreateLedger(LedgerConfiguration ledgerConfiguration, CreateCallback callback, Object context);

public CompletableFuture<LedgerHandle> asyncCreateLedger(LedgerConfiguration ledgerConfiguration);

Migration Plan and Compatibility

No issue about migration and compatibility.

Legacy methods will be retained. No @Deprecated annotation will be added. Futher removal of existing APIs will be evaluated in the future, maybe while starting a new major release, like 5.0.0

Rejected Alternatives

An alternative is not to explicitly add a LedgerConfiguration class but to add some builder method to the BookKeper class, like

BookKeeper bookkeeper =.....;
CompletableFuture<LedgerHandle> lh = bookkeeper.newLedgerBuilderpassword(password)
                                               .ensembleSize(5)
                               .advanced(true|false);
        LedgerHandle ledger =      bookeeper.createLedger(ledgerConfiguration);

...