Graph database_neo4j 底层存储结构分析(4)

3.3.2 DynamicStore 类型

3.3.2.1 AbstractDynamicStore 的存储格式

neo4j 中对于字符串等变长值的保存策略是用一组定长的 block 来保存，block之间用单向链表链接。类 AbstractDynamicStore 实现了该功能，下面是其注释说明。

/**

* An abstract representation of a dynamic store. The difference between a

* normal AbstractStore and a AbstractDynamicStore is

* that the size of a record/entry can be dynamic.

* Instead of a fixed record this class uses blocks to store a record. If a

* record size is greater than the block size the record will use one or more

* blocks to store its data.

* A dynamic store don’t have a IdGenerator because the position of a

* record can’t be calculated just by knowing the id. Instead one should use a

* AbstractStore and store the start block of the record located in the

* dynamic store. Note: This class makes use of an id generator internally for

* managing free and non free blocks.

* Note, the first block of a dynamic store is reserved and contains information

* about the store.

AbstractDynamicStore 类对应的存储文件格式如上图所示, 整个文件是有一个block_size=BLOCK_HEADER_SIZE(8Bytes)+block_content_size的定长数组和一个字符串“StringPropertyStore v0.A.2”或“ArrayPropertyStore v0.A.2”或“SchemaStore v0.A.2”(文件类型描述TYPE_DESCRIPTOR和 neo4j 的 ALL_STORES_VERSION构成)。访问时，可以通过 id 作为数组的下标进行访问。其中，文件的第1个 record 中前4 字节用来保存 block_size。文件的第2个 record开始保存实际的block数据,它由8个字节的block_header和定长的 block_content（可配置）构成. block_header 结构如下:

inUse(1 Byte):第1字节,共分成3部分

[x__ , ] 0: start record, 1: linked record

[ x, ] inUse

[ ,xxxx] high next block bits

第1~4 bit 表示next_block 的高4位
第5 bit表示block 是否在 use;
第8 bit 表示 block 是否是单向链表的第1个 block；0 表示第1个block, 1表示后续 block.

nr_of_bytes(3Bytes)：本 block 中保存的数据的长度。
next_block(4Bytes): next_block 的低 4 个字节，加上 inUse 的第1~4 位，next_block 的实际长度共 36 bit。以数组方式存储的单向链表的指针，指向保存同一条数据的下一个 block 的id.

3.3.2.2 AbstractDynamicStore.java

下面看一下 AbstractDynamicStore.java 中 getRecord() 和readAndVerifyBlockSize() 成员函数，可以帮助理解 DynamicStore 的存储格式。

getRecord( long blockId, PersistenceWindow window, RecordLoad load )

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private DynamicRecord getRecord( long blockId, PersistenceWindow window, RecordLoad load )</pre>

<div>{

DynamicRecord record = new DynamicRecord( blockId );

Buffer buffer = window.getOffsettedBuffer( blockId );

/*

* First 4b

* [x , ][ , ][ , ][ , ] 0: start record, 1: linked record

* [ x, ][ , ][ , ][ , ] inUse

* [ ,xxxx][ , ][ , ][ , ] high next block bits

* [ , ][xxxx,xxxx][xxxx,xxxx][xxxx,xxxx] nr of bytes in the data field in this record

*

*/

long firstInteger = buffer.getUnsignedInt();

boolean isStartRecord = (firstInteger & 0x80000000) == 0;

long maskedInteger = firstInteger & ~0x80000000;

int highNibbleInMaskedInteger = (int) ( ( maskedInteger ) >> 28 );

boolean inUse = highNibbleInMaskedInteger == Record.IN_USE.intValue();

if ( !inUse && load != RecordLoad.FORCE )

{

throw new InvalidRecordException( "DynamicRecord Not in use, blockId[" + blockId + "]" );

}

int dataSize = getBlockSize() - BLOCK_HEADER_SIZE;

int nrOfBytes = (int) ( firstInteger & 0xFFFFFF );

/*

* Pointer to next block 4b (low bits of the pointer)

*/

long nextBlock = buffer.getUnsignedInt();

long nextModifier = ( firstInteger & 0xF000000L ) << 8;

long longNextBlock = longFromIntAndMod( nextBlock, nextModifier );

boolean readData = load != RecordLoad.CHECK;

if ( longNextBlock != Record.NO_NEXT_BLOCK.intValue()

&& nrOfBytes < dataSize || nrOfBytes > dataSize )

{

readData = false;

if ( load != RecordLoad.FORCE )

{

throw new InvalidRecordException( "Next block set[" + nextBlock

+ "] current block illegal size[" + nrOfBytes + "/" + dataSize + "]" );

}

record.setInUse( inUse );

record.setStartRecord( isStartRecord );

record.setLength( nrOfBytes );

record.setNextBlock( longNextBlock );

/*

* Data 'nrOfBytes' bytes

*/

if ( readData )

{

byte byteArrayElement[] = new byte[nrOfBytes];

buffer.get( byteArrayElement );

record.setData( byteArrayElement );

}

return record;

}

readAndVerifyBlockSize()

</pre>

<div>protected void readAndVerifyBlockSize() throws IOException

{

ByteBuffer buffer = ByteBuffer.allocate( 4 );

getFileChannel().position( 0 );

getFileChannel().read( buffer );

buffer.flip();

blockSize = buffer.getInt();

if ( blockSize <= 0 )

{

throw new InvalidRecordException( "Illegal block size: " +

blockSize + " in " + getStorageFileName() );

}

3.3.2.3 类DynamicArrayStore, DynamicStringStore

类SchemaStore，DynamicArrayStore(ArrayPropertyStore), DynamicStringStore(StringPropertyStore)都是继承成自类AbstractDynamicStore，所以与类DynamicArrayStore, DynamicStringStore和 SchemaStore对应文件的存储格式，都是遵循AbstractDynamicStore的存储格式，除了block块的大小（block_size）不同外。

db 文件	存储类型	block_size
neostore.labeltokenstore.db.names	StringPropertyStore	NAME_STORE_BLOCK_SIZE=30
neostore.propertystore.db.index.keys	StringPropertyStore	NAME_STORE_BLOCK_SIZE=30
neostore.relationshiptypestore.db.names	StringPropertyStore	NAME_STORE_BLOCK_SIZE=30
neostore.propertystore.db.strings	StringPropertyStore	string_block_size=120
neostore.nodestore.db.labels	ArrayPropertyStore	label_block_size=60
neostore.propertystore.db.arrays	ArrayPropertyStore	array_block_size=120
neostore.schemastore.db	SchemaStore	BLOCK_SIZE=56

block_size 通过配置文件或缺省值来设置的，下面的代码片段展示了neostore.propertystore.db.strings 文件的创建过程及block_size 的大小如何传入。

1) GraphDatabaseSettings.java

</pre>

<div>public static final Setting string_block_size = setting("string_block_size", INTEGER, "120",min(1));

public static final Setting array_block_size = setting("array_block_size", INTEGER, "120",min(1));

public static final Setting label_block_size = setting("label_block_size", INTEGER, "60",min(1));</div>

<pre>

2) StoreFactory.java的Configuration 类

</pre>

<div>public static abstract class Configuration

{

public static final Setting string_block_size = GraphDatabaseSettings.string_block_size;

public static final Setting array_block_size = GraphDatabaseSettings.array_block_size;

public static final Setting label_block_size = GraphDatabaseSettings.label_block_size;

public static final Setting dense_node_threshold = GraphDatabaseSettings.dense_node_threshold;

}

3) StoreFactory.java的createPropertyStore 函数

</pre>

<div>public void createPropertyStore( File fileName )

{

createEmptyStore( fileName, buildTypeDescriptorAndVersion( PropertyStore.TYPE_DESCRIPTOR ));

int stringStoreBlockSize = config.get( Configuration.string_block_size );

int arrayStoreBlockSize = config.get( Configuration.array_block_size )

createDynamicStringStore(new File( fileName.getPath() + STRINGS_PART), stringStoreBlockSize, IdType.STRING_BLOCK);

createPropertyKeyTokenStore( new File( fileName.getPath() + INDEX_PART ) );

createDynamicArrayStore( new File( fileName.getPath() + ARRAYS_PART ), arrayStoreBlockSize );

}

4) StoreFactory.java的createDynamicStringStore函数

</pre>

<div>private void createDynamicStringStore( File fileName, int blockSize, IdType idType )

{

createEmptyDynamicStore(fileName, blockSize, DynamicStringStore.VERSION, idType);

}

5) StoreFactory.java的createEmptyDynamicStore 函数

</pre>

<div>

/**

* Creates a new empty store. A factory method returning an implementation

* should make use of this method to initialize an empty store. Block size

* must be greater than zero. Not that the first block will be marked as

* reserved (contains info about the block size). There will be an overhead

* for each block of <CODE>AbstractDynamicStore.BLOCK_HEADER_SIZE</CODE>bytes.

*/

public void createEmptyDynamicStore( File fileName, int baseBlockSize,

String typeAndVersionDescriptor, IdType idType)

{

int blockSize = baseBlockSize;

// sanity checks

…

blockSize += AbstractDynamicStore.BLOCK_HEADER_SIZE;

// write the header

try

{

FileChannel channel = fileSystemAbstraction.create(fileName);

int endHeaderSize = blockSize

+ UTF8.encode( typeAndVersionDescriptor ).length;

ByteBuffer buffer = ByteBuffer.allocate( endHeaderSize );

buffer.putInt( blockSize );

buffer.position( endHeaderSize - typeAndVersionDescriptor.length() );

buffer.put( UTF8.encode( typeAndVersionDescriptor ) ).flip();

channel.write( buffer );

channel.force( false );

channel.close();

}

catch ( IOException e )

{

throw new UnderlyingStorageException( "Unable to create store "

+ fileName, e );

}

idGeneratorFactory.create( fileSystemAbstraction, new File( fileName.getPath() + ".id"), 0 );

// TODO highestIdInUse = 0 works now, but not when slave can create store files.

IdGenerator idGenerator = idGeneratorFactory.open(fileSystemAbstraction,

new File( fileName.getPath() + ".id"),idType.getGrabSize(), idType, 0 );

idGenerator.nextId(); // reserve first for blockSize

idGenerator.close();

}