Graph database_neo4j 底层存储结构分析(3)

3.3 通用的Store 类型

3.3.1 id 类型

下面是 neo4j db 中,每种Store都有自己的ID文件(即后缀.id 文件)，它们的格式都是一样的。

[test00]$ls -lh target/neo4j-test00.db/ |grep .id

-rw-r–r–9 04-11 13:28 neostore.id

-rw-r–r–9 04-11 13:28 neostore.labeltokenstore.db.id

-rw-r–r–9 04-11 13:28 neostore.labeltokenstore.db.names.id

-rw-r–r–9 04-11 13:28 neostore.nodestore.db.id

-rw-r–r–9 04-11 13:28 neostore.nodestore.db.labels.id

-rw-r–r–9 04-11 13:28 neostore.propertystore.db.arrays.id

-rw-r–r–9 04-11 13:28 neostore.propertystore.db.id

-rw-r–r–9 04-11 13:28 neostore.propertystore.db.index.id

-rw-r–r–9 04-11 13:28 neostore.propertystore.db.index.keys.id

-rw-r–r–9 04-11 13:28 neostore.propertystore.db.strings.id

-rw-r–r–9 04-11 13:28 neostore.relationshipgroupstore.db.id

-rw-r–r–9 04-11 13:28 neostore.relationshipstore.db.id

-rw-r–r–9 04-11 13:28 neostore.relationshiptypestore.db.id

-rw-r–r–9 04-11 13:28 neostore.relationshiptypestore.db.names.id

-rw-r–r–9 04-11 13:28 neostore.schemastore.db.id

3.3.1.1 ID类型文件的存储格式

neo4j 中后缀为 “.id”的文件格式如上图所示，由文件头（9 Bytes）和 long类型数组 2部分构成：

sticky(1 byte) : if sticky the id generator wasn’t closed properly so it has to berebuilt (go through the node, relationship, property, rel type etc files).
nextFreeId(long) : 保存最大的ID,该值与对应类型的存储数组的数组大小相对应。
reuseId(long):用来保存已经释放且可复用的ID值。通过复用ID ,可以减少资源数组的空洞，提高磁盘利用率。

3.3.1.2 IdGeneratorImpl.java

每一种资源类型的ID 分配 neo4j 中是通过 IdGeneratorImpl 来实现的，其功能是负责ID管理分配和回收复用。对于节点，关系，属性等每一种资源类型，都可以生成一个IdGenerator 实例来负责其ID管理分配和回收复用。

3.3.1.2.1 读取id 文件进行初始化

下面试 IdGeneratorImpl.java 中，读取id 文件进行初始化的过程，IdGeneratorImpl 会从 id 文件中读取grabSize 个可复用的ID (reuseId) 到idsReadFromFile(LinkedList<Long>) 中，在需要申请id 时优先分配 idsReadFromFile中的可复用ID。

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<div>

// initialize the id generator and performs a simple validation

private synchronized void initGenerator()

{

try

{

fileChannel = fs.open( fileName, "rw" );

ByteBuffer buffer = ByteBuffer.allocate( HEADER_SIZE );

readHeader( buffer );

markAsSticky( buffer );

fileChannel.position( HEADER_SIZE );

maxReadPosition = fileChannel.size();

defraggedIdCount = (int) (maxReadPosition - HEADER_SIZE) / 8;

readIdBatch();

}

catch ( IOException e )

{

throw new UnderlyingStorageException(

"Unable to init id generator " + fileName, e );

}

private void readHeader( ByteBuffer buffer ) throws IOException

{

readPosition = fileChannel.read( buffer );

if ( readPosition != HEADER_SIZE )

{

fileChannel.close();

throw new InvalidIdGeneratorException(

"Unable to read header, bytes read: " + readPosition );

}

buffer.flip();

byte storageStatus = buffer.get();

if ( storageStatus != CLEAN_GENERATOR )

{

fileChannel.close();

throw new InvalidIdGeneratorException( "Sticky generator[ " +

fileName + "] delete this id file and build a new one" );

}

this.highId.set( buffer.getLong() );

}

private void readIdBatch()

{

if ( !canReadMoreIdBatches() )

return;

try

{

int howMuchToRead = (int) Math.min( grabSize*8, maxReadPosition-readPosition );

ByteBuffer readBuffer = ByteBuffer.allocate( howMuchToRead );

fileChannel.position( readPosition );

int bytesRead = fileChannel.read( readBuffer );

assert fileChannel.position() <= maxReadPosition;

readPosition += bytesRead;

readBuffer.flip();

assert (bytesRead % 8) == 0;

int idsRead = bytesRead / 8;

defraggedIdCount -= idsRead;

for ( int i = 0; i < idsRead; i++ )

{

long id = readBuffer.getLong();

if ( id != INTEGER_MINUS_ONE )

{

idsReadFromFile.add( id );

}

catch ( IOException e )

{

throw new UnderlyingStorageException(

"Failed reading defragged id batch", e );

}

3.3.1.2.2 释放id(freeId)

用户释放一个 id 后，会先放入 releasedIdList （LinkedList<Long>），当releasedIdList 中回收的 id 个数超过 grabSize 个时，写入到 id 文件的末尾。所以可见，对于一个 IdGeneratorImpl，最多有 2 * grabSize 个 id 缓存(releasedIdList 和 idsReadFromFile)。

<div>

/**

* Frees the <CODE>id</CODE> making it a defragged id that will be

* returned by next id before any new id (that hasn't been used yet) is

* returned.

* <p>

* This method will throw an <CODE>IOException</CODE> if id is negative or

* if id is greater than the highest returned id. However as stated in the

* class documentation above the id isn't validated to see if it really is

* free.

*/

@Override

public synchronized void freeId( long id )

{

if ( id == INTEGER_MINUS_ONE )

{

return;

}

if ( fileChannel == null )

{

throw new IllegalStateException( "Generator closed " + fileName );

}

if ( id < 0 || id >= highId.get() )

{

throw new IllegalArgumentException( "Illegal id[" + id + "]" );

}

releasedIdList.add( id );

defraggedIdCount++;

if ( releasedIdList.size() >= grabSize )

{

writeIdBatch( ByteBuffer.allocate( grabSize*8 ) );

}

3.3.1.2.3 申请id ( nextId)

当用户申请一个 id 时，IdGeneratorImpl 在分配时，有2种分配策略： “正常的分配策略” 和激进分配策略”（aggressiveReuse），可以根据配置进行选择。

n “正常的分配策略”：

a) 首先从idsReadFromFile 中分配; 如果 idsReadFromFile 为空，则先从对应的 id 文件中读取已释放且可复用的 id 到idsReadFromFile.

b) 如果 idsReadFromFile 及 id 文件中没有已释放且可复用的 id了，则分配全新的id,即id = highId.get() 并将highId 加1；

n “激进分配策略”（aggressiveReuse）:

a) 首先从releasedIdList（刚回收的ID List）中分配。

b) releasedIdList分配光了，则从idsReadFromFile 中分配; 如果 idsReadFromFile 为空，则先从对应的 id 文件中读取已释放且可复用的 id 到idsReadFromFile.

c) 如果 idsReadFromFile 及 id 文件中没有已释放且可复用的 id了，则分配全新的id,即id = highId.get() 并将highId 加1；

<div>

/**

* Returns the next "free" id. If a defragged id exist it will be returned

* else the next free id that hasn't been used yet is returned. If no id

* exist the capacity is exceeded (all values <= max are taken) and a

* {@link UnderlyingStorageException} will be thrown.

*/

@Override

public synchronized long nextId()

{

assertStillOpen();

long nextDefragId = nextIdFromDefragList();

if ( nextDefragId != -1 ) return nextDefragId;

long id = highId.get();

if ( id == INTEGER_MINUS_ONE )

{

// Skip the integer -1 (0xFFFFFFFF) because it represents

// special values, f.ex. the end of a relationships/property chain.

id = highId.incrementAndGet();

}

assertIdWithinCapacity( id );

highId.incrementAndGet();

return id;

}