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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
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：
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。
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 validationprivate 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 );}} |
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)。
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<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.*/@Overridepublic 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 ) );}} |
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；
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<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.*/@Overridepublic 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;} |
Graph database_neo4j 底层存储结构分析(3)
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原文地址：http://www.cnblogs.com/gisblogs/p/4545752.html
