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The Apache Ignite can support cache rebalancing as transferring partition files using zero copy algorithm [1] based on an extension of communication SPI and Java NIO API. When the partition file has been transferred to the demander node there are two a few possible approaches can be implemented to preload entries from particular partition file.
The demander node will initialize a preloaded partition file as a new PageStore, make this storage up-to-date by applying previously saved async cache operations and then hot swap it under the checkpoint write lock.
Disadvantages:
Demander node first under checkpoint write lock must swap cache data storage with the temporary one to perform recovery operations under original cache data storage. After partition file has been received from the Supplier node there are to possible cases to make this partition file up-to-date.
Disadvantages:
The demander node will use a preloaded patition file as a new source of cache data entries to load.
Disadvantages:
In the process of balancing data:
The whole process is described in terms of rebalance single cache group and partition files would be rebalanced one-by-one:
In terms of a high-level abstraction, Apache Ignite must support the features described below.
Partition preloader must support cache partition file relocation from one cluster node to another (the zero copy algorithm [1] assume to be used by default). To achieve this, the file transfer machinery must be implemented at Apache Ignite over Communication SPI.
Ignite's Comminication SPI must support to:
After partition is received the historical rebalance must be initiated to load other cache updates.
The swapped temporary storage will log all the cache updates to the temporary WAL storage (per each partition) for further applying them to the corresponding partition file. While the Demander is being receive partition files it must save sequentially all cache entries corresponding to the MOVING partition into a new temporary storage. These entries will be applied later one by one on the newly received cache partition file. All asynchronous operations will be enrolled to the end of temporary storage during storage reads until it becomes fully read. The file-based FIFO approach assumes to be used by this process.
The temporary storage is chosen to be WAL-based. The storage must support to:
Expected problems to be solved
The demander node will use a preloaded patition file as a new source of cache data entries to load.
Disadvantages:
In the process of balancing data:
The whole process is described in terms of rebalancing a single partition file of a cache group. All the other partitions would be rebalanced one-by-one.
In terms of a high-level abstraction, Apache Ignite must support the features described below.
The node partition preloader machinery download cache partition files from cluster nodes which owns desired partitions (the zero copy algorithm [1] assume to be used by default). To achieve this, the file transmission process must be implemented at Apache Ignite over Communication SPI.
IThe Comminication SPI must support to:
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public interface CommunicationListenerEx<T extends Serializable> extends EventListener { /** * @param nodeId Remote node id. * @param initMsg Init channel message. * @param channel Locally created channel endpoint. */ public void onChannelOpened(UUID nodeId, Message initMsg, Channel channel); } |
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The file transmission IO manager must support to:
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/** * */ public interface FileWriter extends CloseableTransmissionHandler { /** * @param fileerr The sourceerr fileof tofail sendhandling atprocess. */ @param offset The positionpublic to start at.void onException(UUID nodeId, Throwable err); /** * @param count The number of bytes to transfer nodeId Remote node id from which request has been received. * @param paramsfileMeta TheFile additional transfer file description keysmeta info. * @param@return plcAbsolute Thepathname policydenoting of handling data on remotea file. */ @throws IgniteCheckedException If fails. public String filePath(UUID nodeId, */TransmissionMeta fileMeta); public void write(/** * <em>Chunk handler</em> Filerepresents file, by itself the way of input data long offset,stream processing. * It accepts within longeach count, chunk a {@link ByteBuffer} with data from input Map<String,for Serializable>further params,processing. * ReadPolicy plc ) throws IgniteCheckedException; } | ||||||||
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/** * */ public interface TransmitSessionHandler { /** * @param nodeId Remote node id from which request has been received. * @param initMeta Initial handler meta info. * @param@return nodeIdInstance Theof remotechunk nodehandler idto connectedprocess from. incoming data * @param sessionId The unique session idby chunks. */ public voidConsumer<ByteBuffer> beginchunkHandler(UUID nodeId, StringTransmissionMeta sessionIdinitMeta); /** * @return<em>File Thehandler</em> instancerepresents ofby readitself handlerthe toway processof incominginput data bystream chunksprocessing. All the data */will public ChunkHandler chunkHandler(); /** * @return The intance of read handler to process incoming data like the {@link FileChannel} manner* be processed under the hood using zero-copy transferring algorithm and only start file processing and * the end of processing will be provided. */ public FileHandler fileHandler(); /** * @param nodeId Remote node id from which request has been received. * The@param endinitMeta ofInitial sessionhandler transmissionmeta processinfo. */ @return Intance of publicread void end(); /** * @param cause The cause of fail handling processhandler to process incoming data like the {@link FileChannel} manner. */ public voidConsumer<File> onException(Throwable causefileHandler(UUID nodeId, TransmissionMeta initMeta); } |
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/** * */ public interface FileHandlerpublic class TransmissionSender implements Closeable { /** * @param namefile TheSource file to namesend transferto fromremote. * @param positionparams The start position pointer of download file in original sourceAdditional transfer file description keys. * @param countplc TotalThe countpolicy of byteshandling readeddata from the original sourceon remote. * @throws @paramIgniteCheckedException paramsIf Thefails. additional transfer file description params.*/ public void send( * @return The absolute pathname string denoting theFile file, or {@code null} if there isMap<String, noSerializable> sense.params, * @throws IOException IfTransmissionPolicy fails.plc */ public String begin(String name, long position, long count, Map<String, Serializable> params) throws IOException;) throws IgniteCheckedException, InterruptedException, IOException { send(file, 0, file.length(), params, plc); } /** * @param file TheSource file withto fullysend downloadedto data intoremote. * @param paramsplc The additional transfer file description params policy of handling data on remote. * @throws IgniteCheckedException If fails. */ public void end(send( File file, Map<String, Serializable> params); } | ||||||||
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/** * */ public interface ChunkHandler { /** TransmissionPolicy plc ) throws IgniteCheckedException, InterruptedException, IOException { * @param name The send(file name on remote., 0, file.length(), new HashMap<>(), plc); } /** @param position The start position* pointer@param offile downloadingSource file into send originalto sourceremote. * @param countoffset TotalPosition countto ofstart bytes to read from the original sourcetrasfer at. * @param cnt Number of bytes to transfer. * @param params The additionalAdditional transfer file description paramskeys. * @param @returnplc The sizepolicy of ofhandling {@linkdata ByteBuffer} to read the input channel intoon remote. * @throws IOExceptionIgniteCheckedException If fails. */ public intvoid beginsend(String name, long position, long count, Map<String, Serializable> params) throws IOException; File file, /** long *offset, @param buff The data filled buffer. long cnt, * @return {@code true} if the chunkMap<String, of data have been successfully accepted.Serializable> params, * @throws IOException IfTransmissionPolicy fails. */ public boolean chunk(ByteBuffer buffplc ) throws IOException; /** * @param params The additional handling channel description params. */ public void end(Map<String, Serializable> params); } |
When the supplier node receives the cache partition file demand request it will send the file over the CommunicationSpi. The cache partition file can be concurrently updated by checkpoint thread during its transmission. To guarantee the file consistency Сheckpointer must use Copy-on-Write [3] tehnique and save a copy of updated chunk into the temporary file.
While the demander node is in the partition file transmission state it must save sequentially all cache entries corresponding to the MOVING partition into a new temporary storage. These entries will be applied later one by one on the newly received cache partition file. All asynchronous operations will be enrolled to the end of temporary storage during storage reads until it becomes fully read. The file-based FIFO approach assumes to be used by this process.
The temporary storage is chosen to be WAL-based. The storage must support to:
Expected problems to be solved
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IgniteCheckedException, InterruptedException, IOException {
// Impl.
}
}
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When the supplier node receives the cache partition file demand request it will send the file over the CommunicationSpi. The cache partition file can be concurrently updated by checkpoint thread during its transmission. To guarantee the file consistency Сheckpointer must use Copy-on-Write [3] tehnique and save a copy of updated chunk into the temporary file.
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The node is ready to become partition owner when partition data is rebalanced and cache indexes are ready. For the message-based cluster rebalancing approach indexes are rebuilding simultaneously with cache data loading. For the file-based rebalancing approach, the index rebuild procedure must be finished before the partition state is set to the OWNING state.
procedure must be finished before the partition state is set to the OWNING state.
Ignite doesn't provide any recovery guarantees for the partitions with the MOVING state. The cache partitions will be fully loaded when the next rebalance procedure occurs.
The node which is beeing rebalancing left the cluster. For such nodes WAL is always disabled (all partitions have MOVING state due to this node is new for the cluster and has no cache data).
Since WAL is disabled we can guarantee that all operations with loaded partition files are safe to be done (renaming partition files, applying async updates) due to a cache directory will be fully dropped on recoveryApache Ignite doesn't provide any recovery guarantees for the partitions with the MOVING state. The cache partitions will be fully loaded when the next rebalance procedure occurs.
Each topology change event JOIN/LEFT/FAILED may or may not change cache affinity assignments of currently rebalacning caches. If assignments is not changed and the node is still needs partitions being rebalanced we can continue the current rebalance process (see for details IGNITE-7165).
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To provide basic recovery guarantees we must to:
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Recovery from different stages:
The SSL must be disabled to take an advantage of Java NIO zero-copy file transmission using FileChannel#transferTo method. If we need to use SSL the file must be splitted on chunks the same way to send them over the socket channel with ByteBuffer. As the SSL engine generally needs a direct ByteBuffer to do encryption we can't avoid copying buffer payload from the kernel level to the application level.
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