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1. The demander node prepares the set of IgniteDhtDemandedPartitionsMap#full cache partitions to fetch;
2. The demander node checks compatibility version (for example, 2.8) and starts recording all incoming cache updates to the new special storage – the temporary WAL;
3. The demander node sends the GridDhtPartitionDemandMessage to the supplier node;
4. When the supplier node receives GridDhtPartitionDemandMessage and starts the new checkpoint process;
5. The supplier node creates empty the temporary cache partition file with .tmp postfix in the same cache persistence directory;
6. The supplier node splits the whole cache partition file into virtual chunks of predefined size (multiply to the PageMemory size);
   
   1. If the concurrent checkpoint thread determines the appropriate cache partition file chunk and tries to flush dirty page to the cache partition file
      1. If rebalance chunk already transferred
         1. Flush the dirty page to the file;
      2. If rebalance chunk not transferred
         1. Write this chunk to the temporary cache partition file;
         2. Flush the dirty page to the file;
   2. The node starts sending to the demander node each cache partition file chunk one by one using FileChannel#transferTo
      1. If the current chunk was modified by checkpoint thread – read it from the temporary cache partition file;
      2. If the current chunk is not touched – read it from the original cache partition file;
7. The demander node starts to listen to new pipe incoming connections from the supplier node on TcpCommunicationSpi;
8. The demander node creates the temporary cache partition file with .tmp postfix in the same cache persistence directory;
9. The demander node receives each cache partition file chunk one by one
   1. The node checks CRC for each PageMemory in the downloaded chunk;
   2. The node flushes the downloaded chunk at the appropriate cache partition file position;
10. When the demander node received the whole cache partition file it cut the .tmp postfixreceives the whole cache partition file
    1. The node stops recording temporary WAL cache data entries;
    2. The node starts applying for cache data entries from temporary storage on .tmp partition file;
    3. All concurrent cache puts are applying both on .tmp and original partition files;
    4. When everything from temporary WAL applied on .tmp cache partition file
       1. Stop applying concurrent cache updates on the partition file;
       2. Cut the .tmp postfix on partition file;
       3. Move the original partition file to .tmp;
       4. Resume applying concurrent cache updates;
       5. Schedule the partition file deletion and temporary WAL storage;
11. The supplier node deletes the temporary cache partition file;The demander node starts applying saved WAL record updates from the temporary storage;
12. The demander node owning the new cache partition file;

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When the supplier node receives the cache partition file demand request it must prepare and provide the cache partition file to transfer over network. The Copy-on-Write [3] tehniques assume to be used to guarantee the data consistency during chunk transfer.  

The checkpointing process description on the supplier node – items 4 - 6 of the Process Overview.

Recording to temp-WAL

During the cache partition file transmitting, the demander node must hold all corresponding data entries on the new temporary WAL storage to apply them later. 

The process description on the demander node – items 2, 10 of the Process OverviewWhen the demander node determines the possibility of peer-2-peer file transfer cache rebalance approach it initiates the new temporary cache update storage. During the cache partition file transfer, the demander node stores WAL records of all cache updates to this storage.

Risks and Assumptions

A few notes can be mentioned:

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