[GitHub] [incubator-iotdb] Genius-pig edited a comment on issue #458: [IOTDB-165][Documents][TsFile] Fix a example and update tsfile documents.

[GitBox]

Genius-pig edited a comment on issue #458: [IOTDB-165][Documents][TsFile] Fix a 
example and update tsfile documents.
URL: https://github.com/apache/incubator-iotdb/pull/458#issuecomment-544153656
 
 
   ## why I delete `position()` method?
   1. This was requested change came from @jt2594838. @qiaojialin and I talked 
personally, then we decided to delete it.
   2. This was literally because every time when you new a 
`TsFileSequenceReader` object and its constructor would call 
`loadMetadataSize()` method, `loadMetadataSize()` would call `position()` 
method. In local mode, it actually called `FileChannel.position()` method. As 
we know, it was a method would move the position pointer. So questions were 
**Why to move the position pointer? It is really necessary to move the position 
pointer when I new a `TsFileSequenceReader` object?**
   ## why to move the position pointer?
   Let's firstly check out TsFile structure. 
   <img width="694" alt="Screen Shot 2019-10-19 at 10 10 39 PM" 
src="https://user-images.githubusercontent.com/26118649/67146402-72c89700-f2bd-11e9-8f09-112a8456799b.png";>
   You can find `MetaMarker.CHUNK_HEADER=0x1` after Magic String. There is a 
method called `readMarker()` to read `MetaMarker.CHUNK_HEADER=0x1`.
   ```
     /**
      * read one byte from the input. <br> this method is not thread safe
      */
     public byte readMarker() throws IOException {
       markerBuffer.clear();
       if (ReadWriteIOUtils.readAsPossible(tsFileInput, markerBuffer) == 0) {
         throw new IOException("reach the end of the file.");
       }
       markerBuffer.flip();
       return markerBuffer.get();
     }
   ```
   There is no `position()` method in `readMarker()`. So it used to use 
`position()` in `loadMetadataSize()` to move the position pointer. But there 
are many tests and examples use `readMarker()`. After you remove `position()` 
in `loadMetadataSize()`, you have to use `position()` before `readMarker()`. 
@qiaojialin suggested me do that and I just done it. 
   
   ## It is really necessary to move the position pointer when I new a 
`TsFileSequenceReader` object?
   No. Move the position pointer when you really need it.
   
   ## What other jobs does this pr have?
   I want to update some TsFile documents in this pr.

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