[Doc] File format documents incorrect schema location

[asfimport]

The documentation for the file format makes it look like the schema is at the beginning of the file, since it "contains" the streaming format (https://github.com/apache/arrow/blob/master/format/IPC.md#file-format), but in reality the schema is in the footer (https://github.com/apache/arrow/blob/master/format/File.fbs#L29)

Reporter: Brian Hulette / @TheNeuralBit
Assignee: Wes McKinney / @wesm

Related issues:

• 0.4.0 release (blocks)

_{Note: This issue was originally created as ARROW-998. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
I think it's in both places

[asfimport]

Brian Hulette / @TheNeuralBit:
Ah, yeah you're right. I think I got tripped up because I expected the Schema Message to be at byte 8, but it looks like the C++ lib is placing it at 64 (js/spec/simple.arrow and struct-example.arrow). It looks like the Java lib is placing the schema header immediately after the magic bytes without any padding though (js/spec/dictionary.arrow).

[asfimport]

Wes McKinney / @wesm:
Looking at the implementation https://github.com/apache/arrow/blob/master/cpp/src/arrow/ipc/writer.cc#L757 I don't think we need the magic to be 64-byte aligned. I'll open a JIRA

[asfimport]

Wes McKinney / @wesm:
ARROW-1002

[asfimport]

Brian Hulette / @TheNeuralBit:
IPC.md suggests it should be 8-byte aligned right now, so seems like 64-byte aligning was technically correct.

[asfimport]

Wes McKinney / @wesm:
PR: #694

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 694
#694