When any file is shared to and from the web server, it uses some or other protocol, and then these protocols also transmit some common information in header section of that particular file. This information provides some basic and important details about the file such as file type, date of creation/modification etc.
The attached information can be of two types;
Protocol defined
and
user defined
.
Protocol defined
information is attached automatically as defined in specifications of that protocol. This information helps the files to be recognized under that protocol domain.
User defined Metadata
on the other hands is specified by the creator of file. This information helps the receiver to understand the file details without the need to open it. We can understand this metadata by the example of “
Content-type
” which is system defined.
“
Content Type
” has different values as per file type. It shows the type of file which is stored on the server.
|
Sr. No |
Value |
Description |
|
1 |
text/plain |
simple text messages |
|
2 |
text/html |
Html content |
|
3 |
image/jpg |
images |
|
4 |
audio/mp3 |
Audio |
|
5 |
video/mp4 |
Video |
|
6 |
application/mswork |
Application |
Amazon S3 also offers both aforementioned
Meta Data
types.Each
Amazon S3 object
has a set of key-value pairs with which it is associated. There are two kinds of metadata: system metadata, and user metadata.
System metadata is used and is sometimes processed by Amazon S3. It depends on API (REST or SOAP) being used. User Meta data has nothing to do with Amazon’s functionality. It is simply a piece of information that creator of file includes to make it clear as far as understanding is concerned.
Meta data added by user can be of any length but must conform to US-ASCII when using REST and UTF-8 when using SOAP or browser-based uploads through POST.
.
For both REST and SOAP requests to Amazon S3, user metadata size is limited to 2k bytes for the total length of all values and keys.
Metadata Interoperability via REST and SOAP:
When user metadata keys get stored using REST, it must begin with "x-amz-meta-" which distinguish them as custom HTTP headers. When this metadata is retrieved via SOAP, the x-amz-meta- prefix is removed. Similarly, metadata stored via SOAP will have x-amz-meta- added as a prefix when it is retrieved via REST or HTTP, except the Content-Type header.
When metadata is retrieved through the REST API, Amazon S3 combines headers that have the same name (ignoring case) into a comma-delimited list. If some metadata contains unprintable characters, it is not returned. Instead, the "x-amz-missing-meta" header is returned with a value of the number of the unprintable metadata entries.
Few of the
custom header
samples are given below:
1.
Expires
suggested expiration time
Syntax:
"Expires" ":" HTTP-date
Example:
Expires: Thu, 31
st
Dec 2009 16:00:00 GMT
2.
From
author(s) or person(s) who taking responsibility for the message
Syntax:
"From" ":" mailbox
Example:
From: info@chambal.com
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