HTTP PUT:
PUT puts a file or resource at a specific URI, and exactly at that URI. If there's already a file or resource at that URI, PUT replaces that file or resource. If there is no file or resource there, PUT creates one. PUT is idempotent, but paradoxically PUT responses are not cacheable.
HTTP 1.1 RFC location for PUT
HTTP POST:
POST sends data to a specific URI and expects the resource at that URI to handle the request. The web server at this point can determine what to do with the data in the context of the specified resource. The POST method is not idempotent, however POST responses are cacheable so long as the server sets the appropriate Cache-Control and Expires headers.
The official HTTP RFC specifies POST to be:
- Annotation of existing resources;
- Posting a message to a bulletin board, newsgroup, mailing list,
or similar group of articles;
- Providing a block of data, such as the result of submitting a
form, to a data-handling process;
- Extending a database through an append operation.
HTTP 1.1 RFC location for POST
Difference between POST and PUT:
The RFC itself explains the core difference:
The fundamental difference between the
POST and PUT requests is reflected in
the different meaning of the
Request-URI. The URI in a POST request
identifies the resource that will
handle the enclosed entity. That
resource might be a data-accepting
process, a gateway to some other
protocol, or a separate entity that
accepts annotations. In contrast, the
URI in a PUT request identifies the
entity enclosed with the request --
the user agent knows what URI is
intended and the server MUST NOT
attempt to apply the request to some
other resource. If the server desires
that the request be applied to a
different URI, it MUST send a 301 (Moved Permanently) response; the user agent MAY then make
its own decision regarding whether or not to redirect the request.
Additionally, and a bit more concisely, RFC 7231 Section 4.3.4 PUT states (emphasis added),
4.3.4. PUT
The PUT method requests that the state of the target resource be
created or replaced with the state defined by the representation
enclosed in the request message payload.
Using the right method, unrelated aside:
One benefit of REST ROA vs SOAP is that when using HTTP REST ROA, it encourages the proper usage of the HTTP verbs/methods. So for example you would only use PUT when you want to create a resource at that exact location. And you would never use GET to create or modify a resource.
URIs identify and URLs locate; however, locators are also identifiers, so every URL is also a URI, but there are URIs which are not URLs.
Examples
This is my name, which is an identifier.
It is like a URI, but cannot be a URL, as it tells you nothing about my location or how to contact me.
In this case it also happens to identify at least 5 other people in the USA alone.
- 4914 West Bay Street, Nassau, Bahamas
This is a locator, which is an identifier for that physical location.
It is like both a URL and URI (since all URLs are URIs), and also identifies me indirectly as "resident of..".
In this case it uniquely identifies me, but that would change if I get a roommate.
I say "like" because these examples do not follow the required syntax.
Popular confusion
From Wikipedia:
In computing, a Uniform Resource Locator (URL) is a subset of the Uniform Resource Identifier (URI) that specifies where an identified resource is available and the mechanism for retrieving it. In popular usage and in many technical documents and verbal discussions it is often incorrectly used as a synonym for URI, ... [emphasis mine]
Because of this common confusion, many products and documentation incorrectly use one term instead of the other, assign their own distinction, or use them synonymously.
URNs
My name, Roger Pate, could be like a URN (Uniform Resource Name), except those are much more regulated and intended to be unique across both space and time.
Because I currently share this name with other people, it's not globally unique and would not be appropriate as a URN. However, even if no other family used this name, I'm named after my paternal grandfather, so it still wouldn't be unique across time. And even if that wasn't the case, the possibility of naming my descendants after me make this unsuitable as a URN.
URNs are different from URLs in this rigid uniqueness constraint, even though they both share the syntax of URIs.
Best Answer
An open TCP socket does not require any communication whatsoever between the two parties (let's call them Alice and Bob) unless actual data is being sent. If Alice has received acknowledgments for all the data she's sent to Bob, there's no way she can distinguish among the following cases:
If Alice hasn't heard from Bob in awhile and wants to distinguish among the above conditions, she can resend her last byte of data, wrapped in a suitable TCP frame to be recognizable as a retransmission, essentially pretending she hasn't heard the acknowledgment. If Bob is unplugged, she'll hear nothing back, even if she repeatedly sends the packet over a period of many seconds. If Bob has rebooted or forgotten the connection, he will immediately respond saying the connection is invalid. If Bob is happy with the connection and simply has nothing to say, he'll respond with an acknowledgment of the retransmission.
The Timeout indicates how long Alice is willing to wait for a response when she sends a packet which demands a reply. The Keepalive time indicates how much time she should allow to lapse before she retransmits her last bit of data and demands an acknowledgment. If Bob goes missing, the sum of the Keepalive and Timeout values will indicate the worst-case time between Alice receiving her last bit of data and her deciding that Bob is dead.