Hi Amit

On 31 December 2010 00:34, amit paliwal <amit.ambitions_at_gmail.com> wrote:
>
> On Thu, Dec 30, 2010 at 5:25 PM, Daniel Stenberg <daniel_at_haxx.se> wrote:
>>
>> On Thu, 30 Dec 2010, amit paliwal wrote:
>>
>>> Which API supports blocking operation with libcurl. I need to get blocked
>>> until Server sends some data, and once it sends data then only do
>>> curl_easy_perform() to fetch that data.
>>
>> curl_easy_perform() is a blocking operation. It will return when the
>> response has been received. As documented.
>
> Reply: Yes, my initial set of operation is happening properly now, but as
> you might remember we had a discussion on server-sent events, so after
> initial set, I will be receiving SSE from Server and I need to fetch it from
> socket. If I do second curl_easy_perform() immediately after the first, it
> does not give time to Server to send SSE, so in principal I need to wait
> until server sends SSE.
>
> How will I come to know that server has sent data???

After reading all of your recent messages I have come to the
conclusion that you have a fundamental misunderstanding of how HTTP
works and how "server-sent events" must work on top of HTTP.

To be clear: It is not possible for the server to send something to
the client unless the client has sent an HTTP request (GET or POST,
etc.). Also, it is not possible for the client to get something from
the server unless it is in response to a previous HTTP request (GET,
POST, etc.)

Also, it seems you may be unclear on exactly what curl_easy_perform()
does. Whenever you call it, a GET or POST etc. request will be sent
to the server (assuming the URL you set was http://... or
https://...). So you cannot use it to "reply" to a "server-sent
event" from the server, but this is not a problem, because the server
cannot send you an unsolicited "server-sent event" and will not expect
a reply to any "server-sent event" it has sent in reply to a prior
request from the client.

So, basically you are mixing up HTTP and "server-sent events" when it
would be easier to think of them as being at separate layers.

You should not be doing a "second curl_easy_perform() immediately
after the first". What you should do is send an initial request and
wait for the reply. Then the server does NOT reply to your initial
request immediately. It waits until it has a "server-sent event"
message that it wants to send you. When it has one, it sends it and
the client should accept and process it. Once the client has finished
doing whatever it wants with the "server-sent event" message, it
should construct a new request to the server containing whatever it
wants to send "in reply" to the "server-sent event". This should NOT
be done from your write callback, or if it is it should be with a new
curl handle. It seems best to me NOT to do it from the callback.

So when the client has a reply it will set whatever options it wants
on the curl handle and then call curl_easy_perform() again. Once
again, the server must NOT send a reply immediately, but only when it
has another "server-sent event" to send to the client.

If the client wants to send a different request that it needs an
immediate response to (i.e. not a "reply" to a "server-sent event")
then it can do that e.g. on a new curl handle and the server must know
that that request needs an immediate response.

I hope that clarifies things for you.

-- 
Michael Wood <esiotrot_at_gmail.com>
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