Re: [libmicrohttpd] chunked data mixed across multiple GETs

[Evgeny Grin]

Hi Conor,

If I've got you right, the question is not about large responses, but 
about simultaneous handling of several responses at the same time.

It is implemented very easily. When your application processes request, 
it should decide how you construct your response: for example, make 
request A for database B or read picture X and convert it to to the 
format Z.
Then you need to put all parameters (the name of database and the 
request or picture location and final format) to the callback parameter 
variable. See chunked_example.c, where 'callback_param' is always 
initialized with the same date. In practice it should be initialized 
with data varied depending on the request.

If you are reusing the same response (struct MHD_Response object) for 
several requests, you must ensure that your callback generates response 
data for response position provided as callback parameter. In this case 
it doesn't matter which request is served, the data is determined by 
response object and position.

Alternatively, if you are using unique responses for each request (as it 
was done in the example), you can put (update) number of last line sent 
to the client into your callback parameter and start your next chunk 
from the next line.

--
Wishes,
Evgeny


On 17.06.2021 18:48, Conor Lennon via libmicrohttpd wrote:
> I have a application that uses libmicrohttpd
>
> Data is stored in an sqlite database and we support GET, POST, PATCH and DELETE 
> HTTP methods to manage the data.
>
> As part of the GET, we call
>
> MHD_create_response_from_callback
>
> with a callback function that reads the next value from the database and sends 
> it back as chunked data.
>
> The problem that we are seeing is that two consecutive GETs can interfere with 
> each other when the first GET has a lot of data to return.
>
> What appears to be happening is that data matching the filter of the second GET 
> is being included in the response for the first GET.
>
> Are there any suggestions on how we can proceed with investigating this?
>
> I see that src/examples/chunked_example.c uses 
> MHD_create_response_from_callback, but I'm not sure how to use that example to 
> test out large data.
>
> Regards,
>
> Conor

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