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While I am configuring my nginx, I found two modules: ngx_http_limit_conn_module and ngx_http_limit_req_module one is for limiting connection per defined key, and one for limiting request.

My question is what is the relationship (and difference) between a HTTP connection and a request. It seems that multiple HTTP requests can use one common HTTP connection, what is the principle under this?

Shankar Narayana Damodaran
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FingerLiu
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3 Answers3

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HTTP connections - client and server introduce themselves.

HTTP requests - client ask something from server.

Making a connection with server involves TCP handshaking and it is basically creating a socket connection with the server. To make a HTTP request you should be already established a connection with the server. If you established connection with a server you can make multiple request using the same connection(HTTP/1.0 by default one request per connection, HTTP/1.1 by default it is keep alive). As most of the web pages need multiple resources from the server(ex: 100 photos to load in the screen). It is a low burden to the server if we keep the connection and request those 100 images using the same connection(No need to go through the connection establishment process 100 times). That is why HTTP/1.0 came up with keep alive as default.

Tharsanan
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Basically connections are established to make requests using it. So for instance endpoint for given key may accept 5 connections per hour from given IP address. But it doesn't mean only 5 requests can be made but much more - if the connection is not closed after a request (from HTTP 1.1 it's by default kept alive).

E.g. an endpoint accepts 5 connections and 10 requests from given IP address. If connection is established for every request only 5 requests overall can be made. If connection is kept alive single client may make all the requests. If there are 5 clients, every establishes a connection and keeps it alive there are 2 request approx. that can be made by each client - however one can make all the request if it's fast enough.

Opal
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  • Thanks a lot. As connection can be kept alive. I am wondering when while the established connection destroy? I am guessing is there a setting in nginx or linux like **max_establish_connection** or sth like that, and when established connection count is beyond that some connections are destroyed? Or, there is just a simple setting like **connection_survival_time**? Which is corret? – FingerLiu Dec 03 '15 at 03:15
  • The most recent versions keep alive by connection by default, unless `Connection: Close` header is sent. No idea how it's configured in nginx but guess there's a kind of timeout for that and/or other settings as well. `keepalive_disable` is one of these settings. – Opal Dec 03 '15 at 14:27
  • @FingerLiu, if you found my answer useful, please accept it. – Opal Dec 08 '15 at 08:09
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A request is a functional execution: "Do something for me, and return the result back to me" - which is made by the client over a channel that the server is listening on, the "connection". Think of it as making a phone call to a restaurant. When the restaurant picks up the phone, you have an established "connection" - and now can place multiple requests over the same connection. The restaurant can handle multiple, simultaneous customer calls, if it has multiple phone lines open to receive the calls. This is your "connection pool" - at any point in time, you can only have as many simultaneous open connections (max) as the size of your connection pool. The number of requests however will vary. Some client may make 3 requests, and hang up, while other client may make 10 requests before hanging up.

The size of your connection pool determines concurrency - how many simultaneous clients can you talk to at any point in time? The length of those conversations will be use case specific.

Kingz
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