HAproxy with session sticky and HA

===================================================1. Simple HTTP load-balancing with cookie insertion===================================================A web application often saturates the front-end server with high CPU loads,due to the scripting language involved. It also relies on a back-end databasewhich is not much loaded. User contexts are stored on the server itself, andnot in the database, so that simply adding another server with simple IP/TCPload-balancing would not work.                +-------+                |clients|  clients and/or reverse-proxy                +---+---+                    |                   -+-----+--------+----                          |       _|_db                       +--+--+   (___)                       | web |   (___)                       +-----+   (___)                   192.168.1.1   192.168.1.2Replacing the web server with a bigger SMP system would cost much more thanadding low-cost pizza boxes. The solution is to buy N cheap boxes and installthe application on them. Install haproxy on the old one which will spread theload across the new boxes.  192.168.1.1    192.168.1.11-192.168.1.14   192.168.1.2 -------+-----------+-----+-----+-----+--------+----        |           |     |     |     |       _|_db     +--+--+      +-+-+ +-+-+ +-+-+ +-+-+    (___)     | LB1 |      | A | | B | | C | | D |    (___)     +-----+      +---+ +---+ +---+ +---+    (___)     haproxy        4 cheap web serversConfig on haproxy (LB1) :-------------------------           listen webfarm 192.168.1.1:80       mode http       balance roundrobin       cookie SERVERID insert indirect       option httpchk HEAD /index.html HTTP/1.0       server webA 192.168.1.11:80 cookie A check       server webB 192.168.1.12:80 cookie B check       server webC 192.168.1.13:80 cookie C check       server webD 192.168.1.14:80 cookie D check       Description :------------- - LB1 will receive clients requests. - if a request does not contain a cookie, it will be forwarded to a valid   server - in return, a cookie "SERVERID" will be inserted in the response holding the   server name (eg: "A"). - when the client comes again with the cookie "SERVERID=A", LB1 will know that   it must be forwarded to server A. The cookie will be removed so that the   server does not see it. - if server "webA" dies, the requests will be sent to another valid server   and a cookie will be reassigned.Flows :-------(client)                           (haproxy)                         (server A)  >-- GET /URI1 HTTP/1.0 ------------> |               ( no cookie, haproxy forwards in load-balancing mode. )                                       | >-- GET /URI1 HTTP/1.0 ---------->                                       | <-- HTTP/1.0 200 OK -------------<               ( the proxy now adds the server cookie in return )  <-- HTTP/1.0 200 OK ---------------< |      Set-Cookie: SERVERID=A           |  >-- GET /URI2 HTTP/1.0 ------------> |      Cookie: SERVERID=A               |      ( the proxy sees the cookie. it forwards to server A and deletes it )                                       | >-- GET /URI2 HTTP/1.0 ---------->                                       | <-- HTTP/1.0 200 OK -------------<   ( the proxy does not add the cookie in return because the client knows it )  <-- HTTP/1.0 200 OK ---------------< |  >-- GET /URI3 HTTP/1.0 ------------> |      Cookie: SERVERID=A               |                                    ( ... )Limits :-------- - if clients use keep-alive (HTTP/1.1), only the first response will have   a cookie inserted, and only the first request of each session will be   analyzed. This does not cause trouble in insertion mode because the cookie   is put immediately in the first response, and the session is maintained to   the same server for all subsequent requests in the same session. However,   the cookie will not be removed from the requests forwarded to the servers,   so the server must not be sensitive to unknown cookies. If this causes   trouble, you can disable keep-alive by adding the following option :        option httpclose - if for some reason the clients cannot learn more than one cookie (eg: the   clients are indeed some home-made applications or gateways), and the   application already produces a cookie, you can use the "prefix" mode (see   below). - LB1 becomes a very sensible server. If LB1 dies, nothing works anymore.   => you can back it up using keepalived (see below) - if the application needs to log the original client's IP, use the   "forwardfor" option which will add an "X-Forwarded-For" header with the   original client's IP address. You must also use "httpclose" to ensure   that you will rewrite every requests and not only the first one of each   session :        option httpclose        option forwardfor   The web server will have to be configured to use this header instead.   For example, on apache, you can use LogFormat for this :        LogFormat "%{X-Forwarded-For}i %l %u %t \"%r\" %>s %b " combined        CustomLog /var/log/httpd/access_log combinedHints :-------Sometimes on the internet, you will find a few percent of the clients whichdisable cookies on their browser. Obviously they have troubles everywhere onthe web, but you can still help them access your site by using the "source"balancing algorithm instead of the "roundrobin". It ensures that a given IPaddress always reaches the same server as long as the number of servers remainsunchanged. Never use this behind a proxy or in a small network, because thedistribution will be unfair. However, in large internal networks, and on theinternet, it works quite well. Clients which have a dynamic address will notbe affected as long as they accept the cookie, because the cookie always hasprecedence over load balancing :    listen webfarm 192.168.1.1:80       mode http       balance source       cookie SERVERID insert indirect       option httpchk HEAD /index.html HTTP/1.0       server webA 192.168.1.11:80 cookie A check       server webB 192.168.1.12:80 cookie B check       server webC 192.168.1.13:80 cookie C check       server webD 192.168.1.14:80 cookie D check       ==================================================================2. HTTP load-balancing with cookie prefixing and high availability==================================================================Now you don't want to add more cookies, but rather use existing ones. Theapplication already generates a "JSESSIONID" cookie which is enough to tracksessions, so we'll prefix this cookie with the server name when we see it.Since the load-balancer becomes critical, it will be backed up with a secondone in VRRP mode using keepalived under Linux.Download the latest version of keepalived from this site and install iton each load-balancer LB1 and LB2 :       http://www.keepalived.org/You then have a shared IP between the two load-balancers (we will still use theoriginal IP). It is active only on one of them at any moment. To allow theproxy to bind to the shared IP on Linux 2.4, you must enable it in /proc :# echo 1 >/proc/sys/net/ipv4/ip_nonlocal_bind    shared IP=192.168.1.1  192.168.1.3  192.168.1.4    192.168.1.11-192.168.1.14   192.168.1.2 -------+------------+-----------+-----+-----+-----+--------+----        |            |           |     |     |     |       _|_db     +--+--+      +--+--+      +-+-+ +-+-+ +-+-+ +-+-+    (___)     | LB1 |      | LB2 |      | A | | B | | C | | D |    (___)     +-----+      +-----+      +---+ +---+ +---+ +---+    (___)     haproxy      haproxy        4 cheap web servers     keepalived   keepalivedConfig on both proxies (LB1 and LB2) :--------------------------------------           listen webfarm 192.168.1.1:80       mode http       balance roundrobin       cookie JSESSIONID prefix       option httpclose       option forwardfor       option httpchk HEAD /index.html HTTP/1.0       server webA 192.168.1.11:80 cookie A check       server webB 192.168.1.12:80 cookie B check       server webC 192.168.1.13:80 cookie C check       server webD 192.168.1.14:80 cookie D check       Notes: the proxy will modify EVERY cookie sent by the client and the server,so it is important that it can access to ALL cookies in ALL requests foreach session. This implies that there is no keep-alive (HTTP/1.1), thus the"httpclose" option. Only if you know for sure that the client(s) will neveruse keep-alive (eg: Apache 1.3 in reverse-proxy mode), you can remove thisoption.Configuration for keepalived on LB1/LB2 :-----------------------------------------    vrrp_script chk_haproxy {           # Requires keepalived-1.1.13        script "killall -0 haproxy"     # cheaper than pidof        interval 2                      # check every 2 secondsweight 2                        # add 2 points of prio if OK    }    vrrp_instance VI_1 {        interface eth0        state MASTER        virtual_router_id 51        priority 101                    # 101 on master, 100 on backup        virtual_ipaddress {            192.168.1.1        }        track_script {            chk_haproxy        }    }Description :------------- - LB1 is VRRP master (keepalived), LB2 is backup. Both monitor the haproxy   process, and lower their prio if it fails, leading to a failover to the   other node. - LB1 will receive clients requests on IP 192.168.1.1. - both load-balancers send their checks from their native IP. - if a request does not contain a cookie, it will be forwarded to a valid   server - in return, if a JESSIONID cookie is seen, the server name will be prefixed   into it, followed by a delimitor ('~') - when the client comes again with the cookie "JSESSIONID=A~xxx", LB1 will   know that it must be forwarded to server A. The server name will then be   extracted from cookie before it is sent to the server. - if server "webA" dies, the requests will be sent to another valid server   and a cookie will be reassigned.Flows :-------(client)                           (haproxy)                         (server A)  >-- GET /URI1 HTTP/1.0 ------------> |               ( no cookie, haproxy forwards in load-balancing mode. )                                       | >-- GET /URI1 HTTP/1.0 ---------->                                       |     X-Forwarded-For: 10.1.2.3                                       | <-- HTTP/1.0 200 OK -------------<                        ( no cookie, nothing changed )  <-- HTTP/1.0 200 OK ---------------< |  >-- GET /URI2 HTTP/1.0 ------------> |    ( no cookie, haproxy forwards in lb mode, possibly to another server. )                                       | >-- GET /URI2 HTTP/1.0 ---------->                                       |     X-Forwarded-For: 10.1.2.3                                       | <-- HTTP/1.0 200 OK -------------<                                       |     Set-Cookie: JSESSIONID=123    ( the cookie is identified, it will be prefixed with the server name )  <-- HTTP/1.0 200 OK ---------------< |      Set-Cookie: JSESSIONID=A~123     |  >-- GET /URI3 HTTP/1.0 ------------> |      Cookie: JSESSIONID=A~123         |       ( the proxy sees the cookie, removes the server name and forwards          to server A which sees the same cookie as it previously sent )                                       | >-- GET /URI3 HTTP/1.0 ---------->                                       |     Cookie: JSESSIONID=123                                       |     X-Forwarded-For: 10.1.2.3                                       | <-- HTTP/1.0 200 OK -------------<                        ( no cookie, nothing changed )  <-- HTTP/1.0 200 OK ---------------< |                                    ( ... )Hints :-------Sometimes, there will be some powerful servers in the farm, and some smallerones. In this situation, it may be desirable to tell haproxy to respect thedifference in performance. Let's consider that WebA and WebB are two oldP3-1.2 GHz while WebC and WebD are shiny new Opteron-2.6 GHz. If yourapplication scales with CPU, you may assume a very rough 2.6/1.2 performanceratio between the servers. You can inform haproxy about this using the "weight"keyword, with values between 1 and 256. It will then spread the load the mostsmoothly possible respecting those ratios :       server webA 192.168.1.11:80 cookie A weight 12 check       server webB 192.168.1.12:80 cookie B weight 12 check       server webC 192.168.1.13:80 cookie C weight 26 check       server webD 192.168.1.14:80 cookie D weight 26 check