HAProxy health check

Layer 4 health check is to establish a TCP connection with backend server at regular interval.  Optionally, one can specify the number of check failure before HAProxy marked the specific backend down.  It can also specify a different interval to check for server up to resume.  This allows a fast fail and slow start configuration.

Layer 7 health check uses HTTP OPTION call to check if backend is processing HTTP request.  A 2xx or 3xx response indicate server is up.  Some http server may not accept OPTION call.  One can configure to use HEAD call for check.

One can also monitor the client traffic on the top of the polling type interval health check.

Health check can be defined end to end.  For example, an app server depends on the database server to be available.  If the database server is down, the service in the app server will be impacted.  In this case, HAProxy can define the app server is depending on database server.  If the database server failed health check, HAProxy can disable the app server. 

Routing in HAProxy

An ACL defines a condition that can be used in HAProxy to route requests satisfying the ACL to a group of special backend. 

The condition can be the full or partial matching part of  url, the search parameter in the url, the hostname part of the url, header value, http verb etc. 

beside routing, HAProxy can also send back redirect response to the client (301, 302 ..etc)

HAProxy

 It is a reverse proxy to load balance traffic to the backend servers. It t operates on either tcp (layer 4) or http (layer X ) mode. To the backend servers, the. Kirby become the proxy. Therefore proxy can be configured to add the client ip by adding it as extra http headers. 

ACL can be used ti define a condition which in turn used in other statement to filter (limited) if a connection is to be forwarded to the backend. 

A HAProxy can be set up to forward different traffic received (interface and port) to different set of backend 

CSS Class

HTML element can define a class attribute.  The class name can be referenced in CSS by prefix it with a dot.  For example, the following refers to the class named "big-text"

.big-text {font: 15}

HTML

 The anchor tag <a> s used to create hypertext link to another document or page. The href (hypertext reference) specifies the url. The target attribute specifies the frame to display the content of the link when clicked on. The value of “_blank” opens the link in a new tab. 

The div (divisions) is used to group a set of elements. The div is used by css to style the set. A span tag further group a subset of element in a div and be applied with a different style. 

Semantic element denotes a specific purpose in a general document. They included header, footer, aside (semantically similar to remark or side notes), article, section and main. 

CSS

Cascade style sheet is designed for to provide a standard look framework for large website with any pages. CSS “cascade the standard style to all the web pages on the site. 

CSS defines a standard formatting spec for a type of elements, or a group of different elements tagged with the same class id, or a single element tagged with an id. These are called element selector, class selector and id selector respectively.

Element selector will be overridden by class selector and in turn overridden by id selector   

Sprite

A sprite is a small image that represent an object of character in game. Sprite can be animated. Sprite is drawn on 2D game and not so common in 3D. In 3D, sprite could be used for HUD or chart window that display message.