Core concepts

Fetch methods

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A fetch method is a function that returns information about the current request, current response, connection, or internal state of the load balancer. You use a fetch by referencing its function name with optional arguments. One example is the path fetch, which returns the requested URL path. In the following example, we check whether path equals /api/bad and, if it does, deny the request:

haproxy
frontend www
bind :80
http-request deny if { path /api/bad/ }
haproxy
frontend www
bind :80
http-request deny if { path /api/bad/ }

The path fetch does not take any arguments. An example of a fetch that does is req.hdr, which takes the name of a request header to collect. Below, we use req.hdr to get the User-Agent header on an http-request capture line. The http-request capture directive adds custom data to the log.

haproxy
frontend www
bind :80
http-request capture req.hdr(user-agent) len 30
haproxy
frontend www
bind :80
http-request capture req.hdr(user-agent) len 30

In the next example, we demonstrate two other fetches. The http-request set-var directive sets a variable named txn.http_version to the value returned by the fetch req.ver, which returns the HTTP version of the request. We also use the http-response add-header directive to add a new response header named Via. Its value holds the HTTP version that we stored in a variable plus the server’s hostname, which we get by using the hostname fetch, which takes no arguments.

haproxy
frontend www
bind :80
http-request set-var(txn.http_version) req.ver
http-response add-header Via "%[var(txn.http_version)] %[hostname]"
haproxy
frontend www
bind :80
http-request set-var(txn.http_version) req.ver
http-response add-header Via "%[var(txn.http_version)] %[hostname]"

This example also demonstrates that some fetches are collected during the request phase of a session and are not available during the response phase. However, you can store the fetch value in a variable by using http-request set-var to make it available throughout the entire transaction. In fact, var(<variable name>) is itself a fetch.

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