Accessing URL parameters

Define placeholders in the trigger URL, then read their values from the HTTP request header inside your function.

A REST API often carries its parameters in the URL path itself, for example:

http://192.168.0.1/guestbook/{name}/{age}

Put parameter placeholders in the value of the --url flag. Fission uses gorilla/mux as its underlying URL router, so you can also write a regular expression to filter out illegal API requests.

$ fission httptrigger create --method GET \
    --url "/guestbook/{name}/{age}" --function restapi-get

$ fission httptrigger create --method GET \
    --url "/guestbook/{name}/{age:[0-9]+}" --function restapi-get

Fission attaches the value of each URL parameter to the HTTP request header, as shown below.

Accept-Encoding: gzip
Host: 172.17.0.25:8888
Connection: close
Accept: */*
User-Agent: curl/7.54.0
Content-Length: 15
Content-Type: application/json
X-Forwarded-For: 172.17.0.1

X-Fission-Function-Uid: 82c95606-9afa-11e8-bbd1-08002720b796
X-Fission-Function-Resourceversion: 480652
X-Fission-Function-Name: reqpayload
X-Fission-Function-Namespace: default

X-Fission-Params-Name: Alice
X-Fission-Params-Age: 23

The headers with key prefix X-Fission-Params- are the fields that hold the URL parameter values.

In some languages, such as Go, the header key is displayed in MIME canonical format. For example:

url: /guestbook/{name}
header key: X-Fission-Params-Name

url: /guestbook/{FooBar}
header key: X-Fission-Params-Foobar

Check the letter case of the header key and convert it if necessary to get the right parameter value.

(In Go, call request.Header.Get() to get the header value without worrying about the key case.)