gmunozfe commented on code in PR #541:
URL: 
https://github.com/apache/incubator-kie-kogito-docs/pull/541#discussion_r1495578655


##########
serverlessworkflow/modules/ROOT/pages/core/custom-functions-support.adoc:
##########
@@ -531,7 +531,61 @@ kogito.sw.functions.greet.timeout=5000 <1>
 ----
 <1> Time in milliseconds
 
-== Custom function types
+== Rest Custom Function 
+
+Serverless Workflow Specification defines the 
xref:service-orchestration/orchestration-of-openapi-based-services.adoc[OpenAPI 
function type], which is the preferred way to interact with existing REST 
servers. 
+However, sometimes a workflow shoud interact with several REST APIs which are 
not described using an OpenAPI specification file. Since generating such file 
for these services might be a tedious exercise, {product_name} offers REST 
custom type as a shorcut. 
+
+When using custom rest, in the function definition you specify the HTTP URI to 
be invoked and the HTTP method (get, post, patch or put) to be used, using 
`operation` string. When the function is actually invoked you pass the request 
arguments as you do when using an OpenAPI function. 
+
+The following example shows the declaration of a `rest` function:
+
+.Example of a `rest` function declaration
+[source,json]
+----
+{
+  "functions": [
+    {
+      "name": "multiplyAllByAndSum", <1>
+      "type": "custom", <2>
+      "operation": "rest:post:/numbers/{multiplier}/multiplyByAndSum" <3>
+    }
+  ]
+}
+----
+
+<1> `multiplyAllAndSum` is the function name
+<2> `custom` is the function type
+<3> `rest:post:/numbers/{multiplier}/multiplyByAndSum` is the custom operation 
definition. In the custom operation definition: 
+* `rest` is the reserved operation keyword that indicates this is a REST call.
+* `post` is the HTTP method. 
+* `/numbers/{multiplier}/multiplyByAndSum` is the relative endpoint. 
+
+When using relative endpoints you must specify the host as a property. The 
format of the host property is `kogito.sw.functions.<function_name>.host`. 
Therefore, in this example, the host property key  is 
`kogito.sw.functions.multiplyAllByAndSum.host`. You might override the default 
port (80) if needed by specifying `kogito.sw.functions.multiplyAllAndSum.port` 
property. 
+
+This particular endpoint expects as body a json object which field `numbers` 
is an array of integers, multiplies each item in the array by `multiplier` and 
returns the sum of all the multiplied items. Therefore, to invoke it, assuming 
the input array is stored in the workflow model as property `inputNumbers`, you 
shoud write:
+
+.Example of a `rest` function call
+[source,json]
+----
+{
+ "functionRef": {
+   "refName": "multiplyAllByAndSum",
+   "arguments": {
+     "numbers": "$.inputNumbers"
+     "multiplier": 3 <1>
+   }
+ }
+}
+----
+<4> you replace path and query parameters by specifying an argument which name 
is equal to the path or query parameter. The query or path parameter to be 
replaced should be enclosed within {} in the endpoint string.  
+
+If `inputNumbers` contains `1`, `2` and `3`, the output of the call will be  
`1*3+2*3+3*3=18. 
+
+In case you want to specify headers in your your HTTP request, you might do it 
by adding arguments starting with `HEADER_` prefix. Therefore if you add 
`"HEADER_ce_id":"123"` to the previous argument set, you will be adding a 
header named `ce_id` with value `123`  to your request. A similar approach 
might be used to add query params to a GET request, in that case you should add 
arguments starting with `QUERY_` prefix. Note that you can also use {} notation 
for replacements of query parameters included directly in the `operation` 
string.  

Review Comment:
   You could add an example of QUERY_ use, but up to you:
   
   `For instance, adding "QUERY_sort": "asc" will append ?sort=asc to the 
endpoint URL.`
   
   Or with the json:
   ```
   {
    "functionRef": {
      "refName": "getProductList",
      "arguments": {
        "category": "electronics",
        "QUERY_sort": "asc"
      }
    }
   }
   
   ```



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