Metricbeat can call Jolokia with GET requests. (elastic#8566) (elasti…

…c#9226)

Add a new `http_method` parameter to Jolokia module which by default
will have the value POST and can accept GET as well.

Fixes elastic#8566.

metricbeat/module/jolokia/jmx/_meta/docs.asciidoc

@@ -20,22 +20,25 @@ configure the following mapping:
   metricsets: ["jmx"]
   hosts: ["localhost:8778"]
   namespace: "testnamespace" <1>
+  http_method: "POST" <2>
   jmx.mappings:
     - mbean: 'java.lang:type=Runtime'
       attributes:
         - attr: Uptime
-          field: uptime <2>
-          event: uptime <3>
+          field: uptime <3>
+          event: uptime <4>
       target:
           url: "service:jmx:rmi:///jndi/rmi://targethost:9999/jmxrmi"
           user: "jolokia"
           password: "s!cr!t"
 ----
 <1> The `namespace` setting is required. This setting is used along with the
 module name to qualify field names in the output event.
-<2> The field where the returned value will be saved. This field will be called
+<2> The `http_method` setting is optional. By default all requests to Jolokia
+are performed using `POST` HTTP method. This setting allows only two values: `POST` or `GET`.
+<3> The field where the returned value will be saved. This field will be called
 `jolokia.testnamespace.uptime` in the output event.  
-<3> The `event` setting is optional. Use this setting to group all attributes
+<4> The `event` setting is optional. Use this setting to group all attributes
 with the same `event` value into the same event when sending data to Elastic. 
 
 If the underlying attribute is an object (such as the `HeapMemoryUsage`
@@ -54,12 +57,55 @@ configure multiple modules.
 When wildcards are used, an event is sent to Elastic for each matching
 MBean, and an `mbean` field is added to the event.
 
+[float]
+=== Accessing Jolokia via POST or GET method
+
+All requests to Jolokia are made by default using HTTP POST method. However, there are specific circumstances 
+on the environment where Jolokia agent is deployed, in which POST method can be unavailable. In this case you can use
+HTTP GET method, by defining `http_method` attribute. In general you can use either POST or GET, but GET has the following
+drawbacks:
+
+1. https://jolokia.org/reference/html/protocol.html#protocol-proxy[Proxy requests]
+are not allowed.
+2. If more than one `jmx.mappings` are defined, then Metricbeat will perform as many GET requests as the mappings defined. 
+   For example the following configuration with 3 mappings will create 3 GET requests, one for every MBean. On the contrary, if you use HTTP POST, Metricbeat will create only 1 request to Jolokia.
+
+[source,yaml]
+----   
+- module: jolokia
+  metricsets: ["jmx"]
+  enabled: true  
+  period: 10s
+  hosts: ["localhost:8080"]
+  namespace: "jolokia_metrics"
+  path: "/jolokia"
+  http_method: 'GET'
+  jmx.mappings:
+    - mbean: 'java.lang:type=Memory'
+      attributes:
+       - attr: HeapMemoryUsage
+         field: memory.heap_usage
+       - attr: NonHeapMemoryUsage
+         field: memory.non_heap_usage
+    - mbean: 'Catalina:name=*,type=ThreadPool'
+      attributes:
+       - attr: port
+         field: catalina.port
+       - attr: maxConnections
+         field: catalina.maxConnections
+    - mbean: 'java.lang:type=Runtime'
+      attributes:
+       - attr: Uptime
+         field: uptime
+----
+
 [float]
 === Limitations
-All Jolokia requests have `canonicalNaming` set to `false`. See the
+1. All Jolokia requests have `canonicalNaming` set to `false`. See the
 https://jolokia.org/reference/html/protocol.html[Jolokia Protocol] documentation
 for more detail about this parameter.
-
+2. If `http_method` is set to `GET`, then https://jolokia.org/reference/html/protocol.html#protocol-proxy[Proxy requests]
+are not allowed. Thus, setting a value to `target` section is going to fail with an error.
 
 [float]
 === Exposed fields, dashboards, indexes, etc.

metricbeat/module/jolokia/jmx/_meta/test/jolokia_get_response.json

@@ -0,0 +1,26 @@
+{
+    "request": {
+        "mbean": "java.lang:type=Memory",
+        "attribute": [
+            "HeapMemoryUsage",
+            "NonHeapMemoryUsage"
+        ],
+        "type": "read"
+    },
+    "value": {
+        "HeapMemoryUsage": {
+            "init": 1073741824,
+            "committed": 1037959168,
+            "max": 1037959168,
+            "used": 227420472
+        },
+        "NonHeapMemoryUsage": {
+            "init": 2555904,
+            "committed": 53477376,
+            "max": -1,
+            "used": 50519768
+        }
+    },
+    "timestamp": 1472298687,
+    "status": 200
+}