Add section to help user understand how logging works

Change-Id: Iebfb747450cc08e930eabd36a87670236b23ffff
Signed-off-by: Yujun Zhang <zhang.yujunz@zte.com.cn>
(cherry picked from commit 8d3f6673e08d6b8c02f604791c6e42a399dc660

1 files changed, 136 insertions, 0 deletions

diff --git a/docs/logging.rst b/docs/logging.rst
index 196ba40..05f3f5b 100644
--- a/docs/logging.rst
+++ b/docs/logging.rst
@@ -26,3 +26,139 @@ It validates the installation with the following criterias
 #. existence of fluented pod
 #. fluentd input is configured correctly
 #. TBD
+
+**************************
+Understanding how it works
+**************************
+
+In clover stack, Istio is configured to automatically gather logs for services
+in a mesh. More specificly, it is configured in `Mixer`_::
+
+- when to log
+- what to log
+- where to log
+
+.. _Mixer: https://istio.io/docs/concepts/policy-and-control/mixer.html
+
+When to log
+===========
+
+Istio defines when to log by creating a custom resource ``rule``. For example:
+
+.. code-block:: yaml
+
+    apiVersion: "config.istio.io/v1alpha2"
+    kind: rule
+    metadata:
+    name: newlogtofluentd
+    namespace: istio-system
+    spec:
+    match: "true" # match for all requests
+    actions:
+    - handler: handler.fluentd
+      instances:
+      - newlog.logentry
+
+This rule specifies that all instances of ``newlog.logentry`` that matches the
+expression will be handled by the specified handler ``handler.fluentd``. We
+shall explain ``instances`` and ``handler`` later. The expression ``true`` means
+whenever a request arrive at Mixer, it will trigger the actions defined belows.
+
+``rule`` is a custom resource definition from `Istio installation`_.
+
+.. code-block:: yaml
+
+    # Rule to send logentry instances to the fluentd handler
+    kind: CustomResourceDefinition
+    apiVersion: apiextensions.k8s.io/v1beta1
+    metadata:
+      name: rules.config.istio.io
+      labels:
+        package: istio.io.mixer
+        istio: core
+    spec:
+      group: config.istio.io
+      names:
+        kind: rule
+        plural: rules
+        singular: rule
+    scope: Namespaced
+    version: v1alpha2
+
+.. _Istio installation: https://github.com/istio/istio/blob/master/install/kubernetes/templates/istio-mixer.yaml.tmpl
+
+What to log
+===========
+
+The instance defines what content to be logged.
+
+> A (request) instance is the result of applying request attributes to the
+> template mapping. The mapping is specified as an instance configuration.
+
+For example:
+
+.. code-block:: yaml
+
+    # Configuration for logentry instances
+    apiVersion: "config.istio.io/v1alpha2"
+    kind: logentry
+    metadata:
+      name: newlog
+      namespace: istio-system
+    spec:
+      severity: '"info"'
+      timestamp: request.time
+      variables:
+        source: source.labels["app"] | source.service | "unknown"
+        user: source.user | "unknown"
+        destination: destination.labels["app"] | destination.service | "unknown"
+        responseCode: response.code | 0
+        responseSize: response.size | 0
+        latency: response.duration | "0ms"
+      monitored_resource_type: '"UNSPECIFIED"'
+
+The keys under ``spec`` should conform to the template. To learn what fields
+are available and valid type, you may need to reference the corresponding
+template, in this case, `Log Entry template`_.
+
+The values of each field could be either `Istio attributes`_ or an expression.
+
+> A given Istio deployment has a fixed vocabulary of attributes that it
+> understands. The specific vocabulary is determined by the set of attribute
+> producers being used in the deployment. The primary attribute producer in
+> Istio is Envoy, although Mixer and services can also introduce attributes.
+
+Refer to the `Attribute Vocabulary`_ to learn the full set.
+
+By the way, ``logentry`` is also a custom resource definition created by Istio.
+
+.. _Istio attributes: https://istio.io/docs/concepts/policy-and-control/attributes.html
+.. _Attribute Vocabulary: https://istio.io/docs/reference/config/mixer/attribute-vocabulary.html
+.. _Log Entry template: https://istio.io/docs/reference/config/template/logentry.html
+
+Where to log
+============
+
+For log, the handler defines where these information will be handled, in this
+example, a fluentd daemon on fluentd-es.logging:24224.
+
+.. code-block:: yaml
+
+    # Configuration for a fluentd handler
+    apiVersion: "config.istio.io/v1alpha2"
+    kind: fluentd
+    metadata:
+      name: handler
+      namespace: istio-system
+    spec:
+      address: "fluentd-es.logging:24224"
+
+In this example, handlers (``handler.fluentd``) configure `Adapters`_
+(``fluentd``) to handle the data delivered from the created instances
+(``newlog.logentry``).
+
+An adapter only accepts instance of specified kind. For example,
+`fluentd adapter`_ accepts logentry but not other kinds.
+
+.. _Adapters: https://istio.io/docs/concepts/policy-and-control/mixer.html#adapters
+.. _fluentd adapter: https://istio.io/docs/reference/config/adapters/fluentd.html