[core] Added dynamic_outputs to Rule

* Now possible to pass dynamic_outputs to the @rule decorator and have
outputs be dynamically configured based on information in the record.
For example, you could use lookup_tables to map an account_id to an
owner which maps to an output

[testing] Updated unit tests and added additional tests for new
dynamic_outputs

[docs] Added dynamic_outputs documentation

Signed-off-by: jack1902 <[email protected]>

docs/source/dynamic-outputs.rst

@@ -0,0 +1,129 @@
+Dynamic Outputs
+===============
+
+Prerequisites
+-------------
+
+* Any output assigned must be added with ``python manage.py output``
+* ``functions`` must return ``None``, ``str`` or ``List[str]`` which maps to an output configured with the above.
+* Only pass ``context`` if the ``rule`` sets context.
+
+Overview
+--------
+
+Adds the ability to have custom logic run to define an ``output`` or ``outputs`` based on information within the ``record``.
+For information on supported outputs and how to add support for additional outputs, see `outputs <outputs.html>`_
+
+As can be seen by the examples below, they are easy to configure, but add a very useful feature to StreamAlert. 
+
+- StreamAlert sends to all outputs defined within a rules ``outputs=[]`` and ``dynamic_outputs=[]`` when sending ``Alerts``.
+- It is also possible to pass ``context`` to the ``dynamic_function`` if the ``rule`` sets it.
+
+.. note::
+  Any ``output`` passed must be configured with ``./manage.py output -h``
+
+
+Dynamic Outputs, Simple
+-----------------------
+
+The below code block is considered a simple ``dynamic_output`` function, because the outputs are dynamically configured, but the information used still lives within the code. It also:
+
+ - allows you to maintain a static list of information inside your code
+ - will return the outputs relevant to the team who "own" the account
+ - ``Alerts`` are sent to the ``aws-sns:security`` output aswell as those returned by the function
+
+.. code-block:: python
+
+  from streamalert.shared.rule import rule
+
+  def map_account_to_team(record):
+      teams = {
+        "team_a": {"accounts": ["123", "456", ...], "outputs": ["aws-sns:team_a"]},
+        "team_b": {"accounts": ["789", ...], "outputs": ["aws-sns:team_b", "slack:team_b"]},
+      }
+
+      account_id = record.get('recipientaccountid')
+
+      for team in teams:
+          if account_id in team["accounts"]:
+              return team["outputs"]
+      # None is guarded against by StreamAlert
+
+  @rule(
+    logs=['cloudwatch:events'],
+    req_subkeys={
+      'detail': ['userIdentity', 'eventType']
+    },
+    outputs=["aws-sns:security"],
+    dynamic_outputs=[map_account_to_team]
+  )
+  def cloudtrail_root_account_usage(rec):
+    # Rule logic
+
+
+Dynamic Outputs, With LookupTables
+----------------------------------
+
+With the simple addition of a `lookup-table <lookup-tables.html>`_ you can take a rule like ``cloudtrail_root_account_usage`` and configure it as such:
+
+.. code-block:: python
+
+  from streamalert.shared.rule import rule
+  from streamalert.shared.lookup_tables.core import LookupTables
+
+  def dynamic_output_with_context(record, context): # pass context only if the rule added context
+      account_id = context["account_id"]
+ 
+      return LookupTables.get(
+        'my_lookup_table',
+        'aws-account-owner:{}'.format(account_id), 
+        None
+      ) # potentially returns [aws-sns:team_a]
+
+  @rule(
+    logs=['cloudwatch:events'],
+    outputs=["aws-sns:security],
+    dynamic_outputs=[dynamic_output_with_context],
+    context={"account_id": "valid_account_id"},
+  )
+  def cloudtrail_root_account_usage(rec):
+      context["account_id"] = record.get('recipientaccountid')
+      # Rule logic
+
+The above has the benefit of using information that lives outside of StreamAlert, which means teams can acquire new accounts and get ``Alerts``
+without having to alter StreamAlert code.
+
+
+Dynamic Outputs, With Other Data Source
+---------------------------------------
+
+.. code-block:: python
+
+  from streamalert.shared.rule import rule
+  import requests
+
+  def dynamic_output(record):
+      account_id = record.get('recipientaccountid')
+
+      # invoke an external API to get data back
+      response = requests.get("API/team_map")
+
+      for team in response.json():
+          if account_id in team["accounts"]:
+              return team["outputs"] # potentially "aws-lambda:team_a"
+
+  @rule(
+    logs=['cloudwatch:events'],
+    outputs=["aws-sns:security],
+    dynamic_outputs=[dynamic_output],
+  )
+  def cloudtrail_root_account_usage(rec):
+      # Rule logic
+
+
+The above example uses an external API to get the output map, which is to be queried with the ``account_id`` on the record.
+This is just an example, but hopefully highlights many ways in which ``dynamic_outputs`` can be used.
+
+.. warning:: 
+  The above example could result in many queries to the API in use and could potentially slow down StreamAlert
+  Lambdas when processing ``Alerts``.

docs/source/index.rst

@@ -77,6 +77,7 @@ Table of Contents
    rules
    testing
    outputs
+   dynamic-outputs
    publishers
    lookup-tables
    metrics

docs/source/rules.rst

@@ -133,6 +133,7 @@ The following table provides an overview of each rule option, with more details
 ``merge_by_keys``      ``List[str]``             List of key names that must match in value before merging alerts
 ``merge_window_mins``  ``int``                   Merge related alerts at this interval rather than sending immediately
 ``outputs``            ``List[str]``             List of alert outputs
+``dynamic_outputs``    ``List[function]``        List of functions which return valid outputs
 ``req_subkeys``        ``Dict[str, List[str]]``  Subkeys which must be present in the record
 =====================  ========================  ===============
 
@@ -252,10 +253,16 @@ but those keys can be nested anywhere in the record structure.
 outputs
 ~~~~~~~
 
-Defines the alert destination if the return value of a rule is ``True``.
+The ``outputs`` keyword argument defines the alert destination if the return value of a rule is ``True``.
 Alerts are always sent to an :ref:`Athena table <athena_user_guide>` which is easy to query.
 Any number of additional `outputs <outputs.html>`_ can be specified.
 
+dynamic_outputs
+~~~~~~~~~~~~~~~
+
+The ``dynamic_outputs`` keyword argument defines additional `outputs <outputs.html>`_ to an Alert which are dynamically generated.
+See `dynamic_outputs <dynamic_outputs.html>`_ for more info
+
 req_subkeys
 ~~~~~~~~~~~
 

streamalert/rules_engine/rules_engine.py

@@ -193,7 +193,7 @@ def _rule_analysis(self, payload, rule):
             return
 
         alert = Alert(
-            rule.name, payload['record'], self._configure_outputs(rule),
+            rule.name, payload['record'], self._configure_outputs(payload['record'], rule),
             cluster=payload['cluster'],
             context=rule.context,
             log_source=payload['log_schema_type'],
@@ -213,14 +213,126 @@ def _rule_analysis(self, payload, rule):
 
         return alert
 
-    def _configure_outputs(self, rule):
+    def _configure_outputs(self, record, rule):
+        """Configure the outputs for the rule
+
+        Args:
+            record (dict): Record to pass through to dynamic_outputs
+            rule (rule.Rule): Attributes for the rule which triggered the alert
+        Returns:
+            set: unique set of outputs, only required outputs if the rule is staged
+        """
         # Check if the rule is staged and, if so, only use the required alert outputs
         if rule.is_staged(self._rule_table):
-            all_outputs = self._required_outputs_set
-        else:  # Otherwise, combine the required alert outputs with the ones for this rule
-            all_outputs = self._required_outputs_set.union(rule.outputs_set)
+            output_sources = [self._required_outputs_set]
+        else:  # Otherwise, combine all outputs into one
+            output_sources = [self._required_outputs_set, rule.outputs_set]
+            if rule.dynamic_outputs:
+                # append dynamic_outputs to output sources if they exist
+                dynamic_outputs = self._configure_dynamic_outputs(record, rule)
+                output_sources.append(dynamic_outputs)
+
+        return {
+            output
+            for output_source in output_sources
+            for output in output_source
+            if self._check_valid_output(output)
+        }
+
+    @classmethod
+    def _configure_dynamic_outputs(cls, record, rule):
+        """Generate list of outputs from dynamic_outputs
+
+        Args:
+            record (dict): Record to pass through to the dynamic_output function
+            rule (rule.Rule): Attributes for the rule which triggered the alert
+        Returns:
+            list: list of additional outputs to append to the current set
+        """
+        args_list = [record]
+        if rule.context:
+            # Pass context to dynamic_output function if context exists
+            args_list.append(rule.context)
+
+        return [
+            output
+            for dynamic_output_function in rule.dynamic_outputs_set
+            for output in cls._call_dynamic_output_function(
+                dynamic_output_function, rule.name, args_list
+            )
+        ]
+
+    @staticmethod
+    def _call_dynamic_output_function(function, rule_name, args_list):
+        """Call the dynamic_output function
+
+        Args:
+            dynamic_output (func): Callable function which returns None, str or List[str]
+            rule_name (str): The name of the rule the functions belong to
+            args_list (list): list of args to be passed to the dynamic function
+                should be (record or record and context)
+        Returns:
+            list: list of additional outputs
+        """
+        LOGGER.debug("invoking function %s", function.__name__)
+
+        outputs = []
+
+        try:
+            outputs = function(*args_list)
+        except Exception:  # pylint: disable=broad-except
+            # Logger error and return []
+            LOGGER.error(
+                "Exception when calling dynamic_output %s for rule %s",
+                function.__name__, rule_name
+            )
+        else:
+            LOGGER.debug("function %s returned: %s", function.__name__, outputs)
+
+            if isinstance(outputs, str):
+                # Case 1: outputs is a string
+                #   return outputs wrapped in a list
+                outputs = [outputs]
+            elif isinstance(outputs, list):
+                # Case 2: outputs is a list
+                #   return outputs
+                pass
+            else:
+                # Case 3: outputs is neither a string or a list
+                #   return an empty list
+                outputs = []
+
+        return outputs
+
+    @staticmethod
+    def _check_valid_output(output):
+        """Verify output is valid
+
+        Args:
+            output (str): The output to check if its valid
+        Returns:
+            True (bool): Output is valid
+            False (bool): Output is invalid
+        """
+        valid = False
+
+        if not isinstance(output, str):
+            # Case 1: output is not a string
+            #   return False
+            LOGGER.warning("Output (%s) is not a string", output)
+            valid = False
+        elif isinstance(output, str) and ":" not in output:
+            # Case 2: output is a string but missing ":"
+            #   Log warning and return False
+            LOGGER.warning("Output (%s) is missing ':'", output)
+
+            valid = False
+        else:
+            # Case 3: output is a string and contains ":"
+            # return True
+            valid = True
 
-        return all_outputs
+        return valid
 
     @classmethod
     def _configure_publishers(cls, rule):

streamalert/shared/rule.py

@@ -59,6 +59,7 @@ def __init__(self, func, **kwargs):
         self.merge_by_keys = kwargs.get('merge_by_keys')
         self.merge_window_mins = kwargs.get('merge_window_mins') or 0
         self.outputs = kwargs.get('outputs')
+        self.dynamic_outputs = kwargs.get('dynamic_outputs')
         self.publishers = kwargs.get('publishers')
         self.req_subkeys = kwargs.get('req_subkeys')
         self.initial_context = kwargs.get('context')
@@ -199,6 +200,11 @@ def description(self, description):
     def outputs_set(self):
         return set(self.outputs or [])
 
+
+    @property
+    def dynamic_outputs_set(self):
+        return set(self.dynamic_outputs or [])
+
     @classmethod
     def disabled_rules(cls):
         return {