Made AbstractConsensusRules use python's abc and enforcing that ConsensusRules plugins subclass it

bigchaindb/__init__.py

@@ -41,9 +41,7 @@ config = {
         'rate': e('BIGCHAIN_STATSD_SAMPLERATE', default=0.01)
     },
     'api_endpoint': 'http://localhost:8008/api/v1',
-    'consensus_plugins': [
-        'base'
-    ]
+    'consensus_plugin': e('BIGCHAIN_CONSENSUS_PLUGIN', default='default')
 }
 
 # We need to maintain a backup copy of the original config dict in case

bigchaindb/config_utils.py

@@ -17,7 +17,7 @@ import json
 import logging
 import collections
 
-from pkg_resources import iter_entry_points
+from pkg_resources import iter_entry_points, ResolutionError
 
 import bigchaindb
 
@@ -103,20 +103,37 @@ def autoconfigure():
         logger.warning('Cannot find your config file. Run `bigchaindb configure` to create one')
 
 
-def get_plugins(plugin_names):
-    if not plugin_names:
-        plugin_names = bigchaindb.config.get('consensus_plugins', [])
+def load_consensus_plugin(name=None):
+    """Find and load the chosen consensus plugin.
 
-    plugins = []
+    Args:
+        name (string): the name of the entry_point, as advertised in the
+            setup.py of the providing package.
 
-    # It's important to maintain plugin ordering as stated in the config file.
-    # e.g. Expensive validation tasks should happen after cheap ones.
-    #
-    # TODO: We might want to add some sort of priority system, but for now we
-    #       simply assume everything in a given plugin is designed to run at the
-    #       same time.
-    for name in plugin_names:
-        for entry_point in iter_entry_points('bigchaindb.plugins', name):
-            plugins.append(entry_point.load())
+    Returns:
+        an uninstantiated subclass of ``bigchaindb.consensus.AbstractConsensusRules``
+    """
+    if not name:
+        name = bigchaindb.config.get('consensus_plugin', 'default')
 
-    return plugins
+    # TODO: This will return the first plugin with group `bigchaindb.plugins`
+    #       and name `name` in the active WorkingSet.
+    #       We should probably support Requirements specs in the config, e.g.
+    #       consensus_plugin: 'my-plugin-package==0.0.1;default'
+    plugin = None
+    for entry_point in iter_entry_points('bigchaindb.consensus', name):
+        plugin = entry_point.load()
+
+    # No matching entry_point found
+    if not plugin:
+        raise ResolutionError(
+            'No plugin found in group `bigchaindb.plugins` with name `{}`'.
+            format(name))
+
+    # Is this strictness desireable?
+    # It will probably reduce developer headaches in the wild.
+    if not issubclass(plugin, (AbstractConsensusRules)):
+        raise TypeError("object of type '{}' does not implement `bigchaindb."
+                        "consensus.AbstractConsensusRules`".format(type(plugin)))
+
+    return plugin

bigchaindb/consensus.py

@@ -1,17 +1,21 @@
+from abc import ABCMeta, abstractmethod
+
 import bigchaindb.exceptions as exceptions
 from bigchaindb import util
-from bigchaindb.crypto import hash_data
+from bigchaindb.crypto import hash_data, PublicKey
 
-# TODO: no real reason to use abc yet, but later we can enforce inheritance from
-#       this class when loading plugins if that's desirable.
-# from abc import ABCMeta
 
-class AbstractConsensusRules:
+class AbstractConsensusRules(metaclass=ABCMeta):
+    """Abstract base class for Bigchain plugins which implement consensus logic.
 
-    # TODO: rather than having plugin-authors inherit and override,
-    #       it'd be cleaner to make a `transactionrule` decorator and etc
-    @classmethod
-    def validate_transaction(cls, bigchain, transaction):
+    A consensus plugin must expose a class inheriting from this one via an
+    entry_point.
+
+    All methods listed below must be implemented.
+    """
+
+    @abstractmethod
+    def validate_transaction(bigchain, transaction):
         """Validate a transaction.
 
         Args:
@@ -26,10 +30,10 @@ class AbstractConsensusRules:
             Descriptive exceptions indicating the reason the transaction failed.
             See the `exceptions` module for bigchain-native error classes.
         """
-        return transaction
+        raise NotImplementedError
 
-    @classmethod
-    def validate_block(cls, bigchain, block):
+    @abstractmethod
+    def validate_block(bigchain, block):
         """Validate a block.
 
         Args: