Add quick start to the replication doc, minor doc fixes.

doc/src/extras.rst

@@ -141,8 +141,81 @@ Logging cursor
 
 .. autoclass:: MinTimeLoggingCursor
 
-Replication cursor
+Replication protocol support
-^^^^^^^^^^^^^^^^^^
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Modern PostgreSQL servers (version 9.0 and above) support replication.  The
+replication protocol is built on top of the client-server protocol and can be
+operated using ``libpq``, as such it can be also operated by ``psycopg2``.
+The replication protocol can be operated on both synchronous and
+:ref:`asynchronous <async-support>` connections.
+
+Server version 9.4 adds a new feature called *Logical Replication*.
+
+.. seealso::
+
+   - PostgreSQL `Streaming Replication Protocol`__
+
+   .. __: http://www.postgresql.org/docs/current/static/protocol-replication.html
+
+
+Logical replication Quick-Start
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+You must be using PostgreSQL server version 9.4 or above to run this quick
+start.
+
+Make sure that replication connections are permitted for user ``postgres`` in
+``pg_hba.conf`` and reload the server configuration.  You also need to set
+``wal_level=logical`` and ``max_wal_senders``, ``max_replication_slots`` to
+value greater than zero in ``postgresql.conf`` (these changes require a server
+restart).  Create a database ``psycopg2test``.
+
+Then run the following code to quickly try the replication support out.  This
+is not production code -- it has no error handling, it sends feedback too
+often, etc. -- and it's only intended as a simple demo of logical
+replication::
+
+  from __future__ import print_function
+  import sys
+  import psycopg2
+  import psycopg2.extras
+
+  conn = psycopg2.connect('dbname=psycopg2test user=postgres',
+     connection_factory=psycopg2.extras.LogicalReplicationConnection)
+  cur = conn.cursor()
+  try:
+      cur.start_replication(slot_name='pytest')
+  except psycopg2.ProgrammingError:
+      cur.create_replication_slot('pytest', output_plugin='test_decoding')
+      cur.start_replication(slot_name='pytest')
+
+  class DemoConsumer(object):
+      def __call__(self, msg):
+          print(msg.payload)
+          msg.cursor.send_feedback(flush_lsn=msg.data_start)
+
+  democonsumer = DemoConsumer()
+
+  print("Starting streaming, press Control-C to end...", file=sys.stderr)
+  try:
+     cur.consume_stream(democonsumer)
+  except KeyboardInterrupt:
+     cur.close()
+     conn.close()
+     print("The slot 'pytest' still exists. Drop it with SELECT pg_drop_replication_slot('pytest'); if no longer needed.", file=sys.stderr)
+     print("WARNING: Transaction logs will accumulate in pg_xlog until the slot is dropped.", file=sys.stderr)
+
+
+You can now make changes to the ``psycopg2test`` database using a normal
+psycopg2 session, ``psql``, etc. and see the logical decoding stream printed
+by this demo client.
+
+This will continue running until terminated with ``Control-C``.
+
+
+Replication connection and cursor classes
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 .. autoclass:: ReplicationConnectionBase
 
@@ -177,17 +250,11 @@ The following replication types are defined:
         phys_cur = phys_conn.cursor()
 
     Both `LogicalReplicationConnection` and `PhysicalReplicationConnection` use
-    `ReplicationCursor` for actual communication on the connection.
+    `ReplicationCursor` for actual communication with the server.
-
-.. seealso::
-
-   - PostgreSQL `Streaming Replication Protocol`__
-
-   .. __: http://www.postgresql.org/docs/current/static/protocol-replication.html
 
 
-The individual messages in the replication stream are presented by
+The individual messages in the replication stream are represented by
-`ReplicationMessage` objects:
+`ReplicationMessage` objects (both logical and physical type):
 
 .. autoclass:: ReplicationMessage
 
@@ -249,7 +316,7 @@ The individual messages in the replication stream are presented by
         replication slot is created by default.  No output plugin parameter is
         required or allowed when creating a physical replication slot.
 
-        In either case, the type of slot being created can be specified
+        In either case the type of slot being created can be specified
         explicitly using *slot_type* parameter.
 
         Replication slots are a feature of PostgreSQL server starting with
@@ -295,25 +362,25 @@ The individual messages in the replication stream are presented by
         replication can be used with both types of connection.
 
         On the other hand, physical replication doesn't require a named
-        replication slot to be used, only logical one does.  In any case,
+        replication slot to be used, only logical replication does.  In any
-        logical replication and replication slots are a feature of PostgreSQL
+        case logical replication and replication slots are a feature of
-        server starting with version 9.4.  Physical replication can be used
+        PostgreSQL server starting with version 9.4.  Physical replication can
-        starting with 9.0.
+        be used starting with 9.0.
 
         If *start_lsn* is specified, the requested stream will start from that
-        LSN.  The default is `!None`, which passes the LSN ``0/0``, causing
+        LSN.  The default is `!None` which passes the LSN ``0/0`` causing
-        replay to begin at the last point at which the server got replay
+        replay to begin at the last point for which the server got flush
-        confirmation from the client for, or the oldest available point for a
+        confirmation from the client, or the oldest available point for a new
-        new slot.
+        slot.
 
         The server might produce an error if a WAL file for the given LSN has
-        already been recycled, or it may silently start streaming from a later
+        already been recycled or it may silently start streaming from a later
         position: the client can verify the actual position using information
-        provided the `ReplicationMessage` attributes.  The exact server
+        provided by the `ReplicationMessage` attributes.  The exact server
         behavior depends on the type of replication and use of slots.
 
-        A *timeline* parameter can only be specified with physical replication
+        The *timeline* parameter can only be specified with physical
-        and only starting with server version 9.3.
+        replication and only starting with server version 9.3.
 
         A dictionary of *options* may be passed to the logical decoding plugin
         on a logical replication slot.  The set of supported options depends
@@ -324,8 +391,9 @@ The individual messages in the replication stream are presented by
         `start_replication_expert()` internally.
 
         After starting the replication, to actually consume the incoming
-        server messages, use `consume_stream()` or implement a loop around
+        server messages use `consume_stream()` or implement a loop around
-        `read_message()` in case of asynchronous connection.
+        `read_message()` in case of :ref:`asynchronous connection
+        <async-support>`.
 
     .. method:: start_replication_expert(command)
 
@@ -343,66 +411,66 @@ The individual messages in the replication stream are presented by
         This method can only be used with synchronous connection.  For
         asynchronous connections see `read_message()`.
 
-        Before calling this method to consume the stream, use
+        Before calling this method to consume the stream use
         `start_replication()` first.
 
-        When called, this method enters an endless loop, reading messages from
+        This method enters an endless loop reading messages from the server
-        the server and passing them to ``consume()``, then waiting for more
+        and passing them to ``consume()``, then waiting for more messages from
-        messages from the server.  In order to make this method break out of
+        the server.  In order to make this method break out of the loop and
-        the loop and return, ``consume()`` can throw a `StopReplication`
+        return, ``consume()`` can throw a `StopReplication` exception.  Any
-        exception (any unhandled exception will make it break out of the loop
+        unhandled exception will make it break out of the loop as well.
-        as well).
 
-        If *decode* is set to `!True`, the messages read from the server are
+        If *decode* is set to `!True` the messages read from the server are
         converted according to the connection `~connection.encoding`.  This
         parameter should not be set with physical replication.
 
-        This method also sends keepalive messages to the server, in case there
+        This method also sends keepalive messages to the server in case there
         were no new data from the server for the duration of
         *keepalive_interval* (in seconds).  The value of this parameter must
-        be equal to at least 1 second, but it can have a fractional part.
+        be set to at least 1 second, but it can have a fractional part.
+
+        The *msg* objects passed to ``consume()`` are instances of
+        `ReplicationMessage` class.
+
+        After processing certain amount of messages the client should send a
+        confirmation message to the server.  This should be done by calling
+        `send_feedback()` method on the corresponding replication cursor.  A
+        reference to the cursor is provided in the `ReplicationMessage` as an
+        attribute.
 
         The following example is a sketch implementation of ``consume()``
         callable for logical replication::
 
             class LogicalStreamConsumer(object):
 
-                def __call__(self, msg):
+                ...
-                    self.store_message_data(msg.payload)
 
-                    if self.should_report_to_the_server_now(msg):
+                def __call__(self, msg):
+                    self.process_message(msg.payload)
+
+                    if self.should_send_feedback(msg):
                         msg.cursor.send_feedback(flush_lsn=msg.data_start)
 
             consumer = LogicalStreamConsumer()
             cur.consume_stream(consumer, decode=True)
 
-        The *msg* objects passed to ``consume()`` are instances of
-        `ReplicationMessage` class.
-
-        After storing certain amount of messages' data reliably, the client
-        should send a confirmation message to the server.  This should be done
-        by calling `send_feedback()` method on the corresponding replication
-        cursor.  A reference to the cursor is provided in the
-        `ReplicationMessage` as an attribute.
-
         .. warning::
 
-            When using replication with slots, failure to properly notify the
+            When using replication with slots, failure to constantly consume
-            server by constantly consuming and reporting success at
+            *and* report success to the server appropriately can eventually
-            appropriate times can eventually lead to "disk full" condition on
+            lead to "disk full" condition on the server, because the server
-            the server, because the server retains all the WAL segments that
+            retains all the WAL segments that might be needed to stream the
-            might be needed to stream the changes via all of the currently
+            changes via all of the currently open replication slots.
-            open replication slots.
 
-            On the other hand, it is not recommended to send a confirmation
+            On the other hand, it is not recommended to send confirmation
-            after every processed message, since that will put an unnecessary
+            after *every* processed message, since that will put an
-            load on network and the server.  A possible strategy is to confirm
+            unnecessary load on network and the server.  A possible strategy
-            after every COMMIT message.
+            is to confirm after every COMMIT message.
 
     .. method:: send_feedback(write_lsn=0, flush_lsn=0, apply_lsn=0, reply=False)
 
         :param write_lsn: a LSN position up to which the client has written the data locally
-        :param flush_lsn: a LSN position up to which the client has stored the
+        :param flush_lsn: a LSN position up to which the client has processed the
                           data reliably (the server is allowed to discard all
                           and every data that predates this LSN)
         :param apply_lsn: a LSN position up to which the warm standby server
@@ -411,7 +479,7 @@ The individual messages in the replication stream are presented by
         :param reply: request the server to send back a keepalive message immediately
 
         Use this method to report to the server that all messages up to a
-        certain LSN position have been stored on the client and may be
+        certain LSN position have been processed on the client and may be
         discarded on the server.
 
         This method can also be called with all default parameters' values to
@@ -433,13 +501,14 @@ The individual messages in the replication stream are presented by
         Returns `!True` if the feedback message was sent successfully,
         `!False` otherwise.
 
-    Low-level methods for asynchronous connection operation.
+    Low-level replication cursor methods for :ref:`asynchronous connection
+    <async-support>` operation.
 
-    With the synchronous connection, a call to `consume_stream()` handles all
+    With the synchronous connection a call to `consume_stream()` handles all
     the complexity of handling the incoming messages and sending keepalive
     replies, but at times it might be beneficial to use low-level interface
-    for better control, in particular to `~select.select()` on multiple
+    for better control, in particular to `~select` on multiple sockets.  The
-    sockets.  The following methods are provided for asynchronous operation:
+    following methods are provided for asynchronous operation:
 
     .. method:: read_message(decode=True)
 
@@ -449,16 +518,16 @@ The individual messages in the replication stream are presented by
         This method should be used in a loop with asynchronous connections
         after calling `start_replication()` once.
 
-        It tries to read the next message from the server, without blocking
+        It tries to read the next message from the server without blocking and
-        and returns an instance of `ReplicationMessage` or `!None`, in case
+        returns an instance of `ReplicationMessage` or `!None`, in case there
-        there are no more data messages from the server at the moment.
+        are no more data messages from the server at the moment.
 
         It is expected that the calling code will call this method repeatedly
-        in order to consume all of the messages that might have been buffered,
+        in order to consume all of the messages that might have been buffered
-        until `!None` is returned.  After receiving a `!None` value from this
+        until `!None` is returned.  After receiving `!None` from this method
-        method, the caller should use `~select.select()` or `~select.poll()`
+        the caller should use `~select.select()` or `~select.poll()` on the
-        on the corresponding connection to block the process until there is
+        corresponding connection to block the process until there is more data
-        more data from the server.
+        from the server.
 
         The server can send keepalive messages to the client periodically.
         Such messages are silently consumed by this method and are never
@@ -480,24 +549,25 @@ The individual messages in the replication stream are presented by
 
     An actual example of asynchronous operation might look like this::
 
-        def consume(msg):
+      def consume(msg):
-            ...
+          ...
 
-        keepalive_interval = 10.0
+      keepalive_interval = 10.0
-        while True:
+      while True:
-            msg = cur.read_message()
+          msg = cur.read_message()
-            if msg:
+          if msg:
-                consume(msg)
+              consume(msg)
-            else:
+          else:
-                now = datetime.now()
+              now = datetime.now()
-                timeout = keepalive_interval - (now - cur.io_timestamp).total_seconds()
+              timeout = keepalive_interval - (now - cur.io_timestamp).total_seconds()
-                if timeout > 0:
+              if timeout > 0:
-                    sel = select.select([cur], [], [], timeout)
+                  sel = select.select([cur], [], [], timeout)
-                else:
+              else:
-                    sel = ([], [], [])
+                  sel = ([], [], [])
 
-                if not sel[0]:
+              if not sel[0]:
-                    cur.send_feedback()
+                  # timed out, send keepalive message
+                  cur.send_feedback()
 
 .. index::
     pair: Cursor; Replication

lib/extras.py

@@ -514,7 +514,7 @@ class StopReplication(Exception):
 
 
 class ReplicationCursor(_replicationCursor):
-    """A cursor used for communication on the replication protocol."""
+    """A cursor used for communication on replication connections."""
 
     def create_replication_slot(self, slot_name, slot_type=None, output_plugin=None):
         """Create streaming replication slot."""

psycopg/replication_message_type.c

@@ -146,7 +146,7 @@ static struct PyGetSetDef replicationMessageObject_getsets[] = {
 /* object type */
 
 #define replicationMessageType_doc \
-"A database replication message."
+"A replication protocol message."
 
 PyTypeObject replicationMessageType = {
     PyVarObject_HEAD_INIT(NULL, 0)