Added documentation about the changes in transaction control

NEWS

@@ -33,6 +33,10 @@ New features:
   (:ticket:`#491`).
 - Added ``async_`` as an alias for ``async`` to support Python 3.7 where
   ``async`` will become a keyword (:ticket:`#495`).
+- Unless in autocommit, do not use :sql:`default_transaction_*` settings to
+  control the session characteristics as it may create problems with external
+  connection pools such as pgbouncer; use :sql:`BEGIN` options instead
+  (:ticket:`#503`).
 
 Bug fixes:
 
@@ -44,6 +48,11 @@ Other changes:
 - Dropped support for Python 2.5.
 - Dropped support for client library older than PostgreSQL 9.1 (but older
   server versions are still supported).
+- `~connection.isolation_level` doesn't read from the database but will return
+  `~psycopg2.extensions.ISOLATION_LEVEL_DEFAULT` if no value was set on the
+  connection.
+- `~connection.set_isolation_level()` will throw an exception if executed
+  inside a transaction; previously it would have silently rolled it back.
 
 
 What's new in psycopg 2.6.3

doc/src/connection.rst

@@ -400,6 +400,32 @@ The ``connection`` class
 
         .. versionadded:: 2.4.2
 
+        .. versionchanged:: 2.7
+            Before this version, the function would have set
+            :sql:`default_transaction_*` attribute in the current session;
+            this implementation has the problem of not playing well with
+            external connection pooling working at transaction level and not
+            resetting the state of the session: changing the default
+            transaction would pollute the connections in the pool and create
+            problems to other applications using the same pool.
+
+            Starting from 2.7, if the connection is not autocommit, the
+            transaction characteristics are issued together with :sql:`BEGIN`
+            and will leave the :sql:`default_transaction_*` settings untouched.
+            For example::
+
+                conn.set_session(readonly=True)
+
+            will not change :sql:`default_transaction_read_only`, but
+            following transaction will start with a :sql:`BEGIN READ ONLY`.
+            Conversely, using::
+
+                conn.set_session(readonly=True, autocommit=True)
+
+            will set :sql:`default_transaction_read_only` to :sql:`on` and
+            rely on the server to apply the read only state to whatever
+            transaction, implicit or explicit, is executed in the connection.
+
 
     .. attribute:: autocommit
 
@@ -428,32 +454,54 @@ The ``connection`` class
         .. versionadded:: 2.4.2
 
 
-    .. attribute:: isolation_level
     .. method:: set_isolation_level(level)
 
         .. note::
 
-            From version 2.4.2, `set_session()` and `autocommit`, offer
+            From version 2.4.2, `set_session()` and `autocommit` offer
             finer control on the transaction characteristics.
 
-        Read or set the `transaction isolation level`_ for the current session.
+        Set the `transaction isolation level`_ for the current session.
         The level defines the different phenomena that can happen in the
         database between concurrent transactions.
 
-        The value set or read is an integer: symbolic constants are defined in
+        The value set is an integer: symbolic constants are defined in
         the module `psycopg2.extensions`: see
         :ref:`isolation-level-constants` for the available values.
 
-        The default level is :sql:`READ COMMITTED`: at this level a
-        transaction is automatically started the first time a database command
-        is executed.  If you want an *autocommit* mode, switch to
-        `~psycopg2.extensions.ISOLATION_LEVEL_AUTOCOMMIT` before
+        The default level is `~psycopg2.extensions.ISOLATION_LEVEL_DEFAULT`:
+        at this level a transaction is automatically started the first time a
+        database command is executed.  If you want an *autocommit* mode,
+        switch to `~psycopg2.extensions.ISOLATION_LEVEL_AUTOCOMMIT` before
         executing any command::
 
             >>> conn.set_isolation_level(psycopg2.extensions.ISOLATION_LEVEL_AUTOCOMMIT)
 
         See also :ref:`transactions-control`.
 
+        .. versionchanged:: 2.7
+
+            the function must be called outside a transaction; previously it
+            would have executed an implicit :sql:`ROLLBACK`; it will now raise
+            an exception.
+
+
+    .. attribute:: isolation_level
+
+        Read the `transaction isolation level`_ for the current session.  The
+        value is one of the :ref:`isolation-level-constants` defined in the
+        `psycopg2.extensions` module.
+
+        .. versionchanged:: 2.7
+
+            the default value for `!isolation_level` is
+            `~psycopg2.extensions.ISOLATION_LEVEL_DEFAULT`; previously the
+            property would have queried the server and returned the real value
+            applied. To know this value you can run a query such as :sql:`show
+            transaction_isolation`. Usually the default value is `READ
+            COMMITTED`, but this may be changed in the server configuration.
+
+
     .. index::
         pair: Client; Encoding
 

doc/src/extensions.rst

@@ -567,15 +567,16 @@ Isolation level constants
 -------------------------
 
 Psycopg2 `connection` objects hold informations about the PostgreSQL
-`transaction isolation level`_.  The current transaction level can be read
-from the `~connection.isolation_level` attribute.  The default isolation
-level is :sql:`READ COMMITTED`.  A different isolation level con be set
-through the `~connection.set_isolation_level()` method.  The level can be
-set to one of the following constants:
+`transaction isolation level`_.  By default Psycopg doesn't change the default
+configuration of the server (`ISOLATION_LEVEL_DEFAULT`); the default for
+PostgreSQL servers is typically :sql:`READ COMMITTED`, but this may be changed
+in the server configuration files.  A different isolation level can be set
+through the `~connection.set_isolation_level()` or `~connection.set_session()`
+methods.  The level can be set to one of the following constants:
 
 .. data:: ISOLATION_LEVEL_AUTOCOMMIT
 
-    No transaction is started when command are issued and no
+    No transaction is started when commands are executed and no
     `~connection.commit()` or `~connection.rollback()` is required.
     Some PostgreSQL command such as :sql:`CREATE DATABASE` or :sql:`VACUUM`
     can't run into a transaction: to run such command use::
@@ -653,11 +654,12 @@ set to one of the following constants:
 
 .. data:: ISOLATION_LEVEL_DEFAULT
 
-    Whatever is defined by the server, either via server configuration or by
-    statements executed within the session outside Pyscopg control; Psycopg
-    will not force an isolation level of its own. If you want to know what the
-    value is you can use a query such as :sql:`show transaction_isolation` or
-    :sql:`show default_transaction_isolation`.
+    A new transaction is started at the first `~cursor.execute()` command, but
+    the isolation level is not explicitly selected by Psycopg: the server will
+    use whatever level is defined in its configuration or by statements
+    executed within the session outside Pyscopg control.  If you want to know
+    what the value is you can use a query such as :sql:`show
+    transaction_isolation`.
 
     .. versionadded:: 2.7
 

doc/src/usage.rst

@@ -676,8 +676,7 @@ commands executed will be immediately committed and no rollback is possible. A
 few commands (e.g. :sql:`CREATE DATABASE`, :sql:`VACUUM`...) require to be run
 outside any transaction: in order to be able to run these commands from
 Psycopg, the connection must be in autocommit mode: you can use the
-`~connection.autocommit` property (`~connection.set_isolation_level()` in
-older versions).
+`~connection.autocommit` property.
 
 .. warning::