113 lines
4.2 KiB
Plaintext
113 lines
4.2 KiB
Plaintext
========================
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Database instrumentation
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========================
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To help you understand and control the queries issued by your code, Django
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provides a hook for installing wrapper functions around the execution of
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database queries. For example, wrappers can count queries, measure query
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duration, log queries, or even prevent query execution (e.g. to make sure that
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no queries are issued while rendering a template with prefetched data).
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The wrappers are modeled after :doc:`middleware </topics/http/middleware>` --
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they are callables which take another callable as one of their arguments. They
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call that callable to invoke the (possibly wrapped) database query, and they
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can do what they want around that call. They are, however, created and
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installed by user code, and so don't need a separate factory like middleware do.
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Installing a wrapper is done in a context manager -- so the wrappers are
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temporary and specific to some flow in your code.
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As mentioned above, an example of a wrapper is a query execution blocker. It
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could look like this::
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def blocker(*args):
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raise Exception('No database access allowed here.')
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And it would be used in a view to block queries from the template like so::
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from django.db import connection
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from django.shortcuts import render
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def my_view(request):
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context = {...} # Code to generate context with all data.
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template_name = ...
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with connection.execute_wrapper(blocker):
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return render(request, template_name, context)
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The parameters sent to the wrappers are:
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* ``execute`` -- a callable, which should be invoked with the rest of the
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parameters in order to execute the query.
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* ``sql`` -- a ``str``, the SQL query to be sent to the database.
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* ``params`` -- a list/tuple of parameter values for the SQL command, or a
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list/tuple of lists/tuples if the wrapped call is ``executemany()``.
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* ``many`` -- a ``bool`` indicating whether the ultimately invoked call is
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``execute()`` or ``executemany()`` (and whether ``params`` is expected to be
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a sequence of values, or a sequence of sequences of values).
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* ``context`` -- a dictionary with further data about the context of
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invocation. This includes the connection and cursor.
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Using the parameters, a slightly more complex version of the blocker could
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include the connection name in the error message::
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def blocker(execute, sql, params, many, context):
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alias = context['connection'].alias
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raise Exception("Access to database '{}' blocked here".format(alias))
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For a more complete example, a query logger could look like this::
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import time
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class QueryLogger:
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def __init__(self):
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self.queries = []
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def __call__(self, execute, sql, params, many, context):
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current_query = {'sql': sql, 'params': params, 'many': many}
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start = time.monotonic()
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try:
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result = execute(sql, params, many, context)
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except Exception as e:
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current_query['status'] = 'error'
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current_query['exception'] = e
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raise
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else:
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current_query['status'] = 'ok'
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return result
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finally:
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duration = time.monotonic() - start
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current_query['duration'] = duration
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self.queries.append(current_query)
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To use this, you would create a logger object and install it as a wrapper::
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from django.db import connection
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ql = QueryLogger()
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with connection.execute_wrapper(ql):
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do_queries()
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# Now we can print the log.
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print(ql.queries)
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.. currentmodule:: django.db.backends.base.DatabaseWrapper
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``connection.execute_wrapper()``
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--------------------------------
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.. method:: execute_wrapper(wrapper)
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Returns a context manager which, when entered, installs a wrapper around
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database query executions, and when exited, removes the wrapper. The wrapper is
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installed on the thread-local connection object.
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``wrapper`` is a callable taking five arguments. It is called for every query
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execution in the scope of the context manager, with arguments ``execute``,
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``sql``, ``params``, ``many``, and ``context`` as described above. It's
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expected to call ``execute(sql, params, many, context)`` and return the return
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value of that call.
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