''Part of DjangoSpecifications''

= Threading improvements in Django =

According to tickets #5632, #6950 and discussions http://groups.google.com/group/django-users/browse_frm/thread/a7d42475b66530bd, http://groups.google.com/group/django-developers/browse_thread/thread/fbcfa88c997d1bb3, at least the following components of Django are not entirely thread-safe:
 * django.template.loader
 * django.db.models
 * django.contrib.sessions

This specification outlines the changes that need to be implemented to solve these issues.

A related task is to identify other components not mentioned here that have threading issues.

See #1442 and http://groups.google.com/group/django-developers/browse_thread/thread/905f79e350525c95 for threading discussions.

== Globals ==

There are three types of globals:
 1. read-only globals that are never assigned to (THREAD-SAFE),
 1. globals assigned to in a function by using the `global` keyword (NOT THREAD-SAFE),
 1. global mutable data structures (lists and dictionaries, also instances) that are assigned to at module level but whose elements are modified in functions and that are accessed without using the `global` keyword (NOT THREAD-SAFE unless never modified).

"Not thread-safe" has two broad subcategories:
 * inefficiencies due to calls meant to occur only once occurring more than once (the general `if not foo: initialize foo`, including `memoize` decorator),
 * errors due to incomplete initialization.

Incomplete initialization errors can generally be avoided by using full
assignment instead of elementwise modification (that's how it is done in the
source mostly). That is,

'''BAD'''
{{{
foo = []
def bar():
   ...
   if not foo:
      for x in y:
	 foo.append(x)
}}}

'''BETTER'''
{{{
foo = []
def bar():
   ...
   global foo
   if not foo:
      tmp = []
      for x in y:
	 tmp.append(x)
      foo = tmp # atomic assignment
}}}

The following modules' use of globals needs review.

See below for raw `grep` results.

||'''Module'''||'''Globals'''||'''Notes'''||
||settings and global_settings||?||not reviewed||
||utils/_decimal.py||lots, including code||not reviewed||
||django/contrib/sites/models.py||SITE_CACHE||minor, one db hit intended, more than one possible||

=== django/contrib/sites/models.py ===

Globals used:
{{{
SITE_CACHE
}}}

=== django/template ===

Globals used:
{{{
context.py: _standard_context_processors
__init__.py: invalid_var_format_string, libraries
loader.py: template_source_loaders
}}}

=== django/utils/translation ===

Globals used:
{{{
trans_real.py: _accepted, _active, _default, _translations
}}}

=== django/contrib/sites/models.py ===

Globals used:
{{{
SITE_CACHE
}}}



{{{
}}}

== Raw `grep` results ==

=== Globals accessed with the `global` keyword ===

{{{
$ grep -r '^[[:space:]]*global ' django/ | egrep -v '(\.svn|\.html|\.css|\.pyc|_doctest\.py)' | sort | uniq
}}}

yields the following results

{{{
django/contrib/sites/models.py:        global SITE_CACHE
django/core/management/__init__.py:    global _commands
django/template/context.py:    global _standard_context_processors
django/template/__init__.py:                    global invalid_var_format_string
django/template/loader.py:    global template_source_loaders
django/utils/translation/trans_real.py:    global _accepted
django/utils/translation/trans_real.py:    global _active
django/utils/translation/trans_real.py:    global _default, _active
django/utils/translation/trans_real.py:        global _translations
django/utils/translation/trans_real.py:    global _translations
}}}

Out of these, `django.core.management` is not used in multi-threading context.

=== Global dictionaries ===

{{{
$ grep -r '^[[:alnum:]_]\+ *= *{' django | egrep -v '(\.svn|_doctest\.py)' | sort
}}}

yields the following results

{{{
django/conf/global_settings.py:ABSOLUTE_URL_OVERRIDES = {}
django/conf/global_settings.py:DATABASE_OPTIONS = {}          # Set to empty dictionary for default.
django/contrib/admin/utils.py:ROLES = {
django/contrib/admin/views/doc.py:DATA_TYPE_MAPPING = {
django/contrib/formtools/tests.py:test_data = {'field1': u'foo',
django/contrib/localflavor/ca/ca_provinces.py:PROVINCES_NORMALIZED = {
django/contrib/localflavor/in_/in_states.py:STATES_NORMALIZED = {
django/contrib/localflavor/us/us_states.py:STATES_NORMALIZED = {
django/contrib/sites/models.py:SITE_CACHE = {}
django/core/cache/__init__.py:BACKENDS = {
django/core/cache/__init__.py:DEPRECATED_BACKENDS = {
django/core/handlers/wsgi.py:STATUS_CODE_TEXT = {
django/core/serializers/__init__.py:BUILTIN_SERIALIZERS = {
django/core/serializers/__init__.py:_serializers = {}
django/core/servers/basehttp.py:_hop_headers = {
django/core/servers/fastcgi.py:FASTCGI_OPTIONS = {
django/core/urlresolvers.py:_callable_cache = {} # Maps view and url pattern names to their view functions.
django/core/urlresolvers.py:_resolver_cache = {} # Maps urlconf modules to RegexURLResolver instances.
django/db/backends/dummy/creation.py:DATA_TYPES = {}
django/db/backends/dummy/introspection.py:DATA_TYPES_REVERSE = {}
django/db/backends/mysql/creation.py:DATA_TYPES = {
django/db/backends/mysql/introspection.py:DATA_TYPES_REVERSE = {
django/db/backends/mysql_old/creation.py:DATA_TYPES = {
django/db/backends/mysql_old/introspection.py:DATA_TYPES_REVERSE = {
django/db/backends/oracle/creation.py:DATA_TYPES = {
django/db/backends/oracle/creation.py:REMEMBER = {}
django/db/backends/oracle/introspection.py:DATA_TYPES_REVERSE = {
django/db/backends/postgresql/creation.py:DATA_TYPES = {
django/db/backends/postgresql/introspection.py:DATA_TYPES_REVERSE = {
django/db/backends/postgresql_psycopg2/introspection.py:DATA_TYPES_REVERSE = {
django/db/backends/sqlite3/creation.py:DATA_TYPES = {
django/db/backends/sqlite3/introspection.py:BASE_DATA_TYPES_REVERSE = {
django/db/models/fields/related.py:pending_lookups = {}
django/db/models/query.py:LEGACY_ORDERING_MAPPING = {'ASC': '_', 'DESC': '-_', 'RANDOM': '?'}
django/db/transaction.py:dirty = {}
django/db/transaction.py:state = {}
django/dispatch/dispatcher.py:connections = {}
django/dispatch/dispatcher.py:senders = {}
django/dispatch/dispatcher.py:sendersBack = {}
django/template/__init__.py:libraries = {}
django/utils/dates.py:MONTHS = {
django/utils/dates.py:MONTHS_3 = {
django/utils/dates.py:MONTHS_3_REV = {
django/utils/dates.py:MONTHS_AP = { # month names in Associated Press style
django/utils/dates.py:WEEKDAYS = {
django/utils/dates.py:WEEKDAYS_ABBR = {
django/utils/dates.py:WEEKDAYS_REV = {
django/utils/_decimal.py:_condition_map = {ConversionSyntax:InvalidOperation,
django/utils/_decimal.py:_infinity_map = {
django/utils/simplejson/decoder.py:BACKSLASH = {
django/utils/simplejson/decoder.py:_CONSTANTS = {
django/utils/simplejson/encoder.py:ESCAPE_DCT = {
django/utils/termcolors.py:opt_dict = {'bold': '1', 'underscore': '4', 'blink': '5', 'reverse': '7', 'conceal': '8'}
django/utils/translation/trans_null.py:TECHNICAL_ID_MAP = {
django/utils/translation/trans_real.py:_accepted = {}
django/utils/translation/trans_real.py:_active = {}
django/utils/translation/trans_real.py:_translations = {}
}}}

Out of these, the following are read-only (i.e. not changed anywhere in code) or otherwise irrelevant: `contrib/admin, formtools tests, localflavor mappings`, `core/cache, core/handlers, core/serializers/__init__.py:BUILTIN_SERIALIZERS`, `core/servers, db/backends, db/models/query.py, utils/dates.py`, `utils/_decimal.py, utils/simplejson, utils/termcolors.py`, `utils/translation/trans_null.py`.

`SITE_CACHE` and everything in `django.utils.translation.trans_real` has already been listed under `globals` above. 

`_callable_cache` and `_resolver_cache` in django/core/urlresolvers.py are used within the memoize decorator, `result = func(*args)` may be called more than once in `utils/functional.py`, but this should generally be a non-issue.

That leaves the following relevant global dicts not listed before:
{{{
django/core/serializers/__init__.py:_serializers = {}
django/db/models/fields/related.py:pending_lookups = {}
django/db/transaction.py:dirty = {}
django/db/transaction.py:state = {}
django/dispatch/dispatcher.py:connections = {}
django/dispatch/dispatcher.py:senders = {}
django/dispatch/dispatcher.py:sendersBack = {}
django/template/__init__.py:libraries = {}
}}}

=== Global lists ===

{{{
$ grep -r '^[[:alnum:]_]\+ *= *\[' django | egrep -v '(\.svn|_doctest\.py|__all__)' | sort
}}}

yields the following results

{{{
django/db/models/fields/__init__.py:BLANK_CHOICE_DASH = [("", "---------")]
django/db/models/fields/__init__.py:BLANK_CHOICE_NONE = [("", "None")]
django/template/__init__.py:builtins = []
django/template/loaders/app_directories.py:app_template_dirs = []
django/utils/_decimal.py:rounding_functions = [name for name in Decimal.__dict__.keys() if name.startswith('_round_')]
django/utils/_decimal.py:_signals = [Clamped, DivisionByZero, Inexact, Overflow, Rounded,
django/utils/html.py:DOTS = ['·', '*', '\xe2\x80\xa2', '•', '•', '•']
django/utils/html.py:LEADING_PUNCTUATION  = ['(', '<', '&lt;']
django/utils/html.py:TRAILING_PUNCTUATION = ['.', ',', ')', '>', '\n', '&gt;']
django/utils/simplejson/decoder.py:ANYTHING = [
}}}

Leaving out the irrelevant read-only ones, the following remain:
{{{
django/template/__init__.py:builtins = []
django/template/loaders/app_directories.py:app_template_dirs = []
}}}

As a matter of style, the read-only ones should really be tuples, not lists (the 'say what you mean' idiom -- if it shouldn't be modified, make it a tuple).