- Set cached session for Django session (refer to
https://docs.djangoproject.com/en/2.0/topics/http/sessions/#using-cached-
sessions)
- On Django app, try to log in or set a language in order to make a
session cookie. At this step, cache nodes should be available.
- To set cache nodes **unavailable**, by changing to wrong node name or
turning the nodes off.
- Reload a page on Django app. Django app responds back with this HTTP
header, which deletes a session cookie:
{{{
Set-Cookie: sessionid=""; expires=Thu, 01-Jan-1970 00:00:00 GMT; Max-
Age=0; Path=/
}}}
The problematic codes are at django/contrib/sessions/middleware.py:22 :
{{{
def process_response(self, request, response):
"""
If request.session was modified, or if the configuration is to
save the
session every time, save the changes and set a session cookie or
delete
the session cookie if the session has been emptied.
"""
try:
accessed = request.session.accessed
modified = request.session.modified
empty = request.session.is_empty()
except AttributeError:
pass
else:
# First check if we need to delete this cookie.
# The session should be deleted only if the session is
entirely empty
if settings.SESSION_COOKIE_NAME in request.COOKIES and empty:
response.delete_cookie(
settings.SESSION_COOKIE_NAME,
path=settings.SESSION_COOKIE_PATH,
domain=settings.SESSION_COOKIE_DOMAIN,
)
...
}}}
I guess the initial intention was to delete a session if there is no
values in it. However, it happens that code execution reaches at that code
without exception, even if cache nodes are unavailable. The reason is, I
already explained above, they just work that way even if the cache backend
failed.
I first met this bug while I was testing failover of AWS Elasticache
(Redis). I was in testing of failover scenario, but Django application got
me logged out repeatedly, even though session data itself is remaining in
the cache replica. (it should, because it was doing failover, not reboot)
IMHO, before checking ''empty'' of session data, there should be a logic
to check cache backend is actually available. I found out
''request.session.cache_key'' can do that function, but it looks less
explicit. Please show be a better way to do this, if you have one.
--
Ticket URL: <https://code.djangoproject.com/ticket/29203>
Django <https://code.djangoproject.com/>
The Web framework for perfectionists with deadlines.
* Attachment "patch_cached_session_deleting_session_cookie.diff" added.
workaround patch
* type: Uncategorized => Bug
--
Ticket URL: <https://code.djangoproject.com/ticket/29203#comment:1>
Old description:
New description:
Some cache backends (AFAIK Memcached and Redis) has a feature to ignore
--
--
Ticket URL: <https://code.djangoproject.com/ticket/29203#comment:2>
--
Ticket URL: <https://code.djangoproject.com/ticket/29203#comment:3>
Old description:
> Some cache backends (AFAIK Memcached and Redis) has a feature to ignore
New description:
Some cache backends (AFAIK Memcached and Redis) has a feature to ignore
connection timeout exception, in order to ensure Django application
working even if cache has failed. This can lead a subtle bug, deleting a
session cookie. Following steps are reproduce this bug:
- Set cached session for Django session (refer to
https://docs.djangoproject.com/en/2.0/topics/http/sessions/#using-cached-
sessions)
- On Django app, try to log in or set a language in order to make a
session cookie. At this step, cache nodes should be available.
- To set cache nodes **unavailable**, by changing to wrong node name or
turning the nodes off.
- Reload a page on Django app. Django app responds back with this HTTP
header, which deletes a session cookie:
{{{
Set-Cookie: sessionid=""; expires=Thu, 01-Jan-1970 00:00:00 GMT; Max-
Age=0; Path=/
}}}
The problematic codes are at django/contrib/sessions/middleware.py:37 :
--
--
Ticket URL: <https://code.djangoproject.com/ticket/29203#comment:4>
Old description:
> Some cache backends (AFAIK Memcached and Redis) has a feature to ignore
> connection timeout exception, in order to ensure Django application
> working even if cache has failed. This can lead a subtle bug, deleting a
> session cookie. Following steps are reproduce this bug:
>
> - Set cached session for Django session (refer to
> https://docs.djangoproject.com/en/2.0/topics/http/sessions/#using-cached-
> sessions)
> - On Django app, try to log in or set a language in order to make a
> session cookie. At this step, cache nodes should be available.
> - To set cache nodes **unavailable**, by changing to wrong node name or
> turning the nodes off.
> - Reload a page on Django app. Django app responds back with this HTTP
> header, which deletes a session cookie:
> {{{
> Set-Cookie: sessionid=""; expires=Thu, 01-Jan-1970 00:00:00 GMT; Max-
> Age=0; Path=/
> }}}
>
> The problematic codes are at django/contrib/sessions/middleware.py:37 :
New description:
Some cache backends (AFAIK Memcached and Redis) has a feature to ignore
connection timeout exception, in order to ensure Django application
working even if cache has failed. This can lead a subtle bug, deleting a
session cookie. Following steps are reproduce this bug:
- Set cached session for Django session (refer to
https://docs.djangoproject.com/en/2.0/topics/http/sessions/#using-cached-
sessions)
- On Django app, try to log in or set a language in order to make a
session cookie. At this step, cache nodes should be available.
- To set cache nodes **unavailable**, by changing to wrong node name or
turning the nodes off.
- Reload a page on Django app. Django app responds back with this HTTP
header, which deletes a session cookie:
{{{
Set-Cookie: sessionid=""; expires=Thu, 01-Jan-1970 00:00:00 GMT; Max-
Age=0; Path=/
}}}
The problematic codes are at django/contrib/sessions/middleware.py:37 :
{{{
def process_response(self, request, response):
"""
If request.session was modified, or if the configuration is to
save the
session every time, save the changes and set a session cookie or
delete
the session cookie if the session has been emptied.
"""
try:
accessed = request.session.accessed
modified = request.session.modified
empty = request.session.is_empty()
except AttributeError:
pass
else:
# First check if we need to delete this cookie.
# The session should be deleted only if the session is
entirely empty
if settings.SESSION_COOKIE_NAME in request.COOKIES and empty:
# NEED TO UPDATE!
response.delete_cookie(
settings.SESSION_COOKIE_NAME,
path=settings.SESSION_COOKIE_PATH,
domain=settings.SESSION_COOKIE_DOMAIN,
)
...
}}}
I guess the initial intention was to delete a session if there is no
values in it. However, it happens that code execution reaches at :37 that
code without exception, even if cache nodes are unavailable. The reason
is, I already explained above, they just work that way even if the cache
backend failed.
I first met this bug while I was testing failover of AWS Elasticache
(Redis). I was in testing of failover scenario, but Django application got
me logged out repeatedly, even though session data itself is remaining in
the cache replica. (it should, because it was doing failover, not reboot)
IMHO, before checking ''empty'' of session data, there should be a logic
to check cache backend is actually available. I found out
''request.session.cache_key'' can do that function, but it looks less
explicit. Please show be a better way to do this, if you have one.
--
--
Ticket URL: <https://code.djangoproject.com/ticket/29203#comment:5>
Comment (by Kenial Sookyum Lee):
When I tested the settings with SESSION_ENGINE =
"django.contrib.sessions.backends.cached_db" (ie. 'write-through cache'),
I didn't get through this bug. Configuration is: Django 1.11, MySQL
5.6.35, mysqlclient 1.3.12, Redis 3.2.7 (x64), django-redis 4.9.0.
--
Ticket URL: <https://code.djangoproject.com/ticket/29203#comment:6>
* version: master => 1.11
Old description:
> Some cache backends (AFAIK Memcached and Redis) has a feature to ignore
> connection timeout exception, in order to ensure Django application
> working even if cache has failed. This can lead a subtle bug, deleting a
> session cookie. Following steps are reproduce this bug:
>
> - Set cached session for Django session (refer to
> https://docs.djangoproject.com/en/2.0/topics/http/sessions/#using-cached-
> sessions)
> - On Django app, try to log in or set a language in order to make a
> session cookie. At this step, cache nodes should be available.
> - To set cache nodes **unavailable**, by changing to wrong node name or
> turning the nodes off.
> - Reload a page on Django app. Django app responds back with this HTTP
> header, which deletes a session cookie:
> {{{
> Set-Cookie: sessionid=""; expires=Thu, 01-Jan-1970 00:00:00 GMT; Max-
> Age=0; Path=/
> }}}
>
> The problematic codes are at django/contrib/sessions/middleware.py:37 :
> {{{
> def process_response(self, request, response):
> """
> If request.session was modified, or if the configuration is to
> save the
> session every time, save the changes and set a session cookie or
> delete
> the session cookie if the session has been emptied.
> """
> try:
> accessed = request.session.accessed
> modified = request.session.modified
> empty = request.session.is_empty()
> except AttributeError:
> pass
> else:
> # First check if we need to delete this cookie.
> # The session should be deleted only if the session is
> entirely empty
> if settings.SESSION_COOKIE_NAME in request.COOKIES and empty:
> # NEED TO UPDATE!
> response.delete_cookie(
> settings.SESSION_COOKIE_NAME,
> path=settings.SESSION_COOKIE_PATH,
> domain=settings.SESSION_COOKIE_DOMAIN,
> )
> ...
> }}}
>
> I guess the initial intention was to delete a session if there is no
> values in it. However, it happens that code execution reaches at :37 that
> code without exception, even if cache nodes are unavailable. The reason
> is, I already explained above, they just work that way even if the cache
> backend failed.
>
> I first met this bug while I was testing failover of AWS Elasticache
> (Redis). I was in testing of failover scenario, but Django application
> got me logged out repeatedly, even though session data itself is
> remaining in the cache replica. (it should, because it was doing
> failover, not reboot)
>
> IMHO, before checking ''empty'' of session data, there should be a logic
> to check cache backend is actually available. I found out
> ''request.session.cache_key'' can do that function, but it looks less
> explicit. Please show be a better way to do this, if you have one.
New description:
Some cache backends (AFAIK Memcached and Redis) has a feature to ignore
connection timeout exception, in order to ensure Django application
working even if cache has failed. This can lead a subtle bug, deleting a
session cookie. Following steps are reproduce this bug:
- Set cached session for Django session (refer to
https://docs.djangoproject.com/en/2.0/topics/http/sessions/#using-cached-
sessions)
- On Django app, try to log in or set a language in order to make a
session cookie. At this step, cache nodes should be available.
- To set cache nodes **unavailable**, by changing to wrong node name or
turning the nodes off.
- Reload a page on Django app. Django app responds back with this HTTP
header, which deletes a session cookie:
{{{
Set-Cookie: sessionid=""; expires=Thu, 01-Jan-1970 00:00:00 GMT; Max-
Age=0; Path=/
}}}
The problematic codes are at django/contrib/sessions/middleware.py:37 :
{{{
def process_response(self, request, response):
"""
If request.session was modified, or if the configuration is to
save the
session every time, save the changes and set a session cookie or
delete
the session cookie if the session has been emptied.
"""
try:
accessed = request.session.accessed
modified = request.session.modified
empty = request.session.is_empty()
except AttributeError:
pass
else:
# First check if we need to delete this cookie.
# The session should be deleted only if the session is
entirely empty
if settings.SESSION_COOKIE_NAME in request.COOKIES and empty:
# NEED TO UPDATE!
response.delete_cookie(
settings.SESSION_COOKIE_NAME,
path=settings.SESSION_COOKIE_PATH,
domain=settings.SESSION_COOKIE_DOMAIN,
)
...
}}}
I guess the initial intention was to delete a session if there is no
values in it. However, it happens that code execution reaches at :37 that
code without exception, even if cache nodes are unavailable. The reason
is, I already explained above, they just work that way even if the cache
backend failed.
I first met this bug while I was testing failover of AWS Elasticache
(Redis). I was in testing of failover scenario, but Django application got
me logged out repeatedly, even though session data itself is remaining in
the cache replica. (it should, because it was doing failover, not reboot)
IMHO, before checking ''empty'' of session data, there should be a logic
to check cache backend is actually available. I found out
''request.session.cache_key'' can do that function, but it looks less
explicit. Please show be a better way to do this, if you have one.
fyi. Configuration is: Django 1.11, MySQL 5.6.35, mysqlclient 1.3.12,
Redis 3.2.7 (x64), and django-redis 4.9.0. I found out this bug on Django
1.11, but it has been changed since. So I believe this bug is applied to
Django 2.x as well.
--
--
Ticket URL: <https://code.djangoproject.com/ticket/29203#comment:7>
Old description:
> Some cache backends (AFAIK Memcached and Redis) has a feature to ignore
> connection timeout exception, in order to ensure Django application
> working even if cache has failed. This can lead a subtle bug, deleting a
> session cookie. Following steps are reproduce this bug:
>
> - Set cached session for Django session (refer to
> https://docs.djangoproject.com/en/2.0/topics/http/sessions/#using-cached-
> sessions)
> - On Django app, try to log in or set a language in order to make a
> session cookie. At this step, cache nodes should be available.
> - To set cache nodes **unavailable**, by changing to wrong node name or
> turning the nodes off.
> - Reload a page on Django app. Django app responds back with this HTTP
> header, which deletes a session cookie:
> {{{
> Set-Cookie: sessionid=""; expires=Thu, 01-Jan-1970 00:00:00 GMT; Max-
> Age=0; Path=/
> }}}
>
> The problematic codes are at django/contrib/sessions/middleware.py:37 :
> {{{
> def process_response(self, request, response):
> """
> If request.session was modified, or if the configuration is to
> save the
> session every time, save the changes and set a session cookie or
> delete
> the session cookie if the session has been emptied.
> """
> try:
> accessed = request.session.accessed
> modified = request.session.modified
> empty = request.session.is_empty()
> except AttributeError:
> pass
> else:
> # First check if we need to delete this cookie.
> # The session should be deleted only if the session is
> entirely empty
> if settings.SESSION_COOKIE_NAME in request.COOKIES and empty:
> # NEED TO UPDATE!
> response.delete_cookie(
> settings.SESSION_COOKIE_NAME,
> path=settings.SESSION_COOKIE_PATH,
> domain=settings.SESSION_COOKIE_DOMAIN,
> )
> ...
> }}}
>
> I guess the initial intention was to delete a session if there is no
> values in it. However, it happens that code execution reaches at :37 that
> code without exception, even if cache nodes are unavailable. The reason
> is, I already explained above, they just work that way even if the cache
> backend failed.
>
> I first met this bug while I was testing failover of AWS Elasticache
> (Redis). I was in testing of failover scenario, but Django application
> got me logged out repeatedly, even though session data itself is
> remaining in the cache replica. (it should, because it was doing
> failover, not reboot)
>
> IMHO, before checking ''empty'' of session data, there should be a logic
> to check cache backend is actually available. I found out
> ''request.session.cache_key'' can do that function, but it looks less
> explicit. Please show be a better way to do this, if you have one.
>
> fyi. Configuration is: Django 1.11, MySQL 5.6.35, mysqlclient 1.3.12,
> Redis 3.2.7 (x64), and django-redis 4.9.0. I found out this bug on Django
> 1.11, but it has been changed since. So I believe this bug is applied to
> Django 2.x as well.
New description:
Some cache backends (AFAIK Memcached and Redis) has a feature to ignore
connection timeout exception, in order to ensure Django application
working even if cache has failed. This can lead a subtle bug, deleting a
session cookie. Following steps are reproduce this bug:
- Set cached session for Django session (refer to
https://docs.djangoproject.com/en/2.0/topics/http/sessions/#using-cached-
sessions)
- On Django app, try to log in or set a language in order to make a
session cookie. At this step, cache nodes should be available.
- To set cache nodes **unavailable**, by changing to wrong node name or
turning the nodes off.
- Reload a page on Django app. Django app responds back with this HTTP
header, which deletes a session cookie:
{{{
Set-Cookie: sessionid=""; expires=Thu, 01-Jan-1970 00:00:00 GMT; Max-
Age=0; Path=/
}}}
The problematic codes are at django/contrib/sessions/middleware.py:37 :
{{{
def process_response(self, request, response):
"""
If request.session was modified, or if the configuration is to
save the
session every time, save the changes and set a session cookie or
delete
the session cookie if the session has been emptied.
"""
try:
accessed = request.session.accessed
modified = request.session.modified
empty = request.session.is_empty()
except AttributeError:
pass
else:
# First check if we need to delete this cookie.
# The session should be deleted only if the session is
entirely empty
if settings.SESSION_COOKIE_NAME in request.COOKIES and empty:
# NEED TO UPDATE!
response.delete_cookie(
settings.SESSION_COOKIE_NAME,
path=settings.SESSION_COOKIE_PATH,
domain=settings.SESSION_COOKIE_DOMAIN,
)
...
}}}
I guess the initial intention was to delete a session if there is no
values in it. However, it happens that code execution reaches at :37 that
code without exception, even if cache nodes are unavailable. The reason
is, I already explained above, they just work that way even if the cache
backend failed.
I first met this bug while I was testing failover of AWS Elasticache
(Redis). I was in testing of failover scenario, but Django application got
me logged out repeatedly, even though session data itself is remaining in
the cache replica. (it should, because it was doing failover, not reboot)
IMHO, before checking ''empty'' of session data, there should be a logic
to check cache backend is actually available. I found out
''request.session.cache_key'' can do that function, but it looks less
explicit. Please show be a better way to do this, if you have one.
fyi. Configuration is: Django 1.11, MySQL 5.6.35, mysqlclient 1.3.12,
Redis 3.2.7 (x64), and django-redis 4.9.0. I found out this bug on Django
1.11 though, but it has not been changed since, so this bug must happen on
Django 2.x as well.
--
--
Ticket URL: <https://code.djangoproject.com/ticket/29203#comment:8>
* has_patch: 1 => 0
Comment:
I'm not sure if this is something that Django should implement. The patch
doesn't look elegant because it's adding an implementation detail
(`cache_key`) of a couple of the backends to code that's designed for all
backends.
As far as I know, Django generally [https://github.com/jazzband/django-
constance/issues/236#issuecomment-346974084) requires the cache to be
available]. It might be that we should document this requirement and
discourage use of the "ignore connection timeout exception" option that
you mentioned.
--
Ticket URL: <https://code.djangoproject.com/ticket/29203#comment:9>
Comment (by Kenial Sookyum Lee):
Replying to [comment:9 Tim Graham]:
> I'm not sure if this is something that Django should implement. The
patch doesn't look elegant because it's adding an implementation detail
(`cache_key`) of a couple of the backends to code that's designed for all
backends.
As long as there is another way to check if cache backend is available,
it's okay. Any idea?
> As far as I know, Django generally [https://github.com/jazzband/django-
constance/issues/236#issuecomment-346974084) requires the cache to be
available]. It might be that we should document this requirement and
discourage use of the "ignore connection timeout exception" option that
you mentioned.
Agreed. But, the thing is, cache downtime itself is inevitable. As I told
you, I found out this bug when I test failover AWS Elasticache - it means,
this can happen during any kind of cache backend failover scenario. If
"CAUTION: failover of cache backend might cause termination of Django
sessions unintentionally" is in documentation, well... It sounds awkward.
--
Ticket URL: <https://code.djangoproject.com/ticket/29203#comment:10>
* stage: Unreviewed => Accepted
Comment:
I'm not sure what to do, but your report seems credible.
--
Ticket URL: <https://code.djangoproject.com/ticket/29203#comment:11>
Comment (by dangelsaurus):
I've confirmed this is still an issue, while load testing my
application\server to the point that the cache took longer than the
timeout (set to 5s in my case, for testing). The big issues for me is
that there is no indication (thrown exception) that this occurred.
--
Ticket URL: <https://code.djangoproject.com/ticket/29203#comment:12>
* version: 1.11 => 3.0
--
Ticket URL: <https://code.djangoproject.com/ticket/29203#comment:13>