There was too much noise removed all those posts :-)
key = filename
> item = memcli:get(key)
> if (! defined item) {
> if (memcli:add(key . "_lock", lock_timeout_time, my_admin_username)) {
> [etc]
> } else {
> # lost the race, handle how you want
> }
> } else if (item.value == my_admin_username) {
> # good to go for that future request
> }
>
In my scenario files are displayed from a certain folder (queue) in an
application to many users at the same time. Each users has a cart where
they can add files they are working upon. They are going to remove it from
the cart or process it from the cart.
In the example pattern you have given,
1) The locking and the action are both happening together. However, we want
lock to move the item to the cart when the user clicks on the file. If
items are there in his cart, he can do anything he wants. Lock out time is
also not needed, user will remove it from the cart. [There is a flaw here
what if someone manipulates dom and add stuff to cart - but we won't worry
about that scenario - your example actually takes care of that since
validation and action if combined]
2) There is a different key called filename + lock which feels a bit
redundant in my case because I can achieve everything I want by entering
the key as filename and like you said username as the value.
I have loads of time to get this code in so I wanted to review this in
detail before suggesting something.
It has one major flaw, memcache.add can also be False because server is
down. I will create a blocker because now users can't add anything -
reducing with productivity at the cost of stopping errors. So I also need
to check if memcache client is up and running, else don't worry about
locks. That code is still not added.
def addToCart(filename, username):
ableToLock = memcache.add(filename, username)
if ableToLock:
# ableToLock can happen if the file is still present
# or if it was already processed.
if os.file.ispath(filename):
# I have a lock and file exists. Think of Cart as a
# JS Object from where you can pick items to "process".
return "Added To Cart"
else:
# I have a lock but looks like file was processed already.
# So removing the residual "key" created.
memcache.delete(filename)
return "Processed by another user."
else:
# "add" can also fail if remote server is down,
# but now we are not handling that now. It will
# block the user's ability to process anything.
user = memcache.get(filename)
if user and os.file.ispath(filename):
# I try my best to show the user processing it.
print "Being processed by %s" % user
else:
# But lost the race to find that.
print "Processed by another user."
def process(source):
shutil.move(source, destination)
# filename => source
memcache.delete(filename)
On Sunday, June 5, 2016 at 1:30:37 AM UTC+5:30, Dormando wrote:
>
> The pattern is identical between the one in the wiki and yours. Simply
> move the delete of the key until you're done using the lock, which would
> be in a separate request.
>
> In your case, you would probably set the contents of the key to be the
> name of the user who has it locked.
>
> In the original pseudocode:
> key = "expensive_frontpage_item"
> item = memcli:get(key)
> if (! defined item) {
> # Oh crap, we have to recache it!
> # Give us 60 seconds to recache the item.
> if (memcli:add(key . "_lock", 60)) {
> item = fetch_expensive_thing_from_database
> memcli:add(key, item, 86400)
> memcli:delete(key . "_lock")
> } else {
> # Lost the race. We can do any number of things:
> # - short sleep, then re-fetch.
> # - try the above a few times, then slow-fetch and return the item
> # - show the user a page without this expensive content
> # - show some less expensive content
> # - throw an error
> }
> }
> return item
>
> In yours:
> key = filename
> item = memcli:get(key)
> if (! defined item) {
> if (memcli:add(key . "_lock", lock_timeout_time, my_admin_username)) {
> [etc]
> } else {
> # lost the race, handle how you want
> }
> } else if (item.value == my_admin_username) {
> # good to go for that future request
> }
>
> Then when you're done holding the lock, delete the key.
>
> On Sat, 4 Jun 2016, Nishant Varma wrote:
>
> > I am reading
> https://github.com/memcached/memcached/wiki/ProgrammingTricks#ghetto-central-locking,
>
> it seems to deal with a slightly different lock scenario of getting some
> > expensive item from Database to avoid "Stampeding"
> > In my case its slightly different lock that I need. I show regular files
> from a folder in a web application to many users. So, to "lock" a file
> using memcache isn't this
> > simple API sufficient or I still need that pattern :-)?
> > def access(filename):
> > if memcache.add(filename, timestamp):
> > return "Access Granted. Lock Obtained" # Normally this results
> in checking HTML checkbox against the filename so User can do actions with
> that/
> > else:
> > return "Access Denied" # Normally this leads to an alert saying
> that someone else is working on this.
> >
> > Isn't this simple API using add good enough in my case? I am sorry if I
> am repeating this, but I could not really relate the "fetching expensive
> item from Database" to my
> > scenario which is why I even wrote a simple script to test the validity
> of the claim etc.
> >
> > Can you please let me know?
> >
> >
> > On Saturday, June 4, 2016 at 6:42:35 PM UTC+5:30, Nishant Varma wrote:
> > Excellent I rely on you. I guess this is the reason you say I am
> over-engineering this problem. Makes sense :-) I will again check the link
> you gave me. I will go
> > through the documentation this weekend.
> >
> > On Saturday, June 4, 2016 at 1:33:04 PM UTC+5:30, Dormando wrote:
> > Hey,
> >
> > You really don't need to test this: I'm telling you flatly,
> as an author
> > of this software and all of the documentation for it, that
> you should
> > absolutely not rely on that pattern. I'm trying to save you
> some time.
> >
> > The pattern that is slightly better is written explicitly in
> pseudocode in
> > the link I gave you several times in the issue. Please use
> it.
> >
> > Thanks,
> > -Dormando
> >
> > On Fri, 3 Jun 2016, Nishant Varma wrote:
> >
> > > Can anyone help me peer review this script
> https://gist.github.com/varmanishant/0129286d41038cc21471652a6460a5ff
> that demonstrate potential problems
> > with get set if it is used
> > > to implement distributed locking. I was suggested to
> modify from get set to add in this thread
> https://github.com/memcached/memcached/issues/163.
> > However I wanted a small
> > > simulation to demonstrate this.
> > >
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