6.3.2 Fair semaphores
Because we can’t assume that all system clocks are exactly the same on all systems, our
earlier basic counting semaphore will have issues where clients on systems with slower
system clocks can steal the semaphore from clients on systems with faster clocks. Any
time there’s this kind of sensitivity, locking itself becomes unfair. We want to reduce
the effect of incorrect system times on acquiring the semaphore to the point where as
long as systems are within 1 second, system time doesn’t cause semaphore theft or
early semaphore expiration.
In order to minimize problems with inconsistent system times, we’ll add a counter
and a second ZSET. The counter creates a steadily increasing timer-like mechanism
that ensures that whoever incremented the counter first should be the one to get the
semaphore. We then enforce our requirement that clients that want the semaphore
who get the counter first also get the semaphore by using an “owner” ZSET with the
counter-produced value as the score, checking our identifier’s rank in the new ZSET to
determine which client got the semaphore. The new owner ZSET appears in figure 6.7.
We continue to handle timeouts the same way as our basic semaphore, by removing
entries from the system time ZSET. We propagate those timeouts to the new owner
ZSET by the use of ZINTERSTORE and the WEIGHTS argument.
Bringing it all together in listing 6.14, we first time out an entry by removing old
entries from the timeout ZSET and then intersect the timeout ZSET with the owner
ZSET, saving to and overwriting the owner ZSET. We then increment the counter and
add our counter value to the owner ZSET, while at the same time adding our current
system time to the timeout ZSET. Finally, we check whether our rank in the owner ZSET
is low enough, and if so, we have a semaphore. If not, we remove our entries from the
owner and timeout ZSETs.
This function has a few different pieces. We first clean up timed-out semaphores,
updating the owner ZSET and fetching the next counter ID for this item. After we’ve
added our time to the timeout ZSET and our counter value to the owner ZSET, we’re
ready to check to see whether our rank is low enough.
FAIR SEMAPHORES ON 32-BIT PLATFORMSOn 32-bit Redis platforms, integer
counters are limited to 231 - 1, the standard signed integer limit. An overflow
situation could occur on heavily used semaphores roughly once every 2 hours
in the worst case. Though there are a variety of workarounds, the simplest is
to switch to a 64-bit platform for any machine using any counter-based ID.
Let’s look at figure 6.8, which shows the sequence of operations that are performed
when process ID 8372 wants to acquire the semaphore at time 1326437039.100 when
there’s a limit of 5.
Releasing the semaphore is almost as easy as before, only now we remove our identifier
from both the owner and timeout ZSETs, as can be seen in this next listing.
If we wanted to be lazy, in most situations we could just remove our semaphore identifier
from the timeout ZSET; one of our steps in the acquire sequence is to refresh the
owner ZSET to remove identifiers that are no longer in the timeout ZSET. But by only
removing our identifier from the timeout ZSET, there’s a chance (rare, but possible)
that we removed the entry, but the acquire_fair_semaphore() was between the part
where it updated the owner ZSET and when it added its own identifiers to the timeout
and owner ZSETs. If so, this could prevent it from acquiring the semaphore when it
should’ve been able to. To ensure correct behavior in as many situations as possible,
we’ll stick to removing the identifier from both ZSETs.
Now we have a semaphore that doesn’t require that all hosts have the same system
time, though system times do need to be within 1 or 2 seconds in order to ensure that
semaphores don’t time out too early, too late, or not at all.