[erlang-questions] Does erlang:now() guarantee that subsequent calls to this BIF returns continuously increasing values even in erlang cluster?

Michael Turner michael.eugene.turner@REDACTED
Wed Apr 22 02:59:57 CEST 2015

"slapping "lamport clock" on it is reductive."

-- it was just a suggestion, in case something like that might work. My
suggestion was not intended to be all-inclusive or a panacea. Slapping
"reductive" on my suggestion is ... well, reductive?

"Not come with a pet solution to push through."

It's not my "pet solution". I don't have a "pet solution." I don't even
know what this guy's problem is, and you don't either. So how can I have a
pet solution if there's no way to know what the solution is in the first

I just happen to know that Erlang/OTP has this feature that's been in
"beta" for what seems to be a decade or more, one that exposes a kind of
Lamport clock functionality (even though the documentation fails to call it
that, which might be why it's still waiting for enough user input to refine
the interface -- people who go looking for something like that in
Erlang/OTP are not finding it.) seq_trace might be part of /a/ solution to
his problem. But since we don't know what his problem really is, I'm just
making suggestions.


Michael Turner
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On Tue, Apr 21, 2015 at 9:20 PM, Fred Hebert <mononcqc@REDACTED> wrote:

> On 04/21, Michael Turner wrote:
>> "Lamport/vector clocks and other similar ones operate on *causality*, but
>> this partial ordering is not the only one available or workable."
>> Whether it's "workable" depends on what's desired. Sorting by {Node,
>> Timestamp} is not accurate if causality matters and clocks have drifted
>> out
>> of synch. As they will. Hence Lamport's work, and the work of others. And
>> if causality doesn't matter, well, I wonder: why bother? Unless you just
>> want a rough idea of when certain things happened, in which case {Node,
>> Timestamp} can give you a /total/ order that's, if anything, more accurate
>> than what you need.
> That's not necessarily true. Let's see for different options and when they
> can be useful.
> - Lamport/vector clocks: causality. I wan to track the logical
> dependencies of changes.
> - `{Node, Timestamp}`: I have lots of local events (say HTTP requests  and
> responses in logs) and want to see *when* they happen and how far  apart.
> The timestmap might need to be monotonic, but the per-node  value lets me
> impose a logical order, track some density over time  (assuming I at least
> have NTP working), and so on.
> - {Shard, Timestamp}: I require a total order, but for events within a
> sharded data set.
> - {Cluster, Timestamp}: Each cluster I run might belong to specific
> customers or whatever, or run a specific set of hardware, or be a  logical
> division. In any case, it's possible they have their own time  or id
> service and I may want a partial or total order based on the  events within
> that cluster, without worrying I might want to compare  cross-cluster
> activity.
> - {Region, Timestamp}: Similar to the above, but by geographical area. I
> might decide that I need a total order on some form of transactions  and
> will run a service, but for latency (and if real world allows it),  I won't
> try to synchronize my time across data-centers or large  geographical areas.
> All of these 'labelled timestamps' *are* a partial order. They only define
> it on some label. I.e. you can sort all timestamps within a
> node/shard/cluster/region, but can't do it across boundaries.
> There are other avenues that even combine some of them; One interesting
> case is inspired by Google's Chubby and CRDTs: You use a timestamp
> synchronized by NTP, guaranteeing you a maximal drift interval. You then
> add in a lamport clock whenever two events happen within too close of an
> interval that we cannot guarantee from the system clocks they truly
> happened apart.
> The lamport clock is mergeable in a deterministic way that is also
> commutative and idempotent (that's a CRDT!), and acts as a tie-breaker
> between events that happen at too close together.
> This way you get reliable timestamps when you can, and when you suddenly
> can't, you get a form of causality (or global monotonicity) to break things
> up.
> slapping "lamport clock" on it is reductive. It's a good way to track some
> levels of causality, but has its limitations. If you only *need* node-local
> accuracy and you have access to a monotonic clock, it might be far less
> work to just slap the monotonic clock into things than weave the logical
> clock through everything, and obtain the same logical result in the end
> (plus more information). Maybe it's not the best solution either.
> But really, if we want to make good recommendations, we have to ask what
> the user needs. Not come with a pet solution to push through.
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