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From Pablo Estrada <pabl...@google.com>
Subject Re: About the Gauge metric API
Date Fri, 06 Apr 2018 18:35:30 GMT
Nobody wants to get rid of Gauges. I see that we have three separate themes
being discussed here, and I think it's useful to point them out and address
them independently:

1. Whether Gauges should change to hold string values.
2. If Gauges are to support string values, whether Gauges should also
continue to have an int API.
3. Whether Beam should support some sort of label / tag / worker-id for
aggregation of Gauges (maybe other metrics?)

-P.

On Fri, Apr 6, 2018 at 11:21 AM Ben Chambers <bchambers@apache.org> wrote:

> Gauges are incredibly useful for exposing the current state of the system.
> For instance, number of elements in a queue, current memory usage, number
> of RPCs in flight, etc. As mentioned above, these concepts exist in
> numerous systems for monitoring distributed environments, including
> Stackdriver Monitoring. The key to making them work is the addition of
> labels or tags, which as an aside are also useful for *all* metric types,
> not just Gauges.
>
> If Beam gets rid of Gauges, how would we go about exporting "current"
> values like memory usage, RPCs in flight, etc.?
>
> -- Ben
>
> On Fri, Apr 6, 2018 at 11:13 AM Kenneth Knowles <klk@google.com> wrote:
>
>> Just naively - the use cases that Gauge addresses seem relevant, and the
>> information seems feasible to gather and present. The bit that doesn't seem
>> to make sense is aggregating gauges by clobbering each other. So I think
>> that's just +1 Ben?
>>
>> On Fri, Apr 6, 2018 at 10:26 AM Raghu Angadi <rangadi@google.com> wrote:
>>
>>> I am not opposed to removing other data types, though they are extra
>>> convenience for user.
>>>
>>> In Scott's example above, if the metric is a counter, what are the
>>> guarantees provided? E.g. would it match the global count using GBK? If
>>> yes, then gauges (especially per-key gauges) can be very useful too (e.g.
>>> backlog for each Kafka partition/split).
>>>
>>> On Fri, Apr 6, 2018 at 10:01 AM Robert Bradshaw <robertwb@google.com>
>>> wrote:
>>>
>>>> A String API makes it clear(er) that the values will not be aggregated
>>>> in any way across workers. I don't think retaining both APIs (except for
>>>> possibly some short migration period) worthwhile. On another note, I still
>>>> find the distributed gague API to be a bit odd in general.
>>>>
>>>> On Fri, Apr 6, 2018 at 9:46 AM Raghu Angadi <rangadi@google.com> wrote:
>>>>
>>>>> I would be in favor of replacing the existing Gauge.set(long) API with
>>>>>> the String version and removing the old one. This would be a breaking
>>>>>> change. However this is a relatively new API and is still marked
>>>>>> @Experimental. Keeping the old API would retain the potential confusion.
>>>>>> It's better to simplify the API surface: having two APIs makes it
less
>>>>>> clear which one users should choose.
>>>>>
>>>>>
>>>>> Supporting additional data types sounds good. But the above states
>>>>> string API will replace the existing API. I do not see how string API
makes
>>>>> the semantics more clear.  Semantically both are same to the user.
>>>>>
>>>>> On Fri, Apr 6, 2018 at 9:31 AM Pablo Estrada <pabloem@google.com>
>>>>> wrote:
>>>>>
>>>>>> Hi Ben : D
>>>>>>
>>>>>> Sure, that's reasonable. And perhaps I started the discussion in
the
>>>>>> wrong direction. I'm not questioning the utility of Gauge metrics.
>>>>>>
>>>>>> What I'm saying is that Beam only supports integers,, but Gauges
are
>>>>>> aggregated by dropping old values depending on their update times;
so it
>>>>>> might be desirable to not restrict the data type to just integers.
>>>>>>
>>>>>> -P.
>>>>>>
>>>>>> On Fri, Apr 6, 2018 at 9:19 AM Ben Chambers <bchambers@apache.org>
>>>>>> wrote:
>>>>>>
>>>>>>> See for instance how gauge metrics are handled in Prometheus,
>>>>>>> Datadog and Stackdriver monitoring. Gauges are perfect for use
in
>>>>>>> distributed systems, they just need to be properly labeled. Perhaps
we
>>>>>>> should apply a default tag or allow users to specify one.
>>>>>>>
>>>>>>> On Fri, Apr 6, 2018, 9:14 AM Ben Chambers <bchambers@apache.org>
>>>>>>> wrote:
>>>>>>>
>>>>>>>> Some metrics backend label the value, for instance with the
worker
>>>>>>>> that sent it. Then the aggregation is latest per label. This
makes it
>>>>>>>> useful for holding values such as "memory usage" that need
to hold current
>>>>>>>> value.
>>>>>>>>
>>>>>>>> On Fri, Apr 6, 2018, 9:00 AM Scott Wegner <swegner@google.com>
>>>>>>>> wrote:
>>>>>>>>
>>>>>>>>> +1 on the proposal to support a "String" gauge.
>>>>>>>>>
>>>>>>>>> To expand a bit, the current API doesn't make it clear
that the
>>>>>>>>> gauge value is based on local state. If a runner chooses
to parallelize a
>>>>>>>>> DoFn across many workers, each worker will have its own
local Gauge metric
>>>>>>>>> and its updates will overwrite other values. For example,
from the API it
>>>>>>>>> looks like you could use a gauge to implement your own
element count metric:
>>>>>>>>>
>>>>>>>>> long count = 0;
>>>>>>>>> @ProcessElement
>>>>>>>>> public void processElement(ProcessContext c) {
>>>>>>>>>   myGauge.set(++count);
>>>>>>>>>   c.output(c.element());
>>>>>>>>> }
>>>>>>>>>
>>>>>>>>> This looks correct, but each worker has their own local
'count'
>>>>>>>>> field, and gauge metric updates from parallel workers
will overwrite each
>>>>>>>>> other rather than get aggregated. So the final value
would be "the number
>>>>>>>>> of elements processed on one of the workers". (The correct
implementation
>>>>>>>>> uses a Counter metric).
>>>>>>>>>
>>>>>>>>> I would be in favor of replacing the existing Gauge.set(long)
API
>>>>>>>>> with the String version and removing the old one. This
would be a breaking
>>>>>>>>> change. However this is a relatively new API and is still
marked
>>>>>>>>> @Experimental. Keeping the old API would retain the potential
confusion.
>>>>>>>>> It's better to simplify the API surface: having two APIs
makes it less
>>>>>>>>> clear which one users should choose.
>>>>>>>>>
>>>>>>>>> On Fri, Apr 6, 2018 at 8:28 AM Pablo Estrada <pabloem@google.com>
>>>>>>>>> wrote:
>>>>>>>>>
>>>>>>>>>> Hello all,
>>>>>>>>>> As I was working on adding support for Gauges in
Dataflow, some
>>>>>>>>>> noted that Gauge is a fairly unusual kind of metric
for a distributed
>>>>>>>>>> environment, since many workers will report different
values and stomp on
>>>>>>>>>> each other's all the time.
>>>>>>>>>>
>>>>>>>>>> We also looked at Flink and Dropwizard Gauge metrics
[1][2], and
>>>>>>>>>> we found that these use generics, and Flink explicitly
mentions that a
>>>>>>>>>> toString implementation is required[3].
>>>>>>>>>>
>>>>>>>>>> With that in mind, I'm thinking that it might make
sense to 1)
>>>>>>>>>> expand Gauge to support string values (keep int-based
API for backwards
>>>>>>>>>> compatibility), and migrate it to use string behind
the covers.
>>>>>>>>>>
>>>>>>>>>> What does everyone think about this?
>>>>>>>>>>
>>>>>>>>>> Best
>>>>>>>>>> -P.
>>>>>>>>>>
>>>>>>>>>> 1 -
>>>>>>>>>> https://ci.apache.org/projects/flink/flink-docs-release-1.3/monitoring/metrics.html#metric-types
>>>>>>>>>> 2 - https://metrics.dropwizard.io/3.1.0/manual/core/#gauges
>>>>>>>>>> 3 -
>>>>>>>>>> https://github.com/apache/flink/blob/master/docs/monitoring/metrics.md#gauge
>>>>>>>>>> JIRA issue for Gauge metrics -
>>>>>>>>>> https://issues.apache.org/jira/browse/BEAM-1616
>>>>>>>>>> --
>>>>>>>>>> Got feedback? go/pabloem-feedback
>>>>>>>>>> <https://goto.google.com/pabloem-feedback>
>>>>>>>>>>
>>>>>>>>> --
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> Got feedback? http://go/swegner-feedback
>>>>>>>>>
>>>>>>>> --
>>>>>> Got feedback? go/pabloem-feedback
>>>>>> <https://goto.google.com/pabloem-feedback>
>>>>>>
>>>>> --
Got feedback? go/pabloem-feedback

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