Github user tillrohrmann commented on the issue:
https://github.com/apache/flink/pull/2112
We decided to add the histogram metric, because users requested this
feature. The important thing is that the histograms are not used by the runtime
per default, because they indeed don't come for free. But if the user decides
to use such a metric, then he should be able to explicitly register a histogram.
Additionally, we don't want to add a strict dependency on dropwizard from
flink-core. Therefore, I introduced the `Histogram` interface which effectively
hides the concrete implementation. So later we can easily swap the
implementation out.
One of these implementations is the `DropwizardHistogramWrapper` which
allows you to use Dropwizard histograms in Flink. The reason for this is that
Dropwizard's `Histogram` has already a lot of functionality (different
reservoirs for sampling streams, for example) and it would be a lot of
duplicate work to reimplement it.
Since the assumption is that `Histograms` are a user metric, it seems to be
ok for me to not ship an implementation of the interface with flink-core but to
require the user to include an additional module such as
flink-metrics-dropwizard. This module would add the dropwizard dependency
anyway. If necessary, then we can also add our own histogram implementation
later.
The PR does not introduce a Dropwizard compatibility layer. Dropwizard is
only one of many possible implementation for our metrics (counter, gauge and
histogram so far). It is up to the community to decide which other metrics to
add.
I agree that the naming of `HistogramWrapper` is not so distinctive. I
actually sticked to the naming scheme for `Counter` and `Gauge` which are
called `CounterWrapper` and `GaugeWrapper`. This can be easily changed. Do you
have a proposal?
I'm sorry if you feel angry because you already did some of this work which
was later removed. But please keep in mind that not everyone was involved in
the metrics PR.
---
If your project is set up for it, you can reply to this email and have your
reply appear on GitHub as well. If your project does not have this feature
enabled and wishes so, or if the feature is enabled but not working, please
contact infrastructure at [email protected] or file a JIRA ticket
with INFRA.
---
