@grobie who might be most keen on this.
@juliusv whom I gave a short preview in person.
But then probably everybody else is interested, @fabxc , @brian-brazil , …
General context and approch
This is the first part of the long awaited wider refurbishment of
client_golang/prometheus/.... After a lot of struggling, I decided
to not go for one breaking big-bang, but cut things into smaller steps
after all, mostly to keep the changes manageable and easy to
review. I'm aiming for having the invasive breaking changes
concentrated in as few steps as possible (ideally one). Some steps
will not be breaking at all, but typically there will be breaking
changes that only affect quite special cases so that 95+% of users
will not be affected. This first step is an example for that, see
details below.
What's happening in this commit?
This step is about finally creating an exported registry
interface. This could not be done by simply export the existing
internal implementation because the interface would be way too
fat. This commit introduces a qutie lean Registry interface
(compared to the previous interval implementation). The functions that
act on the default registry are retained (with very few exceptions) so
that most use cases won't see a change. However, several of those are
deprecated now to clean up the namespace in the future.
The default registry is kept in the public variable
DefaultRegistry. This follows the example of the http package in the
standard library (cf. http.DefaultServeMux, http.DefaultClient)
with the same implications. (This pattern is somewhat disputed within
the Go community but I chose to go with the devil you know instead of
creating something more complex or even disallowing any changes to the
default registry. The current approach gives everybody the freedom to
not touch DefaultRegistry or to do everything with a custom registry
to play save.)
Another important part in making the registry lean is the extraction
of the HTTP exposition, which also allows for customization of the
HTTP exposition. Note that the separation of metric collection and
exposition has the side effect that managing the MetricFamily and
Metric protobuf objects in a free-list or pool isn't really feasible
anymore. By now (with better GC in more recent Go versions), the
returns were anyway dimisishing. To be effective at all, scrapes had
to happen more often than GC cycles, and even then most elements of
the protobufs (everything excetp the MetricFamily and Metric structs
themselves) would still cause allocation churn. In a future breaking
change, the signature of the Write method in the Metric interface will
be adjusted accordingly. In this commit, avoiding breakage is more
important.
The following issues are fixed by this commit (some solved "on the
fly" now that I was touching the code anyway and it would have been
stupid to port the bugs):
#46
#100
#170
#205
Documentation including examples have been amended as required.
What future changes does this commit enable?
The following items are not yet implemented, but this commit opens the
possibility of implementing these independently.
- The separation of the HTTP exposition allows the implementation of
other exposition methods based on the Registry interface, as known
from other Prometheus client libraries, e.g. sending the metrics to
Graphite.
Cf. #197
- The public
Registry interface allows the implementation of
convenience tools for testing metrics collection. Those tools can
inspect the collected MetricFamily protobufs and compare them to
expectation. Also, tests can use their own testing instance of a
registry.
Cf. #58
Notable non-goals of this commit
Non-goals that will be tackled later
The following two issues are quite closely connected to the changes in
this commit but the line has been drawn deliberately to address them
in later steps of the refurbishment:
InstrumentHandler has many known problems. The plan is to create a
saner way to conveniently intrument HTTP handlers and remove the old
InstrumentHandler altogether. To keep breakage low for now, even
the default handler to expose metrics is still using the old
InstrumentHandler. This leads to weird naming inconsistencies but
I have deemed it better to not break the world right now but do it
in the change that provides better ways of instrumenting HTTP
handlers.
Cf. #200- There is work underway to make the whole handling of metric
descriptors (
Desc) more intuitive and transparent for the user
(including an ability for less strict checking,
cf. #47). That's
quite invasive from the perspective of the internal code, namely the
registry. I deliberately kept those changes out of this commit.
- While this commit adds new external dependency, the effort to vendor
anything within the library that is not visible in any exported
types will have to be done later.
Non-goals that might be tackled later
There is a strong and understandable urge to divide the prometheus
package into a number of sub-packages (like registry, collectors,
http, metrics, …). However, to not run into a multitude of
circular import chains, this would need to break every single existing
usage of the library. (As just one example, if the ubiquitious
prometheus.MustRegister (with more than 2,000 uses on GitHub alone)
is kept in the prometheus package, but the other registry concerns
go into a new registry package, then the prometheus package would
import the registry package (to call the actual register method),
while at the same time the registry package needs to import the
prometheus package to access Collector, Metric, Desc and
more. If we moved MustRegister into the registry package,
thousands of code lines would have to be fixed (which would be easy if
the world was a mono repo, but it is not). If we moved everything else
the proposed registry package needs into packages of their own, we
would break thousands of other code lines.)
The main problem is really the top-level functions like
MustRegister, Handler, …, which effectively pull everything into
one package. Those functions are however very convenient for the easy
and very frequent use-cases.
This problem has to be revisited later.
For now, I'm trying to keep the amount of exported names in the
package as low as possible (e.g. I unexported expvarCollector in this
commit because the NewExpvarCollector constructor is enough to export,
and it is now consistent with other collectors, like the goCollector).
Non-goals that won't be tackled anytime soon
Something that I have played with a lot is "streaming collection",
i.e. allow an implementation of the Registry interface that collects
metrics incrementally and serves them while doing so. As it has turned
out, this has many many issues and makes the Registry interface very
clunky. Eventually, I made the call that it is unlikely we will really
implement streaming collection; and making the interface more clunky
for something that might not even happen is really a big no-no. Note
that the Registry interface only creates the in-memory
representation of the metric family protobufs in one go. The
serializaton onto the wire can still be handled in a streaming fashion
(which hasn't been done so far, without causing any trouble, but might
be done in the future without breaking any interfaces).
What are the breaking changes?
- Signatures of functions pushing to Pushgateway have changed to allow
arbitrary grouping (which was planned for a long time anyway, and
now that I had to work on the Push code anyway for the registry
refurbishment, I finally did it,
cf. #100).
With the gained insight that pushing to the default registry is almost
never the right thing, and now that we are breaking the Push call
anyway, all the Push functions were moved to their own package,
which cleans up the namespace and is more idiomatic (pushing
Collectors is now literally done by
push.Collectors(...)).
- The registry is doing more consistency checks by default now. Past
creators of inconsistent metrics could have masked the problem by
not setting
EnableCollectChecks. Those inconsistencies will now be
detected. (But note that a "best effort" metrics collection is now
possible with HandlerOpts.ErrorHandling = ContinueOnError.)
EnableCollectChecks is gone. The registry is now performing some
of those checks anyway (see previous item), and a registry with all
of those checks can now be created with NewPedanticRegistry (only
used for testing).PanicOnCollectError is gone. This behavior can now be configured
when creating a custom HTTP handler.
This change is 
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