#openmetrics #prometheus #pushgateway

app gravel-gateway

A Prometheus Push Gateway for FAAS applications

7 stable releases

1.6.1 Jan 22, 2023
1.6.0 Sep 26, 2022
1.5.1 Jul 11, 2022
1.3.0 May 15, 2022
1.1.0 Oct 25, 2021

#2 in #aggregation

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Gravel Gateway


Gravel Gateway is a Prometheus Push Gateway for FAAS applications. In particular it allows aggregation to be controlled by the incoming metrics, and thus provides much more flexibility in the semantics that your metrics can follow. In general, the Gravel Gateway functions as a standard aggregating push gateway - by default, everything except Gauges are sumed, so e.g. if you push

# TYPE value_total counter
value_total 1
# TYPE value2 gauge
value2 1

three times, then Prometheus will scrape

# TYPE value_total counter
value_total 3
# TYPE value2 gauge
value2 1

Where the Gravel Gateway differs, is that it allows you to specify a special clearmode label to dictate how metrics are aggregated.

We currently support three different values of clearmode - aggregate (the default for non gauges), replace (the default for gauges), and family which provides info like semantics. As a practical example, if we push:

# TYPE value_total counter
value_total 1
# TYPE value2 gauge
value2{clearmode="aggregate"} 1
# TYPE version gauge
version{version="0.0.1",clearmode="family"} 1

and then

# TYPE value_total counter
value_total 3
# TYPE value2 gauge
value2{clearmode="aggregate"} 1
# TYPE version gauge
version{version="0.0.2",clearmode="family"} 1

(note the changed version label), Prometheus will scrape:

# TYPE version gauge
version{version="0.0.2"} 1
# TYPE value2 gauge
value2 2
# TYPE value_total counter
value_total 4

With the counter value being replaced, the gauge value being sumed, and the version value completely replacing the old version. You'll also note that the clearmode label is removed by the gateway - it's not included in the metrics exposed to the Prometheus scrape. In that way, this aggregating process is completely transparent to Prometheus.


Prometheus Gravel Gateway 

    gravel-gateway [FLAGS] [OPTIONS]

        Whether or not to enable clustering

    -h, --help               
            Prints help information

    -V, --version            
            Prints version information

        --basic-auth-file <basic-auth-file>    
            The file to use for basic authentication validation.
                            This should be a path to a file of bcrypt hashes, one per line,
                            with each line being an allowed hash.
    -l <listen>                                
            The address/port to listen on [default: localhost:4278]

        --peer <peers>...                      
            The address/port of a peer to connect to

        --peers-file <peers-file>              
            The SRV record to look up to discover peers

        --peers-srv <peers-srv>                
            The SRV record to look up to discover peers

        --tls-cert <tls-cert>                  
            The certificate file to use with TLS

        --tls-key <tls-key>                    
            The private key file to use with TLS

To use, run the gateway:


You can then make POSTs to /metrics to push metrics:

echo '# TYPE value_total counter
value_total{clearmode="replace"} 3
# TYPE value2 gauge
value2{clearmode="aggregate"} 1
# TYPE version gauge
version{version="0.0.2",clearmode="family"} 1' | curl --data-binary @- localhost:4278/metrics

And point Prometheus at it to scrape:

  scrape_interval: 15s
  evaluation_interval: 30s
  - job_name: prometheus
    honor_labels: true
      - targets: [""]


Gravel Gateway supports (pseudo) Basic authentication (with the auth feature). To use, populate a file with bcrypt hashes, 1 per line, e.g.

htpasswd -bnBC 10 "" supersecrets | tr -d ':\n' > passwords

and then start gravel-gateway pointing to that file:

gravel-gateway --basic-auth-file ./passwords

Requests to the POST /metrics endpoint will then be rejected unless they contain a valid Authorization header:

curl http://localhost:4278/metrics -vvv --data-binary @metrics.txt -u :supersecrets


TLS is provided by the tls-key and tls-cert args. Both are required to start a TLS server, and represent the private key, and the certificate that is presented respectively.


To horizonally scale the gateway, you can use clustering. The Gravel Gateway support clustering by maintaining a hash ring of peers, provided by either a static list, an SRV record, or a file. When a request comes in, if clustering is enabled, the job label is hashed to produce an "authoritive" node for that job, and the request is forwarded accordingly. That node thus becomes the only node that will expose metrics for the given job.

To enable clustering, use the cluster-enabled flag, and provide a discovery mechanism. For example:

./gravel-gateway --cluster-enabled --peer localhost:4279 --peer localhost:4280
./gravel-gateway --cluster-enabled -l localhost:4279 --peer localhost:4278 --peer localhost:4280
./gravel-gateway --cluster-enabled -l localhost:4280 --peer localhost:4278 --peer localhost:4279

starts three gravel gateway instances, clustered such that they will forward requests between each other


Some times, for Gauges, you don't want to track just one of your values (the default for Gauges is "replace"). If we have, say, a new release that doubles the memory usage, then we probably want to know about that increase without it being pulled down by weeks of the previous version. For this usecase, the Gravel Gateway supports "pebbles". Pebbles are effectively a circular buffer of time based buckets. Each bucket represents a distinct timeslice, and tracks a pre-aggregated value inside that time slice. The final value for the metric is the same aggregation applied over each bucket.

This means that we actually get an "aggregate of aggregates" out the other end, which does lose some precision compared to storing all the raw data but in practice this hasn't affected us much.

You can start a pebble using a clearmode in the form <aggregation><time> e.g. {clearmode="mean5m"} will take a mean over the last 5 minutes of incoming data. Available aggregations at the moment include "sum" and "mean", but "median" is coming soon, and maybe "percentile" would be a good PR.


I recently wrote about my frustrations with trying to orchestrate Prometheus in an FAAS (Functions-As-A-Service) system that will rename nameless. My key frustration was that the number of semantics I was trying to extract from my Prometheus metrics was too much for the limited amount of data you can ship with them. In particular, there was three semantics I was trying to drive:

  1. Aggregated Counters - Things like request counts. FAAS applications only process one request (in general), so each sends a 1 to the gateway and I want to aggregate that into a total request count across all the invocations
  2. Non aggregated Gauges - It doesn't really make sense to aggregate Gauges in the general case, so I want to be able to send gauge values to the gateway and have them replace the old value
  3. Info values - Things like the build information. When a new labelset comes along for these metrics, I want to be able to replace all the old labelsets, e.g. upgrading from {version="0.1"} to {version="0.2"} should replace the {version="0.1"} labelset

Existing gateways, like the prom-aggregation-gateway, or pushgateway are all or nothing in regards to aggregation - the pushgateway does not aggregate at all, completly replacing values as they come in. The aggregation gateway is the opposite here - it aggregates everything. What I wanted was something that allows more flexibility in how metrics are aggregated. To that end, I wrote the Gravel Gateway


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