docs: describe purpose in the readme

README.md

@@ -1 +1,91 @@
-# cryptoprom
+# Crypto::Prom
+
+[![Docker Repository on Quay](https://quay.io/repository/linki/cryptoprom/status "Docker Repository on Quay")](https://quay.io/repository/linki/cryptoprom)
+[![GitHub release](https://img.shields.io/github/release/linki/cryptoprom.svg)](https://github.com/linki/cryptoprom/releases)
+
+CryptoProm is a Prometheus metrics exporter for Cryptocurrency market prices.
+
+When you run it, it queries the Coinbase API once a minute to get the current
+market price of Bitcoin and exports that data via HTTP so that Prometheus can
+scrape it.
+
+You can then use your favorite graphing tool, e.g. Grafana, to visualize the
+data from Prometheus however you want.
+
+![CryptoProm overview](img/cryptoprom1.png "CryptoProm overview")
+
+## Setup
+
+You need to install Ruby 2.4 and Bundler, however other versions might work as well. Then install dependent Gems via Bundler and start the Rack server.
+
+You have to provide your Coinbase API credentials in order to retrieve data from their API:
+
+```console
+$ bundle install
+$ export COINBASE_API_KEY="<your-api-key>"
+$ export COINBASE_API_SECRET="<your-api-secret>"
+$ rackup
+```
+
+Test that it works in a separate shell with curl:
+
+```console
+$ curl -sS localhost:9292/metrics | grep cryptoprom
+...
+cryptoprom_cryptocurrency_rates{currency="AUD",denominator="USD"} 0.7462686567164178
+...
+cryptoprom_cryptocurrency_rates{currency="BTC",denominator="USD"} 1335.113484646195
+cryptoprom_cryptocurrency_rates{currency="ETH",denominator="USD"} 59.990017661061195
+cryptoprom_cryptocurrency_rates{currency="EUR",denominator="USD"} 1.0869565217391304
+cryptoprom_cryptocurrency_rates{currency="GBP",denominator="USD"} 1.282051282051282
+...
+cryptoprom_cryptocurrency_rates{currency="ZWL",denominator="USD"} 0.0031018331834113963
+```
+
+This is Prometheus syntax and shows you the current value of each
+currency Coinbase knows about in USD, including BTC, ETH and various fiat currencies.
+
+You can then use PromQL to query that data from Grafana. For instance, to create a
+chart displaying the current BTC price in USD over time you could use the
+following query:
+
+```
+cryptoprom_cryptocurrency_rates{currency="BTC"}
+```
+
+Which will result in the following graph:
+
+![CryptoProm detail](img/cryptoprom2.png "CryptoProm detail")
+
+You can also do more advances queries. The following takes both the BTC and ETH
+prices in USD and relates them to each other. It basically shows what the "real"
+price for ETH in BTC should be, when only given their prices in USD.
+
+```
+cryptoprom_cryptocurrency_rates{currency="ETH"} / ignoring(currency) cryptoprom_cryptocurrency_rates{currency="BTC"}
+```
+
+Last but not least, you can monitor CryptoProm itself. It exports metrics about
+requests to its own metrics endpoint as well as its requests to the Coinbase API.
+
+```
+http_server_requests_total{code="200",method="get",path="/metrics"} 2.0
+cryptoprom_coinbase_api_requests_total 14.0
+```
+
+You could use that to create a graph for CryptoProm's request rate against the
+Coinbase API to validate that it behaves and you don't run into rate limiting
+issues. You could create a Gauge in Grafana with the following PromQL query:
+
+```
+rate(cryptoprom_coinbase_api_requests_total[1h]) * 60
+```
+
+Which will display like this:
+
+![CryptoProm request rate](img/cryptoprom3.png "CryptoProm request rate")
+
+## Deploying
+
+The `manifest` folder contains Kubernetes manifests that describe how CryptoProm
+can be deployed. A Docker image can be found at `quay.io/linki/cryptoprom`.

manifests/cryptoprom-depl.yaml

@@ -25,7 +25,11 @@ spec:
               name: cryptoprom
               key: COINBASE_API_SECRET
         ports:
-        - containerPort: 9099
+        - containerPort: 8080
         resources:
           requests:
             cpu: 10m
+            memory: 64Mi
+          limits:
+            cpu: 100m
+            memory: 256Mi

manifests/cryptoprom-svc.yaml

@@ -6,7 +6,7 @@ metadata:
     app: cryptoprom
 spec:
   ports:
-  - port: 9099
-    targetPort: 9099
+  - port: 8080
+    targetPort: 8080
   selector:
     app: cryptoprom