First working version

This commit adds a simple Python script and a Dockerfile to create a
minimal container image around it. The script expects to run as part of
a Pod in Kubernetes with the stock CouchDB container. When it runs it
will query DNS looking for the SRV record associated with a headless
service called "couchdb". It will then iterate over the targets of that
SRV record and create well-formed membership documents for each one in
the local nodes DB.

The end result should be that all the Pods backing a StatefulSet
associated with the headless Service will be automatically joined up
into a cluster on startup.

Author: kocolosk

Dockerfile

@@ -0,0 +1,12 @@
+FROM python:alpine
+
+MAINTAINER Adam Kocoloski [email protected]
+
+RUN pip install requests dnspython
+
+COPY mem3_helper.py /opt/mem3_helper/
+
+WORKDIR /opt/mem3_helper
+
+CMD ["mem3_helper.py"]
+ENTRYPOINT ["python"]

mem3_helper.py

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+# This file grabs the list of pod names behind the headless service
+# called "couchdb" and feeds those as `couchdb@HOSTNAME` nodes to mem3.
+
+import json
+import os
+import requests
+import time
+import dns.resolver
+
+def discover_peers_dns(retries=5):
+    # Assumes svc.cluster.local is in the DNS search space ...
+    resource = '_couchdb._tcp.{0}.{1}'.format(
+        os.environ.get("COUCHDB_SERVICE", "couchdb"),
+        os.environ.get("POD_NAMESPACE", "default")
+    )
+    if retries > 0:
+        try:
+            answers = dns.resolver.query(resource, 'SRV')
+        except dns.resolver.NXDOMAIN:
+            print('DNS query for SRV records failed with NXDOMAIN error')
+            time.sleep(5)
+            discover_peers_dns(retries - 1)
+        # Erlang requires that we drop the trailing period from the absolute DNS
+        # name to form the hostname used for the Erlang node. This feels hacky
+        # but not sure of a more official answer
+        hostnames = [rdata.target.to_text()[:-1] for rdata in answers]
+        print(hostnames)
+        connect_the_dots(hostnames)
+    else:
+        print('Could not resolve resource ', resource)
+
+def connect_the_dots(names, retries=5):
+    if retries > 0:
+        for name in names:
+            uri = "http://127.0.0.1:5986/_nodes/couchdb@{0}".format(name)
+            doc = {}
+            try:
+                r = requests.put(uri, data = json.dumps(doc))
+                print(name, r.status_code)
+            except requests.exceptions.ConnectionError:
+                print('CouchDB admin port not up yet')
+                time.sleep(5)
+                connect_the_dots(names, retries - 1)
+        sleep_forever()
+    else:
+        print('Could not connect to local admin port to supply node names')
+
+def sleep_forever():
+    while True:
+        time.sleep(5)
+
+if __name__ == '__main__':
+    print(os.environ)
+    discover_peers_dns()