Implement json prototype of LTsDb

doc/multiple-timeseries

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+PoC, not optimized for performance.
+Store data as JSON
+Use one file per timeseries
+Metadata is a dict of dimension/value pairs.
+We can find each file quickly by using a hash of the metadata as the
+filename
+Can we find all timeseries which match only some of the dimensions
+(e.g. response times of a particular service across all nodes)?
+Opening each file is going to be slow pretty fast. So we need an index.
+We don't expect new timeseries to spring into existence all that often,
+so I guess for now we can just rewrite the whole index when a new
+timeseries is added. Structure is pretty simple: Two levels of
+dict (dimension, value) and then a list of matching timeseries.
+
+Using an RDBMS doesn't seem like a good idea,
+
+I'll design an efficient binary format later.

ltsdb_json.py

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+#!/usr/bin/python3
+
+import fcntl
+import glob
+import hashlib
+import json
+import time
+
+class LTS:
+    base_dir = "data"
+    limit = 1000
+
+    def __init__(self, description):
+        # Oh, I think we need to be able to load by hash, too
+        canonical_description = {x: description[x] for x in sorted(description.keys())}
+        self.description = canonical_description
+        serialized_description = json.dumps(canonical_description)
+        m = hashlib.sha256()
+        m.update(bytes(serialized_description, encoding="UTF-8"))
+        self.filename = self.base_dir + "/" + m.hexdigest()
+        try:
+            with open(self.filename, "r") as fh:
+                fcntl.flock(fh, fcntl.LOCK_SH)
+                d = json.load(fh)
+            self.new = False
+            self.data = d["data"]
+        except FileNotFoundError as e:
+            self.new = True
+            self.data = []
+            # Create the file immediately. Makes saving later simpler if we can
+            # assume it exists
+            with open(self.filename, "x+") as fh:
+                fcntl.flock(fh, fcntl.LOCK_EX)
+                json.dump({"description": self.description, "data": self.data}, fh)
+            self.rebuild_index()
+
+    def add(self, ts, value):
+        while len(self.data) >= self.limit:
+            r = random.randrange(0, self.limit)
+            self.data.pop(r)
+
+        if len(self.data) == 0 or ts >= self.data[-1][0]:
+            self.data.append((ts, value,))
+        else:
+            # Shouldn't happen that often, so I do a simple linear search instead
+            # of a binary search
+            for i in range(len(self.data)):
+                if self.data[i][0] >= ts:
+                    break
+            self.data.insert(i, (ts, value,))
+
+    def save(self):
+        with open(self.filename, "r+") as fh:
+            fcntl.flock(fh, fcntl.LOCK_EX)
+            json.dump({"description": self.description, "data": self.data}, fh)
+            fh.truncate()
+
+    def rebuild_index(self):
+        t0 = time.time()
+        index  = {}
+        for fn in glob.glob(self.base_dir + "/*"):
+            (_, _, hash) = fn.rpartition("/")
+            with open(fn, "r") as fh:
+                fcntl.flock(fh, fcntl.LOCK_SH)
+                d = json.load(fh)
+            for k, v in d["description"].items():
+                d1 = index.setdefault(k, {})
+                d2 = d1.setdefault(v, [])
+                d2.append(hash)
+        with open(self.base_dir + "/.index", "r+") as fh:
+            fcntl.flock(fh, fcntl.LOCK_EX)
+            json.dump(index, fh)
+        t1 = time.time()
+        print("index rebuilt in", t1 - t0, "seconds")
+
+    @classmethod
+    def find(self, match):
+        with open(self.base_dir + "/.index", "r") as fh:
+            fcntl.flock(fh, fcntl.LOCK_SH)
+            index = json.dump(fh)
+

ltsdb_test

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+#!/usr/bin/python3
+
+from ltsdb_json import LTS
+
+ts1 = LTS({"hostname": "rorschach.hjp.at", "measure": "uptime"})
+ts1.add(1661026122, 4)
+ts1.save()
+
+ts1 = LTS({"hostname": "rorschach.hjp.at", "website": "i12e.hjp.at", "measure": "rtt"})
+ts1.add(1661026122, 0.06)
+ts1.save()
+
+ts1 = LTS({"hostname": "rorschach.hjp.at", "measure": "uptime"})
+ts1.add(1661026361, 5)
+ts1.save()
+
+ts1 = LTS({"hostname": "charly.wsr.ac.at", "website": "www.wifo.ac.at", "measure": "rtt"})
+ts1.add(1661026122, 0.347)
+ts1.save()
+