Ep 029: Problem Unknown: Log Lines

Functional Design in Clojure

Nate is dropped in the middle of a huge log file and hunts for the source of the errors.

• We dig into the world of DonutGram.
• "We are the devops."
• Problem: we start seeing errors in the log.
• "The kind of error that makes total sense to whoever wrote the log line."
• (04:10) We want to use Clojure to characterize the errors.
• Why not grep?
• Well...we grep out the lines, count them, and we get 10,351 times. Is that a lot? Hard to say. We need more info.
• (05:50) Developers think it's safe to ignore, but it's bugging us.
• We want to come up with an incident count by user.
• "Important thing we do a lot in the devops area: we see data and then we have to figure out, what does it mean?"
• "What story is the data telling us?"
• The "forensic data" versus "operating data" (Ep 022, 027)
• "With forensic data, you're trying to save the state of the imprint of the known universe over time."
• (07:40) The log file is the most common forensic data for applications.
• To characterize, we need to manipulate the data at a higher level than grep.
• First goal: make the data useful in Clojure
• Get a nice sample from the production log, put it in a file, load up the REPL, and start writing code to parse it.
• Need to parse out the data from the log file and turn it into something more structured.
• (12:05) Let's make a function, parse-line that separates out all the common elements for each line:
  • timestamp
  • log level
  • thread name
  • package name
  • freeform "message"
• "A message has arrived from the developers, lovingly bundled in a long log line."
• (13:10) We can parse every line generically, but we need to parse the message and make sense of that.
• "We want to lift up the unstructured data into structured data that we can map, filter, and reduce on."
• We will have different kinds of log lines, so we need to detect them and parse appropriately.
• We want to amplify the map to include the details for the specific kind of log line.
• We'll use a kind field to identify which kind of log line it is.
• There are two steps:
  1. recognize which kind of log line the "message" is for
  2. parse the data out of that message
• "Maybe we'll have a podcast that's constantly about constantly. It's not just juxt."
• (18:45) How do we do this concisely?
• Let's use cond:
  • flatten out all the cases
  • code to detect kind (the condition)
  • code to parse the details (the expression)
• Can use includes? in the condition to detect the kind and re-matches to parse.
• (20:00) Why not use the regex to detect the kind too?
• Can avoid writing the regex twice by using a def.
• It's less code, but now we're running the regex twice: once in the condition and once to parse.
• We can't capture the result of the test in cond. No "cond-let" macro.
• We could write our own cond-let macro, but should we?
• "When you feel like you should write a macro, you should step back and assess your current state of being. Am I tired? Did I have a bad day? Do I really need this?"
• (24:05) New goals for our problem:
  • one regex literal
  • only run the regex once
  • keep the code that uses the matches close to the regex
• Similar problem to "routes" for web endpoints: want the route definition next to the code that executes using the data for that route.
• Feels like an index or table of options.
• (25:20) Let's make a "table". A vector of regex and handler-code pairs.
• We need a coffee mug: "Clojure. It's just a list."
• The code can be a function literal that takes the matches and returns a map of parsed data.
• Write a function parse-details which goes through each regex until one matches and then invokes the handler for that one. (See below.)
• (30:15) Once we have higher-level data, it's straight forward to filter and group-by to get our user count.
• Once again, the goal is to take unstructured data and turn it into structured data.
• "You have just up-leveled unstructured information into a sequence of structured information."
• Can slurp, split, map, filter, and then aggregate.
• (32:10) What happens when we try to open a 10 GB log file?
• Sounds like a problem for next week.
• "Fixing production is always a problem for right now."
• "If a server falls over and no one outside of the devops team knows about it, did the server really fall over?"
• "Who watches the watchers?"

Related episodes:

• 020: Data Dessert
• 022: Evidence of Attempted Posting
• 027: Collected Context

Clojure in this episode:

• #""
• re-matches
• case
• cond
• if-let
• ->>
• slurp
• filter
• group-by
• def
• constantly
• juxt
• clojure.string/
  • includes?
  • split

Related links:

• GraalVM

Code sample from this episode:

(ns devops.week-01  (:require  [clojure.java.io :as io]  [clojure.string :as string]  ))  ;; General parsing  (def general-re #"(\d\d\d\d-\d\d-\d\d)\s+(\d\d:\d\d:\d\d)\s+\|\s+(\S+)\s+\|\s+(\S+)\s+\|\s+(\S+)\s+\|\s(.*)")  (defn parse-line  [line]  (when-let [[whole dt tm thread-name level ns message] (re-matches general-re line)]  {:raw/line whole  :log/date dt  :log/time tm  :log/thread thread-name  :log/level level  :log/namespace ns  :log/message message  }))  (defn general-parse  [lines]  (->> lines  (map parse-line)  (filter some?)))   ;; Detailed parsing  (def detail-specs  [[#"transaction failed while updating user ([^:]+): code 357"  (fn [[_whole user]] {:kind :code-357 :code-357/user user})]  ])  (defn try-detail-spec  [message [re fn]]  (when-some [matches (re-matches re message)]  (fn matches)))  (defn parse-details  [entry]  (let [{:keys [log/message]} entry]  (if-some [extra (->> detail-specs  (map (partial try-detail-spec message))  (filter some?)  (first))]  (merge entry extra)  entry)))   ;; Log analysis  (defn lines  [filename]  (->> (slurp filename)  (string/split-lines)))  (defn summarize  [filename calc]  (->> (lines filename)  (general-parse)  (map parse-details)  (calc)))   ;; data summarizing  (defn code-357-by-user  [entries]  (->> entries  (filter #(= :code-357 (:kind %)))  (map :code-357/user)  (frequencies)))   (comment  (summarize "sample.log" code-357-by-user)  )

Log file sample:

2019-05-14 16:48:55 | process-Poster | INFO | com.donutgram.poster | transaction failed while updating user joe: code 357 2019-05-14 16:48:56 | process-Poster | INFO | com.donutgram.poster | transaction failed while updating user sally: code 357 2019-05-14 16:48:57 | process-Poster | INFO | com.donutgram.poster | transaction failed while updating user joe: code 357