5: Match with Crux – Saturn Assignment

NOTICE: Crux has been renamed to XTDB. This tutorial is now available at https://nextjournal.com/xtdb-tutorial instead. Please consider the following tutorial deprecated.

Introduction

This is the match installment of the Crux tutorial.

Setup

You need to get Crux running before you can use it.

{:deps
 {org.clojure/clojure {:mvn/version "1.10.0"}
  org.clojure/tools.deps.alpha
  {:git/url "https://github.com/clojure/tools.deps.alpha.git"
   :sha "f6c080bd0049211021ea59e516d1785b08302515"}
  juxt/crux-core {:mvn/version "RELEASE"}}}
Extensible Data Notation
(require '[crux.api :as crux])
12.1s

Arrival on Saturn

As you pass through the innermost ring of Saturn, A warning light appears on your communications panel. You open the message. It’s from Space Customs and reads:

"We extend you the warmest welcome.
We must check papers before we can give you permission to land."
— Cronus Peaceful Nations
Clojure

They are asking to see your flight manifest.

Choose your path:

You have your manifest : You have permission to land, continue to the space port.

You do not have your manifest : You do not have permission to land. You can either return to Neptune or continue at your own risk.

Space Port

As you prepare to land you open your Crux manual to the page on match

"Currently there are only four transaction operations in Crux: put, delete, match and evict.
		Transaction 	(Description)
    put    		(Writes a version of a document)
    delete    (Deletes a version of a document)
    match     (Stops a transaction if the precondition is not met.)
    evict    	(Removes an document entirely)
match:
match checks the current state of an entity - if the entity doesn’t match the provided doc, the transaction will not continue. You can also pass nil to check that the entity doesn’t exist prior to your transaction.
A match transaction takes the entity id, along with an expected document. Optionally you can provide a valid time.
Time in Crux is denoted #inst 'yyyy-MM-ddThh:mm:ss'. For example, 9:30 pm on January 2nd 1999 would be written: #inst \"1999-01-02T21:30:00\".
A complete match transaction has the form:
    [:crux.tx/match entity-id expected-doc valid-time]
Note that if there is no old-doc in the system, you can provide `nil` in its place."
— Crux Manual
Clojure

Read More

You are happy with what you have read, and in anticipation of the assignment you define the standalone node.

(def crux (crux/start-node {}))
6.3s

Assignment

As you land on the surface of Saturn the job ticket for this assignment is unlocked.

Ticket

Task 			"Secure trading system"
Company		"Cronus Market Technologies"
Contact 			"Ubuku Eppimami"
Submitted "2115-02-23T13:38:20"
Additional information:
"We need to be shown how to ensure no trades are done without the buyer and seller having the necessary funds or stock respectively"
Clojure
example_data.txt
Download

The next shuttle to the CMT office leaves in 5 Earth minutes. While you wait you use your easy ingest function you created on Pluto to put the example data into your system. 

(defn easy-ingest
  "Uses Crux put transaction to add a vector of 
  documents to a specified node"
  [node docs]
  (crux/submit-tx node
                  (vec (for [doc docs]
                         [:crux.tx/put doc]))))
(def data
  [{:crux.db/id :gold-harmony
    :company-name "Gold Harmony"
    :seller? true
    :buyer? false
    :units/Au 10211
    :credits 51}
   {:crux.db/id :tombaugh-resources
    :company-name "Tombaugh Resources Ltd."
    :seller? true
    :buyer? false
    :units/Pu 50
    :units/N 3
    :units/CH4 92
    :credits 51}
   {:crux.db/id :encompass-trade
    :company-name "Encompass Trade"
    :seller? true
    :buyer? true
    :units/Au 10
    :units/Pu 5
    :units/CH4 211
    :credits 1002}
   {:crux.db/id :blue-energy
    :seller? false
    :buyer? true
    :company-name "Blue Energy"
    :credits 1000}])
(easy-ingest crux data)
0.3s

You also decide to make some Clojure functions so you can easily show Ubuku the stock and fund levels after the trades.

(defn stock-check
  [company-id item]
  {:result (crux/q (crux/db crux)
                   {:find '[name funds stock]
                    :where ['[e :company-name name]
                            '[e :credits funds]
                            ['e item 'stock]]
                    :args [{'e company-id}]})
   :item item})
(defn format-stock-check
  [{:keys [result item] :as stock-check}]
  (for [[name funds commod] result]
    (str "Name: " name ", Funds: " funds ", " item " " commod)))
0.1s

Just as you are finishing off your shuttle arrives.

After a short journey through the icy lower clouds of Saturn you are met by a friendly faced Ubuku.

"Hello friend.
We have been using Crux for a short time now and think it is great. The problem is a human one. Occasionally we process trades without checking that are enough funds in the buyers account.
I know there is a way that we can stop this happening in Crux.
I sent you some example data in the job ticket for you to use, I trust you found it.
Do you think you can help us?"
— Ubuku Eppimami
Clojure

Choose your path:

"Yes, I'll give it a go." : Continue to complete the assignment.

"I'm not even sure how to begin" : Take some time to read through the Crux manual again. If you're still unsure then you can follow along anyway and see if things become clear.

Assignment

You explain to Ubuku that all they need to do to solve their problem is to use the match operation instead of put when they are processing their trades.

You show Ubuku the match operation for a valid transaction. You move 10 units of Methane (:units/CH4) each at the cost of 100 credits to Blue Energy:

(crux/submit-tx
 crux
 [[:crux.tx/match
   :blue-energy
   {:crux.db/id :blue-energy
    :seller? false
    :buyer? true
    :company-name "Blue Energy"
    :credits 1000}]
  [:crux.tx/put
   {:crux.db/id :blue-energy
    :seller? false
    :buyer? true
    :company-name "Blue Energy"
    :credits 900
    :units/CH4 10}]
  [:crux.tx/match
   :tombaugh-resources
   {:crux.db/id :tombaugh-resources
    :company-name "Tombaugh Resources Ltd."
    :seller? true
    :buyer? false
    :units/Pu 50
    :units/N 3
    :units/CH4 92
    :credits 51}]
  [:crux.tx/put
   {:crux.db/id :tombaugh-resources
    :company-name "Tombaugh Resources Ltd."
    :seller? true
    :buyer? false
    :units/Pu 50
    :units/N 3
    :units/CH4 82
    :credits 151}]])
0.1s

You explain that because the old doc is as expected for both the buyer and the seller that the transaction goes through.

You show Ubuku the result of the trade using the function you created earlier:

(format-stock-check (stock-check :tombaugh-resources :units/CH4))
0.5s
(format-stock-check (stock-check :blue-energy :units/CH4))
0.1s

They are happy that this works as he sees the 1000 credits move from Blue energy to Tombaugh Resources Ltd. and 10 units of Methane the other way.

Ubuku asks if you can show them what would happen if there was not enough funds in the account of a buyer. You show him a trade where the old doc is not as expected for Encompass trade, to buy 10,000 units of Gold from Gold Harmony.

(crux/submit-tx
 crux
 [[:crux.tx/match
   :gold-harmony
   {:crux.db/id :gold-harmony
    :company-name "Gold Harmony"
    :seller? true
    :buyer? false
    :units/Au 10211
    :credits 51}]
  [:crux.tx/put
   {:crux.db/id :gold-harmony
    :company-name "Gold Harmony"
    :seller? true
    :buyer? false
    :units/Au 211
    :credits 51}]
  [:crux.tx/match
   :encompass-trade
   {:crux.db/id :encompass-trade
    :company-name "Encompass Trade"
    :seller? true
    :buyer? true
    :units/Au 10
    :units/Pu 5
    :units/CH4 211
    :credits 100002}]
  [:crux.tx/put
   {:crux.db/id :encompass-trade
    :company-name "Encompass Trade"
    :seller? true
    :buyer? true
    :units/Au 10010
    :units/Pu 5
    :units/CH4 211
    :credits 1002}]])
0.1s
(format-stock-check (stock-check :gold-harmony :units/Au))
0.1s
(format-stock-check (stock-check :encompass-trade :units/Au))
0.1s

You explain to Ubuku that this time, because you have both match operations in the same transaction, the trade does not go through. The accounts remain the same, even though the failing match was the second operation.

Ubuku thanks you. This is just what they are looking for. You head back to the space station to see if there is another assignment waiting for you.

Space Port

Back at the spaceship there is a light waiting for you on your communications panel.

"Well done, you’ve had a productive week. We have one final task for you to do before you finish for the week
You need to go to Jupiter and meet Kaarlang, it’s his last day working for us and he needs to delete his trade clients from his personal Crux node for data protection."
— Helios Banking Inc.
Clojure

You update your manifest with your most recent badge.

(crux/submit-tx
 crux
 [[:crux.tx/put 
   {:crux.db/id :manifest
    :pilot-name "Johanna"
    :id/rocket "SB002-sol"
    :id/employee "22910x2"
    :badges ["SETUP" "PUT" "DATALOG-QUERIES" "MATCH"]
    :cargo ["stereo" "gold fish" "slippers" "secret note"]}]])
0.1s

As you do so, you check to see if you still have the note that the porter gave you for Kaarlang back on Earth.

(crux/q (crux/db crux)
          {:find '[belongings]
           :where '[[e :cargo belongings]]
           :args [{'belongings "secret note"}]})
0.1s

Feeling a bit apprehensive, you enter countdown for lift off to Jupiter. See you soon.

Runtimes (1)