Peter Wilkins

Testing Against XTDB

May 4, 2022


Testing pure functions is straightforward but most applications need to persist state.

Traditionally, testing against a database introduces some incidental complexities that, while acceptable for a while, can become major problems as applications grow to a significant size:

  • The database is an extra dependency to manage during setup.

  • The test suite is slowed down when managing external database state.

  • Database state needs to be reset between each test to maintain test isolation.

  • Care is needed running tests in parallel to avoid tests affecting each other inadvertently.

Common strategies to avoid these problems include:

  • Use of a Data Access Layer to abstract away calls to an external database.

  • Using mocks to assert that the correct calls were made.

  • Using a 'similar' in-memory database and relying on the SQL implementations to behave the same way.

  • Having fewer 'integration' tests that confirm basic functionality of the data layer.

However, there are drawbacks to using these strategies:

  • Test accuracy is compromised as critical sections of code are not covered by unit tests.

  • The ability to use database specific features is sacrificed when limiting code to a subset of SQL.

  • Decisions need to be made on what to test at unit or integration level.

  • The temptation to use more 'reliable' integration tests can eventually lead to the situation where the test suite becomes the bottleneck in Continuous Integration.

However, there are databases that don’t have these limitations. Our database, XTDB, allows you to avoid these complexities and also provides some interesting 'time travel' abilities.

We’ll talk about how XTDB provides consistent behaviour with swappable backends (including in-memory storage), time-travelling tests, the ability to reproduce bugs by trivially testing against specific database snapshots, bisecting our data’s history, and efficiently executing tests with many assertions without writing the results to disk.

Consistent behaviour with Different Storage Backends

The architecture of XTDB allows for seamless swapping of storage backends with no change to the query and transaction APIs. Using the in-memory backend for tests provides a consistent feedback loop without the risk of tests grinding to a halt in the future.

Call xt/start-node with empty an options map {} to start an in-memory node:

(ns xtdb-test
  (:require [xtdb.api :as xt]))

    (with-open [node (xt/start-node {})]
      (xt/await-tx node
                     [[::xt/put {:xt/id "account1" :balance 100}]]))))
"Elapsed time: 9.958169 msecs"
; => #:xtdb.api{:tx-time #inst"2022-03-09T10:00:42.245-00:00", :tx-id 0}

Testing behaviour across time

Tests may need to make assertions on a system at different dates and times. As XTDB is a bitemporal Database it provides the ability to write and query data "as of" different times in its API. This greatly simplifies application code handling time.

Data can be transacted with valid time arguments, signifying when the data is visible in the system.

A consistent query across all the data can be made at any arbitrary point in time (between the year 0 and 9999).

In the fictional example below, account holders are given a bonus of 2% of their current balance at the start of every year. There is also a "bonus funds" account that the amount is transferred from.

The transfer-bonus transaction function retrieves the current balance of the recipient account at the valid time and updates it by 2%, deducting the same amount from the funds account.

In the test, transfer-bonus is called twice at the start of 2023 and 2024 and the test asserts that two accounts will balance after these future events.

(ns xtdb-test
  (:require [xtdb.api :as xt]
            [clojure.test :refer :all]))

(defn transact-sync [node data]
  (xt/await-tx node (xt/submit-tx node data)))

(defn insert-transaction-fn [node]
       {:xt/id :transfer-bonus
        :xt/fn '(fn [ctx recipient-id valid-time]
                  (let [db (xtdb.api/db ctx {::xt/valid-time valid-time})
                        bonus-funds-account (xtdb.api/entity db "bonus-funds")
                        recipient (xtdb.api/entity db recipient-id)
                        bonus (* (:balance recipient) 0.02)]
                    [[::xt/put (update recipient :balance #(+ % bonus)) valid-time]
                     [::xt/put (update bonus-funds-account :balance #(- % bonus)) valid-time]]))}]]))

(defn insert-accounts [node]
     [[::xt/put {:xt/id "bonus-funds" :balance 1000} #inst "2020-01-01T00:00:00"]
      [::xt/put {:xt/id "account1" :balance 100} #inst "2020-01-01T00:00:00"]]))

(deftest transfer-bonus-balanced
  (with-open [node (xt/start-node {})]
    (insert-accounts node)
    (insert-transaction-fn node)
    (let [year-start-2023 #inst "2023-01-01T00:00:00"
          year-start-2024 #inst "2024-01-01T00:00:00"]
          [[::xt/fn :transfer-bonus "account1" year-start-2023]
           [::xt/fn :transfer-bonus "account1" year-start-2024]]))
      (let [db (xt/db node {::xt/valid-time year-start-2024})]
        (is (= (float (+ 100 1000))
               (+ (:balance (xt/entity db "account1"))
                  (:balance (xt/entity db "bonus-funds"))))))))

Git Checkout and Git Bisect for data

XTDB records all previous states of the database similarly to how Git records all versions of a code repository.

Calling xt/db with a transaction id is the equivalent of git checkout <commit-hash> for your data.

(xt/db node {:xtdb.api/tx {::xt/tx-id transaction-id}})

If you are looking for the point at which a bug was introduced to your Git repo, you may reach for git bisect. It allows you to run a test across the previous versions of a codebase to find the failure point.

XTDB allows you to programmatically switch to any previous state of the database. Therefore, it is possible to replicate the behaviour of git bisect but bisecting the state of the data rather than the code.

In this example, the boolean :true-equals-false? is just a placeholder for a more realistic assertion used to check the consistency of data. The predicate function passed into bisect "checks out" the database at the midpoint of a range of transactions.

If the predicate fails then we know the issue started later, if it passes, earlier. By recurring, bisect quickly finds the exact transaction where the issue started.

(ns xtdb-test
  (:require [xtdb.api :as xt]))

(defn bisect [pred low high]
  (if (< low high)
    (let [mid (quot (+ low high) 2)]
      (if (pred mid)
        (recur pred low mid)
        (recur pred (inc mid) high)))

(with-open [node (xt/start-node {})]
  ; submit 50 transactions
  (let [bug-introduced-at (rand-int 50)]
    (doseq [n (range 50)]
        [[::xt/put {:xt/id "foo" :true-equals-false? (>= n bug-introduced-at)}]]))))
  (bisect (fn [cut-at]
            (:true-equals-false? (xt/entity (xt/db node {:xtdb.api/tx {::xt/tx-id cut-at}}) "foo")))
          (::xt/tx-id (xt/latest-completed-tx node))))

The persistent nature of XTDB makes it possible to cheaply capture the state of a database in bug reports. It might even be reasonable to make a snapshot of the database available on CI and use it to create failing test cases for complex bugs.

Speculative Transactions

Speculative Transactions allow a transaction to be run against a database to see the resulting value without actually applying it to storage or indexing.

This enables faster property testing where thousands of test cases can be run without the cost of resetting fixtures for each test.

In this example, the transfer-bonus Transaction Function from above is tested with different initial balances for the account holder. Thousands of different values are tested in less than a second.

(ns xtdb-test
  (:require [clojure.instant :refer [read-instant-date]]
            [clojure.test :refer :all]
            [clojure.test.check :as tc]
            [clojure.test.check.generators :as gen]
            [ :as prop]
            [xtdb.api :as xt]))

(defn accounts-balance? [db valid-time initial-balance]
  (let [transactions [[::xt/put {:xt/id "account1" :balance initial-balance} valid-time]
                      [::xt/fn :transfer-bonus "account1" valid-time]]
        new-db-value (xt/with-tx db transactions)]
    (= (float (+ initial-balance 1000))
       (+ (:balance (xt/entity new-db-value "account1"))
          (:balance (xt/entity new-db-value "bonus-funds"))))))

(defn accounts-balance-property [node]
  (let [valid-time #inst "2023-01-01T00:00:00"
        db (xt/db node {::xt/valid-time valid-time})]
    (prop/for-all [amount (gen/choose 0 10000)]
      (accounts-balance? db valid-time amount))))

(with-open [node (xt/start-node {})]
  (is node)
  (insert-fixtures node)
  (tc/quick-check 1000 (accounts-balance-property node)))
; => {:result true, :pass? true, :num-tests 1000, :time-elapsed-ms 688, :seed 1647508942240}

One limitation with xt/with-tx is that the returned database value is set at the valid time and tx-time of the database value passed in. Therefore, the scope for using speculative transactions with temporal aspects is limited.


Testing is hard, especially when changing state and temporal aspects are considered. XTDB provides stable, isolated and navigable state storage that won’t slow you down in the future.

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