yoltq

An opinionated Datomic queue for building (more) reliable systems. Implements the transactional outbox pattern. Supports retries, backoff, ordering and more. On-prem only.

Installation

1-minute example

(require '[com.github.ivarref.yoltq :as yq])

(def conn (datomic.api/connect "..."))

; Initialize system
(yq/init! {:conn conn})

; Add a queue consumer that will intentionally fail on the first attempt
(yq/add-consumer! :q
                 (let [cnt (atom 0)]
                   (fn [payload]
                     (when (= 1 (swap! cnt inc))
                       ; A consumer throwing an exception is considered a queue job failure
                       (throw (ex-info "failed" {})))
                     ; Anything else than a throwing exception is considered a queue job success
                     ; This includes nil, false and everything else.
                     (log/info "got payload" payload))))

; Start threadpool that picks up queue jobs
(yq/start!)

; Queue a job
@(d/transact conn [(yq/put :q {:work 123})])

; On your console you will see something like this:
; 17:29:54.598 DEBUG queue item 613... for queue :q is pending status :init
; 17:29:54.602 DEBUG queue item 613... for queue :q now has status :processing
; 17:29:54.603 DEBUG queue item 613... for queue :q is now processing
; 17:29:54.605 WARN  queue-item 613... for queue :q now has status :error after 1 try in 4.8 ms
; 17:29:54.607 WARN  error message was: "failed" for queue-item 613...
; 17:29:54.615 WARN  ex-data was: {} for queue-item 613...
; The item is so far failed...

; But after approximately 10 seconds have elapsed, the item will be retried:
; 17:30:05.596 DEBUG queue item 613... for queue :q now has status :processing
; 17:30:05.597 DEBUG queue item 613... for queue :q is now processing
; 17:30:05.597 INFO  got payload {:work 123}
; 17:30:05.599 INFO  queue-item 613... for queue :q now has status :done after 2 tries in 5999.3 ms
; And then it has succeeded.

Rationale

Integrating with external systems that may be unavailable can be tricky. Imagine the following code:

(defn post-handler [user-input]
  (let [db-item (process user-input)
        ext-ref (clj-http.client/post ext-service {:connection-timeout 3000 ; milliseconds 
                                                   :socket-timeout     10000 ; milliseconds
                                                   ...})] ; may throw exception
    @(d/transact conn [(assoc db-item :some/ext-ref ext-ref)])))

What if the POST request fails? Should it be retried? For how long? Should it be allowed to fail? How do you then process failures later?

PS: If you do not set connection/socket-timeout, there is a chance that clj-http/client will wait for all eternity in the case of a dropped TCP connection.

The queue way to solve this would be:

(defn get-ext-ref [{:keys [id]}]
  (let [ext-ref (clj-http.client/post ext-service {:connection-timeout 3000  ; milliseconds 
                                                   :socket-timeout     10000 ; milliseconds
                                                   ...})] ; may throw exception
    @(d/transact conn [[:db/cas [:some/id id]
                        :some/ext-ref
                        nil
                        ext-ref]])))

(yq/add-consumer! :get-ext-ref get-ext-ref {:allow-cas-failure? true})

(defn post-handler [user-input]
  (let [{:some/keys [id] :as db-item} (process user-input)]
    @(d/transact conn [db-item
                       (yq/put :get-ext-ref {:id id})])))

Here post-handler will always succeed as long as the transaction commits.

get-ext-ref may fail multiple times if ext-service is down. This is fine as long as it eventually succeeds.

There is a special case where get-ext-ref succeeds, but saving the new queue job status to the database fails. Thus get-ext-ref and any queue consumer should tolerate to be executed successfully several times.

For get-ext-ref this is solved by using the database function :db/cas (compare-and-swap) to achieve a write-once behaviour. The yoltq system treats cas failures as job successes when a consumer has :allow-cas-failure? set to true in its options.

How it works

Queue jobs

Creating queue jobs is done by @(d/transact conn [...other data... (yq/put :q {:work 123})]). Inspecting (yq/put :q {:work 123})] you will see something like this:

#:com.github.ivarref.yoltq{:id #uuid"614232a8-e031-45bb-8660-be146eaa32a2", ; Queue job id 
                           :queue-name :q, ; Destination queue                                 
                           :status :init, ; Status
                           :payload "{:work 123}", ; Payload persisted to the database with pr-str
                           :bindings "{}", ; Bindings that will be applied before executing consumer function
                           :lock #uuid"037d7da1-5158-4243-8f72-feb1e47e15ca", ; Lock to protect from multiple consumers
                           :tries 0, ; How many times the job has been executed
                           :init-time 4305758012289 ; Time of initialization (System/nanoTime)
                           }

This is the queue job as it will be stored into the database. You can see that the payload, i.e. the second argument of yq/put, is persisted into the database. Thus the payload must be pr-str-able (unless you have specified custom :encode and :decode functions that override this).

A queue job will initially have status :init. It will then transition to the following statuses:

:processing: When the queue job begins processing in the queue consumer function.
:done: If the queue consumer function returns normally.
:error: If the queue consumer function throws an exception.

Queue consumers

Queue jobs will be consumed by queue consumers. A consumer is a function taking a single argument, the payload. It can be added like this:

(yq/add-consumer! 
  :q ; Queue to consume  
  (fn [payload] (println "got payload:" payload)) ; Queue consumer function
  ; An optional map of queue opts
  {:allow-cas-failure? true ; Treat [:db.cas ...] failures as success. This is one way for the
                            ; consumer function to ensure idempotence.
   :valid-payload? (fn [payload] (some? (:id payload))) ; Function that verifies payload.
                                                        ; Should return truthy for valid payloads.
                                                        ; The default function always returns true.
   :max-retries 10})        ; Specify maximum number of times an item will be retried. Default: 10000.
                            ; If :max-retries is given as 0, the job will ~always be retried, i.e.
                            ; 9223372036854775807 times (Long/MAX_VALUE).

The payload will be deserialized from the database using clojure.edn/read-string before invocation, i.e. you will get back what you put into yq/put.

The yoltq system treats a queue consumer function invocation as successful if it does not throw an exception. Any return value, be it nil, false, true, etc. is considered a success.

Listening for queue jobs

When (yq/start!) is invoked, a threadpool is started.

One thread is permanently allocated for listening to the tx-report-queue and responding to changes. This means that yoltq will respond and process newly created queue jobs fairly quickly. This also means that queue jobs in status :init will almost always be processed without any type of backoff.

The threadpool also schedules polling jobs that will check for various statuses regularly:

Jobs in status :error that have waited for at least :error-backoff-time (default: 5 seconds) will be retried.
Jobs that have been in :processing for at least :hung-backoff-time (default: 30 minutes) will be considered hung and retried.
Old :init-backoff-time (default: 1 minute) :init jobs that have not been processed. Queue jobs can be left in status :init during application restart/upgrade, and thus the need for this strategy.

Retry and backoff strategy

Yoltq assumes that if a queue consumer throws an exception for one item, it will also do the same for another item in the immediate future, assuming the remote system that the queue consumer represents is still down. Thus if there are ten failures for queue :q, it does not make sense to retry all of them at once.

The retry polling job that runs regularly (:poll-delay, default: every 10 seconds) thus stops at the first failure. Each queue have their own polling job, so if one queue is down, it will not stop other queues from retrying.

The retry polling job will continue to eagerly process queue jobs as long as it encounters only successes.

While the :error-backoff-time of default 5 seconds may seem short, in practice if there is a lot of failed items and the external system is still down, the actual backoff time will be longer.

Stuck threads and stale jobs

A single thread is dedicated to monitoring how much time a queue consumer spends on a single job. If this exceeds :max-execute-time (default: 5 minutes) the stack trace of the offending consumer will be logged as :ERROR.

If a job is found stale, that is if the database spent time exceeds :hung-backoff-time (default: 30 minutes), the job will either be retried or marked as :error. This case may happen if the application is shut down abruptly during processing of queue jobs.

Giving up

A queue job will remain in

Yoltq

Install / Use

README