Superlifter: A DataLoader for Clojure
Efficient fetching from decomplected code

Image credit: Iain M Banks / Orbit Books

No beating around the bush here, you’re going to have to give up whatever job you have and plough your life savings into a pet store. This is convenient for me, because it will help you understand this blog post. Hope you like animals!

Naturally you are going to computerise everything you can. You’ve heard that graphs are cool, and that cool means good. GraphQL gives you an abstraction over your data. This must also be cool! The information about your pet store could look like this:

pet store graph

Each node in the graph is a bit of data you have, and the edges are how you access it. Most GraphQL implementations take care of the edges for you so your only job is to write resolvers that can return the data for each node. Let’s tell a story.

Someone comes in to your store and asks what kinds of animal you have. You first go and get a plan showing you where all the animals are located, and bring it back to the counter along with a blank sheet of paper. Then you walk to the first location, find a golden retriever and walk back to the counter to write down "dog" on the sheet of paper. Then you go to the second location, see a Maine coon, come back to the desk and add "cat" to the sheet before walking off to the third location…​ did you know platypuses lay eggs? By the time you’ve finished the list you’ve walked all over the shop, covering the same ground multiple times. Isn’t there a more efficient way?

In the diagram above, each coloured node represents an invocation of a resolver. If you have worked on a GraphQL server such as lacinia you will probably have come across what I call the 1+n problem when resolving a list of nodes, which is where you resolve your list of n pets and then call the pet details resolver for each of them, n times in total. When your data is remote this becomes especially painful. This post will introduce the DataLoader pattern and a Clojure library called superlifter which implements it. The examples in this post are based on implementing lacinia resolvers but the techniques apply to resolving any non-trivial graph of data.

Talking about a resolution

Imagine you have a REST service called PetDB that serves a list of your pets. It has two endpoints:

  • /api/pets/ returns a list of the ids of your pets

  • /api/pets/:id returns details of your pet with the given id

If you implement the lacinia GraphQL service over the top, your graph would be described like this:

(require '[com.walmartlabs.lacinia.schema :as schema])

(defn compile-schema []
  (schema/compile
   {:objects {:Pet {:fields {:id {:type 'String}
                             :kind {:type 'String
                                    :resolve resolve-pet-kind}
                             :name {:type 'String
                                    :resolve resolve-pet-name}}}}
    :queries {:pets
              {:type '(list :Pet)
               :resolve resolve-pets}}}))

And your resolvers would look like this:

(defn resolve-pets [context arguments parent]
  (-> (http/get "https://petdb.com/api/pets/") :body :ids))

(defn resolve-pet-kind [context arguments id]
  (-> (http/get (str "https://petdb.com/pets/" id)) :body :kind))

(defn resolve-pet-name [context arguments id]
  (-> (http/get (str "https://petdb.com/pets/" id)) :body :name))

So far, so simple. That’s because the resolvers are only given a single job to resolve one piece of data. They are decoupled and therefore reusable which is great when you have a graph where nodes can be reached from many edges.

To get the names and kinds of all your pets you issue the following query:

{
  pets {
    kind
    name
  }
}

You have n pets (your pet store is very successful) but that means the query is taking quite a long time to resolve. You are making 1+n calls to PetDB and paying the overhead of a call 1+n times in serial.

naive sequence diagram

If you queried for both kind and name of the pets you would make 1+2n calls and the performance would be twice as bad.

The incredible bulk

You discover that PetDB has a bulk endpoint where you could fetch details for multiple pets in one call.

  • /api/pets/:id0,:id1,:id2…​

The problem is there’s no straightforward way to take advantage of it. Your resolvers for the pet details are run in isolation from each other and the only hook you have is the top level pets resolver. What if you did all the work in there?

(defn resolve-pets [context arguments parent]
  (let [ids (->> (http/get "https://petdb.com/api/pets/") :body :ids)
        details (->> (http/get (str "https://petdb.com/api/pets/" (str/join "," ids))) :body)]
    (map (fn [id details]
           (assoc details :id id))
         ids details)))

(defn resolve-pet-kind [context arguments parent]
  parent)

(defn resolve-pet-name [context arguments parent]
  parent)

This works, but you’ve complected your pets resolver with the work of the children. You can’t reuse the child resolvers from another edge in the graph now without duplicating this work in all those other parent resolvers.

n wrongs make a right

The DataLoader pattern has an answer, allowing you to keep your resolvers simple and reusable yet still take advantage of the bulk endpoint.

DataLoader allows you to decouple unrelated parts of your application without sacrificing the performance of batch data-loading. While the loader presents an API that loads individual values, all concurrent requests will be coalesced and presented to your batch loading function. This allows your application to safely distribute data fetching requirements throughout your application and maintain minimal outgoing data requests.

The way it works is by providing a bucket into which you can throw all the fetches that your resolvers want to do, and returning a promise for each one. Then, usually after a given time interval, the fetches are coalesced (deduplicated and combined) and performed, delivering the individual results back to the promises.

dataloader sequence diagram

superlifter

superlifter is a Clojure implementation of the DataLoader pattern. It has the following features:

  • Fast, simple implementation of DataLoader pattern

  • Bucketing by time or by queue size

  • Asynchronous fetching

  • Batching of fetches

  • Shared cache for all fetches in a session

  • Guarantees consistent results

  • Avoids duplicating work

  • Access to the cache allows longer-term persistence

Here’s how to set it up.

Start a superlifter per query

A superlifter should live for the lifetime of a query, ensuring consistent results for duplicate reads and capturing and executing all fetches. In lacinia the queries are processed using interceptor chains, so superlifter provides an interceptor that starts a superlifter with your options:

(require '[superlifter.api :as s])
(require '[superlifter.lacinia :refer [inject-superlifter with-superlifter]])
(require '[com.walmartlabs.lacinia.pedestal :as lacinia])

(def lacinia-opts {:graphiql true})

(def superlifter-args
  {:buckets {:default {:triggers {:interval {:interval 10}}}}
   :urania-opts {:env {:db {"abc-123" {:name "Lyra"
                                       :age 11}
                            "def-234" {:name "Pantalaimon"
                                       :age 11}
                            "ghi-345" {:name "Iorek"
                                       :age 41}}}}})

(def service (lacinia/service-map
              (compile-schema)
              (assoc lacinia-opts
                     :interceptors (into [(inject-superlifter superlifter-args)]
                                         (lacinia/default-interceptors (compile-schema) lacinia-opts)))))

Note also that we are injecting data so you can see the data we are working with, but normally this would be a database connection or http library. Our default bucket performs a fetch every 10ms of everything that’s in it at the time. You can add buckets later at runtime for specific purposes - I’ll cover this later on.

Describe your fetches

We now need a way to describe our fetches before we perform them, so that we have a chance of introspecting them in order to deduplicate and batch before executing. The urania library is used for this, so you can implement those protocols if you want the full power of urania, but superlifter provides a abstraction over them with def-fetcher and def-superfetcher that gives a simpler API.

;; def-fetcher - a convenience macro like defrecord for things which cannot be combined
(s/def-fetcher FetchPets []
  (fn [_this env]
    (map (fn [id] {:id id}) (keys (:db env)))))

;; def-superfetcher - a convenience macro like defrecord for combinable things
(s/def-superfetcher FetchPet [id]
  (fn [many env]
    (log/info "Combining request for" (count many) "pets")
    (map (:db env) (map :id many))))

See how the def-superfetcher implementation has the opportunity to combine multiple FetchPet requests into one. This is where we take advantage of any bulk APIs to speed things up!

Chuck 'em in the (time) bucket

We can now write our resolvers to put our fetches into the superlifter bucket:

(defn- resolve-pets [context _args _parent]
  (with-superlifter context
    (s/enqueue! (->FetchPets))))

(defn- resolve-pet-details [context _args {:keys [id]}]
  (with-superlifter context
    (s/enqueue! :pet-details (->FetchPet id))))

The resolve-pets resolver puts a fetch into the bucket which will be performed within the next 10ms. It returns 3 pet ids, after which the resolve-pet-details resolver will run 3 times. Instead of performing 3 separate fetches, those three fetches go into the bucket and are combined into one fetch which will be performed within the next 10ms.

A time bucket of 10ms means you will add up to 10ms to each layer of the query. 10ms might be relatively small but will add up quickly if you query is many layers deep, but making it smaller may mean your fetches span multiple buckets and you lose some efficiency. One of the worst-case scenarios is shown below, with yellow indicating wasted time.

time bucket sequence diagram

Size buckets

In some cases (like when fetching our pet details) we already know how many fetches we are going to make because our parent has given us a list. You can create a new bucket in superlifter for this purpose with a particular size, rather than time window. The fetch is triggered when the number of items in the bucket meets the threshold, saving the cost of waiting for an interval to elapse:

(def superlifter-args
  {:buckets {:default {:triggers {:queue-size {:threshold 1}}}}})

(defn- resolve-pets [context _args _parent]
  (with-superlifter context
    (-> (s/enqueue! (->FetchPets))
        (s/add-bucket! :pet-details
                       (fn [pet-ids]
                         {:triggers {:queue-size {:threshold (count pet-ids)}
                                     :interval {:interval 50}}})))))

(defn- resolve-pet-details [context _args {:keys [id]}]
  (with-superlifter context
    (s/enqueue! :pet-details (->FetchPet id))))

The default bucket has a queue size of 1. The resolve-pets resolver puts its fetch in the default bucket, which will be performed immediately, and then adds a new bucket called :pet-details which has a size of 3, because we know we will fetch details for the 3 pets we have found.

The resolve-pet-details resolver then puts all its fetches into the :pet-details bucket and as soon as the third resolver has run the combined fetch will be performed immediately. No time lost and maximum efficiency!

size bucket sequence diagram

More features

superlifter exposes the cache that is used for fetches, allowing you to prepopulate it from a long-term or shared store before execution and read the results of new fetches after execution to write them to the store. This gives you even finer grained control over how often you perform fetches.

The existing triggers (interval and size) are implemented using multimethods. You can implement your own by participating in them! There is also a "manual mode" without any triggers, allowing you to let the bucket fill and perform the fetches on demand (for example in response to a user pressing a button). You can use any permutation of triggers at the same time - the fetch will be performed when any are satisfied.

In addition, you can create as many buckets as you want, each with their own triggers, and put fetches into their most appropriate bucket. This lets you achieve the best performance and efficiency possible but still keep your resolvers decoupled.

superlifter is on Github with the code for this post in the example project. Feedback is very welcome, along with any features you’d like to see or issues. I have future plans to support Clojurescript, add a "debounced time" bucket (which triggers after a fixed quiet period) and improve the API around creating buckets dynamically.

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