Sending APQ to subgraphs

6. Sending APQ to subgraphs

10m

Overview

The router can also play the role of client—when it sends individual, smaller queries to each subgraph. Accordingly, it can also use APQ to shorten its requests to subgraphs.

In this lesson, we will:

Configure the router to send APQ to subgraphs
Test out APQ in a request to the listings subgraph

Sending APQ to subgraphs

We can use the router-config.yaml file to customize how the router handles APQ with its subgraphs.

We'll need a top-level apq key, with another key nested below called subgraph. (If you added a router key from the previous step, the subgraph key can sit on the same level.)

router-config.yaml

apq:
  subgraph:
  router: # optional configuration for client APQ behavior

Under the subgraph key, we can specify settings for all subgraphs, or override the rules for a specific subgraph. Let's try this out, disabling APQ globally while enabling it for a single subgraph: listings.

router-config.yaml

apq:
  subgraph:
    all:
      enabled: false
    subgraphs:
      listings:
        enabled: true
  router: # router configuration

Then, restart the router to make use of these new settings; make sure to include the --log flag, and pass it a value of trace.

APOLLO_KEY=<APOLLO_KEY> \
APOLLO_GRAPH_REF=<APOLLO_GRAPH_REF> \
  ./router --config router-config.yaml \
  --log trace

Now, when communicating with the listings subgraph, the router can send it the operation identifier of the smaller operation string it would normally send. Provided that the listings subgraph server is configured to understand this hash and use it correctly, that's all that's needed! And in our case, because the listings subgraph is powered by Apollo Server, it can handle the APQ without any trouble at all.

Let's return to the query plan we inspected in the last lesson. It consists of two components that can run in sequence: one query sent to the listings subgraph first, followed by a query to reviews.

QueryPlan {
  Sequence {
    Fetch(service: "listings") {
      {
        listing(id: $listingId) {
          __typename
          id
          title
          description
          numOfBeds
        }
      }
    },
    Flatten(path: "listing") {
      Fetch(service: "reviews") {
        {
          ... on Listing {
            __typename
            id
          }
        } =>
        {
          ... on Listing {
            overallRating
            reviews {
              id
              text
            }
          }
        }
      },
    },
  },
}

Let's zoom in on the request to listings. For the given $listingId, the listings service needs to provide a number of fields.

{
  listing(id: $listingId) {
    __typename
    id
    title
    description
    numOfBeds
  }
}

Return to Explorer, and let's rerun this query to see what happens under the hood of the router.

query GetListingAndReviews($listingId: ID!) {
  listing(id: $listingId) {
    title
    description
    numOfBeds
    overallRating
    reviews {
      id
      text
    }
  }
}

And in the Variables panel.

{
  "listingId": "listing-1"
}

After running the query, return to the router's terminal. We could scroll through its output to find the exact line we're looking for, but let's expedite things by searching directly for the following line, which references the SHA-256 hash generated for the query being sent to listings.

\"sha256Hash\":\"8c6389470a25e2b57141aeb1aee624f77b8310e90563a17a11ebced119756fe8\\"

This brings us to a specific bit of output that starts with "our JSON body:". Here we can find both the query string, along with the SHA-256 hash that was generated for it.

our JSON body: "{
  \"query\":\"query GetListingAndReviews__listings__0($listingId:ID!){listing(id:$listingId){__typename id title description numOfBeds}}\",
  \"operationName\":\"GetListingAndReviews__listings__0\",
  \"variables\":{\"listingId\":\"listing-1\"},
  \"extensions\":{
    \"persistedQuery\":{
      \"version\":1,
      \"sha256Hash\":\"8c6389470a25e2b57141aeb1aee624f77b8310e90563a17a11ebced119756fe8\"
    }
  }
}"

The first time the router sends this request to the listings subgraph, it includes the full operation string along with the hash. This enables us to send just the hash on subsequent requests.

We'll send the listings service another separate request to demonstrate this. Open a new terminal, and build the following curl command to the listings service at https://rt-airlock-subgraphs-listings.herokuapp.com/. This time, we won't send the query string—just the hash. (But note that we do still need to provide a value for our variable, listingId!)

curl --get https://rt-airlock-subgraphs-listings.herokuapp.com/ \
  --header 'content-type: application/json' \
  --data-urlencode 'variables={"listingId":"listing-1"}' \
  --data-urlencode 'extensions={"persistedQuery":{"version":1,"sha256Hash":"8c6389470a25e2b57141aeb1aee624f77b8310e90563a17a11ebced119756fe8"}}'

Note: To provide the variables object for our query, we use an additional --data-urlencode flag that sets the variables property with the appropriate value.

Now that the listings service has already received the operation string this hash refers to, it knows exactly what to do the second time we send it. The hash, along with the appropriate variables, is enough to get data back!

The response from listings

{
  "data": {
    "listing": {
      "__typename": "Listing",
      "id": "listing-1",
      "title": "Cave campsite in snowy MoundiiX",
      "description": "Enjoy this amazing cave campsite in snow MoundiiX, where you'll be one with the nature and wildlife in this wintery planet. All space survival amenities are available. We have complementary dehydrated wine upon your arrival. Check in between 34:00 and 72:00. The nearest village is 3AU away, so please plan accordingly. Recommended for extreme outdoor adventurers.",
      "numOfBeds": 2
    }
  }
}

And because we used query variables ($listingId) in our original operation, we can change the value we provide on the fly; the same hash will continue to work.

Try it out with a different variable!

curl --get https://rt-airlock-subgraphs-listings.herokuapp.com/ \
  --header 'content-type: application/json' \
  --data-urlencode 'variables={"listingId":"listing-3"}' \
  --data-urlencode 'extensions={"persistedQuery":{"version":1,"sha256Hash":"8c6389470a25e2b57141aeb1aee624f77b8310e90563a17a11ebced119756fe8"}}'

You might have also noticed router logs detailing the component of the query that was sent to the reviews subgraph: this request did NOT include a hash of the query. Per our configuration, we enabled APQ exclusively for the listings subgraph!

Practice

Which of the following statements describes router-to-subgraph APQ?

The router sends the operation string, query variables, and the operation's SHA-256 hash. Subsequent requests can omit the operation string.The router cannot send APQ to subgraphs; it can only receive APQ from clients.The router sends the operation string, query variables, and the operation's SHA-256 hash. Subsequent requests can omit the SHA-256 hash.The router sends only the query variables, because the subgraph already has the operation string and the hash.

Key takeaways

The router can also use APQ in the queries it sends to subgraphs. This feature can be enabled globally or on an individual subgraph basis. Subgraphs built with Apollo Server support APQ out of the box.

Up next

With automatic persisted queries, we can reduce the amount of data we send over the wire; but caching them still relies upon the router's (or subgraph server's) in-memory cache. In the next lesson, we'll take our caching to the next level with a distributed cache multiple router instances can share.

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