Add optional operationTypeHeader to Apollo client options

When set, HTTP requests carry the GraphQL operation type (query/mutation)
as the value of the given header, so a load balancer can route queries to
a read replica. Opt-in via new createClient'/createSubscriptionClient';
existing constructors are unchanged.

Author: purefunctor

src/GraphQL/Client/BaseClients/Apollo.js

@@ -2,7 +2,6 @@ import { createClient as createWsClient } from "graphql-ws";
 import {
   gql,
   split,
-  HttpLink,
   createHttpLink,
   InMemoryCache,
   ApolloClient,
@@ -11,6 +10,37 @@ import { getMainDefinition } from "@apollo/client/utilities/index.js";
 import { setContext } from "@apollo/client/link/context/index.js";
 import { GraphQLWsLink } from "@apollo/client/link/subscriptions/index.js";
 
+// Build the HTTP link. When opts.operationTypeHeader is set, requests are
+// split by operation type and the type ("query" or "mutation") is sent as
+// the value of that header, so eg. a load balancer can route queries to a
+// read replica.
+const mkHttpLink = function (opts) {
+  if (!opts.operationTypeHeader) {
+    return createHttpLink({
+      uri: opts.url,
+    });
+  }
+
+  const linkForOperationType = function (operationType) {
+    return createHttpLink({
+      uri: opts.url,
+      headers: { [opts.operationTypeHeader]: operationType },
+    });
+  };
+
+  return split(
+    function ({ query }) {
+      const definition = getMainDefinition(query);
+      return (
+        definition.kind === "OperationDefinition" &&
+        definition.operation === "query"
+      );
+    },
+    linkForOperationType("query"),
+    linkForOperationType("mutation"),
+  );
+};
+
 const createClientWithoutWebsockets = function (opts) {
   const authLink = setContext(function (_, { headers }) {
     // get the authentication token from local storage if it exists
@@ -25,9 +55,7 @@ const createClientWithoutWebsockets = function (opts) {
     };
   });
 
-  const httpLink = createHttpLink({
-    uri: opts.url,
-  });
+  const httpLink = mkHttpLink(opts);
 
   return new ApolloClient({
     link: authLink.concat(httpLink),
@@ -40,13 +68,7 @@ const createClientWithoutWebsockets = function (opts) {
 };
 
 const createClientWithWebsockets = function (opts) {
-  const httpLink = new HttpLink({
-    uri: opts.url,
-    options: {
-      authToken: opts.authToken,
-      reconnect: true,
-    },
-  });
+  const httpLink = mkHttpLink(opts);
 
   const wsLink = new GraphQLWsLink(
     createWsClient({

src/GraphQL/Client/BaseClients/Apollo.purs

@@ -1,13 +1,17 @@
 -- | Creates GraphQL Apollo clients
 module GraphQL.Client.BaseClients.Apollo
   ( ApolloClientOptions
+  , ApolloClientOptions'
   , ApolloSubClientOptions
+  , ApolloSubClientOptions'
   , ApolloClient
   , ApolloSubClient
   , MutationOpts
   , QueryOpts
   , createClient
+  , createClient'
   , createSubscriptionClient
+  , createSubscriptionClient'
   , class IsApollo
   , updateCacheJson
   , updateCache
@@ -45,13 +49,37 @@ type ApolloClientOptions =
   , headers :: Array RequestHeader
   }
 
+-- | As `ApolloClientOptions`, plus `operationTypeHeader`: when set, HTTP
+-- | requests include the GraphQL operation type ("query" or "mutation") as
+-- | the value of the header with this name, so eg. a load balancer can route
+-- | queries to a read replica.
+type ApolloClientOptions' =
+  { url :: URL
+  , authToken :: Maybe String
+  , headers :: Array RequestHeader
+  , operationTypeHeader :: Maybe String
+  }
+
 type ApolloSubClientOptions =
   { url :: URL
   , websocketUrl :: URL
   , authToken :: Maybe String
   , headers :: Array RequestHeader
   }
 
+-- | As `ApolloSubClientOptions`, plus `operationTypeHeader`: when set, HTTP
+-- | requests include the GraphQL operation type ("query" or "mutation") as
+-- | the value of the header with this name, so eg. a load balancer can route
+-- | queries to a read replica. Subscriptions go over the websocket and are
+-- | unaffected (custom headers are not supported in websockets).
+type ApolloSubClientOptions' =
+  { url :: URL
+  , websocketUrl :: URL
+  , authToken :: Maybe String
+  , headers :: Array RequestHeader
+  , operationTypeHeader :: Maybe String
+  }
+
 -- | Apollo client to make graphQL queries and mutations.
 -- | From the @apollo/client npm module
 foreign import data ApolloClient :: Type
@@ -91,41 +119,60 @@ createClient
   :: forall schema
    . ApolloClientOptions
   -> Effect (Client ApolloClient schema)
-createClient = clientOptsToForeign >>> createClientImpl >>> map Client
+createClient { url, authToken, headers } =
+  createClient' { url, authToken, headers, operationTypeHeader: Nothing }
+
+createClient'
+  :: forall schema
+   . ApolloClientOptions'
+  -> Effect (Client ApolloClient schema)
+createClient' = clientOptsToForeign >>> createClientImpl >>> map Client
 
 createSubscriptionClient
   :: forall schema
    . ApolloSubClientOptions
   -> Effect (Client ApolloSubClient schema)
-createSubscriptionClient = clientOptsToForeign >>> createSubscriptionClientImpl >>> map Client
+createSubscriptionClient { url, websocketUrl, authToken, headers } =
+  createSubscriptionClient' { url, websocketUrl, authToken, headers, operationTypeHeader: Nothing }
+
+createSubscriptionClient'
+  :: forall schema
+   . ApolloSubClientOptions'
+  -> Effect (Client ApolloSubClient schema)
+createSubscriptionClient' = clientOptsToForeign >>> createSubscriptionClientImpl >>> map Client
 
 clientOptsToForeign
   :: forall r
    . { authToken :: Maybe String
      , headers :: Array RequestHeader
+     , operationTypeHeader :: Maybe String
      | r
      }
   -> { authToken :: Nullable String
      , headers :: Object String
+     , operationTypeHeader :: Nullable String
      | r
      }
 clientOptsToForeign opts =
   opts
     { authToken = toNullable opts.authToken
     , headers = Object.fromFoldable $ opts.headers <#> \h -> Tuple (name h) (value h)
+    , operationTypeHeader = toNullable opts.operationTypeHeader
     }
 
 type ApolloClientOptionsForeign =
   { url :: URL
   , authToken :: Nullable String
   , headers :: Object String
+  , operationTypeHeader :: Nullable String
   }
 
 type ApolloSubApolloClientOptionsForeign =
   { url :: URL
   , websocketUrl :: URL
   , authToken :: Nullable String
   , headers :: Object String
+  , operationTypeHeader :: Nullable String
   }
 
 instance queryClient ::