Gateway composition in GraphQL - Step-by-Step Execution

Concept Flow - Gateway composition

Client sends query

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Gateway receives query

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Gateway splits query into subqueries

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Send subqueries to respective services

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Services process subqueries

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Services return partial results

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Gateway merges partial results

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Gateway returns combined result to client

The gateway receives a client query, splits it into parts for different services, collects their responses, merges them, and sends back a single combined result.

Execution Sample

GraphQL

query {
  user(id: "1") {
    name
    posts {
      title
    }
  }
}

A client requests a user’s name and their posts’ titles; the gateway splits this to user and post services and combines the results.

Execution Table

Step	Action	Query/Response	Service	Result
1	Receive full query	{ user(id: "1") { name posts { title } } }	Gateway	Full query received
2	Split query	{ user(id: "1") { name } }	Gateway	User subquery created
3	Split query	{ posts(userId: "1") { title } }	Gateway	Post subquery created
4	Send subquery	{ user(id: "1") { name } }	User Service	Request sent
5	Send subquery	{ posts(userId: "1") { title } }	Post Service	Request sent
6	Receive response	{ name: "Alice" }	User Service	User data received
7	Receive response	{ posts: [{ title: "Hello World" }, { title: "GraphQL Rocks" }] }	Post Service	Post data received
8	Merge responses	{ user: { name: "Alice", posts: [{ title: "Hello World" }, { title: "GraphQL Rocks" }] } }	Gateway	Combined result created
9	Return combined result	{ user: { name: "Alice", posts: [{ title: "Hello World" }, { title: "GraphQL Rocks" }] } }	Gateway	Result sent to client
10	End			Query completed

💡 All subqueries processed and merged; final combined result returned to client.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 6	After Step 7	Final
fullQuery	null	{ user(id: "1") { name posts { title } } }	{ user(id: "1") { name posts { title } } }	{ user(id: "1") { name posts { title } } }	{ user(id: "1") { name posts { title } } }	null
userSubquery	null	{ user(id: "1") { name } }	{ user(id: "1") { name } }	{ name: "Alice" }	{ name: "Alice" }	{ name: "Alice" }
postSubquery	null	null	{ posts(userId: "1") { title } }	null	{ posts: [{ title: "Hello World" }, { title: "GraphQL Rocks" }] }	{ posts: [{ title: "Hello World" }, { title: "GraphQL Rocks" }] }
combinedResult	null	null	null	null	null	{ user: { name: "Alice", posts: [{ title: "Hello World" }, { title: "GraphQL Rocks" }] } }

Key Moments - 3 Insights

Why does the gateway split the query into subqueries?

Because each service owns part of the data, the gateway splits the query so each service can handle its part independently, as shown in steps 2 and 3 of the execution_table.

How does the gateway combine results from different services?

After receiving partial results from services (steps 6 and 7), the gateway merges them into one combined response (step 8), so the client gets a single unified answer.

What happens if one service is slow or fails?

The gateway waits for all services to respond before merging. If a service is slow or fails, the gateway may delay or return an error. This is why step 9 only happens after all responses are received.

Visual Quiz - 3 Questions

Look at the execution_table, what query does the gateway send to the Post Service?

A { user(id: "1") { name posts { title } } }

B { posts(userId: "1") { title } }

C { user(id: "1") { name } }

D { posts { title } }

💡 Hint

Check step 3 and 5 in the execution_table where the post subquery is created and sent.

At which step does the gateway merge the partial results?

A Step 6

B Step 7

C Step 8

D Step 9

💡 Hint

Look for the step labeled 'Merge responses' in the execution_table.

If the user service returned no name, how would the combined result change?

A The combined result would have user name as null or missing

B The combined result would not include posts

C The gateway would skip merging

D The gateway would send an error to the client immediately

💡 Hint

Refer to variable_tracker's combinedResult and how partial results affect it.

Concept Snapshot

Gateway Composition in GraphQL:
- Gateway receives full client query
- Splits query into subqueries per service
- Sends subqueries to respective services
- Receives partial results
- Merges results into one response
- Returns combined data to client

Full Transcript

Gateway composition in GraphQL works by the gateway receiving a full client query. It then splits this query into smaller subqueries, each targeting a specific service responsible for part of the data. The gateway sends these subqueries to the respective services. Each service processes its subquery and returns partial results. The gateway waits for all responses, merges them into a single combined result, and sends this unified response back to the client. This process allows multiple services to work together seamlessly to fulfill a single client request.