Persisted queries in GraphQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the result of executing a persisted query with a missing hash?

Consider a GraphQL server that uses persisted queries identified by SHA-256 hashes. If a client sends a request with a hash that does not match any stored query, what will the server respond with?

A The server ignores the hash and executes the query text sent by the client.

B The server executes a default fallback query instead.

C The server returns an error indicating the query was not found.

D The server caches the new query under the given hash automatically.

💡 Hint

Think about what happens when the server cannot find a query matching the client's hash.

Persisted queries rely on the server having the query stored under the given hash. If the hash is unknown, the server cannot execute the query and returns an error.

🧠 Conceptual

intermediate

Why use persisted queries in GraphQL?

Which of the following is the main benefit of using persisted queries in GraphQL?

A To reduce the size of requests by sending only query hashes instead of full query text.

B To allow clients to send multiple queries in a single request.

C To enable real-time subscriptions over WebSocket connections.

D To automatically generate schema documentation.

💡 Hint

Think about network efficiency and bandwidth savings.

Persisted queries reduce request size by sending only a small hash instead of the full query text, improving performance especially on slow networks.

📝 Syntax

advanced

Which option correctly shows how to register a persisted query in a GraphQL server?

Given a GraphQL server setup, which snippet correctly registers a persisted query with hash abc123 and query { user { id name } }?

A persistedQueries.set('abc123', '{ user { id name } }');

B persistedQueries.add('{ user { id name } }', 'abc123');

C persistedQueries['abc123'] = '{ user { id name } }';

D persistedQueries.register('{ user { id name } }', 'abc123');

💡 Hint

Consider common JavaScript Map methods for storing key-value pairs.

The set method is used to add a key-value pair to a Map object, where the key is the hash and the value is the query string.

❓ optimization

advanced

How do persisted queries improve caching efficiency?

Which statement best explains how persisted queries help with caching on the server or CDN?

A Persisted queries disable caching to ensure fresh data every time.

B Persisted queries compress the query text to reduce cache size.

C Persisted queries require clients to send full query text, improving cache hits.

D Persisted queries use fixed hashes, allowing caches to store and identify queries by hash instead of varying query text.

💡 Hint

Think about how identical keys help caches recognize repeated requests.

Using a fixed hash as a key means caches can easily identify repeated queries and serve cached responses, improving performance.

🔧 Debug

expert

What error occurs if a client sends a persisted query hash but no query text and the hash is unknown?

A client sends a GraphQL request with only a persisted query hash that the server does not recognize, and no query text. What error will the server produce?

A TimeoutError because the server waits for query text indefinitely.

B PersistedQueryNotFound error indicating the hash is unknown.

C AuthenticationError because the client is unauthorized.

D SyntaxError due to missing query text.

💡 Hint

Consider the server's response when it cannot find the query for the given hash.

The server returns a specific PersistedQueryNotFound error when the hash is unknown and no query text is provided to fallback on.

Practice

1. What is the main benefit of using persisted queries in GraphQL?

easy

A. The server stores user credentials for faster login.

B. Clients send only a unique ID instead of the full query, saving bandwidth.

C. Queries are automatically optimized by the client.

D. It allows clients to write queries without validation.

Solution

1. Step 1: Understand what persisted queries do

   Persisted queries store the full GraphQL query on the server with a unique ID.

2. Step 2: Identify the client-server interaction

   Clients send only the ID to run the query, reducing the data sent over the network.

3. Final Answer:

   Clients send only a unique ID instead of the full query, saving bandwidth. -> Option B

4. Quick Check:

   Persisted queries reduce data sent = Clients send only a unique ID instead of the full query, saving bandwidth. [OK]

Hint: Persisted queries send IDs, not full queries [OK]

Common Mistakes:

• Thinking clients send full queries every time
• Confusing persisted queries with client-side caching
• Believing persisted queries store user data

2. Which of the following is the correct way to send a persisted query request in GraphQL?

easy

A. { "id": "12345", "variables": { "userId": "1" } }

B. { "query": "{ user(id: 1) { name } }" }

C. { "mutation": "updateUser" }

D. { "headers": { "Authorization": "token" } }

Solution

1. Step 1: Identify the persisted query request format

   Persisted queries send the unique query ID and variables, not the full query string.

2. Step 2: Match the correct JSON structure

   { "id": "12345", "variables": { "userId": "1" } } sends an ID and variables, which is the correct persisted query format.

3. Final Answer:

   { "id": "12345", "variables": { "userId": "1" } } -> Option A

4. Quick Check:

   Persisted query request = ID + variables [OK]

Hint: Persisted queries use ID field, not full query [OK]

Common Mistakes:

• Sending full query instead of ID
• Using mutation key instead of id
• Confusing headers with query payload

3. Given the following persisted query setup, what will the server return when the client sends { "id": "abc123", "variables": { "limit": 5 } } if the query with ID abc123 fetches the first limit users?

Assume the database has 10 users named User1 to User10.

Options:

medium

A. []

B. [{"name": "User6"}, {"name": "User7"}, {"name": "User8"}, {"name": "User9"}, {"name": "User10"}]

C. [{"name": "User1"}, {"name": "User2"}, {"name": "User3"}, {"name": "User4"}, {"name": "User5"}]

D. Error: Query ID not found

Solution

1. Step 1: Understand the query and variables

   The query with ID 'abc123' fetches users limited by the 'limit' variable, which is 5 here.

2. Step 2: Determine the expected result

   Since the database has users User1 to User10, fetching the first 5 returns User1 to User5.

3. Final Answer:

   [{"name": "User1"}, {"name": "User2"}, {"name": "User3"}, {"name": "User4"}, {"name": "User5"}] -> Option C

4. Quick Check:

   Limit 5 users returns first 5 users [OK]

Hint: Variables control query output; check their values [OK]

Common Mistakes:

• Assuming query returns last users
• Thinking ID is invalid
• Ignoring variables in query

4. A developer tries to use persisted queries but gets an error: Query ID not found. What is the most likely cause?

medium

A. The client sent a query ID that the server does not recognize.

B. The client sent the full query instead of the ID.

C. The server does not support GraphQL.

D. The client forgot to include variables.

Solution

1. Step 1: Analyze the error message

   The error 'Query ID not found' means the server cannot find the query matching the sent ID.

2. Step 2: Identify the cause

   This usually happens if the client sends an ID that was never registered or stored on the server.

3. Final Answer:

   The client sent a query ID that the server does not recognize. -> Option A

4. Quick Check:

   Unknown query ID causes 'Query ID not found' error [OK]

Hint: Check if query ID is registered on server [OK]

Common Mistakes:

• Assuming full query sent causes this error
• Blaming missing variables for this error
• Thinking server lacks GraphQL support

5. You want to secure your GraphQL API by allowing only persisted queries. Which approach best achieves this?

hard

A. Disable persisted queries and use API keys instead.

B. Allow all queries but log those without IDs for review.

C. Require clients to send full queries and IDs together.

D. Reject any request that does not include a valid persisted query ID.

Solution

1. Step 1: Understand API security with persisted queries

   Allowing only persisted queries means the server accepts requests only if they have a valid stored query ID.

2. Step 2: Identify the best enforcement method

   Rejecting requests without valid IDs ensures only approved queries run, improving security.

3. Final Answer:

   Reject any request that does not include a valid persisted query ID. -> Option D

4. Quick Check:

   Only accept valid persisted query IDs to secure API [OK]

Hint: Block requests missing valid persisted query IDs [OK]

Common Mistakes:

• Allowing all queries weakens security
• Sending full queries defeats persisted query purpose
• Relying only on API keys without query control