API Testing Interview Questions and Answers(Level-3)

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41. What is the upper limit for a payload in the POST method?

Answer:
Theoretically, there is no limit on the size of the payload you can send using the POST method.
Unlike GET requests, which append data to the URL and are restricted by maximum URL length limitations (e.g., ~2048 characters in most browsers), POST payloads are transmitted in the body and do not have size constraints at the protocol level.

However, in real-world applications:

• Web servers, proxies, and application frameworks often impose practical limits (e.g., 2 MB, 5 MB, 10 MB) to avoid resource exhaustion and performance bottlenecks.
  
• Sending a very large payload can lead to increased bandwidth consumption, high server memory usage, longer processing times, and higher chances of request timeouts.
  

Best Practice:

• Use streaming uploads for very large payloads.
  
• Always consider splitting large data into multiple smaller requests.
  
• Configure server-side limits properly (e.g., in Nginx, Apache, Spring Boot).
  

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42. What is the caching mechanism in web APIs?

Answer:
Caching refers to temporarily storing data for faster future retrieval without making redundant network requests to the server.
In the context of API testing and API design:

• Client-side caching stores data locally (e.g., browser cache, mobile app memory) to reduce API calls.
  
• Server-side caching stores previously computed responses in high-speed storage (memory, in-memory databases like Redis) and returns them for identical requests.
  

Key Benefits:

• Reduces server load.
  
• Improves response time for users.
  
• Saves bandwidth and cost.
  

Common HTTP Cache Headers:

• Cache-Control (public, private, no-cache, max-age)
  
• ETag (entity tag for versioning)
  
• Expires (absolute expiry date)
  
• Last-Modified (timestamp of last change)
  

Best Practice:

• Cache static resources aggressively.
  
• Use validation tokens like ETags to handle cache updates efficiently.
  

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43. What are SOAP Web Services?

Answer:
SOAP (Simple Object Access Protocol) is an XML-based protocol for exchanging structured information in web services over networks.
SOAP enables different systems (written in different languages and running on different platforms) to communicate seamlessly by using standardized XML messages.

Key Features:

• Platform and Language Independent
  
• Works over HTTP, SMTP, TCP
  
• Supports WS-Security, Transactions, and Messaging Patterns
  
• Describes operations using WSDL (Web Services Description Language)
  

Use Cases:

• Enterprise-level, mission-critical web services
  
• Complex transactions requiring high security and reliability
  

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44. How does SOAP work internally?

Answer:
The SOAP communication process includes:

• Client Application generates a SOAP Request (XML-based) that contains the operation name and parameters.
  
• Request is transmitted over the network using a transport protocol (commonly HTTP POST).
  
• SOAP Server parses the request, processes it, executes necessary operations, and generates a SOAP Response (again XML).
  
• The response is transmitted back to the client.
  

Key Points:

• SOAP messages must be wrapped inside an Envelope.
  
• Data exchange happens in XML format.
  
• Transport can also happen using SMTP, JMS, etc.
  

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45. When should you use SOAP API?

Answer:
You should prefer SOAP API when:

• Advanced Security is required (e.g., WS-Security, XML Encryption, SAML Tokens).
  
• Reliable Messaging and Transactional Support are necessary (e.g., financial, banking systems).
  
• Formal Contracts are important (WSDL definitions).
  
• Need stateful operations with strict request/response structures.
  
• Interoperability with legacy systems that are SOAP-based.
  

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46. How do users utilize SOAP facilities?

Answer:
SOAP web services expose several functionalities, for example:

• PutAddress() → Submits address data.
  
• PutListing() → Submits complete XML documents into the system.
  
• GetAddress() → Retrieves address information based on a search query.
  
• GetAllListing() → Fetches all available listings in XML format.
  

Each operation encapsulates request and response in XML, allowing platform-independent interactions.

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47. What is the major obstacle users face with SOAP?

Answer:
The biggest obstacle is firewall security:

• Many firewalls block non-HTTP protocols and unknown ports.
  
• SOAP, when using HTTP port 80, can pass through, but performance suffers due to its large XML payload.
  
• SOAP mixes transport layer and message structure specifications, making debugging and scaling harder.
  

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48. What are the different approaches for developing SOAP Web Services?

Answer:
Two approaches:

• Contract-First (Top-Down):
  
  • Define WSDL first.
    
  • Generate Java classes later.
    
  • Ensures strong API contract before coding.
    
  • Ideal for enterprise-grade services.
    
• Contract-Last (Bottom-Up):
  
  • Create Java classes first.
    
  • Auto-generate WSDL.
    
  • Easier and faster, but harder to control WSDL.
    

Note:
Contract-First is generally preferred for high-quality web services.

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49. What are the elements of a SOAP message structure?

Answer:
SOAP messages have:

• Envelope – Root element, defines the start and end.
  
• Header – (Optional) Meta-information like authentication, transaction management.
  
• Body – Contains the actual API request/response payload.
  
• Fault – Error handling block describing problems that occurred during processing.
  

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50. What are the syntax rules for a SOAP message?

Answer:

• Must use well-formed encoded XML.
  
• Must include Envelope namespace.
  
• Encoding style must be defined.
  
• Must not contain DTD references.
  

Must not include XML processing instructions (<? ?>).

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