400 Bad Request in REST APIs: Best Practices
Introduction
A 400 Bad Request error indicates that the server could not understand the client's request due to invalid syntax, missing required parameters, or other formatting issues. Properly handling 400 errors is crucial for building robust, developer-friendly APIs. Ungracefully handled 400s lead to frustrating user experiences that erode API adoption and retention.
This article will provide REST API design best practices to avoid 400 errors, and graceful error handling techniques for when 400s inevitably occur. Following these guidelines will help create resilient APIs that return meaningful 400 responses developers can quickly debug. The end goal is happy API consumers who can easily integrate with your services.
API Design Best Practices to Avoid 400 Errors
Carefully designing your API's interface upfront can prevent many 400 errors by validating inputs, documenting properly, and accounting for edge cases. Here are some best practices to bake into your API that will head off bad requests down the line.
Validate Required Parameters
One of the most common triggers for 400 errors is missing required parameters in the request. All server-side endpoints should programmatically check for the presence of required parameters and return a 400 specifying exactly which ones are missing or invalid.
For example, an endpoint expecting an email address parameter should validate it matches an expected pattern like user@example.com. Relying solely on frontend validation is not sufficient, as not all consumers may use your UI. Performing validation at the API layer ensures all clients will get pre-emptively warned about missing data.
Provide clear and consistent error messages indicating the names of missing params. Avoid generic “missing required fields” texts.
Consider making some parameters optional with sensible defaults rather than strictly required. This avoids 400s when consumers don’t have values for optional data.
Trim and sanitize input values before validating to handle whitespace and special characters that may cause false negatives.
Use Semantic Versioning
Following semantic versioning practices avoids breaking existing API functionality for consumers when releasing updates.
Increment the major version number for changes that alter or remove existing endpoints. Minor versions add new functionality in a backwards-compatible way. Patch versions contain only bug fixes.
Avoid suddenly deprecating or altering endpoints without warning. Use versioning to introduce changes gradually and avoid unexpectedly breaking consumers' integrations.
Write Clear Documentation
Thoroughly documenting your API's expected request parameters,formats, headers, and example responses goes a long way towards avoiding bad requests.
Provide annotated sample requests showing exactly how to structure queries, path variables, and POST/PUT body contents.
Document all required and optional parameters with data types and constraints. Explain any default values assumed for missing optional params.
Keep documentation in sync with evolving API functionality to avoid outdated examples that no longer work.
When returning 400 errors, reference the relevant section of the docs to guide developers to solutions.
Consider using an interactive documentation generator like Swagger UI that lets developers try out requests.
Set Reasonable Limits
Setting appropriate string length limits, maximum array sizes, and allowed integer ranges when validating user input also heads off bad requests.
Restrict strings like names, emails, etc. to reasonable maximum lengths. Validate array sizes fall under sane limits for your use case. Constrain numeric values to appropriate ranges.
This avoids resource exhaustion DOS attacks from excessively large payloads as well as typos that provide unusably large values.
Encode Carefully
Properly encoding path, query, and header values avoids parsing issues that lead to 400 errors.
Use percent encoding for all special characters in paths and query strings. Be cautious when decoding percent encoded values on the server.
Always encode paths even if working locally during development to match real world usage.
Consider allowing '.' characters in path segments for greater flexibility if appropriate.
Cover Common Edge Cases
Accounting for edge cases in your validation logic avoids unexpected 400s down the line.
Check for duplicate parameters and combinations of values that don't make logical sense in your API. If certain endpoints are only meant to accept certain HTTP methods, check the method and return a 405 Method Not Allowed if incorrect.
Consider using a schema language like OpenAPI to define formal request/response formats. This forces enumerating edge cases upfront.
The more validation logic, the lower the probability of letting bad requests through. But beware overly restrictive validation that blocks acceptable requests. Find the right balance for your API.
Handling 400 Errors Gracefully
Even with excellent validation logic, some 400 errors will inevitably occur once your API is in the wild. Handling 400s thoughtfully improves the developer experience and builds API resilience.
Provide Actionable Error Messages
Return error messages that clearly explain the issue so developers know exactly how to fix the problem.
Specify the validation that failed like missing required parameters. List all missing params in the message.
Avoid generic "400 Bad Request" body text - customize the message for the specific error scenario at hand.
Suggest fixes in the message text where possible, like including an example of the correct parameter format expected.
But don't expose sensitive implementation details in production environments - log details internally only.
Use Unique Error Codes
Define a set of application-specific error codes that map to each 400 error scenario, like INVALID_CUSTOMER_ID_FORMAT.
Allows programmatic handling without parsing and extracting texts from responses.
Can group similar errors under a common parent code for easier handling.
Pick intuitive codes using your team's naming conventions. Thoroughly document all codes.
Log and Monitor 400s
Logging 400 errors lets you identify clients generating excessive bad requests that potentially need to be blacklisted or rate limited at some threshold.
Can reveal patterns and common problems in requests to further improve validations proactively.
Configure alerting for spikes in 400s that may indicate an issue or attack.
Log as much context about requests as possible - client, URLs, times, frequency etc.
Retry Intermittently
For suspected transient errors, retry the same request 1-2 times with jittered delays between attempts.
But set reasonable limits on number and frequency of retries to avoid overloading backend with repeat attempts.
Carefully choose which 400s are likely safe to retry versus permanent errors that won't succeed with a retry.
Return 400 Fast
Avoid expensive payload parsing and processing for requests that are clearly invalid from the first few checks.
Perform just enough validation to identify core issue then return 400 error immediately to avoid wasted resources.
Set sane limits on parsing/buffering sizes for malformed payloads.
The faster you can return 400 errors, the quicker developers can fix and resend requests. Slow 400s waste developer time waiting on bad requests.
Conclusion
Carefully designing your API interface and input validation helps avoid bad requests, while gracefully handling 400 errors improves developer experience when they occur. Following these best practices leads to satisfied developers who can build quickly and productively. This creates ecosystem value around your API. Monitor 400 errors to identify improvements that will further reduce bad requests in the future. Well-crafted APIs supported by platforms like DevHunt make developers happy and lead to successful integrations.