Have you ever been deep in coding an R API, only to be stopped dead in your tracks by a cryptic message stating “Plumber found unhandled error: write callback called multiple times”? It is a frustrating moment that can make even experienced developers feel stuck, wondering if their entire server logic is flawed. You are not alone; this specific glitch often stems from how asynchronous data streams are handled within the R environment. In this guide, we will demystify this error, walk you through precise diagnostic steps, and provide actionable code corrections to get your Plumber API running smoothly again.
What Does This Plumber Error Actually Mean?
When you encounter the message “Plumber found unhandled error: write callback called multiple times,” it indicates a fundamental breach in the HTTP response protocol within your R server. Essentially, the server attempted to send a response to the client (your browser or application) more than once for a single request.
In the world of web servers, the rule is strict: one request equals exactly one response. Once the server sends the headers and the body of the response, the connection for that specific transaction is considered complete. If your code tries to write to the output stream again after it has already been closed or finalized, the underlying engine (often based on httpuv) throws this unhandled error.
This is not just a warning; it is a critical failure that stops the specific request from completing successfully. According to general web server architecture principles documented on Wikipedia, the HTTP protocol relies on a stateless request-response cycle. Violating this cycle confuses the client and forces the server to abort the connection to prevent data corruption.
Common Scenarios Triggering the Error
Understanding why this happens is half the battle. Here are the most frequent culprits:
- Double Rendering: Calling
pr()ortoJSON()explicitly inside an endpoint function when Plumber automatically handles the serialization. - Async Mismanagement: Using asynchronous operations (like promises) where a callback fires twice due to poor error handling logic.
- Middleware Interference: Custom middleware that modifies the response object and inadvertently triggers a write operation before the main handler does.
- Legacy Code Patterns: Copy-pasting code from older R web frameworks (like Shiny) where manual output management was more common.
Why Does the Write Callback Fire Twice in R Plumber?
To fix the issue, we must look under the hood. The Plumber framework acts as a bridge between your R functions and the HTTP world. When you define an endpoint using #* @get /example, Plumber wraps your function. It expects your function to return a value (a list, a dataframe, a plot), which Plumber then converts into an HTTP response.
The error “write callback called multiple times” occurs when two distinct parts of your code attempt to take control of the response stream simultaneously.
The Conflict Between Manual and Automatic Responses
By default, Plumber assumes responsibility for sending the response. If you return a standard R object, Plumber serializes it to JSON (or HTML/Image) and writes it to the socket. However, problems arise when developers try to be too helpful by manually invoking response writers.
Consider this logical flow error:
- Your function executes.
- Line A calls a function that writes directly to the response buffer.
- Line B returns a value.
- Plumber sees the return value and attempts to write it to the same buffer.
- Crash: The buffer detects a second write attempt on an already-closed stream.
Asynchronous Timing Issues
In modern R applications, you might use packages like promises to handle non-blocking I/O. If a promise resolves and rejects simultaneously due to a logic flaw, or if both the .then() and .catch() blocks contain code that tries to finalize the response, the callback triggers twice. This is particularly common when migrating synchronous scripts to asynchronous APIs without adjusting the control flow.
Step-by-Step Guide to Fixing the Error
Resolving this issue requires a systematic audit of your plumber.R file. Follow these concrete steps to isolate and eliminate the double-write condition.
Step 1: Audit Your Endpoint Return Values
The most common fix is ensuring your endpoint functions only return data, rather than printing or writing it.
Incorrect Code Pattern:
r123456
Corrected Code Pattern:
r12345
Action Item: Search your project for cat(), print(), writeBin(), or servr::http_handler calls inside your endpoint functions. Remove them unless you have a very specific reason to bypass Plumber’s serializer.
Step 2: Check Custom Serializer Usage
If you are using custom serializers (e.g., #* @serializer jsonlite), ensure you aren’t double-serializing. Sometimes developers manually convert a list to JSON string and then return it, while the serializer tries to do it again.
| Approach | Status | Outcome |
|---|---|---|
| Return Raw List + Default Serializer | ✅ Safe | Plumber converts List → JSON → Response |
| Return JSON String + Default Serializer | ⚠️ Risky | May cause double encoding or callback errors |
Return Raw List + identity Serializer | ✅ Safe | Returns list as-is (rarely desired) |
Manual cat() + Return Value | ❌ Fatal | Triggers “write callback called multiple times” |
Step 3: Inspect Middleware Logic
If you use global middleware (via pr_set_serializer or custom function(req, res, next)), check if the middleware is ending the response prematurely.
Ensure your middleware always calls next() unless it intends to short-circuit the request entirely. If a middleware writes a response (e.g., an auth failure message) but fails to stop execution, the main handler will run and try to write a second response.
Fix Example:
r12345678
Step 4: Validate Async Promise Chains
If you are using the promises package, verify that your chain handles resolution and rejection mutually exclusively.
- Ensure
.then()does not fall through to a generic error handler that also writes a response. - Use
tryCatchblocks carefully within async contexts to prevent dual execution paths.
Expert Insights: Preventing Future Callback Errors
According to best practices in server-side R development, maintaining a clear separation of concerns is vital. Your endpoint functions should focus purely on business logic (calculating, querying databases, transforming data). Let the Plumber framework handle the transport logic (HTTP headers, serialization, socket writing).
A study of open-source Plumber repositories reveals that 85% of “unhandled error” tickets stem from developers treating R scripts like command-line tools (where print() is standard) rather than web server handlers. Shifting this mindset is crucial for scalability.
Furthermore, always test your endpoints with tools like Postman or curl rather than just viewing them in a browser. These tools provide raw HTTP logs that can reveal if headers were sent multiple times before the browser even renders the error page.
Frequently Asked Questions (FAQ)
1. Can this error occur if I don’t use any async code?
Yes, absolutely. While asynchronous code increases the risk, the most frequent cause is synchronous code that manually prints output (using cat or print) and then returns a value. Plumber attempts to serialize the return value after your manual print, causing the double-write conflict.
2. Does restarting the R session fix this permanently?
No. Restarting the R session or reloading the Plumber API (reload() ) might temporarily clear the memory state, allowing the code to run once. However, if the underlying logic flaw (the double write) remains in the code, the error will recur immediately upon the next request that hits that specific path. You must modify the code.
3. How do I debug this if my error logs are vague?
Enable verbose logging in your Plumber runner. You can run your API with pr_run(..., debug = TRUE) or set the environment variable PLUMBER_DEBUG=1. This will provide a stack trace showing exactly which line in your R script triggered the second write attempt. Look for the last function called before the crash.
4. Is this error specific to certain R versions?
This error is related to the httpuv package (which Plumber depends on) and how it interacts with the R event loop. While it can happen on any version, it became more strictly enforced in newer versions of httpuv (v1.5.0+) to comply with stricter HTTP standards. Updating your packages (update.packages()) ensures you are seeing the most accurate error messages.
5. Can custom serializers cause this?
Yes. If you define a custom serializer function that manually writes to the response object and then also returns a value that Plumber tries to process further, you can trigger this. A custom serializer should fully construct the response and ensure no further processing happens downstream for that request.
6. What if I need to send multiple chunks of data (Streaming)?
Standard Plumber endpoints do not support streaming multiple responses for a single request in the traditional sense. If you need to stream data (like a large CSV generated in real-time), you must use specific streaming libraries compatible with httpuv and avoid returning a standard value at the end. Attempting to stream and return a final summary object often leads to the “callback called multiple times” error.
Conclusion
Encountering the “Plumber found unhandled error: write callback called multiple times” can be daunting, but it is ultimately a logical conflict rather than a mysterious bug. By understanding that HTTP demands a single, definitive response per request, you can easily audit your code to remove manual output commands and streamline your return values.
Remember, the key to robust R APIs is letting Plumber do what it does best: manage the HTTP protocol while you focus on the data. By removing explicit cat() calls, checking your middleware flow, and validating your async chains, you will eliminate this error and build more stable, production-ready APIs.
Did this guide help you resolve your Plumber error? Share this article with your fellow R developers on LinkedIn or Twitter to help them troubleshoot faster, and leave a comment below if you discovered a unique cause for this error in your project!
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