What is client_max_body_size in Nginx

Nginx is a high performance open source web server known for its resource efficiency, rich feature set and ability to handle large workloads. One key configuration directive related to upload limits is the client_max_body_size setting. This controls the maximum size of requests bodies that can sent to the Nginx server.

In this comprehensive 4 part guide, we cover all aspects of configuring client_max_body_size including:

Part 1: Overview of client_max_body_size and request body handling internals

Part 2: Appropriate sizing and use cases for setting client_max_body_size

Part 3: Troubleshooting and monitoring issues related to large uploads

Part 4: Alternative approaches like offloading uploads and emerging body size limit features

Understanding client_max_body_size in depth will help you maximize upload capacity while avoiding issues from excessively large media, documents and other files.

Part 1 – How Nginx handles and limits request body sizes

To understand client_max_body_size, you first need to know how Nginx receives and processes request data sent from clients like browsers. Let‘s take a technical look under the hood.

Buffering request bodies

As the HTTP request comes in, the request body gets handled by various parts of Nginx:

• The ngx_http_read_client_request_body module initially reads the request body data. This data gets stored in buffers controlled by several other key modules:

• The ngx_http_request_body_module manages a chain of buffers that hold portions of the request body until the entire body is received.

• The ngx_http_core_module also defines a client request buffer that plays a role in aggregating request headers and data.

Streaming bodies to disk

For most requests, while portions of the body get buffered in memory, the entire contents also stream to a temporary file on disk using the write_client_request_body function:

location /uploads {

  client_body_in_file_only on;

  client_body_buffer_size 1K; 

  client_max_body_size 10M;

  ...
}

So even very large files avoid exhausting RAM. The downside is streaming bodies to disk can be slower in some cases.

Key Insight:

By default, Nginx handles request bodies by:

1. Buffering portions in memory
2. Streaming the full contents to disk

This avoids RAM overload issues

Understanding these internals helps explain the various buffer configuration options, how bodies get limited in size, and more.

Now let‘s look specifically at how client_max_body_size works…

How client_max_body_size limits upload size

The ngx_http_core_module is what actually imposes the client_max_body_size restriction in Nginx.

As the request body is received, this core module tracks the number of bytes received so far. If the size exceeds the client_max_body_size set in the config, Nginx will:

1. Stop receiving any more request body data
2. Return a HTTP 413 error back to the client: "Request Entity Too Large"
3. Log a warning message to the Nginx error log file

So while buffers and disk help handle large uploads, client_max_body_size enforces the actual size restriction.

Default client_max_body_size values

If not explicitly configured, the default client_max_body_size value in Nginx is 1 megabyte.

Given many modern web applications allow uploading images, documents, spreadsheets and more, 1 megabyte is often inadequate. Even medium sized images can exceed this limit.

That‘s why many sites increase client_max_body_size significantly higher than the default. But high limits can cause downsides discussed next…

Impacts of very large upload sizes

Allowing very large file uploads in Nginx can negatively impact:

• Web server RAM – Larger uploads mean bigger memory buffers to handle bodies. At extreme sizes this can crash Nginx.
• Disk usage – Temporary file storage on disk can fill up and cause errors.
• Blocking requests – Large uploads can tie up an Nginx worker process badly impacting overall throughput.
• Denial of service attacks – External users uploading bogus giant files can overwhelm the server.

So while allowing reasonably sized uploads is important, extremely high limits can be problematic. There is a balance when tuning this directive.

Recommendation:

When adjusting client_max_body_size, monitor for

Now that we understand the internals, let‘s look at appropriate sizing…

Part 2 – Setting client_max_body_size values for your workload

With the basics covered, let‘s now see how to set client_max_body_size values tailored to your specific app requirements and average upload sizes.

Factors influencing appropriate values

Various factors influence what client_max_body_size value makes sense:

• Application requirements – Do you allow file uploads? Large JSON/XML API payloads?
• Typical media sizes – Video and images have grown larger over years.
• User expectations – Accomodate reasonable user content sizes.
• Server resources – Memory, CPU, disk and network capacity all play a role.

Additionally, keep in mind:

• Workflow steps like resizing images after upload lessen server impact.
• CDNs and object storage can alleviate the app server‘s burden.

With those in mind, let‘s see some common use cases and average file sizes…

Average upload sizes by media type

To help choose the right client_max_body_size values, here are average sizes for common media types from 2021 studies:

File Type	Average Size	Percentile Distribution
Documents
PDF	6.4 MB	90% < 30 MB
Word	2.6 MB	90% < 10 MB
PowerPoint	7 MB	90% < 30 MB
Images
JPG	2.1 MB	90% < 10 MB
PNG	6.3 MB	90% < 15 MB
Video
1080p	268 MB	4K video can exceed 1 GB
720p	150 MB	90% < 750 MB
YouTube, Vimeo	25 – 800+ MB	Highly variable

Key Stat:

• 90% of Word docs under 10 MB
• 90% of JPG images under 10 MB
• 720p video averages 150 MB

Understanding the distribution helps set sane defaults.

Now let‘s look at some common configurations…

Use case examples

Here are good starting client_max_body_size values for various cases:

1. Default content site

For blogs, news sites, and content sites allowing some image uploads, a 64-128 MB limit offers a good balance:

client_max_body_size 128M; 

This accommodates even very large images, docs and charts without too much burden.

2. Ecommerce product images

For retail sites allowing product images along with buyer-submitted content, 256 MB is more appropriate:

client_max_body_size 256M;

High resolution product photos with large counts can demand more headroom.

3. Social media

For social sites allowing very large media uploads like Facebook/YouTube, a higher 512 MB limit may be warranted:

client_max_body_size 512M; 

This covers the majority of user-generated mobile photos and video clips.

4. Video / rich media sites

User uploaded video sites or rich media apps might need ~2 GB client limits:

client_max_body_size 2G;

This accommodates very large media while keeping within server capacity limits for average videos.

Recommendation

Choose the lowest client_max_body_size that meets your site‘s needs.

Start small, monitor traffic patterns and increase as truly necessary.

Now let‘s tackle tuning, troubleshooting and monitoring related to uploads.

Part 3 – Troubleshooting and monitoring large uploads

In addition to picking appropriate sizes, it’s critical to monitor upload traffic and tune OS limits that impact Nginx memory and disk usage.

Here are key things to address:

1. Tune operating system upload limits

Linux and Unix put various limits on processes that constrain Nginx upload handling:

File handles

• Check the max open file handles with ulimit -Hn
• Nginx + disk files need this set adequately high

Memory limits

• Per process memory limits impact Nginx memory buffers
• Adjust with PAM os settings

Disk partitions

• Separate partition for Nginx temp upload files recommended
• Helps avoid filling up OS partitions

Pro Tip:

Tune your OS limits to align with Nginx client_max_body_size!

2. Benchmark to find bottlenecks

Load test your upload forms and APIs to identify bottlenecks before production use:

• Slow memory allocs?
• Disk latency issues?
• Network saturation points?

Address these proactively through tuning or architectural changes.

3. Monitor uploads with observability

Robust monitoring helps spot issues rapidly:

• Metrics: Graph upload request counts, body sizes, errors. Trends aid capacity planning.
• Logging: Send Nginx upload logs to a tool like the ELK stack.
• Traces: Correlate uploads with backend processing using distributed tracing.

4. Tune timeouts for large uploads

Large uploads take more time. Tune associated timeouts accordingly:

• client_body_timeout – Increase to allow more time for full body to POST
• client_header_timeout – Avoid header delays aborting uploads
• proxy_read_timeout – Raise for slow upstream transfers

Pitfall:

Don‘t let default conservative timeouts disrupt large uploads!

5. Handle upstream read errors carefully

For proxied uploads, backend disruptions can result in NGINX read() failed (104: Connection reset by peer) while reading upstream type errors inside Nginx:

• Return 5xx errors not 4xx to avoid browser retry loops
• Log details to analyze root cause offline
• Consider more robust upstream protocols like gRPC

Logging details aids troubleshooting without end user impact.

6. Revisit buffering, timeouts and zones for microservices

Large uploads with microservices need more care:

• With multiple services involved, adjust timeouts at each hop
• Reassess buffering approaches – should uploads transit directly to storage?
• Request size limit zones can constrain specific services without impacting others

See our in-depth microservices upload guide for more details.

In summary, actively manage uploads via tuning, monitoring and architecture.

Now let‘s shift gears to options that help offload handling large uploads…

Part 4 – Advanced handling of large uploads

Beyond adjusting client_max_body_size values directly, employing some more advanced techniques can help manage uploads smoothly:

Offload handling via object storage

Rather than transit uploads through Nginx itself, object stores like Amazon S3 can simplify handling:

POST /upload -> Client library uploads directly to S3  
                       |
                       v    
          Storage on S3 

App downloads media from S3 for actual business logic

Benefits include:

• Avoid Nginx resource usage during uploads
• Persistent storage handled natively
• Fine grained access controls

Just be sure to validate uploads logically afterwards.

Architecture option:
Object storage upload handling relieves app servers.

Distribute incoming requests with zones

For very large scale uploads, new request body size limit zones in Nginx help:

     Internet
       | 
       | Requests  
       v
[ Size Limit Zone with API Servers ] -> [ App Zone with 512MB limit ]
       |
      S3

Benefits include:

• Limits apply individually across zones
• Zone 1 takes burden of small requests
• Zone 2 (apps) sized appropriately

See our zones guide for more details.

Consider CDNs for caching and delivery

CDNs like Fastly, Cloudflare and Akamai can also improve upload performance:

• Geo-distributed edge caching reduces upload round trips
• Traffic routing load balances across app instances
• Media optimization (images, video) lowers storage footprint

But CDN cache facilities tend to focus on GET vs POST uploads.

Efficiency option:
CDN caching helps performance, but many specialize in read optimization vs writes.

Conclusion and recommendations

Appropriately configuring client_max_body_size ensures your Nginx instance can handle application file uploads, media synchronization and API data sizes without exhaustion issues.

Based on our in-depth 4 part guide covering request body internals, sizing guidelines, monitoring practices and architectural options, here are our core recommendations:

• Learn exactly how Nginx receives, buffers and proxies request bodies – this clarifies how client_max_body_size works.
• Review typical file sizes by type for your site (images, docs etc) to choose sensible upload limits.
• Start small with conservative client_max_body_size values, then scale up intentionally based on data and monitoring, rather than arbitrary large limits.
• Actively graph upload metrics to validate capacity planning and spot issues early.
• Tune OS level limits in tandem with client_max_body_size to ensure harmony.
• Consider advanced approaches like S3 offloading and CDN caching if uploads tax app servers.

We hope this comprehensive 4 part guide gives you confidence in setting client_max_body_size appropriately for your workload! Please share any lessons learned or additional best practices.