> ## Documentation Index
> Fetch the complete documentation index at: https://mintlify.com/fastapi/fastapi/llms.txt
> Use this file to discover all available pages before exploring further.

# GZipMiddleware

> Enable GZip compression for response data in your FastAPI application

# GZipMiddleware

The `GZipMiddleware` handles GZip compression for responses when the client supports it (indicated by an `Accept-Encoding` header that includes "gzip").

## Usage

```python theme={null}
from fastapi import FastAPI
from fastapi.middleware.gzip import GZipMiddleware

app = FastAPI()

app.add_middleware(GZipMiddleware, minimum_size=1000)
```

## Parameters

<ParamField path="minimum_size" type="int" default="500">
  The minimum response size in bytes before compression is applied. Responses smaller than this value will not be compressed. Default is 500 bytes.
</ParamField>

<ParamField path="compresslevel" type="int" default="9">
  The GZip compression level, ranging from 0 to 9. Higher values provide better compression but use more CPU. Default is 9 (maximum compression).

  * `0`: No compression
  * `1`: Fastest compression, least CPU usage
  * `9`: Best compression, most CPU usage
</ParamField>

## How It Works

The middleware automatically:

1. Checks if the client supports GZip compression via the `Accept-Encoding` header
2. Compresses the response body if it exceeds the `minimum_size` threshold
3. Sets the `Content-Encoding: gzip` header on compressed responses
4. Removes the `Content-Length` header (as the compressed size differs)

## Example with Custom Configuration

```python theme={null}
from fastapi import FastAPI
from fastapi.middleware.gzip import GZipMiddleware

app = FastAPI()

# Compress responses larger than 2KB with medium compression
app.add_middleware(
    GZipMiddleware,
    minimum_size=2000,
    compresslevel=6,
)

@app.get("/large-data")
async def get_large_data():
    return {"data": "x" * 10000}  # Will be compressed

@app.get("/small-data")
async def get_small_data():
    return {"data": "small"}  # Won't be compressed
```

## Performance Considerations

* **Minimum Size**: Set `minimum_size` appropriately to avoid compressing small responses where compression overhead exceeds benefits
* **Compression Level**: Balance between compression ratio and CPU usage:
  * Use lower values (4-6) for CPU-constrained environments
  * Use higher values (7-9) when bandwidth is more expensive than CPU
* **Content Types**: GZip works best with text-based content (JSON, HTML, CSS, JavaScript)
* **Pre-compressed Assets**: Consider pre-compressing static assets rather than compressing on every request

## When to Use

GZipMiddleware is beneficial when:

* Serving large JSON responses or text-based data
* Bandwidth is limited or expensive
* Clients primarily access your API over slower networks
* Response sizes regularly exceed 1KB

## When to Avoid

* Binary data (images, videos) that are already compressed
* Very small responses (\< 500 bytes) where compression adds overhead
* CPU resources are constrained
* You're already using compression at the reverse proxy/CDN level