cpp-httplib Streaming API

This document describes the streaming extensions for cpp-httplib, providing an iterator-style API for handling HTTP responses incrementally with true socket-level streaming.

Important Notes:
No Keep-Alive: Each stream::Get() call uses a dedicated connection that is closed when the returned stream::Result is destroyed. For connection reuse, use Client::Get().
Single iteration only: The next() method can only iterate through the body once.
Result is not thread-safe: While stream::Get() can be called from multiple threads simultaneously, the returned stream::Result must be used from a single thread only.

Overview

The streaming API allows you to process HTTP response bodies chunk by chunk using an iterator-style pattern. Data is read directly from the network socket, enabling low-memory processing of large responses. This is particularly useful for:

LLM/AI streaming responses (e.g., ChatGPT, Claude, Ollama)
Server-Sent Events (SSE)
Large file downloads with progress tracking (mind the payload size limit)
Reverse proxy implementations

Quick Start

#include "httplib.h"

int main() {
    httplib::Client cli("http://localhost:8080");
    
    // Get streaming response
    auto result = httplib::stream::Get(cli, "/stream");
    
    if (result) {
        // Process response body in chunks
        while (result.next()) {
            std::cout.write(result.data(), result.size());
        }
    }
    
    return 0;
}

API Layers

cpp-httplib provides multiple API layers for different use cases:

┌─────────────────────────────────────────────┐
│  SSEClient                        │  ← SSE-specific, parsed events
│  - on_message(), on_event()                 │
│  - Auto-reconnect, Last-Event-ID            │
├─────────────────────────────────────────────┤
│  stream::Get() / stream::Result             │  ← Iterator-based streaming
│  - while (result.next()) { ... }            │
├─────────────────────────────────────────────┤
│  open_stream() / StreamHandle               │  ← General-purpose streaming
│  - handle.read(buf, len)                    │
├─────────────────────────────────────────────┤
│  Client::Get()                              │  ← Traditional, full buffering
└─────────────────────────────────────────────┘

Use Case	Recommended API
SSE with auto-reconnect	`SSEClient` (see README-sse.md)
LLM streaming (JSON Lines)	`stream::Post()` or `stream::Get()`
Large file download	`stream::Get()` or `open_stream()`
Reverse proxy	`open_stream()`
Small responses with Keep-Alive	`Client::Get()`

API Reference

Low-Level API: `StreamHandle`

The StreamHandle struct provides direct control over streaming responses. It takes ownership of the socket connection and reads data directly from the network.

Note: When using open_stream(), the connection is dedicated to streaming and Keep-Alive is not supported. For Keep-Alive connections, use client.Get() instead.

// Open a stream (takes ownership of socket)
httplib::Client cli("http://localhost:8080");
auto handle = cli.open_stream("GET", "/path");

// Check validity
if (handle.is_valid()) {
    // Access response headers immediately
    int status = handle.response->status;
    auto content_type = handle.response->get_header_value("Content-Type");
    
    // Read body incrementally
    char buf[4096];
    ssize_t n;
    while ((n = handle.read(buf, sizeof(buf))) > 0) {
        process(buf, n);
    }
}

StreamHandle Members

Member	Type	Description
`response`	`std::unique_ptr<Response>`	HTTP response with headers
`error`	`Error`	Error code if request failed
`is_valid()`	`bool`	Returns true if response is valid
`read(buf, len)`	`ssize_t`	Read up to `len` bytes directly from socket
`get_read_error()`	`Error`	Get the last read error
`has_read_error()`	`bool`	Check if a read error occurred

For a chunked response, the trailers are in response->trailers once read() has reached the end of the body.

High-Level API: `stream::Get()` and `stream::Result`

The httplib.h header provides a more ergonomic iterator-style API.

#include "httplib.h"

httplib::Client cli("http://localhost:8080");
...

// Simple GET
auto result = httplib::stream::Get(cli, "/path");

// GET with custom headers
httplib::Headers headers = {{"Authorization", "Bearer token"}};
auto result = httplib::stream::Get(cli, "/path", headers);

// Process the response
if (result) {
    while (result.next()) {
        process(result.data(), result.size());
    }
}

// Or read entire body at once
auto result2 = httplib::stream::Get(cli, "/path");
if (result2) {
    std::string body = result2.read_all();
}

Helpers

There is a helper for each HTTP method. The arguments in brackets can be left out, and every helper takes a trailing size_t chunk_size = 8192, the most bytes one next() returns.

Helper	Arguments
`stream::Get()`	`(cli, path, [params], [headers], [chunk_size])`
`stream::Head()`	`(cli, path, [params], [headers], [chunk_size])`
`stream::Options()`	`(cli, path, [params], [headers], [chunk_size])`
`stream::Post()`	`(cli, path, [params], [headers], body, content_type, [chunk_size])`
`stream::Put()`	`(cli, path, [params], [headers], body, content_type, [chunk_size])`
`stream::Patch()`	`(cli, path, [params], [headers], body, content_type, [chunk_size])`
`stream::Delete()`	`(cli, path, [params], [headers], [chunk_size])`
`stream::Delete()`	`(cli, path, [params], [headers], body, content_type, [chunk_size])`

params is a Params, headers a Headers, and body and content_type are std::strings.

// POST a JSON body and stream the response in 4KB chunks
auto result3 = httplib::stream::Post(cli, "/api/generate", headers,
                                     R"({"prompt": "hello"})",
                                     "application/json", 4096);

stream::Result Members

Member	Type	Description
`operator bool()`	`bool`	Returns true if response is valid
`is_valid()`	`bool`	Same as `operator bool()`
`status()`	`int`	HTTP status code
`headers()`	`const Headers&`	Response headers
`get_header_value(key, def)`	`std::string`	Get header value (with optional default)
`has_header(key)`	`bool`	Check if header exists
`next()`	`bool`	Read next chunk, returns false when done
`data()`	`const char*`	Pointer to current chunk data
`size()`	`size_t`	Size of current chunk
`read_all()`	`std::string`	Read entire remaining body into string
`error()`	`Error`	Get the connection/request error
`read_error()`	`Error`	Get the last read error
`has_read_error()`	`bool`	Check if a read error occurred

Usage Examples

Example 1: SSE (Server-Sent Events) Client

#include "httplib.h"
#include <iostream>

int main() {
    httplib::Client cli("http://localhost:1234");
    
    auto result = httplib::stream::Get(cli, "/events");
    if (!result) { return 1; }
    
    while (result.next()) {
        std::cout.write(result.data(), result.size());
        std::cout.flush();
    }
    
    return 0;
}

For a complete SSE client with auto-reconnection and event parsing, see example/ssecli-stream.cc.

Example 2: LLM Streaming Response

#include "httplib.h"
#include <iostream>

int main() {
    httplib::Client cli("http://localhost:11434");  // Ollama
    
    auto result = httplib::stream::Get(cli, "/api/generate");
    
    if (result && result.status() == 200) {
        while (result.next()) {
            std::cout.write(result.data(), result.size());
            std::cout.flush();
        }
    }
    
    // Check for connection errors
    if (result.read_error() != httplib::Error::Success) {
        std::cerr << "Connection lost\n";
    }
    
    return 0;
}

Example 3: Large File Download with Progress

#include "httplib.h"
#include <fstream>
#include <iostream>

int main() {
    httplib::Client cli("http://example.com");
    cli.set_payload_max_length(0);  // No limit (the default is 100MB)
    auto result = httplib::stream::Get(cli, "/large-file.zip");
    
    if (!result || result.status() != 200) {
        std::cerr << "Download failed\n";
        return 1;
    }
    
    std::ofstream file("download.zip", std::ios::binary);
    size_t total = 0;
    
    while (result.next()) {
        file.write(result.data(), result.size());
        total += result.size();
        std::cout << "\rDownloaded: " << (total / 1024) << " KB" << std::flush;
    }
    
    if (result.has_read_error()) {
        std::cerr << "\nDownload failed: "
                  << httplib::to_string(result.read_error()) << "\n";
        return 1;
    }

    std::cout << "\nComplete!\n";
    return 0;
}

Example 4: Reverse Proxy Streaming

#include "httplib.h"

// Keeps the client and its stream alive until the response has been sent.
struct Upstream {
    httplib::Client cli{"http://backend:8080"};
    httplib::ClientImpl::StreamHandle handle;
};

httplib::Server svr;

svr.Get("/proxy/(.*)", [](const httplib::Request& req, httplib::Response& res) {
    auto upstream = std::make_shared<Upstream>();
    upstream->handle = upstream->cli.open_stream("GET", "/" + req.matches[1].str());

    if (!upstream->handle.is_valid()) {
        res.status = 502;
        return;
    }

    res.status = upstream->handle.response->status;
    res.set_chunked_content_provider(
        upstream->handle.response->get_header_value("Content-Type"),
        [upstream](size_t, httplib::DataSink& sink) {
            char buf[8192];
            auto n = upstream->handle.read(buf, sizeof(buf));
            if (n > 0) {
                sink.write(buf, static_cast<size_t>(n));
                return true;
            }
            sink.done();
            return true;
        }
    );
});

svr.listen("0.0.0.0", 3000);

Comparison with Existing APIs

Feature	`Client::Get()`	`open_stream()`	`stream::Get()`
Headers available	After complete	Immediately	Immediately
Body reading	All at once	Direct from socket	Iterator-based
Memory usage	Full body in RAM	Minimal (controlled)	Minimal (controlled)
Keep-Alive support	✅ Yes	❌ No	❌ No
Compression	Auto-handled	Auto-handled	Auto-handled
Best for	Small responses, Keep-Alive	Low-level streaming	Easy streaming

Features

True socket-level streaming: Data is read directly from the network socket
Low memory footprint: Only the current chunk is held in memory
Compression support: Automatic decompression for gzip, brotli, and zstd
Chunked transfer: Full support for chunked transfer encoding
SSL/TLS support: Works with HTTPS connections

Important Notes

Keep-Alive Behavior

The streaming API (stream::Get() / open_stream()) takes ownership of the socket connection for the duration of the stream. This means:

Keep-Alive is not supported for streaming connections
The socket is closed when StreamHandle is destroyed
For Keep-Alive scenarios, use the standard client.Get() API instead

// Use for streaming (no Keep-Alive)
auto result = httplib::stream::Get(cli, "/large-stream");
while (result.next()) { /* ... */ }

// Use for Keep-Alive connections
auto res = cli.Get("/api/data");  // Connection can be reused

Payload Size Limit

open_stream() and the stream:: helpers apply the client's set_payload_max_length() (100MB by default, CPPHTTPLIB_PAYLOAD_MAX_LENGTH) to the body, both as received and after decompression. Past the limit, read() and next() stop, and the read error is Error::ExceedMaxPayloadSize. To download something larger, raise the limit, or call cli.set_payload_max_length(0) for no limit.

Issue #2269 - Original feature request
example/ssecli-stream.cc - SSE client with auto-reconnection