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65 lines
2.2 KiB
Markdown
65 lines
2.2 KiB
Markdown
---
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title: "S15. Log Requests on the Server"
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order: 34
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status: "draft"
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---
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To log the requests the server receives and the responses it returns, use `Server::set_logger()`. The callback fires once per completed request, making it the foundation for access logs and metrics collection.
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## Basic usage
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```cpp
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svr.set_logger([](const httplib::Request &req, const httplib::Response &res) {
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std::cout << req.remote_addr << " "
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<< req.method << " " << req.path
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<< " -> " << res.status << std::endl;
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});
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```
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The log callback receives both the `Request` and the `Response`. You can grab the method, path, status, client IP, headers, body — whatever you need.
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## Access-log style format
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Here's an Apache/Nginx-ish access log format.
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```cpp
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svr.set_logger([](const auto &req, const auto &res) {
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auto now = std::time(nullptr);
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char timebuf[32];
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std::strftime(timebuf, sizeof(timebuf), "%Y-%m-%d %H:%M:%S",
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std::localtime(&now));
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std::cout << timebuf << " "
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<< req.remote_addr << " "
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<< "\"" << req.method << " " << req.path << "\" "
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<< res.status << " "
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<< res.body.size() << "B"
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<< std::endl;
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});
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```
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## Measure request time
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To include request duration in the log, stash a start timestamp in `res.user_data` from a pre-routing handler, then subtract in the logger.
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```cpp
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svr.set_pre_routing_handler([](const auto &req, auto &res) {
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res.user_data.set("start", std::chrono::steady_clock::now());
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return httplib::Server::HandlerResponse::Unhandled;
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});
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svr.set_logger([](const auto &req, const auto &res) {
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auto *start = res.user_data.get<std::chrono::steady_clock::time_point>("start");
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auto elapsed = start
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? std::chrono::duration_cast<std::chrono::milliseconds>(
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std::chrono::steady_clock::now() - *start).count()
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: 0;
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std::cout << req.method << " " << req.path
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<< " " << res.status << " " << elapsed << "ms" << std::endl;
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});
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```
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For more on `user_data`, see [S12. Pass data between handlers with `res.user_data`](../s12-user-data).
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> **Note:** The logger runs synchronously on the same thread as request processing. Heavy work inside it hurts throughput — push it to a queue and process asynchronously if you need anything expensive.
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