Semantic conventions for HTTP spans

Status: Experimental

This document defines semantic conventions for HTTP client and server Spans. They can be used for http and https schemes and various HTTP versions like 1.1, 2 and SPDY.

Name

HTTP spans MUST follow the overall guidelines for span names. Many REST APIs encode parameters into URI path, e.g. /api/users/123 where 123 is a user id, which creates high cardinality value space not suitable for span names. In case of HTTP servers, these endpoints are often mapped by the server frameworks to more concise HTTP routes, e.g. /api/users/{user_id}, which are recommended as the low cardinality span names. However, the same approach usually does not work for HTTP client spans, especially when instrumentation is provided by a lower-level middleware that is not aware of the specifics of how the URIs are formed. Therefore, HTTP client spans SHOULD be using conservative, low cardinality names formed from the available parameters of an HTTP request, such as "HTTP {METHOD_NAME}". Instrumentation MUST NOT default to using URI path as span name, but MAY provide hooks to allow custom logic to override the default span name.

Status

Span Status MUST be left unset if HTTP status code was in the 1xx, 2xx or 3xx ranges, unless there was another error (e.g., network error receiving the response body; or 3xx codes with max redirects exceeded), in which case status MUST be set to Error.

For HTTP status codes in the 4xx range span status MUST be left unset in case of SpanKind.SERVER and MUST be set to Error in case of SpanKind.CLIENT.

For HTTP status codes in the 5xx range, as well as any other code the client failed to interpret, span status MUST be set to Error.

Don’t set the span status description if the reason can be inferred from http.status_code.

Common Attributes

[1]: If net.transport is not specified, it can be assumed to be IP.TCP except if http.flavor is QUIC, in which case IP.UDP is assumed.

[2]: If different than inet and if any of net.sock.peer.addr or net.sock.host.addr are set. Consumers of telemetry SHOULD accept both IPv4 and IPv6 formats for the address in net.sock.peer.addr if net.sock.family is not set. This is to support instrumentations that follow previous versions of this document.

[3]: If available and different from net.peer.name and if net.sock.peer.addr is set.

[4]: If defined for the address family and if different than net.peer.port and if net.sock.peer.addr is set.

http.flavor has the following list of well-known values. If one of them applies, then the respective value MUST be used, otherwise a custom value MAY be used.

Following attributes MUST be provided at span creation time (when provided at all), so they can be considered for sampling decisions:

• http.method

It is recommended to also use the general socket-level attributes - net.sock.peer.addr when available, net.sock.peer.name and net.sock.peer.port when don’t match net.peer.name and net.peer.port (if intermediary is detected).

HTTP request and response headers

[1]: Instrumentations SHOULD require an explicit configuration of which headers are to be captured. Including all request/response headers can be a security risk - explicit configuration helps avoid leaking sensitive information.

The User-Agent header is already captured in the http.user_agent attribute. Users MAY explicitly configure instrumentations to capture them even though it is not recommended.

[2]: The attribute value MUST consist of either multiple header values as an array of strings or a single-item array containing a possibly comma-concatenated string, depending on the way the HTTP library provides access to headers.

HTTP client

This span type represents an outbound HTTP request.

For an HTTP client span, SpanKind MUST be Client.

If set, http.url must be the originally requested URL, before any HTTP-redirects that may happen when executing the request.

[1]: http.url MUST NOT contain credentials passed via URL in form of https://username:password@www.example.com/. In such case the attribute’s value should be https://www.example.com/.

[2]: Determined by using the first of the following that applies

• Host identifier of the request target if it’s sent in absolute-form
• Host identifier of the Host header

SHOULD NOT be set if capturing it would require an extra DNS lookup.

[3]: When request target is absolute URI, net.peer.name MUST match URI port identifier, otherwise it MUST match Host header port identifier.

[4]: If not default (80 for http scheme, 443 for https).

Following attributes MUST be provided at span creation time (when provided at all), so they can be considered for sampling decisions:

• http.url
• net.peer.name
• net.peer.port

Note that in some cases host and port identifiers in the Host header might be different from the net.peer.name and net.peer.port, in this case instrumentation MAY populate Host header on http.request.header.host attribute even if it’s not enabled by user.

HTTP request retries and redirects

Retries and redirects cause more than one physical HTTP request to be sent. A CLIENT span SHOULD be created for each one of these physical requests. No span is created corresponding to the “logical” (encompassing) request.

For retries, http.retry_count attribute SHOULD be added to each retry span with the value that reflects the ordinal number of request retry attempt.

See examples for more details.

HTTP server

To understand the attributes defined in this section, it is helpful to read the “Definitions” subsection.

HTTP server definitions

This section gives a short summary of some concepts in web server configuration and web app deployment that are relevant to tracing.

Usually, on a physical host, reachable by one or multiple IP addresses, a single HTTP listener process runs. If multiple processes are running, they must listen on distinct TCP/UDP ports so that the OS can route incoming TCP/UDP packets to the right one.

Within a single server process, there can be multiple virtual hosts. The HTTP host header (in combination with a port number) is normally used to determine to which of them to route incoming HTTP requests.

The host header value that matches some virtual host is called the virtual hosts’s server name. If there are multiple aliases for the virtual host, one of them (often the first one listed in the configuration) is called the primary server name. See for example, the Apache ServerName or NGINX server_name directive or the CGI specification on SERVER_NAME (RFC 3875). In practice the HTTP host header is often ignored when just a single virtual host is configured for the IP.

Within a single virtual host, some servers support the concepts of an HTTP application (for example in Java, the Servlet JSR defines an application as “a collection of servlets, HTML pages, classes, and other resources that make up a complete application on a Web server” – SRV.9 in JSR 53; in a deployment of a Python application to Apache, the application would be the PEP 3333 conformant callable that is configured using the WSGIScriptAlias directive of mod_wsgi).

An application can be “mounted” under some application root (also know as context root context prefix, or document base) which is a fixed path prefix of the URL that determines to which application a request is routed (e.g., the server could be configured to route all requests that go to an URL path starting with /webshop/ at a particular virtual host to the com.example.webshop web application).

Some servers allow to bind the same HTTP application to multiple (virtual host, application root) pairs.

TODO: Find way to trace HTTP application and application root (opentelemetry/opentelementry-specification#335)

HTTP Server semantic conventions

This span type represents an inbound HTTP request.

For an HTTP server span, SpanKind MUST be Server.

Given an inbound request for a route (e.g. "/users/:userID?") the name attribute of the span SHOULD be set to this route. If the route does not include the application root, it SHOULD be prepended to the span name.

If the route cannot be determined, the name attribute MUST be set as defined in the general semantic conventions for HTTP.

[1]: ‘http.route’ MUST NOT be populated when this is not supported by the HTTP server framework as the route attribute should have low-cardinality and the URI path can NOT substitute it.

[2]: This is not necessarily the same as net.sock.peer.addr, which would identify the network-level peer, which may be a proxy.

This attribute should be set when a source of information different from the one used for net.sock.peer.addr, is available even if that other source just confirms the same value as net.sock.peer.addr. Rationale: For net.sock.peer.addr, one typically does not know if it comes from a proxy, reverse proxy, or the actual client. Setting http.client_ip when it’s the same as net.sock.peer.addr means that one is at least somewhat confident that the address is not that of the closest proxy.

[3]: Determined by using the first of the following that applies

• The primary server name of the matched virtual host. MUST only include host identifier.
• Host identifier of the request target if it’s sent in absolute-form.
• Host identifier of the Host header

SHOULD NOT be set if only IP address is available and capturing name would require a reverse DNS lookup.

[4]: Determined by using the first of the following that applies

• Port identifier of the primary server host of the matched virtual host.
• Port identifier of the request target if it’s sent in absolute-form.
• Port identifier of the Host header

[5]: If not default (80 for http scheme, 443 for https).

[6]: If defined for the address family and if different than net.host.port and if net.sock.host.addr is set.

Following attributes MUST be provided at span creation time (when provided at all), so they can be considered for sampling decisions:

• http.scheme
• http.target
• net.host.name
• net.host.port

http.route MUST be provided at span creation time if and only if it’s already available. If it becomes available after span starts, instrumentation MUST populate it anytime before span ends.

Note that in some cases host and port identifiers in the Host header might be different from the net.host.name and net.host.port, in this case instrumentation MAY populate Host header on http.request.header.host attribute even if it’s not enabled by user.

HTTP client-server example

As an example, if a browser request for https://example.com:8080/webshop/articles/4?s=1 is invoked from a host with IP 192.0.2.4, we may have the following Span on the client side:

Span name: HTTP GET

The corresponding server Span may look like this:

Span name: /webshop/articles/:article_id.

HTTP retries examples

Example of retries in the presence of a trace started by an inbound request:

request (SERVER, trace=t1, span=s1)
  |
  -- GET / - 500 (CLIENT, trace=t1, span=s2)
  |   |
  |   --- server (SERVER, trace=t1, span=s3)
  |
  -- GET / - 500 (CLIENT, trace=t1, span=s4, http.retry_count=1)
  |   |
  |   --- server (SERVER, trace=t1, span=s5)
  |
  -- GET / - 200 (CLIENT, trace=t1, span=s6, http.retry_count=2)
      |
      --- server (SERVER, trace=t1, span=s7)

Example of retries with no trace started upfront:

GET / - 500 (CLIENT, trace=t1, span=s1)
 |
 --- server (SERVER, trace=t1, span=s2)

GET / - 500 (CLIENT, trace=t2, span=s1, http.retry_count=1)
 |
 --- server (SERVER, trace=t2, span=s2)

GET / - 200 (CLIENT, trace=t3, span=s1, http.retry_count=2)
 |
 --- server (SERVER, trace=t3, span=s1)

HTTP redirects examples

Example of redirects in the presence of a trace started by an inbound request:

request (SERVER, trace=t1, span=s1)
  |
  -- GET / - 302 (CLIENT, trace=t1, span=s2)
  |   |
  |   --- server (SERVER, trace=t1, span=s3)
  |
  -- GET /hello - 200 (CLIENT, trace=t1, span=s4)
      |
      --- server (SERVER, trace=t1, span=s5)

Example of redirects with no trace started upfront:

GET / - 302 (CLIENT, trace=t1, span=s1)
 |
 --- server (SERVER, trace=t1, span=s2)

GET /hello - 200 (CLIENT, trace=t2, span=s1)
 |
 --- server (SERVER, trace=t2, span=s2)