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

AttributeTypeDescriptionExamplesRequired
http.methodstringHTTP request method.GET; POST; HEADYes
http.urlstringFull HTTP request URL in the form scheme://host[:port]/path?query[#fragment]. Usually the fragment is not transmitted over HTTP, but if it is known, it should be included nevertheless. [1]https://www.foo.bar/search?q=OpenTelemetry#SemConvNo
http.targetstringThe full request target as passed in a HTTP request line or equivalent./path/12314/?q=ddds#123No
http.hoststringThe value of the HTTP host header. An empty Host header should also be reported, see note. [2]www.example.orgNo
http.schemestringThe URI scheme identifying the used protocol.http; httpsNo
http.status_codeintHTTP response status code.200If and only if one was received/sent.
http.flavorstringKind of HTTP protocol used. [3]1.0No
http.user_agentstringValue of the HTTP User-Agent header sent by the client.CERN-LineMode/2.15 libwww/2.17b3No
http.request_content_lengthintThe size of the request payload body in bytes. This is the number of bytes transferred excluding headers and is often, but not always, present as the Content-Length header. For requests using transport encoding, this should be the compressed size.3495No
http.request_content_length_uncompressedintThe size of the uncompressed request payload body after transport decoding. Not set if transport encoding not used.5493No
http.response_content_lengthintThe size of the response payload body in bytes. This is the number of bytes transferred excluding headers and is often, but not always, present as the Content-Length header. For requests using transport encoding, this should be the compressed size.3495No
http.response_content_length_uncompressedintThe size of the uncompressed response payload body after transport decoding. Not set if transport encoding not used.5493No
http.retry_countintThe ordinal number of request re-sending attempt.3If and only if a request was retried.
net.peer.ipstringRemote address of the peer (dotted decimal for IPv4 or RFC5952 for IPv6)127.0.0.1No
net.peer.portintRemote port number.80; 8080; 443No

[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]: When the header is present but empty the attribute SHOULD be set to the empty string. Note that this is a valid situation that is expected in certain cases, according the aforementioned section of RFC 7230. When the header is not set the attribute MUST NOT be set.

[3]: 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.

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.

ValueDescription
1.0HTTP/1.0
1.1HTTP/1.1
2.0HTTP/2
3.0HTTP/3
SPDYSPDY protocol.
QUICQUIC protocol.

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

It is recommended to also use the general network attributes, especially net.peer.ip. 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.

HTTP request and response headers

AttributeTypeDescriptionExamplesRequired
http.request.header.<key>string[]HTTP request headers, <key> being the normalized HTTP Header name (lowercase, with - characters replaced by _), the value being the header values. [1] [2]http.request.header.content_type=["application/json"]; http.request.header.x_forwarded_for=["1.2.3.4", "1.2.3.5"]No
http.response.header.<key>string[]HTTP response headers, <key> being the normalized HTTP Header name (lowercase, with - characters replaced by _), the value being the header values. [1] [2]http.response.header.content_type=["application/json"]; http.response.header.my_custom_header=["abc", "def"]No

[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.

Some HTTP headers - Host and User-Agent - are already captured in the http.host and http.user_agent attributes. 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 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 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.

AttributeTypeDescriptionExamplesRequired
net.peer.namestringRemote hostname or similar, see note below. [1]example.comSee below

[1]: net.peer.name SHOULD NOT be set if capturing it would require an extra DNS lookup.

Additional attribute requirements: At least one of the following sets of attributes is required:

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

Note that in some cases http.host might be different from the net.peer.name used to look up the net.peer.ip that is actually connected to. In that case it is strongly recommended to set the net.peer.name attribute in addition to http.host.

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.

AttributeTypeDescriptionExamplesRequired
http.server_namestringThe primary server name of the matched virtual host. This should be obtained via configuration. If no such configuration can be obtained, this attribute MUST NOT be set ( net.host.name should be used instead). [1]example.comSee below
http.routestringThe matched route (path template)./users/:userID?No
http.client_ipstringThe IP address of the original client behind all proxies, if known (e.g. from X-Forwarded-For). [2]83.164.160.102No

[1]: http.url is usually not readily available on the server side but would have to be assembled in a cumbersome and sometimes lossy process from other information (see e.g. open-telemetry/opentelemetry-python/pull/148). It is thus preferred to supply the raw data that is available.

[2]: This is not necessarily the same as net.peer.ip, 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.peer.ip, is available even if that other source just confirms the same value as net.peer.ip. Rationale: For net.peer.ip, 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.peer.ip means that one is at least somewhat confident that the address is not that of the closest proxy.

Additional attribute requirements: At least one of the following sets of attributes is required:

Of course, more than the required attributes can be supplied, but this is recommended only if they cannot be inferred from the sent ones. For example, http.server_name has shown great value in practice, as bogus HTTP Host headers occur often in the wild.

It is strongly recommended to set http.server_name to allow associating requests with some logical server entity.

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

Attribute nameValue
http.method"GET"
http.flavor"1.1"
http.url"https://example.com:8080/webshop/articles/4?s=1"
net.peer.ip"192.0.2.5"
http.status_code200

The corresponding server Span may look like this:

Span name: /webshop/articles/:article_id.

Attribute nameValue
http.method"GET"
http.flavor"1.1"
http.target"/webshop/articles/4?s=1"
http.host"example.com:8080"
http.server_name"example.com"
net.host.port8080
http.scheme"https"
http.route"/webshop/articles/:article_id"
http.status_code200
http.client_ip"192.0.2.4"
net.peer.ip"192.0.2.5" (the client goes through a proxy)
http.user_agent"Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:72.0) Gecko/20100101 Firefox/72.0"

Note that following the recommendations above, http.url is not set in the above example. If set, it would be "https://example.com:8080/webshop/articles/4?s=1" but due to http.scheme, http.host and http.target being set, it would be redundant. As explained above, these separate values are preferred but if for some reason the URL is available but the other values are not, URL can replace http.scheme, http.host and http.target.

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)