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Semantic Conventions for HTTP Spans

Status: Experimental, Feature-freeze

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.

Warning Existing HTTP instrumentations that are using v1.20.0 of this document (or prior):

  • SHOULD NOT change the version of the HTTP or networking attributes that they emit until the HTTP semantic conventions are marked stable (HTTP stabilization will include stabilization of a core set of networking attributes which are also used in HTTP instrumentations).
  • SHOULD introduce an environment variable OTEL_SEMCONV_STABILITY_OPT_IN in the existing major version which is a comma-separated list of values. The only values defined so far are:
    • http - emit the new, stable HTTP and networking attributes, and stop emitting the old experimental HTTP and networking attributes that the instrumentation emitted previously.
    • http/dup - emit both the old and the stable HTTP and networking attributes, allowing for a seamless transition.
    • The default behavior (in the absence of one of these values) is to continue emitting whatever version of the old experimental HTTP and networking attributes the instrumentation was emitting previously.
  • SHOULD maintain (security patching at a minimum) the existing major version for at least six months after it starts emitting both sets of attributes.
  • SHOULD drop the environment variable in the next major version (stable next major version SHOULD NOT be released prior to October 1, 2023).

Name

HTTP spans MUST follow the overall guidelines for span names. HTTP server span names SHOULD be {http.request.method} {http.route} if there is a (low-cardinality) http.route available. HTTP server span names SHOULD be {http.request.method} if there is no (low-cardinality) http.route available. HTTP client spans have no http.route attribute since client-side instrumentation is not generally aware of the "route", and therefore HTTP client spans SHOULD use {http.request.method}. 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.response.status_code.

Common Attributes

The common attributes listed in this section apply to both HTTP clients and servers in addition to the specific attributes listed in the HTTP client and HTTP server sections below.

Attribute Type Description Examples Requirement Level
http.response.status_code int HTTP response status code. 200 Conditionally Required: If and only if one was received/sent.
http.request.method_original string Original HTTP method sent by the client in the request line. GeT; ACL; foo Conditionally Required: [1]
http.request.body.size int The 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. 3495 Recommended
http.response.body.size int The 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. 3495 Recommended
http.request.method string HTTP request method. [2] GET; POST; HEAD Required
network.protocol.name string OSI Application Layer or non-OSI equivalent. The value SHOULD be normalized to lowercase. http; spdy Recommended: if not default (http).
network.protocol.version string Version of the application layer protocol used. See note below. [3] 1.0; 1.1; 2.0 Recommended
network.transport string OSI Transport Layer or Inter-process Communication method. The value SHOULD be normalized to lowercase. tcp; udp Conditionally Required: [4]
network.type string OSI Network Layer or non-OSI equivalent. The value SHOULD be normalized to lowercase. ipv4; ipv6 Recommended
user_agent.original string Value of the HTTP User-Agent header sent by the client. CERN-LineMode/2.15 libwww/2.17b3 Recommended

[1]: If and only if it's different than http.request.method.

[2]: HTTP request method value SHOULD be "known" to the instrumentation. By default, this convention defines "known" methods as the ones listed in RFC9110 and the PATCH method defined in RFC5789.

If the HTTP request method is not known to instrumentation, it MUST set the http.request.method attribute to _OTHER and, except if reporting a metric, MUST set the exact method received in the request line as value of the http.request.method_original attribute.

If the HTTP instrumentation could end up converting valid HTTP request methods to _OTHER, then it MUST provide a way to override the list of known HTTP methods. If this override is done via environment variable, then the environment variable MUST be named OTEL_INSTRUMENTATION_HTTP_KNOWN_METHODS and support a comma-separated list of case-sensitive known HTTP methods (this list MUST be a full override of the default known method, it is not a list of known methods in addition to the defaults).

HTTP method names are case-sensitive and http.request.method attribute value MUST match a known HTTP method name exactly. Instrumentations for specific web frameworks that consider HTTP methods to be case insensitive, SHOULD populate a canonical equivalent. Tracing instrumentations that do so, MUST also set http.request.method_original to the original value.

[3]: network.protocol.version refers to the version of the protocol used and might be different from the protocol client's version. If the HTTP client used has a version of 0.27.2, but sends HTTP version 1.1, this attribute should be set to 1.1.

[4]: If not default (tcp for HTTP/1.1 and HTTP/2, udp for HTTP/3).

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

  • http.request.method

http.request.method 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.

Value Description
CONNECT CONNECT method.
DELETE DELETE method.
GET GET method.
HEAD HEAD method.
OPTIONS OPTIONS method.
PATCH PATCH method.
POST POST method.
PUT PUT method.
TRACE TRACE method.
_OTHER Any HTTP method that the instrumentation has no prior knowledge of.

network.transport 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.

Value Description
tcp TCP
udp UDP
pipe Named or anonymous pipe. See note below.
unix Unix domain socket

network.type 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.

Value Description
ipv4 IPv4
ipv6 IPv6

HTTP request and response headers

Attribute Type Description Examples Requirement Level
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"] Opt-In
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"] Opt-In

[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 user_agent.original 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. There are two ways this can be achieved in an instrumentation:

  1. Instrumentations SHOULD create an HTTP span for each attempt to send an HTTP request over the wire. In case the request is resent, the resend attempts MUST follow the HTTP resend spec. In this case, instrumentations SHOULD NOT (also) emit a logical encompassing HTTP client span.

  2. If for some reason it is not possible to emit a span for each send attempt (because e.g. the instrumented library does not expose hooks that would allow this), instrumentations MAY create an HTTP span for the top-most operation of the HTTP client. In this case, the url.full MUST be the absolute URL that was originally requested, before any HTTP-redirects that may happen when executing the request.

For an HTTP client span, SpanKind MUST be Client.

Attribute Type Description Examples Requirement Level
http.resend_count int The ordinal number of request resending attempt (for any reason, including redirects). [1] 3 Recommended: if and only if request was retried.
server.address string Host identifier of the "URI origin" HTTP request is sent to. [2] example.com Required
server.port int Port identifier of the "URI origin" HTTP request is sent to. [3] 80; 8080; 443 Conditionally Required: [4]
server.socket.address string Physical server IP address or Unix socket address. If set from the client, should simply use the socket's peer address, and not attempt to find any actual server IP (i.e., if set from client, this may represent some proxy server instead of the logical server). 10.5.3.2 Recommended: If different than server.address.
server.socket.domain string The domain name of an immediate peer. [5] proxy.example.com Recommended: If different than server.address.
server.socket.port int Physical server port. 16456 Recommended: If different than server.port.
url.full string Absolute URL describing a network resource according to RFC3986 [6] https://www.foo.bar/search?q=OpenTelemetry#SemConv; //localhost Required

[1]: The resend count SHOULD be updated each time an HTTP request gets resent by the client, regardless of what was the cause of the resending (e.g. redirection, authorization failure, 503 Server Unavailable, network issues, or any other).

[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

If an HTTP client request is explicitly made to an IP address, e.g. http://x.x.x.x:8080, then server.address SHOULD be the IP address x.x.x.x. A DNS lookup SHOULD NOT be used.

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

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

[5]: Typically observed from the client side, and represents a proxy or other intermediary domain name.

[6]: For network calls, URL usually has scheme://host[:port][path][?query][#fragment] format, where the fragment is not transmitted over HTTP, but if it is known, it should be included nevertheless. url.full MUST NOT contain credentials passed via URL in form of https://username:password@www.example.com/. In such case username and password should be redacted and attribute's value should be https://REDACTED:REDACTED@www.example.com/. url.full SHOULD capture the absolute URL when it is available (or can be reconstructed) and SHOULD NOT be validated or modified except for sanitizing purposes.

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 host and port identifiers in the Host header might be different from the server.address and server.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 span duration

There are some minimal constraints that SHOULD be honored:

  • HTTP client spans SHOULD start sometime before the first request byte is sent. This may or may not include connection time.
  • HTTP client spans SHOULD end sometime after the HTTP response headers are fully read (or when they fail to be read). This may or may not include reading the response body.

If there is any possibility for application code to not fully read the HTTP response (and for the HTTP client library to then have to clean up the HTTP response asynchronously), the HTTP client span SHOULD NOT be ended in this cleanup phase, and instead SHOULD end at some point after the HTTP response headers are fully read (or fail to be read). This avoids the span being ended asynchronously later on at a time which is no longer directly associated with the application code which made the HTTP request.

Because of the potential for confusion around this, HTTP client library instrumentations SHOULD document their behavior around ending HTTP client spans.

HTTP request retries and redirects

Retries and redirects cause more than one physical HTTP request to be sent. A request is resent when an HTTP client library sends more than one HTTP request to satisfy the same API call. This may happen due to following redirects, authorization challenges, 503 Server Unavailable, network issues, or any other.

Each time an HTTP request is resent, the http.resend_count attribute SHOULD be added to each repeated span and set to the ordinal number of the request resend attempt.

See the examples for more details about:

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 an application root (also known as a 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 cannot be determined, the name attribute MUST be set as defined in the general semantic conventions for HTTP.

Attribute Type Description Examples Requirement Level
http.route string The matched route (path template in the format used by the respective server framework). See note below [1] /users/:userID?; {controller}/{action}/{id?} Conditionally Required: If and only if it's available
client.address string Client address - unix domain socket name, IPv4 or IPv6 address. [2] 83.164.160.102 Recommended
client.port int The port of the original client behind all proxies, if known (e.g. from Forwarded or a similar header). Otherwise, the immediate client peer port. [3] 65123 Recommended
client.socket.address string Immediate client peer address - unix domain socket name, IPv4 or IPv6 address. /tmp/my.sock; 127.0.0.1 Recommended: If different than client.address.
client.socket.port int Immediate client peer port number 35555 Recommended: If different than client.port.
server.address string Name of the local HTTP server that received the request. [4] example.com Recommended
server.port int Port of the local HTTP server that received the request. [5] 80; 8080; 443 Recommended: [6]
server.socket.address string Local socket address. Useful in case of a multi-IP host. 10.5.3.2 Opt-In
server.socket.port int Local socket port. Useful in case of a multi-port host. 16456 Opt-In
url.path string The URI path component [7] /search Required
url.query string The URI query component [8] q=OpenTelemetry Conditionally Required: If and only if one was received/sent.
url.scheme string The URI scheme component identifying the used protocol. http; https Required

[1]: 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. SHOULD include the application root if there is one.

[2]: The IP address of the original client behind all proxies, if known (e.g. from Forwarded, X-Forwarded-For, or a similar header). Otherwise, the immediate client peer address.

[3]: When observed from the server side, and when communicating through an intermediary, client.port SHOULD represent client port behind any intermediaries (e.g. proxies) if it's available.

[4]: 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.

[5]: 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

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

[7]: When missing, the value is assumed to be /

[8]: Sensitive content provided in query string SHOULD be scrubbed when instrumentations can identify it.

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

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 server.address and server.port, in this case instrumentation MAY populate Host header on http.request.header.host attribute even if it's not enabled by user.

Examples

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: GET

Attribute name Value
http.request.method "GET"
network.protocol.version "1.1"
url.full "https://example.com:8080/webshop/articles/4?s=1"
server.address example.com
server.port 8080
server.socket.address "192.0.2.5"
http.response.status_code 200

The corresponding server Span may look like this:

Span name: GET /webshop/articles/:article_id.

Attribute name Value
http.request.method "GET"
network.protocol.version "1.1"
url.path "/webshop/articles/4"
url.query "?s=1"
server.address "example.com"
server.port 8080
url.scheme "https"
http.route "/webshop/articles/:article_id"
http.response.status_code 200
client.address "192.0.2.4"
client.socket.address "192.0.2.5" (the client goes through a proxy)
user_agent.original "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:72.0) Gecko/20100101 Firefox/72.0"

HTTP client 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.resend_count=1)
  |   |
  |   --- server (SERVER, trace=t1, span=s5)
  |
  -- GET / - 200 (CLIENT, trace=t1, span=s6, http.resend_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.resend_count=1)
 |
 --- server (SERVER, trace=t2, span=s2)

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

HTTP client authorization retry examples

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

request (SERVER, trace=t1, span=s1)
  |
  -- GET /hello - 401 (CLIENT, trace=t1, span=s2)
  |   |
  |   --- server (SERVER, trace=t1, span=s3)
  |
  -- GET /hello - 200 (CLIENT, trace=t1, span=s4, http.resend_count=1)
      |
      --- server (SERVER, trace=t1, span=s5)

Example of retries with no trace started upfront:

GET /hello - 401 (CLIENT, trace=t1, span=s1)
 |
 --- server (SERVER, trace=t1, span=s2)

GET /hello - 200 (CLIENT, trace=t2, span=s1, http.resend_count=1)
 |
 --- server (SERVER, trace=t2, span=s2)

HTTP client 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, http.resend_count=1)
      |
      --- 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, http.resend_count=1)
 |
 --- server (SERVER, trace=t2, span=s2)