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Semantic conventions for FaaS spans

This document defines how to describe an instance of a function that runs without provisioning or managing of servers (also known as serverless) with spans.

General Attributes

Span name should be set to the function name being executed. Depending on the value of the faas.trigger attribute, additional attributes MUST be set. For example, an http trigger SHOULD follow the HTTP Server semantic conventions. For more information, refer to the Function Trigger Type section.

If Spans following this convention are produced, a Resource of type faas MUST exist following the Resource semantic convention.

Attribute name Notes and examples Required?
faas.trigger Type of the trigger on which the function is executed.
It SHOULD be one of the following strings: "datasource", "http", "pubsub", "timer", or "other".		 See below.
faas.execution String containing the execution id of the function. E.g. af9d5aa4-a685-4c5f-a22b-444f80b3cc28 No

On FaaS instances, faas.trigger MUST be set on incoming invocations. Clients invoking FaaS instances MUST set faas.trigger on outgoing invocations, if it is known to the client. This is, for example, not the case, when the transport layer is abstracted in a FaaS client framework without access to its configuration.

Function Name

There are 2 locations where the function's name can be recorded: the span name and the faas.name Resource attribute.

It is guaranteed that if faas.name attribute is present it will contain the function name, since it is defined in the semantic convention strictly for that purpose. It is also highly likely that Span name will contain the function name (e.g. for Span displaying purposes), but it is not guaranteed (since it is a weaker "SHOULD" requirement). Consumers that needs such guarantee can use faas.name attribute as the source.

Difference between execution and instance

For performance reasons (e.g. AWS lambda, or Azure functions), FaaS providers allocate an execution environment for a single instance of a function that is used to serve multiple requests. Developers exploit this fact to solve the cold start issue, caching expensive resource computations between different function executions. Furthermore, FaaS providers encourage this behavior, e.g. Google functions. This field MAY be set to help correlate function executions that belong to the same execution environment. The span attribute faas.execution differs from the resource attribute faas.instance in the following:

  • faas.execution refers to the current request ID handled by the function;
  • faas.instance refers to the execution environment ID of the function.

Incoming Invocations

This section describes incoming FaaS invocations as they are reported by the FaaS instance itself.

For incoming FaaS spans, the span kind MUST be Server.

Attribute Type Description Required
faas.coldstart boolean Indicates that the serverless function is executed for the first time (aka cold start). No

Outgoing Invocations

This section describes outgoing FaaS invocations as they are reported by a client calling a FaaS instance.

For outgoing FaaS spans, the span kind MUST be Client.

The values reported by the client for the attributes listed below SHOULD be equal to the corresponding FaaS resource attributes and Cloud resource attributes, which the invoked FaaS instance reports about itself, if it's instrumented.

Attribute Corresponding resource attribute Type Description Example Required
faas.invoked_name faas.name string The name of the invoked function. my-function Yes
faas.invoked_provider cloud.provider string The cloud provider of the invoked function. aws Yes
faas.invoked_region cloud.region string The cloud region of the invoked function. eu-central-1 See below

For some cloud providers, like AWS or GCP, the region in which a function is hosted is essential to uniquely identify the function and also part of its endpoint. Since it's part of the endpoint being called, the region is always known to clients. In these cases, faas.invoked_region MUST be set accordingly. If the region is unknown to the client or not required for identifying the invoked function, setting faas.invoked_region is optional.

Function Trigger Type

This section describes how to handle the span creation and additional attributes based on the value of the attribute faas.trigger.

Datasource

A datasource function is triggered as a response to some data source operation such as a database or filesystem read/write. For faas spans with trigger datasource, it is recommended to set the following attributes.

Attribute name Notes and examples Required?
faas.document.collection The name of the source on which the operation was perfomed. For example, in Cloud Storage or S3 corresponds to the bucket name, and in Cosmos DB to the database name. Yes
faas.document.operation Describes the type of the operation that was performed on the data.
It SHOULD be one of the following strings: "insert", "edit", "delete".		 Yes
faas.document.time A string containing the time when the data was accessed in the ISO 8601 format expressed in UTC. E.g. "2020-01-23T13:47:06Z" Yes
faas.document.name The document name/table subjected to the operation.
For example, in Cloud Storage or S3 is the name of the file, and in Cosmos DB the table name.		 No

HTTP

The function responsibility is to provide an answer to an inbound HTTP request. The faas span SHOULD follow the recommendations described in the HTTP Server semantic conventions.

PubSub

A function is set to be executed when messages are sent to a messaging system. In this case, multiple messages could be batch and forwarded at once to the same function execution. Therefore, a different root span of type faas MUST be created for each message processed by the function, following the Messaging systems semantic conventions. This way, it is possible to correlate each individual message with its execution sender.

Timer

A function is scheduled to be executed regularly. The following additional attributes are recommended.

Attribute name Notes and examples Required?
faas.time A string containing the function invocation time in the ISO 8601 format expressed in UTC. E.g. "2020-01-23T13:47:06Z" Yes
faas.cron A string containing the schedule period as Cron Expression. E.g. "0/5 * * * ? *" No

Other

Function as a Service offers such flexibility that it is not possible to fully cover with semantic conventions. When a function does not satisfy any of the aforementioned cases, a span MUST set the attribute faas.trigger to "other". In this case, it is responsibility of the framework or instrumentation library to define the most appropriate attributes.

Example

This example shows the FaaS attributes for a (non-FaaS) process hosted on Google Cloud Platform (Span A with kind Client), which invokes a Lambda function called "my-lambda-function" in Amazon Web Services (Span B with kind Server).

Attribute Kind Attribute Span A (Client, GCP) Span B (Server, AWS Lambda)
Resource cloud.provider "gcp" "aws"
Resource cloud.region "europe-west3" "eu-central-1"
Span faas.invoked_name "my-lambda-function" n/a
Span faas.invoked_provider "aws" n/a
Span faas.invoked_region "eu-central-1" n/a
Span faas.trigger n/a "http"
Span faas.execution n/a "af9d5aa4-a685-4c5f-a22b-444f80b3cc28"
Span faas.coldstart n/a true
Resource faas.name n/a "my-lambda-function"
Resource faas.id n/a "arn:aws:lambda:us-west-2:123456789012:function:my-lambda-function"
Resource faas.version n/a "semver:2.0.0"
Resource faas.instance n/a "my-lambda-function:instance-0001"