Semantic Conventions for FaaS Spans

Status: Experimental

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

See also the additional instructions for instrumenting AWS Lambda.

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	Type	Description	Examples	Requirement Level	Stability
cloud.resource_id	string	Cloud provider-specific native identifier of the monitored cloud resource (e.g. an ARN on AWS, a fully qualified resource ID on Azure, a full resource name on GCP) [1]	arn:aws:lambda:REGION:ACCOUNT_ID:function:my-function; //run.googleapis.com/projects/PROJECT_ID/locations/LOCATION_ID/services/SERVICE_ID; /subscriptions/<SUBSCRIPTION_GUID>/resourceGroups/<RG>/providers/Microsoft.Web/sites/<FUNCAPP>/functions/<FUNC>	Recommended
faas.invocation_id	string	The invocation ID of the current function invocation.	af9d5aa4-a685-4c5f-a22b-444f80b3cc28	Recommended
faas.trigger	string	Type of the trigger which caused this function invocation. [2]	datasource; http; pubsub	Recommended

[1]: On some cloud providers, it may not be possible to determine the full ID at startup, so it may be necessary to set cloud.resource_id as a span attribute instead.

The exact value to use for cloud.resource_id depends on the cloud provider. The following well-known definitions MUST be used if you set this attribute and they apply:

• AWS Lambda: The function ARN. Take care not to use the “invoked ARN” directly but replace any alias suffix with the resolved function version, as the same runtime instance may be invocable with multiple different aliases.
• GCP: The URI of the resource
• Azure: The Fully Qualified Resource ID of the invoked function, not the function app, having the form /subscriptions/<SUBSCRIPTION_GUID>/resourceGroups/<RG>/providers/Microsoft.Web/sites/<FUNCAPP>/functions/<FUNC>. This means that a span attribute MUST be used, as an Azure function app can host multiple functions that would usually share a TracerProvider.

[2]: For the server/consumer span on the incoming side, faas.trigger MUST be set.

Clients invoking FaaS instances usually cannot set faas.trigger, since they would typically need to look in the payload to determine the event type. If clients set it, it should be the same as the trigger that corresponding incoming would have (i.e., this has nothing to do with the underlying transport used to make the API call to invoke the lambda, which is often HTTP).

faas.trigger 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	Stability
datasource	A response to some data source operation such as a database or filesystem read/write
http	To provide an answer to an inbound HTTP request
other	If none of the others apply
pubsub	A function is set to be executed when messages are sent to a messaging system
timer	A function is scheduled to be executed regularly

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 invocation 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 invocations. Furthermore, FaaS providers encourage this behavior, e.g. Google functions. The faas.instance resource attribute MAY be set to help correlate function invocations that belong to the same execution environment. The span attribute faas.invocation_id differs from the resource attribute faas.instance in the following:

• faas.invocation_id refers to the ID of the current invocation of 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.

Incoming FaaS Span attributes

Attribute	Type	Description	Examples	Requirement Level	Stability
faas.trigger	string	Type of the trigger which caused this function invocation. [1]	datasource; http; pubsub	Required
faas.coldstart	boolean	A boolean that is true if the serverless function is executed for the first time (aka cold-start).		Recommended

[1]: For the server/consumer span on the incoming side, faas.trigger MUST be set.

Clients invoking FaaS instances usually cannot set faas.trigger, since they would typically need to look in the payload to determine the event type. If clients set it, it should be the same as the trigger that corresponding incoming would have (i.e., this has nothing to do with the underlying transport used to make the API call to invoke the lambda, which is often HTTP).

faas.trigger 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	Stability
datasource	A response to some data source operation such as a database or filesystem read/write
http	To provide an answer to an inbound HTTP request
other	If none of the others apply
pubsub	A function is set to be executed when messages are sent to a messaging system
timer	A function is scheduled to be executed regularly

Resource attributes as incoming FaaS span attributes

In addition to the attributes listed above, any FaaS or cloud resource attributes MAY instead be set as span attributes on incoming FaaS invocation spans: In some FaaS environments some of the information required for resource attributes is only readily available in the context of an invocation (e.g. as part of a “request context” argument) and while a separate API call to look up the resource information is often possible, it may be prohibitively expensive due to cold start duration concerns. The cloud.resource_id and cloud.account.id attributes on AWS are some examples. In principle, the above considerations apply to any resource attribute that fulfills the criteria above (not being readily available without some extra effort that could be expensive).

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	Type	Description	Examples	Requirement Level	Stability
faas.invoked_name	string	The name of the invoked function. [1]	my-function	Required
faas.invoked_provider	string	The cloud provider of the invoked function. [2]	alibaba_cloud; aws; azure	Required
faas.invoked_region	string	The cloud region of the invoked function. [3]	eu-central-1	Conditionally Required [4]

[1]: SHOULD be equal to the faas.name resource attribute of the invoked function.

[2]: SHOULD be equal to the cloud.provider resource attribute of the invoked function.

[3]: SHOULD be equal to the cloud.region resource attribute of the invoked function.

[4]: 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.

faas.invoked_provider 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	Stability
alibaba_cloud	Alibaba Cloud
aws	Amazon Web Services
azure	Microsoft Azure
gcp	Google Cloud Platform
tencent_cloud	Tencent Cloud

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. FaaS instrumentations that produce faas spans with trigger datasource, SHOULD use the following set of attributes.

Attribute	Type	Description	Examples	Requirement Level	Stability
faas.document.collection	string	The name of the source on which the triggering operation was performed. For example, in Cloud Storage or S3 corresponds to the bucket name, and in Cosmos DB to the database name.	myBucketName; myDbName	Required
faas.document.operation	string	Describes the type of the operation that was performed on the data.	insert; edit; delete	Required
faas.document.name	string	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.	myFile.txt; myTableName	Recommended
faas.document.time	string	A string containing the time when the data was accessed in the ISO 8601 format expressed in UTC.	2020-01-23T13:47:06Z	Recommended

faas.document.operation 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	Stability
delete	When an object is deleted.
edit	When an object is modified.
insert	When a new object is created.

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 invocation. 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 invocation sender.

Timer

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

Attribute	Type	Description	Examples	Requirement Level	Stability
faas.cron	string	A string containing the schedule period as Cron Expression.	0/5 * * * ? *	Recommended
faas.time	string	A string containing the function invocation time in the ISO 8601 format expressed in UTC.	2020-01-23T13:47:06Z	Recommended

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).