SDK Trace: Adding span collection limits

CHANGELOG.md

@@ -23,6 +23,8 @@ New:
   ([#697](https://github.com/open-telemetry/opentelemetry-specification/pull/697))
   * API was extended to allow adding arbitrary event attributes ([#874](https://github.com/open-telemetry/opentelemetry-specification/pull/874))
   * `exception.escaped` was added ([#784](https://github.com/open-telemetry/opentelemetry-specification/pull/784))
+- Add default limits to the number of events in SDK Spans
+  ([#942](https://github.com/open-telemetry/opentelemetry-specification/pull/942))
 
 Updates:
 

spec-compliance-matrix.md

@@ -14,6 +14,8 @@ status of the feature is not known.
 |[TracerProvider](https://github.com/open-telemetry/opentelemetry-specification/blob/master/specification/trace/api.md#tracerprovider-operations)|
 |Create TracerProvider                         | + | +  | + | +    | +  | +    | + | +  | + | +  |
 |Get a Tracer                                  | + | +  | + | +    | +  | +    | + | +  | + | +  |
+|Set a collection limit                        |   |    |   |      |    |      |   |    |   |    |
+|Get a collection limit                        |   |    |   |      |    |      |   |    |   |    |
 |Safe for concurrent calls                     | + | +  | + |      | +  | +    | + | +  | + | +  |
 |[Tracer](https://github.com/open-telemetry/opentelemetry-specification/blob/master/specification/trace/api.md#tracer-operations)|
 |Create a new Span                             | + | +  | + | +    | +  | +    | + | +  | + | +  |
@@ -37,6 +39,8 @@ status of the feature is not known.
 |IsRecording                                   | + | +  | + | +    | +  | +    | + |    | + | +  |
 |Set status                                    | + | +  | + | +    | +  | +    | + | +  | + | +  |
 |Safe for concurrent calls                     | + | +  | + |      | +  | +    | + | +  | + | +  |
+|Set a collection limit                        |   |    |   |      |    |      |   |    |   |    |
+|Discard events greater than collection limit  |   |    |   |      |    |      |   |    |   |    |
 |[Span attributes](https://github.com/open-telemetry/opentelemetry-specification/blob/master/specification/trace/api.md#set-attributes)|
 |SetAttribute                                  | + | +  | + | +    | +  | +    | + | +  | + | +  |
 |Set order preserved                           | + | -  | + | +    | +  | +    | + | +  | + | +  |

specification/trace/sdk.md

@@ -7,6 +7,7 @@
 * [Sampling](#sampling)
 * [Tracer Creation](#tracer-creation)
 * [Additional Span Interfaces](#additional-span-interfaces)
+* [Limits on Span Collections](#limits-on-span-collections)
 * [Span Processor](#span-processor)
 * [Span Exporter](#span-exporter)
 
@@ -215,7 +216,7 @@ Thus, the SDK specification defines sets of possible requirements for
   It must also be able to reliably determine whether the Span has ended
   (some languages might implement this by having an end timestamp of `null`,
   others might have an explicit `hasEnded` boolean).
-  
+
   A function receiving this as argument might not be able to modify the Span.
 
   Note: Typically this will be implemented with a new interface or
@@ -236,7 +237,34 @@ Thus, the SDK specification defines sets of possible requirements for
   that the [span creation API](api.md#span-creation) returned (or will return) to the user
   (for example, the `Span` could be one of the parameters passed to such a function,
   or a getter could be provided).
-
+
+## Limits on Span Collections
+
+Erroneous code can add unintended attributes, events, and links to a span. If
+these collections are unbounded, they can quickly exhaust available memory,
+resulting in crashes that are difficult to recover from safely.
+
+To protect against such errors, SDK Spans MUST discard attributes, links, and
+events that would increase the number of elements of each collection beyond a
+limit configured in the `TracerProvider`.
+
+There SHOULD be a log message emitted by the SDK for the first time a span is
+discarded due to a limit. This message should be emitted once for
+attributes, events, and links, and MUST be configurable on or off, with
+a default of on.
+
+The following interfaces MUST exist:
+
+- `TracerProvider`: a method to set a maximum collection count
+- `TracerProvider`: a method to retrieve the maximum collection count
+
+The default for this limit SHOULD be 1000 elements. A value of -1 indicates no
+limit, while a value of 0 would result in no events being collected.
+
+Implementations MAY also enable configuration for the maximum element count of
+individual collections: attributes, events, and links. These limits would take
+precedence over the general collection limit.
+
 ## Span processor
 
 Span processor is an interface which allows hooks for span start and end method