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api-distributedcontext.md

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DistributedContext API

An Entry is used to label anything that is associated with a specific operation, such as an HTTP request.

DistributedContext is an abstract data type that represents a collection of entries. Each key of DistributedContext is associated with exactly one value. DistributedContext is serializable, to facilitate propagating it not only inside the process but also across process boundaries. DistributedContext is used to annotate telemetry with the name:value pair Entry. Those values can be used to add dimension to the metric or additional context properties to logs and traces.

DistributedContext is a recommended name but languages can have more language-specific names like dctx.

Entry

An Entry consists of EntryMetadata, EntryKey, and EntryValue.

EntryKey

EntryKey is the name of the Entry. EntryKey along with EntryValue can be used to aggregate and group stats, annotate traces and logs, etc.

Restrictions

  • Must contain only printable ASCII (codes between 32 and 126 inclusive)
  • Must have length greater than zero and less than 256.
  • Must not be empty.

Create

Creates a new EntryKey with the given name in string. This is a static method.

Required parameter:

Name of the EntryKey.

GetName

Returns the name of the EntryKey.

EntryValue

EntryValue is a string. It MUST contain only printable ASCII (codes between 32 and 126)

Create

Creates a new EntryValue with the given value in string. This is a static method.

Required parameter:

String value of the EntryValue.

AsString

Returns the string value of the EntryValue.

EntryMetadata

EntryMetadata contains properties associated with an Entry. For now only the property EntryTTL is defined. In future, additional properties may be added to address specific situations.

The creator of entries determines metadata of an entry it creates.

Create

Creates a new EntryMetadata with the EntryTTL. This is a static method.

Required parameter:

EntryTTL that represents number of hops an entry can propagate.

GetEntryTTL

Returns the EntryTTL.

EntryTTL

EntryTTL is an integer that represents number of hops an entry can propagate. Anytime a sender serializes an entry, sends it over the wire and receiver deserializes the entry then the entry is considered to have travelled one hop. There could be one or more proxy(ies) between sender and receiver. Proxies are treated as transparent entities and they may not create additional hops. Every propagation implementation should support an option decrementTTL (default set to true) that allows proxies to set it to false.

For now, ONLY special values (0 and -1) are supported.

Special Values

  • NO_PROPAGATION (0): An Entry with EntryTTL value of zero is considered to have local scope and is used within the process it created.

  • UNLIMITED_PROPAGATION (-1): An Entry with EntryTTL value of -1 can propagate unlimited hops. EntryTTL value of -1 is typical used to represent a request, processing of which may span multiple entities.

Example for EntryTTL > 0

On a server side typically there is no information about the caller besides ip/port, but in every process there is a notion of "service_name" entry that is added as a "caller" entry before serialization when a RPC/HTTP call is made. For the "caller" entry, desirable EntryTTL value is 1.

Note that EntryTTL value of 1 is not supported at this time. The example is listed here simply to show a possible use case for EntryTTL > 0.

Processing at Receiver and Sender

For now, limited processing is required on Sender and Receiver. However, for the sake of completeness, future processing requirement is also listed here. These requirements are marked with "(future)".

This processing is done as part of entry propagator.

At Receiver

Upon receiving an entry from a remote entity, an entry extractor:

  • MUST decrement the value of EntryTTL by one if it is greater than zero. (future)
  • MUST treat the value of EntryTTL as -1 if it is not present.
  • MUST discard the Entry for any other value of EntryTTL. (future)
At Sender

Upon preparing to send an entry to a remote entity, an entry injector:

  • MUST send the entry AND include EntryTTL if its value is greater than 0. (future)
  • MUST send the entry without EntryTTL if its value is -1. Absence of EntryTTL on the wire is treated as having EntryTTL of -1. This is to optimize on-the-wire representation of common case.
  • MUST not send the entry if the value of EntryTTL is 0.

Entry Conflict Resolution

If a new entry conflicts with an existing entry then the new entry takes precedence. Entire Entry along with EntryValue and EntryMetadata is replaced by the most recent entry (regardless of it is locally generated or received from a remote peer). Replacement is limited to a scope in which the conflict arises. When the scope is closed the original value and metadata prior to the conflict is restored. For example,

T# - Entry keys
V# - Entry Values
M# - Entry Metadata

Enter Scope 1
   Current Entries E1=V1/M1, E2=V2/M2
    Enter Scope 2
      Add Entries E3=V3/M3, E2=V4/M4
      Current Entries E1=V1/M1, E2=V4/M4, E3=V3/M3 <== Value/Metadata of E2 is replaced by V4/M4.
    Close Scope 2
   Current Entries E1=V1/M1, E2=V2/M2  <== E2 is restored.
Close Scope 1

DistributedContext

GetEntries

Returns the entries in this DistributedContext. The order of entries is not significant. Based on the language specification, the returned value can be either an immutable collection or an immutable iterator to the collection of entries in this DistributedContext.

GetEntryValue

Returns the EntryValue associated with the given EntryKey, or null if the given EntryKey is not present.

Required parameter:

EntryKey entry key to return the value for.

Limits

Combined size of all entries should not exceed 8192 bytes before encoding. The size restriction applies to the deserialized entries so that the set of decoded DistributedContexts is independent of the encoding format.

DistributedContext Propagation

DistributedContext may be propagated across process boundaries or across any arbitrary boundaries (process, $OTHER_BOUNDARY1, $OTHER_BOUNDARY2, etc) for various reasons. For example, one may propagate 'project-id' Entry across all micro-services to break down metrics by 'project-id'. Not all entries in a DistributedContext should be propagated and not all entries in a DistributedContext should be accepted from a remote peer. Hence, DistributedContext propagator must allow specifying an optional list of ordered EntryPropagationFilters for receiving entries or for forwarding entries or for both. An EntryPropagationFilter list for receiving MAY be different than that for forwarding.

If no filter is specified for receiving then all entries are received. If no filter is specified for forwarding then all entries are forwarded except those that have EntryTTL of 0.

EntryPropagationFilter

Entry Propagation Filter consists of an action (EntryPropagationFilterAction) and a condition (EntryPropagationFilterMatchOperator and EntryPropagationFilterMatchString). An EntryKey is evaluated against condition of each EntryPropagationFilter in order. If the condition is evaluated to true then action is taken according to EntryPropagationFilterAction and filter processing is stopped. If the condition is evaluated to false then the EntryKey is processed against next EntryPropagationFilter in the ordered list. If none of the condition is evaluated to true then the default action is Exclude.

EntryPropagationFilterAction

This is an interface. Implementation of this interface takes appropriate action on the Entry if the condition (EntryPropagationFitlerMatchOperator and EntryPropagationFilterMatchString) is evaluated to true. At a minimum, Exclude and Include actions MUST be implemented.

Exclude If the EntryPropagationFilterAction is Exclude then any Entry whose EntryKey evaluates to true with the condition (EntryPropagationFitlerMatchOperator and EntryPropagationFilterMatchString) MUST be excluded.

Include If the EntryPropagationFilterAction is Include then any Entry whose EntryKey evaluates to true with the condition (EntryPropagationFitlerMatchOperator and EntryPropagationFilterMatchString) MUST be included.

EntryPropagationFilterMatchOperator
Operator Description
EQUAL The condition is evaluated to true if EntryKey is exactly same as EntryPropagationFilterMatchString
NOTEQUAL The condition is evaluated to true if EntryKey is NOT exactly same as EntryPropagationFilterMatchString
HAS_PREFIX The condition is evaluated to true if EntryKey begins with EntryPropagationFilterMatchString
EntryPropagationFilterMatchString

It is a string to compare against EntryKey using EntryPropagationFilterMatchOperator in order to include or exclude an Entry.