Add specs on Propagator API. (#147)

* Add specs on Propagator API.

Fixes #72.

* Fix a bracket.

* Update TOC.

specification/propagators-api.md

@@ -0,0 +1,137 @@
+# Propagators API
+
+<details>
+<summary>
+Table of Content
+</summary>
+
+- [Binary Format](#binary-format)
+  - [ToBytes](#tobytes)
+  - [FromBytes](#frombytes)
+- [HTTP Text Format](#http-text-format)
+  - [Fields](#fields)
+  - [Inject](#inject)
+  - [Setter](#setter)
+    - [Put](#put)
+  - [Extract](#extract)
+  - [Getter](#getter)
+    - [Get](#get)
+
+</details>
+
+Propagators API consists of two main formats:
+- `BinaryFormat` is used to serialize and deserialize a value into a binary representation.
+- `HTTPTextFormat` is used to inject and extract a value as text into carriers that travel
+in-band across process boundaries.
+
+## Binary Format
+
+`BinaryFormat` is a formatter to serialize and deserialize a value into a binary format.
+
+`BinaryFormat` MUST expose the APIs that serializes values into bytes,
+and deserializes values from bytes.
+
+### ToBytes
+
+Serializes the given value into the on-the-wire representation.
+
+Required arguments:
+- the value to serialize, can be `SpanContext` or `DistributedContext`.
+
+Returns the on-the-wire byte representation of the value.
+
+### FromBytes
+
+Creates a value from the given on-the-wire encoded representation.
+
+If the value could not be parsed, the underlying implementation SHOULD decide to return ether
+an empty value, an invalid value, or a valid value.
+
+Required arguments:
+- on-the-wire byte representation of the value.
+
+Returns a value deserialized from bytes.
+
+## HTTP Text Format
+
+`HTTPTextFormat` is a formatter that injects and extracts a value as text into carriers that 
+travel in-band across process boundaries.
+
+Encoding is expected to conform to the HTTP Header Field semantics. Values are often encoded as
+RPC/HTTP request headers.
+
+The carrier of propagated data on both the client (injector) and server (extractor) side is
+usually an http request. Propagation is usually implemented via library-specific request
+interceptors, where the client-side injects values and the server-side extracts them.
+
+`HTTPTextFormat` MUST expose the APIs that injects values into carriers,
+and extracts values from carriers.
+
+### Fields
+
+The propagation fields defined. If your carrier is reused, you should delete the fields here
+before calling [inject](#inject).
+
+For example, if the carrier is a single-use or immutable request object, you don't need to
+clear fields as they couldn't have been set before. If it is a mutable, retryable object,
+successive calls should clear these fields first.
+
+The use cases of this are:
+- allow pre-allocation of fields, especially in systems like gRPC Metadata
+- allow a single-pass over an iterator
+
+Returns list of fields that will be used by this formatter.
+
+### Inject
+
+Injects the value downstream. For example, as http headers.
+
+Required arguments:
+- the value to be injected, can be `SpanContext` or `DistributedContext`.
+- the carrier that holds propagation fields. For example, an outgoing message or http request.
+- the setter invoked for each propagation key to add or remove.
+
+### Setter
+
+`Setter` allows a `HTTPTextFormat` to set propagated fields into a carrier.
+
+`Setter` MUST be stateless and allowed to be saved as a constant to avoid runtime allocations.
+
+#### Put
+
+Replaces a propagated field with the given value.
+
+Required arguments:
+- the carrier holds propagation fields. For example, an outgoing message or http request.
+- the key of the field.
+- the value of the field.
+
+### Extract
+
+Extracts the value from upstream. For example, as http headers.
+
+If the value could not be parsed, the underlying implementation will decide to return an
+object representing either an empty value, an invalid value, or a valid value. Implementation
+MUST not return null.
+
+Required arguments:
+- the carrier holds propagation fields. For example, an outgoing message or http request.
+- the getter invoked for each propagation key to get.
+
+Returns the non-null extracted value.
+
+### Getter
+
+`Getter` allows a `HttpTextFormat` to read propagated fields from a carrier.
+
+`Getter` MUST be stateless and allowed to be saved as a constant to avoid runtime allocations.
+
+#### Get
+
+Returns the first value of the given propagation key or returns null if the key doesn't exist.
+
+Required arguments:
+- the carrier of propagation fields, such as an http request.
+- the key of the field.
+
+Returns the first value of the given propagation key or returns null if the key doesn't exist.

terminology.md

@@ -227,6 +227,15 @@ for an example.
 **TODO**: Better describe the difference between the resource and a Node
 https://github.com/open-telemetry/opentelemetry-proto/issues/17
 
+## Propagators
+
+OpenTelemetry uses `Propagators` to serialize and deserialize `SpanContext` and `DistributedContext`
+into a binary or text format. Currently there are two types of propagators:
+
+- `BinaryFormat` which is used to serialize and deserialize a value into a binary representation.
+- `HTTPTextFormat` which is used to inject and extract a value as text into carriers that travel
+in-band across process boundaries.
+
 ## Agent/Collector
 
 The OpenTelemetry service is a set of components that can collect traces,