Add requirements for probability sampler

Signed-off-by: Bogdan Drutu <bogdandrutu@gmail.com>

specification/trace/sdk.md

@@ -90,7 +90,7 @@ It produces an output called `SamplingResult` which contains:
 
 Returns the sampler name or short description with the configuration. This may
 be displayed on debug pages or in the logs. Example:
-`"ProbabilitySampler{0.000100}"`.
+`"TraceIdRatioBased{0.000100}"`.
 
 Description MUST NOT change over time and caller can cache the returned value.
 
@@ -109,15 +109,30 @@ The default sampler is `ParentOrElse(AlwaysOn)`.
 * Returns `NOT_RECORD` always.
 * Description MUST be `AlwaysOffSampler`.
 
-#### Probability
+#### TraceIdRatioBased
 
-* The `ProbabilitySampler` MUST ignore the parent `SampledFlag`.
-  To respect the parent `SampledFlag`, the `ProbabilitySampler` should be used as a delegate of the `ParentOrElse` sampler specified below.
-* Description MUST be `ProbabilitySampler{0.000100}`.
+* The `TraceIdRatioBased` MUST ignore the parent `SampledFlag`.
+  To respect the parent `SampledFlag`, the `TraceIdRatioBased` should be used as a delegate of the `ParentOrElse` sampler specified below.
+* Description MUST be `TraceIdRatioBased{0.000100}`.
 
-TODO: Add details about how the `ProbabilitySampler` is implemented as a function
+TODO: Add details about how the `TraceIdRatioBased` is implemented as a function
 of the `TraceID`.
 
+##### Requirements for `TraceIdRatioBased` sampler algorithm
+
+* Ensures that sampling algorithm is deterministic. A trace identified by a 
+given `TraceId` is sampled or not independent of language, time, etc. To achieve
+this implementations MUST use the random part of the `TraceId` when computing
+the sampling decision.
+  * By ensuring this, even if the user configures the `TraceIdRatioBased`
+  sampler to run for all the `Spans` the same number of traces will be sampled.
+* A `TraceIdRatioBased` sampler with a given sampling rate should sample all
+traces that any `TraceIdRatioBased` sampler with a lower sampling rate would
+sample.
+  * This is important when a backend System may want to run with a higher
+  sampling rate than the frontend System, this way all frontend traces will
+  still be sampled and extra traces will be sampled on the backend only.
+
 #### ParentOrElse
 
 * This is a composite sampler. `ParentOrElse(delegateSampler)` either respects the parent span's sampling decision or delegates to  `delegateSampler` for root spans.