Profiles: optimize profile stack trace representation.

opentelemetry/proto/profiles/v1development/profiles.proto

@@ -132,8 +132,66 @@ message ProfilesData {
 
   // Represents a mapping between Attribute Keys and Units.
   repeated AttributeUnit attribute_units = 8;
-}
 
+  // Stack tree encoded as two parallel arrays: one for the parent array,
+  // another for the ProfilesData.location_table index of the location
+  // corresponding to the given stack tree node. The two arrays must have the
+  // same length. The parent array uses -1 for the root nodes.
+  //
+  // For example, the following locations and stack tree data:
+  //
+  // +----------------+----------------+
+  // | location index | location_table |
+  // +----------------+----------------+
+  // |       0        | ""             |
+  // |       1        | "main"         |
+  // |       2        | "foo"          |
+  // |       3        | "bar"          |
+  // |       4        | "baz1"         |
+  // |       5        | "baz2"         |
+  // +----------------+----------------+
+  //
+  // +-------------+--------------------+----------------------+
+  // | stack index | stack_parent_array | stack_location_index |
+  // +-------------+--------------------+----------------------+
+  // |      0      |        -1          |          1           |
+  // |      1      |         0          |          2           |
+  // |      2      |         1          |          3           |
+  // |      3      |         2          |          4           |
+  // |      4      |         2          |          5           |
+  // |      5      |         1          |          5           |
+  // +-------------+--------------------+----------------------+
+  //
+  // are encoding these stacks (the number is the stack index):
+  //
+  // main (0)
+  // └── foo (1)
+  //     ├── bar (2)
+  //     │   ├── baz1 (3)
+  //     │   └── baz2 (4)
+  //     └── baz2 (5)
+  //
+  // And here is an example of the operations to decode the stack trace
+  // "main->foo->bar->baz2" at stack index 4.
+  //
+  // 1. Lookup stack_location_index[4]  = 5
+  // 2. Lookup location_table[5]        = "baz2"
+  // 3. Lookup stack_parent_array[4]    = 2
+  //
+  // 4. Lookup stack_location_index[2]  = 3
+  // 5. Lookup location_table[3]        = "bar"
+  // 6. Lookup stack_parent_array[2]    = 1
+  //
+  // 7. Lookup stack_location_index[1]  = 2
+  // 8. Lookup location_table[2]        = "foo"
+  // 9. Lookup stack_parent_array[1]    = 0
+  //
+  // 10. Lookup stack_location_index[0] = 1
+  // 11. Lookup location_table[1]       = "main"
+  // 12. Lookup stack_parent_array[0]   = -1
+  repeated int32 stack_parent_array = 9;
+  repeated int32 stack_location_index = 10;
+}
 
 // A collection of ScopeProfiles from a Resource.
 message ResourceProfiles {
@@ -217,9 +275,6 @@ message Profile {
   // The set of samples recorded in this profile.
   repeated Sample sample = 2;
 
-  // References to locations in ProfilesData.location_table.
-  repeated int32 location_indices = 3;
-
   // The following fields 4-14 are informational, do not affect
   // interpretation of results.
 
@@ -379,11 +434,9 @@ message ValueType {
 // augmented with auxiliary information like the thread-id, some
 // indicator of a higher level request being handled etc.
 message Sample {
-  // locations_start_index along with locations_length refers to to a slice of locations in Profile.location_indices.
-  int32 locations_start_index = 1;
-  // locations_length along with locations_start_index refers to a slice of locations in Profile.location_indices.
-  // Supersedes location_index.
-  int32 locations_length = 2;
+  // Index into ProfilesData.{stack_parent_array, stack_location_index} stack
+  // table, representing the stack tree node of the sample.
+  int32 stack_index = 1;
   // The type and unit of each value is defined by the corresponding
   // entry in Profile.sample_type. All samples must have the same
   // number of values, the same as the length of Profile.sample_type.