diff --git a/.github/workflows/ci.yaml b/.github/workflows/ci.yaml
index 4acdda5..5807ad9 100644
--- a/.github/workflows/ci.yaml
+++ b/.github/workflows/ci.yaml
@@ -52,5 +52,6 @@
       - name: 'Run Tests'
         run: |
           cd lib/skywalking
-          lua id_generator_test.lua
+          lua util_test.lua
+          lua span_test.lua 
           lua tracing_context_test.lua 
\ No newline at end of file
diff --git a/LICENSE b/LICENSE
new file mode 100644
index 0000000..9c8f3ea
--- /dev/null
+++ b/LICENSE
@@ -0,0 +1,201 @@
+                                 Apache License
+                           Version 2.0, January 2004
+                        http://www.apache.org/licenses/
+
+   TERMS AND CONDITIONS FOR USE, REPRODUCTION, AND DISTRIBUTION
+
+   1. Definitions.
+
+      "License" shall mean the terms and conditions for use, reproduction,
+      and distribution as defined by Sections 1 through 9 of this document.
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+      outstanding shares, or (iii) beneficial ownership of such entity.
+
+      "You" (or "Your") shall mean an individual or Legal Entity
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+      form, that is based on (or derived from) the Work and for which the
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+      of this License, Derivative Works shall not include works that remain
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+      "Contribution" shall mean any work of authorship, including
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+      on behalf of whom a Contribution has been received by Licensor and
+      subsequently incorporated within the Work.
+
+   2. Grant of Copyright License. Subject to the terms and conditions of
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+      meet the following conditions:
+
+      (a) You must give any other recipients of the Work or
+          Derivative Works a copy of this License; and
+
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+
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+          notices within Derivative Works that You distribute, alongside
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+
+      You may add Your own copyright statement to Your modifications and
+      may provide additional or different license terms and conditions
+      for use, reproduction, or distribution of Your modifications, or
+      for any such Derivative Works as a whole, provided Your use,
+      reproduction, and distribution of the Work otherwise complies with
+      the conditions stated in this License.
+
+   5. Submission of Contributions. Unless You explicitly state otherwise,
+      any Contribution intentionally submitted for inclusion in the Work
+      by You to the Licensor shall be under the terms and conditions of
+      this License, without any additional terms or conditions.
+      Notwithstanding the above, nothing herein shall supersede or modify
+      the terms of any separate license agreement you may have executed
+      with Licensor regarding such Contributions.
+
+   6. Trademarks. This License does not grant permission to use the trade
+      names, trademarks, service marks, or product names of the Licensor,
+      except as required for reasonable and customary use in describing the
+      origin of the Work and reproducing the content of the NOTICE file.
+
+   7. Disclaimer of Warranty. Unless required by applicable law or
+      agreed to in writing, Licensor provides the Work (and each
+      Contributor provides its Contributions) on an "AS IS" BASIS,
+      WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
+      implied, including, without limitation, any warranties or conditions
+      of TITLE, NON-INFRINGEMENT, MERCHANTABILITY, or FITNESS FOR A
+      PARTICULAR PURPOSE. You are solely responsible for determining the
+      appropriateness of using or redistributing the Work and assume any
+      risks associated with Your exercise of permissions under this License.
+
+   8. Limitation of Liability. In no event and under no legal theory,
+      whether in tort (including negligence), contract, or otherwise,
+      unless required by applicable law (such as deliberate and grossly
+      negligent acts) or agreed to in writing, shall any Contributor be
+      liable to You for damages, including any direct, indirect, special,
+      incidental, or consequential damages of any character arising as a
+      result of this License or out of the use or inability to use the
+      Work (including but not limited to damages for loss of goodwill,
+      work stoppage, computer failure or malfunction, or any and all
+      other commercial damages or losses), even if such Contributor
+      has been advised of the possibility of such damages.
+
+   9. Accepting Warranty or Additional Liability. While redistributing
+      the Work or Derivative Works thereof, You may choose to offer,
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+      on Your own behalf and on Your sole responsibility, not on behalf
+      of any other Contributor, and only if You agree to indemnify,
+      defend, and hold each Contributor harmless for any liability
+      incurred by, or claims asserted against, such Contributor by reason
+      of your accepting any such warranty or additional liability.
+
+   END OF TERMS AND CONDITIONS
+
+   APPENDIX: How to apply the Apache License to your work.
+
+      To apply the Apache License to your work, attach the following
+      boilerplate notice, with the fields enclosed by brackets "{}"
+      replaced with your own identifying information. (Don't include
+      the brackets!)  The text should be enclosed in the appropriate
+      comment syntax for the file format. We also recommend that a
+      file or class name and description of purpose be included on the
+      same "printed page" as the copyright notice for easier
+      identification within third-party archives.
+
+   Copyright {yyyy} {name of copyright owner}
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
\ No newline at end of file
diff --git a/NOTICE b/NOTICE
new file mode 100644
index 0000000..e646f51
--- /dev/null
+++ b/NOTICE
@@ -0,0 +1,5 @@
+Apache SkyWalking
+Copyright 2017-2020 The Apache Software Foundation
+
+This product includes software developed at
+The Apache Software Foundation (http://www.apache.org/).
diff --git a/README.md b/README.md
index cb674c5..c9eedf1 100644
--- a/README.md
+++ b/README.md
@@ -14,6 +14,18 @@
 # Setup Doc
 TODO
 
+# APIs
+
+# Set up dev env
+All codes in the `lib/skywalking` require the `*_test.lua` to do the UnitTest. To run that, you need to install
+- Lua 5.3
+- LuaRocks
+
+The following libs are required in runtime or test cases, please use `LuaRocks` to install them.
+- luaunit
+- luasocket
+
+
 # Download
 Have no release yet.
 
diff --git a/lib/skywalking/span.lua b/lib/skywalking/span.lua
index e69de29..207be6e 100644
--- a/lib/skywalking/span.lua
+++ b/lib/skywalking/span.lua
@@ -0,0 +1,68 @@
+-- 
+-- Licensed to the Apache Software Foundation (ASF) under one or more
+-- contributor license agreements.  See the NOTICE file distributed with
+-- this work for additional information regarding copyright ownership.
+-- The ASF licenses this file to You under the Apache License, Version 2.0
+-- (the "License"); you may not use this file except in compliance with
+-- the License.  You may obtain a copy of the License at
+-- 
+--    http://www.apache.org/licenses/LICENSE-2.0
+-- 
+-- Unless required by applicable law or agreed to in writing, software
+-- distributed under the License is distributed on an "AS IS" BASIS,
+-- WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+-- See the License for the specific language governing permissions and
+-- limitations under the License.
+-- 
+
+local spanLayer = require("span_layer")
+local Context = require("tracing_context")
+local Util = require('util')
+
+Span = {
+    span_id,
+    parent_span_id,
+    operation_name,
+    tags,
+    logs,
+    layer = spanLayer.NONE,
+    isEntry = false,
+    isExit = false,
+    peer,
+    start_time,
+    end_time,
+    error_occurred = false,
+}
+
+-- Create an entry span, represent the HTTP incoming request.
+function Span:createEntrySpan(operationName, context, parent)
+    local span = self:new(operationName, context, parent)
+    span.isEntry = true
+
+    return span
+end
+
+-- Create a default span.
+-- Usually, this method wouldn't be called by outside directly.
+-- Read newEntrySpan, newExitSpan and newLocalSpan for more details
+function Span:new(operationName, context, parent)
+    local o = {}
+    setmetatable(o, self)
+    self.__index = self
+
+    o.operation_name = operationName
+    o.span_id = context.internal:nextSpanID()
+    if parent == nil then
+        -- As the root span, the parent span id is -1
+        o.parent_span_id = -1
+    else
+        o.parent_span_id = parent.span_id
+    end 
+
+    context.internal:addActive(o)
+    o.start_time = Util.timestamp()
+
+    return o
+end
+
+return Span
diff --git a/lib/skywalking/id_generator.lua b/lib/skywalking/span_layer.lua
similarity index 75%
rename from lib/skywalking/id_generator.lua
rename to lib/skywalking/span_layer.lua
index 3054774..7cdec08 100644
--- a/lib/skywalking/id_generator.lua
+++ b/lib/skywalking/span_layer.lua
@@ -15,16 +15,13 @@
 -- limitations under the License.
 -- 
 
-socket=require'socket'
+Layer = {
+    NONE = {name = "NONE", value=0},
+    DB = {name = "DB", value=1},
+    RPC_FRAMEWORK = {name = "RPC_FRAMEWORK", value=2},
+    HTTP = {name = "HTTP", value=3},
+    MQ = {name = "MQ", value=4},
+    CACHE = {name = "CACHE", value=5},
+}
 
-IDGenerator = {}
-MAX_ID_PART2 = 1000000000
-MAX_ID_PART3 = 100000
-SEQ = 1
-
-function IDGenerator:newID()
-    SEQ = SEQ + 1
-    return {socket.gettime(), math.random( 0, MAX_ID_PART2), math.random( 0, MAX_ID_PART3) + SEQ}
-end
-
-return IDGenerator
\ No newline at end of file
+return Layer
\ No newline at end of file
diff --git a/lib/skywalking/span_test.lua b/lib/skywalking/span_test.lua
new file mode 100644
index 0000000..0523b63
--- /dev/null
+++ b/lib/skywalking/span_test.lua
@@ -0,0 +1,63 @@
+-- 
+-- Licensed to the Apache Software Foundation (ASF) under one or more
+-- contributor license agreements.  See the NOTICE file distributed with
+-- this work for additional information regarding copyright ownership.
+-- The ASF licenses this file to You under the Apache License, Version 2.0
+-- (the "License"); you may not use this file except in compliance with
+-- the License.  You may obtain a copy of the License at
+-- 
+--    http://www.apache.org/licenses/LICENSE-2.0
+-- 
+-- Unless required by applicable law or agreed to in writing, software
+-- distributed under the License is distributed on an "AS IS" BASIS,
+-- WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+-- See the License for the specific language governing permissions and
+-- limitations under the License.
+-- 
+
+local lu = require('luaunit')
+local TC = require('tracing_context')
+local Span = require('span')
+local SpanLayer = require("span_layer")
+
+TestSpan = {}
+    function TestSpan:testNewEntry()
+        local context = TC:new()
+        lu.assertNotNil(context)
+
+        local span1 = Span:createEntrySpan("operation_name", context, nil)
+        lu.assertNotNil(span1)
+        lu.assertEquals(span1.isEntry, true)
+        lu.assertEquals(span1.isExit, false)
+        lu.assertEquals(span1.layer, SpanLayer.NONE)
+
+        lu.assertEquals(#(context.internal.active_spans), 1)
+    end
+
+    function TestSpan:testNew()
+        local context = TC:new()
+        lu.assertNotNil(context)
+
+        local span1 = Span:new("operation_name", context, nil)
+        lu.assertNotNil(span1)
+        lu.assertEquals(span1.parent_span_id, -1)
+        lu.assertEquals(span1.span_id, 0)
+        lu.assertEquals(span1.operation_name, "operation_name")
+        local span2 = Span:new("operation_name", context, span1)
+        lu.assertEquals(span2.parent_span_id, 0)
+        lu.assertEquals(span2.span_id, 1)
+        lu.assertNotNil(span2.start_time)
+
+        -- Use new context to check again
+        context = TC:new()
+        lu.assertNotNil(context)
+
+        span1 = Span:new("operation_name", context, nil)
+        lu.assertNotNil(span1)
+        lu.assertEquals(span1.parent_span_id, -1)
+        lu.assertEquals(span1.span_id, 0)
+    end
+-- end TestSpan
+
+
+os.exit( lu.LuaUnit.run() )
\ No newline at end of file
diff --git a/lib/skywalking/tracing_context.lua b/lib/skywalking/tracing_context.lua
index fa42f9a..1e122e2 100644
--- a/lib/skywalking/tracing_context.lua
+++ b/lib/skywalking/tracing_context.lua
@@ -15,26 +15,69 @@
 -- limitations under the License.
 -- 
 
-local idGen = require('id_generator')
+local Util = require('util')
 
 TracingContext = {
-    span_id_seq = 0,
     trace_id,
     segment_id,
-    -- Linked Lists
-    finished_spans = nil,
+    is_noop = false,
+
+    internal,
 }
 
-function TracingContext:new(o)
-    o = o or {} 
+function TracingContext:new()
+    local o = {}
     setmetatable(o, self)
+    self.__index = self
 
-    o.trace_id = idGen.newID();
+    o.trace_id = Util:newID();
     o.segment_id = o.trace_id
+    o.internal = Internal:new()
+    o.internal.owner = o
     return o
 end
 
 function TracingContext:createEntrySpan(operationName)
 end
 
+-------------- Internal Object-------------
+-- Internal Object hosts the methods for SkyWalking LUA internal APIs only.
+Internal = {
+    -- span id starts from 0
+    span_id_seq,
+    -- Owner means the Context instance holding this Internal object.
+    owner,
+    -- Lists
+    -- Created span and still active
+    active_spans,
+    -- Finished spans
+    finished_spans,
+}
+
+-- Create an internal instance
+function Internal:new()
+    local o = {}
+    setmetatable(o, self)
+    self.__index = self
+
+    o.span_id_seq = 0
+    o.active_spans = {}
+    o.finished_spans = {}
+
+    return o
+end
+
+function Internal:addActive(span)
+    table.insert(self.active_spans, span)
+    return self.owner
+end
+
+-- Generate the next span ID.
+function Internal:nextSpanID()
+    local nextSpanId = self.span_id_seq
+    self.span_id_seq = self.span_id_seq + 1;
+    return nextSpanId
+end
+---------------------------------------------
+
 return TracingContext
\ No newline at end of file
diff --git a/lib/skywalking/tracing_context_test.lua b/lib/skywalking/tracing_context_test.lua
index 9de8c7d..1b07dc1 100644
--- a/lib/skywalking/tracing_context_test.lua
+++ b/lib/skywalking/tracing_context_test.lua
@@ -16,18 +16,33 @@
 -- 
 
 local lu = require('luaunit')
-local tc = require('tracing_context')
+local TC = require('tracing_context')
 
 TestTracingContext = {}
     function TestTracingContext:testNew()
-        local context = tc.new()
-
+        local context = TC:new()
+        lu.assertNotNil(context)
         lu.assertNotNil(context.segment_id[1])
         lu.assertNotNil(context.segment_id[2])
         lu.assertNotNil(context.segment_id[3])
 
         lu.assertEquals(context.trace_id, context.segment_id)
     end
+
+    function TestTracingContext:testInternal_NextSpanSeqID()
+        local context = TC:new()
+
+        lu.assertEquals(context.internal:nextSpanID(), 0)
+    end
+
+    function TestTracingContext:testInternal_addActive()
+        local context = TC:new()
+
+        local mockSpan = {field = "ws"}
+        context.internal:addActive(mockSpan)
+
+        lu.assertEquals(#(context.internal.active_spans), 1)
+    end
 -- end TestTracingContext
 
 
diff --git a/lib/skywalking/id_generator.lua b/lib/skywalking/util.lua
similarity index 60%
copy from lib/skywalking/id_generator.lua
copy to lib/skywalking/util.lua
index 3054774..ecabe7d 100644
--- a/lib/skywalking/id_generator.lua
+++ b/lib/skywalking/util.lua
@@ -15,16 +15,29 @@
 -- limitations under the License.
 -- 
 
-socket=require'socket'
-
-IDGenerator = {}
+Util = {}
 MAX_ID_PART2 = 1000000000
 MAX_ID_PART3 = 100000
 SEQ = 1
 
-function IDGenerator:newID()
+function Util:newID()
     SEQ = SEQ + 1
-    return {socket.gettime(), math.random( 0, MAX_ID_PART2), math.random( 0, MAX_ID_PART3) + SEQ}
+    return {Util.timestamp(), math.random( 0, MAX_ID_PART2), math.random( 0, MAX_ID_PART3) + SEQ}
 end
 
-return IDGenerator
\ No newline at end of file
+-- A simulation implementation of Java's System.currentTimeMillis() by following the SkyWalking protocol.
+-- Return the difference as string, measured in milliseconds, between the current time and midnight, January 1, 1970 UTC.
+-- But in using os.clock(), I am not sure whether it is accurate enough.
+function Util:timestamp()
+    local a,b = math.modf(os.clock())
+    if b==0 then 
+        b='000' 
+    else 
+        b=tostring(b):sub(3,5) 
+    end
+
+    return os.time() * 1000 + b
+end
+
+
+return Util
\ No newline at end of file
diff --git a/lib/skywalking/id_generator_test.lua b/lib/skywalking/util_test.lua
similarity index 80%
rename from lib/skywalking/id_generator_test.lua
rename to lib/skywalking/util_test.lua
index a536bed..e620017 100644
--- a/lib/skywalking/id_generator_test.lua
+++ b/lib/skywalking/util_test.lua
@@ -16,17 +16,22 @@
 -- 
 
 local lu = require('luaunit')
-local idGen = require('id_generator')
+local Util = require('util')
 
-TestIDGenerator = {}
-    function TestIDGenerator:test()
-        local id = idGen.newID()
+TestUtil = {}
+    function TestUtil:testNewID()
+        local id = Util.newID()
 
         lu.assertNotNil(id[1])
         lu.assertNotNil(id[2])
         lu.assertNotNil(id[3])
     end
--- end TestIDGenerator
+
+    function TestUtil:testTimestamp()
+        local id = Util.timestamp()
+        lu.assertNotNil(id)
+    end
+-- end TestUtil
 
 
 os.exit( lu.LuaUnit.run() )
\ No newline at end of file