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+# SONiC FlexCounter Refactor #
+
+## Table of Content
+
+### Revision
+
+ | Rev | Date | Author | Change Description |
+ |:---:|:-----------:|:------------------:|-----------------------------------|
+ | 0.1 | | Junchao Chen | Initial version |
+
+### Scope
+
+This document is the design document for refactoring FlexCounter class in syncd of sonic-sairedis.
+
+### Definitions/Abbreviations
+
+N/A
+
+### Overview
+
+FlexCounter class is a representation of flex counter group which supports querying multiple types of statistic/attributes. It supports multiple statistic/attribute types such as port counter, port debug counter, queue counter, queue attribute and so on. For each statistic/attribute type, it defines several member functions:
+
+- setXXXCounterList: e.g. setPortCounterList, setPortDebugCounterList
+- removeXXX: e.g. removePort, removeQueue
+- collectXXXCounters: e.g. collectPortCounters, collectQueueCounters
+- collectXXXAttr: e.g. collectQueueAttrs, collectPriorityGroupAttrs
+- so on
+
+For different statistic/attribute types, these functions have very similar logic. This design document describes a proposal about how to refactor FlexCounter class by moving duplicate/similar logic to a single place.
+
+#### Challenge
+
+The major challenge of this refactor is that, different statistic/attributes types may have different SAI type. E.g. type of port counter is sai_port_stat_t, type of queue counter is sai_queue_stat_t, type of queue attribute is sai_port_attr_t. C++ does not allow to put different type of object into the same container.
+
+#### Benefit
+
+A POC shows that, the refactor brings a few benefits:
+
+- Code lines of FlexCounter.cpp change from ~4000 to ~1000
+- Code lines of FlexCounter.h change from ~600 to ~200
+- Supporting new flex counter would requires only change a few places instead of implementing all those setXXXCounterList/removeXXX/collectXXXCounter and so on
+
+### Requirements
+
+- The change shall be limited in FlexCounter class
+- Public interfaces of FlexCounter class shall keep the same functional
+- Other class/component of SONiC shall treat this change as transparent
+
+### Architecture Design
+
+Current design of FlexCounter class:
+
+.
+
+New design:
+
+.
+
+### High-Level Design
+
+Changes shall be made into sonic-sairedis. Affected files are: FlexCounter.cpp, FlexCounter.h. This chapter will describe that how the new architecture can perfectly replace the old functions.
+
+> Note: Code present in this design document is only for demonstrating the design idea, it is not production code.
+
+#### sonic-sairedis
+
+##### Store Counter Related Data
+
+Currently, there are several types of counter related data.
+
+- Counter IDs map: stores object vid, rid and a list of counter IDs that will be queried. E.g. m_portCounterIdsMap, m_queueCounterIdsMap...
+- Plugins: stores redis lua SHA values that will be execute by redis. E.g. m_portPlugins, m_queuePlugins...
+- Supported counters: stores supported counter IDs. E.g. m_supportedPortCounters, m_supportedQueueCounters...
+
+In the new design, each statistic/attribute type is represented by an instance of CounterContext/AttrContext which will be stored in counter_context_map. These data members are moved to CounterContext and AttrContext as data members:
+
+- object_ids_map
+- plugins
+- supported_counters
+
+##### Counter IDs Structure
+
+Currently, a few similar structures are defined in FlexCounter to represent rid, counter IDs as well as stats mode.
+
+```cpp
+struct QueueCounterIds
+{
+ QueueCounterIds(
+ _In_ sai_object_id_t queue,
+ _In_ const std::vector &queueIds);
+
+ sai_object_id_t queueId;
+ std::vector queueCounterIds;
+};
+
+struct PortCounterIds
+{
+ PortCounterIds(
+ _In_ sai_object_id_t port,
+ _In_ const std::vector &portIds);
+
+ sai_object_id_t portId;
+ std::vector portCounterIds;
+};
+
+struct BufferPoolCounterIds
+{
+ BufferPoolCounterIds(
+ _In_ sai_object_id_t bufferPool,
+ _In_ const std::vector &bufferPoolIds,
+ _In_ sai_stats_mode_t statsMode);
+
+ sai_object_id_t bufferPoolId;
+ sai_stats_mode_t bufferPoolStatsMode;
+ std::vector bufferPoolCounterIds;
+};
+
+...
+```
+
+In new design, two template structures are defined to replaces above structures. The second structure is for those statistic/attribute type which support per instance stats mode; the first structure is for other statistic/attribute type which use counter group level stats mode.
+
+```cpp
+template
+struct CounterIds
+{
+ CounterIds(
+ _In_ sai_object_id_t rid,
+ _In_ const std::vector &counter_ids
+ ): rid(rid), counter_ids(counter_ids) {}
+ sai_object_id_t rid;
+ std::vector counter_ids;
+};
+
+// CounterIds structure contains stats mode, now buffer pool is the only one
+// has member stats_mode.
+template
+struct CounterIds::value>::type >
+{
+ CounterIds(
+ _In_ sai_object_id_t rid,
+ _In_ const std::vector &counter_ids
+ ): rid(rid), counter_ids(counter_ids) {}
+ sai_object_id_t rid;
+ std::vector counter_ids;
+ sai_stats_mode_t stats_mode;
+};
+```
+
+##### Handle Counter Object Change
+
+Each statistic/attribute type can have multiple counter objects. For example, Ethernet0 is a counter object of port statistic; Ethernet0 queue0 is a counter object of queue statistic. The most important information of a counter object are:
+
+- rid: SAI ID of the object (Real ID)
+- vid: Syncd ID of the object (Virtual ID)
+- counter_ids: SAI statistic IDs
+
+FlexCounter handles counter object change so that it queries relevant statistic/attribute from SAI.
+
+##### Add/Update Counter Object
+
+Currently, the flow of adding/updating counter object is like this:
+
+1. FlexCounterManager calls FlexCounter::addCounter
+2. In `addCounter`, it calls setXXXCounterList according to the input parameters
+3. In `setXXXCounterList`, it updates supported counters
+4. In `updateXXXSuportedCounters`, it query statistic capability via SAI API
+5. If step 4 fails, it fall back to `getXXXSupportedCounters`, in which is query statistic value for each input counter IDs
+6. If supported counters is not empty, update the counter object to a proper counter IDs map
+
+In the new design, step 3 ~ step 6 are moved to CounterContext class, but the flow/function does not change at all. Here is the chart of the new design(the chart shows the design idea, it does not contain all details):
+
+.
+
+For different statistic type, the flow is similar, but not exactly the same. So, there are special cases shall be handled in implementation:
+
+- Some statistic type query statistic capability via SAI API, while some do not. Need a flag in BaseCounterContext to handle this.
+- Some statistic type always query statistic capability, while some only do it once. Need a flag in BaseCounterContext to handle this.
+- Some statistic type use `getStats` querying statistic data, while some use `getStatsExt`. Need a flag in BaseCounterContext to handle this.
+- Some statistic support per counter object stats mode, while some do not. Plan to use `if constexpr` to handle this.
+
+##### Remove Counter Object
+
+Currently, FlexCounter just remove counter object from m_XXXCounterIdsMap. In the new design, the flow/function is the same, but remove operation is moved to CounterContext/AttrContext. In new design, flow is like this:
+
+.
+
+##### Handle Plugin Change
+
+Currently, FlexCounter just add plugin to m_XXXPlugins. In new design, the flow/function is the same, but add operation is moved to BaseCounterContext. In new design, the flow is like this:
+
+.
+
+##### Handle Statistic/Attribute Collect
+
+Currently, FlexCounter collect counters by saving collectXXXCounters function pointer to a map and loop the map. In new design, that map is not needed anymore. New way:
+
+```cpp
+void FlexCounter::collectCounters(
+ _In_ swss::Table &countersTable)
+{
+ for (const auto &it : m_counterContext)
+ {
+ it->second->collectData(countersTable);
+ }
+
+ countersTable.flush();
+}
+```
+
+Currently, FlexCounter run redis plugins by looping each m_XXXPlugins one by one. New way:
+
+```cpp
+void FlexCounter::runPlugins(
+ _In_ swss::DBConnector& counters_db)
+{
+ const std::vector argv =
+ {
+ std::to_string(counters_db.getDbId()),
+ COUNTERS_TABLE,
+ std::to_string(m_pollInterval)
+ };
+
+ for (const auto &it : m_counterContext)
+ {
+ it->second->runPlugin(counters_db, argv);
+ }
+}
+```
+
+### SAI API
+
+N/A
+
+### Configuration and management
+
+N/A
+
+### Warmboot and Fastboot Design Impact
+
+N/A
+
+### Restrictions/Limitations
+
+N/A
+
+### Testing Requirements/Design
+
+#### Unit Test cases
+
+TBD
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