Add readme.md for database shim layer

Add some documentation related to how the mongo backend is
implemented

Issue-ID: MULTICLOUD-666
Change-Id: I9dae429b1da004fbaa7ea694e6e58fa2d337a042
Signed-off-by: Kiran Kamineni <kiran.k.kamineni@intel.com>

1 files changed, 123 insertions, 0 deletions

diff --git a/src/k8splugin/internal/db/README.md b/src/k8splugin/internal/db/README.md
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+# Database Abstraction Layer
+
+This package contains implementations of the Database interface defined in `store.go`
+Any database can be used as the backend as long as the following interface is implemented;
+
+```go
+type Store interface {
+	// Returns nil if db health is good
+	HealthCheck() error
+
+	// Unmarshal implements any unmarshaling needed for the database
+	Unmarshal(inp []byte, out interface{}) error
+
+	// Creates a new master table with key and links data with tag and
+	// creates a pointer to the newly added data in the master table
+	Create(table string, key Key, tag string, data interface{}) error
+
+	// Reads data for a particular key with specific tag.
+	Read(table string, key Key, tag string) ([]byte, error)
+
+	// Update data for particular key with specific tag
+	Update(table string, key Key, tag string, data interface{}) error
+
+	// Deletes a specific tag data for key.
+	// TODO: If tag is empty, it will delete all tags under key.
+	Delete(table string, key Key, tag string) error
+
+	// Reads all master tables and data from the specified tag in table
+	ReadAll(table string, tag string) (map[string][]byte, error)
+}
+```
+
+Therefore, `mongo.go`, `consul.go` implement the above interface and can be used as the backend as needed based on initial configuration.
+
+## Details on Mongo Implementation
+
+`mongo.go` implements the above interface using the `go.mongodb.org/mongo-driver` package.
+The code converts incoming binary data and creates a new document in the database.
+
+### Create
+
+Arguments:
+```go
+collection string
+key interface
+tag string
+data []byte
+```
+
+Create inserts the provided `data` into the `collection` which returns an auto-generated (by `mongodb`) ID which we then associate with the `key` that is provided as one of the arguments.
+
+We use the `FindOneAndUpdate` mongo API to achieve this with the `upsert` option set to `true`.
+We create the following documents in mongodb for each new definition added to the database:
+
+There is a Master Key document that contains references to other documents which are related to this `key`.
+
+#### Master Key Entry
+```json
+{
+    "_id" : ObjectId("5e0a8554b78a15f71d2dce7e"),
+    "key" : { "rbname" : "edgex", "rbversion" : "v1"},
+    "defmetadata" : ObjectId("5e0a8554be261ecb57f067eb"),
+    "defcontent" : ObjectId("5e0a8377bcfcdd0f01dc7b0d")
+}
+```
+#### Metadata Key Entry
+```json
+{
+    "_id" : ObjectId("5e0a8554be261ecb57f067eb"),
+    "defmetadata" : { "rbname" : "edgex", "rbversion" : "v1", "chartname" : "", "description" : "", "labels" : null }
+}
+```
+#### Definition Content
+```json
+{
+    "_id" : ObjectId("5e0a8377bcfcdd0f01dc7b0d"),
+    "defcontent" : "H4sICCVd3FwAA3Byb2ZpbGUxLnRhcgDt1NEKgjAUxvFd7ylG98aWOsGXiYELxLRwJvj2rbyoIPDGiuD/uzmwM9iB7Vvruvrgw7CdXHsUn6Ejm2W3aopcP9eZLYRJM1voPN+ZndAm16kVSn9onheXMLheKeGqfdM0rq07/3bfUv9PJUkiR9+H+tSVajRymM6+lEqN7njxoVSbU+z2deX388r9nWzkr8fGSt5d79pnLOZfm0f+dRrzb7P4DZD/LyDJAAAAAAAAAAAAAAAA/+0Ksq1N5QAoAAA="
+}
+```
+
+### Unmarshal
+
+Data in mongo is stored as `bson` which is a compressed form of `json`. We need mongo to convert the stored `bson` data to regular `json`
+that we can use in our code when returned.
+
+We just use the `bson.Unmarshal` API to achieve this.
+
+### Read
+
+Arguments:
+```go
+collection string
+key interface
+tag string
+```
+
+Read is straight forward and it uses the `FindOne` API to find our Mongo document based on the provided `key` and then gets the corresponding data for the given `tag`. It will return []byte which can then be passed to the `Unmarshal` function to get the desired GO object.
+
+### Delete
+
+Delete is similar to Read and deletes all the objectIDs being stored for a given `key` in the collection.
+
+## Testing Interfaces
+
+The following interface exists to allow for the development of unit tests which don't require mongo to be running.
+It is mentioned so in the code as well.
+
+```go
+// MongoCollection defines the a subset of MongoDB operations
+// Note: This interface is defined mainly for mock testing
+type MongoCollection interface {
+	InsertOne(ctx context.Context, document interface{},
+		opts ...*options.InsertOneOptions) (*mongo.InsertOneResult, error)
+	FindOne(ctx context.Context, filter interface{},
+		opts ...*options.FindOneOptions) *mongo.SingleResult
+	FindOneAndUpdate(ctx context.Context, filter interface{},
+		update interface{}, opts ...*options.FindOneAndUpdateOptions) *mongo.SingleResult
+	DeleteOne(ctx context.Context, filter interface{},
+		opts ...*options.DeleteOptions) (*mongo.DeleteResult, error)
+	Find(ctx context.Context, filter interface{},
+		opts ...*options.FindOptions) (*mongo.Cursor, error)
+}
+```
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