docs/code-flow : Add code flow documentation for add cmd.

docs/add-code-flow.md

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+# IPFS : The `Add` command demystified
+
+The goal of this document is to capture the code flow for adding a file (see the `coreapi` package) using the IPFS CLI, in the process exploring some datastructures and packages like `ipld.Node` (aka `dagnode`), `FSNode`, `MFS`, etc.
+
+**Try this yourself**
+> 
+> ```
+> # Convert a file to the IPFS format.
+> echo "Hello World" > new-file
+> ipfs add new-file
+> added QmWATWQ7fVPP2EFGu71UkfnqhYXDYH566qy47CnJDgvs8u new-file
+> 12 B / 12 B [=========================================================] 100.00%
+>
+> # Add a file to the MFS.
+> NEW_FILE_HASH=$(ipfs add new-file -Q)
+> ipfs files cp /ipfs/$NEW_FILE_HASH /new-file
+> 
+> # Get information from the file in MFS.
+> ipfs files stat /new-file
+> # QmWATWQ7fVPP2EFGu71UkfnqhYXDYH566qy47CnJDgvs8u
+> # Size: 12
+> # CumulativeSize: 20
+> # ChildBlocks: 0
+> # Type: file
+> 
+> # Retrieve the contents.
+> ipfs files read /new-file
+> # Hello World
+> ```
+
+---
+
+# Code Flow
+
+**[`UnixfsAPI.Add()`](https://github.com/ipfs/go-ipfs/blob/v0.4.18/core/coreapi/unixfs.go#L31)** - *Entrypoint into the `Unixfs` package*
+
+The `UnixfsAPI.Add()` acts on the input data or files, to build a _merkledag_ node (in essence it is the entire tree represented by the root node) and adds it to the _blockstore_.
+Within the function, a new `Adder` is created with the configured `Blockstore` and __DAG service__`. 
+
+- **[`adder.AddAllAndPin(files)`](https://github.com/ipfs/go-ipfs/blob/v0.4.18/core/coreunix/add.go#L403)** - *Entrypoint to the `Add` logic*
+    encapsulates a lot of the underlying functionality that will be investigated in the following sections. 
+
+    Our focus will be on the simplest case, a single file, handled by `Adder.addFile(file files.File)`. 
+
+  - **[`adder.addFile(file files.File)`](https://github.com/ipfs/go-ipfs/blob/v0.4.18/core/coreunix/add.go#L450)** - *Create the _DAG_ and add to `MFS`*
+
+      The `addFile(file)` method takes the data and converts it into a __DAG__ tree and adds the root of the tree into the `MFS`.
+
+      https://github.com/ipfs/go-ipfs/blob/v0.4.18/core/coreunix/add.go#L508-L521
+
+      There are two main methods to focus on -
+
+      1. **[`adder.add(io.Reader)`](https://github.com/ipfs/go-ipfs/blob/v0.4.18/core/coreunix/add.go#L115)** - *Create and return the **root** __DAG__ node*
+
+          This method converts the input data (`io.Reader`) to a __DAG__ tree, by splitting the data into _chunks_ using the `Chunker` and organizing them in to a __DAG__ (with a *trickle* or *balanced* layout. See [balanced](https://github.com/ipfs/go-unixfs/blob/6b769632e7eb8fe8f302e3f96bf5569232e7a3ee/importer/balanced/builder.go) for more info). 
+
+          The method returns the **root** `ipld.Node` of the __DAG__.
+
+      2. **[`adder.addNode(ipld.Node, path)`](https://github.com/ipfs/go-ipfs/blob/v0.4.18/core/coreunix/add.go#L366)** - *Add **root** __DAG__ node to the `MFS`*
+
+          Now that we have the **root** node of the `DAG`, this needs to be added to the `MFS` file system. 
+          Fetch (or create, if doesn't already exist) the `MFS` **root** using `mfsRoot()`. 
+
+          > NOTE: The `MFS` **root** is an ephemeral root, created and destroyed solely for the `add` functionality.
+
+          Assuming the directory already exists in the MFS file system, (if it doesn't exist it will be created using `mfs.Mkdir()`), the **root** __DAG__ node is added to the `MFS` File system using the `mfs.PutNode()` function.
+
+          - **[MFS] [`PutNode(mfs.Root, path, ipld.Node)`](https://github.com/ipfs/go-mfs/blob/v0.1.18/ops.go#L86)** - *Insert node at path into given `MFS`*
+
+              The `path` param is used to determine the `MFS Directory`, which is first looked up in the `MFS` using `lookupDir()` function. This is followed by adding the **root** __DAG__ node (`ipld.Node`) in to this `Directory` using `directory.AddChild()` method.
+
+          - **[MFS] Add Child To `UnixFS`**
+            - **[`directory.AddChild(filename, ipld.Node)`](https://github.com/ipfs/go-mfs/blob/v0.1.18/dir.go#L350)** - *Add **root** __DAG__ node under this directory*
+
+                Within this method the node is added to the `Directory`'s __DAG service__ using the `dserv.Add()` method, followed by adding the **root** __DAG__ node with the given name, in the `directory.addUnixFSChild(directory.child{name, ipld.Node})` method.
+
+            - **[MFS] [`directory.addUnixFSChild(child)`](https://github.com/ipfs/go-mfs/blob/v0.1.18/dir.go#L375)** - *Add child to inner UnixFS Directory*
+
+                The node is then added as a child to the inner `UnixFS` directory using the `(BasicDirectory).AddChild()` method.
+
+                > NOTE: This is not to be confused with the `directory.AddChild(filename, ipld.Node)`, as this operates on the `UnixFS` `BasicDirectory` object.
+
+            - **[UnixFS] [`(BasicDirectory).AddChild(ctx, name, ipld.Node)`](https://github.com/ipfs/go-unixfs/blob/v1.1.16/io/directory.go#L137)** - *Add child to `BasicDirectory`*
+
+                > IMPORTANT: It should be noted that the `BasicDirectory` object uses the `ProtoNode` type object which is an implementation of the `ipld.Node` interface, seen and used throughout this document. Ideally the `ipld.Node` should always be used, unless we need access tp specific functions from `ProtoNode` (like `Copy()`) that are not available in the interface.
+
+                This method first attempts to remove any old links (`ProtoNode.RemoveNodeLink(name)`) to the `ProtoNode` prior to adding a link to the newly added `ipld.Node`, using `ProtoNode.AddNodeLink(name, ipld.Node)`.
+
+                - **[Merkledag] [`AddNodeLink()`](https://github.com/ipfs/go-merkledag/blob/v1.1.15/node.go#L99)**
+
+                  The `AddNodeLink()` method is where an `ipld.Link` is created with the `ipld.Node`'s `CID` and size in the `ipld.MakeLink(ipld.Node)` method, and is then appended to the `ProtoNode`'s links in the `ProtoNode.AddRawLink(name)` method.
+
+  - **[`adder.Finalize()`](https://github.com/ipfs/go-ipfs/blob/v0.4.18/core/coreunix/add.go#L200)** - *Fetch and return the __DAG__ **root** from the `MFS` and `UnixFS` directory*
+
+      The `Finalize` method returns the `ipld.Node` from the `UnixFS` `Directory`.
+
+  - **[`adder.PinRoot()`](https://github.com/ipfs/go-ipfs/blob/v0.4.18/core/coreunix/add.go#L171)** - *Pin all files under the `MFS` **root***
+
+    The whole process ends with `PinRoot` recursively pinning all the files under the `MFS` **root**
+
+# Also see
+1. https://github.com/ipfs/go-ipfs/#development
+2. [Concept document about files](https://github.com/ipfs/docs/issues/133)