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uncloud_etcd_based/docs/source/theory/summary.rst

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+Summary
+=======
+
+.. image:: /images/ucloud.svg
+
+.. code-block::
+
+                              <cli>
+                                |
+                                |
+                                |
+    +-------------------------<api>
+    |                           |
+    |           |```````````````|```````````````|
+    |           |               |               |
+    |     <file_scanner>    <scheduler>   <image_scanner>
+    |                           |
+    |                           |
+    +-------------------------<host>
+                                |
+                                |
+                                |
+                         Virtual Machine------<init>------<metadata>
+
+
+
+**ucloud-cli** interact with **ucloud-api** to do the following operations:
+
+- Create/Delete/Start/Stop/Migrate/Probe (Status of) Virtual Machines
+- Create/Delete Networks
+- Add/Get/Delete SSH Keys
+- Create OS Image out of a file (tracked by file_scanner)
+- List User's files/networks/vms
+- Add Host
+
+ucloud can currently stores OS-Images on
+
+* File System
+* `CEPH <https://ceph.io/>`_
+
+
+**ucloud-api** in turns creates appropriate Requests which are taken
+by suitable components of ucloud. For Example, if user uses ucloud-cli
+to create a VM, **ucloud-api** would create a **ScheduleVMRequest** containing
+things like pointer to VM's entry which have specs, networking
+configuration of VMs.
+
+**ucloud-scheduler** accepts requests for VM's scheduling and
+migration. It finds a host from a list of available host on which
+the incoming VM can run and schedules it on that host.
+
+**ucloud-host** runs on host servers i.e servers that
+actually runs virtual machines, accepts requests
+intended only for them. It creates/delete/start/stop/migrate
+virtual machines. It also arrange network resources needed for the
+incoming VM.
+
+**ucloud-filescanner** keep tracks of user's files which would be needed
+later for creating OS Images.
+
+**ucloud-imagescanner** converts images files from qcow2 format to raw
+format which would then be imported into image store.
+
+* In case of **File System**, the converted image would be copied to
+  :file:`/var/image/` or the path referred by :envvar:`IMAGE_PATH`
+  environement variable mentioned in :file:`/etc/ucloud/ucloud.conf`.
+
+* In case of **CEPH**, the converted image would be imported into
+  specific pool (it depends on the image store in which the image
+  belongs) of CEPH Block Storage.
+
+**ucloud-metadata** provides metadata which is used to contextualize
+VMs. When, the VM is created, it is just clone (duplicate) of OS
+image from which it is created. So, to differentiate between my
+VM and your VM, the VM need to be contextualized. This works
+like the following
+
+.. note::
+    Actually, ucloud-init makes the GET request. You can also try it
+    yourself using curl but ucloud-init does that for yourself.
+
+* VM make a GET requests http://metadata which resolves to actual
+  address of metadata server. The metadata server looks at the IPv6
+  Address of the requester and extracts the MAC Address which is possible
+  because the IPv6 address is
+  `IPv6 EUI-64 <https://community.cisco.com/t5/networking-documents/understanding-ipv6-eui-64-bit-address/ta-p/3116953>`_.
+  Metadata use this MAC address to find the actual VM to which it belongs
+  and its owner, ssh-keys and much more. Then, metadata return these
+  details back to the calling VM in JSON format. These details are
+  then used be the **ucloud-init** which is explained next.
+
+**ucloud-init** gets the metadata from **ucloud-metadata** to contextualize
+the VM. Specifically, it gets owner's ssh keys (or any other keys the
+owner of VM added to authorized keys for this VM) and put them to ssh
+server's (installed on VM) authorized keys so that owner can access
+the VM using ssh. It also install softwares that are needed for correct
+behavior of VM e.g rdnssd (needed for `SLAAC <https://en.wikipedia.org/wiki/IPv6#Stateless_address_autoconfiguration_(SLAAC)>`_).
+