Update user technical documentation

Summary: Document durability and RPM installation in user docs

Reviewers: buda, mtomic, ipaljak

Reviewed By: buda, mtomic

Differential Revision: https://phabricator.memgraph.io/D1345

docs/user_technical/README.md

@@ -14,7 +14,7 @@ data structures, multi-version concurrency control and asynchronous IO.
   * [Quick Start](quick-start.md)
   * [Examples](examples.md)
   * [Drivers](drivers.md)
-  * [Storable Data Types](data-types.md)
+  * [Data Storage](storage.md)
   * [openCypher Query Language](open-cypher.md)
   * [Import Tools](import-tools.md)
   * [Upcoming Features](upcoming-features.md)

docs/user_technical/examples.md

@@ -39,9 +39,9 @@ when starting Memgraph using the `--durability-directory` option.
   --durability-enabled=false --snapshot-on-exit=false
 ```
 
-When using Memgraph installed from DEB package, you may need to stop the
-currently running Memgraph server before you can import the example. Use the
-following command:
+When using Memgraph installed from DEB or RPM package, you may need to stop
+the currently running Memgraph server before you can import the example. Use
+the following command:
 
 ```
 systemctl stop memgraph

docs/user_technical/open-cypher.md

@@ -253,7 +253,7 @@ You can still use the `RETURN` clause to produce results after writing, but it
 is not mandatory.
 
 Details on which kind of data can be stored in *Memgraph* can be found in
-**Storable Data Types** chapter.
+**Data Storage** chapter.
 
 #### CREATE
 

docs/user_technical/quick-start.md

@@ -7,11 +7,16 @@ basic queries against the database.
 
 The Memgraph binary is offered as:
 
-  * Debian package for Debian 9 (Stretch) and
+  * Debian package for Debian 9 (Stretch);
+  * RPM package for CentOS 7 and
   * Docker image.
 
 After downloading the binary, proceed to the corresponding section below.
 
+NOTE: Currently, newer versions of Memgraph are not backward compatible with
+older versions. This is mainly noticeable by unsupported loading of storage
+snapshots between different versions.
+
 #### Docker Installation
 
 Before proceeding with the installation, please install the Docker engine on
@@ -92,6 +97,47 @@ systemctl stop memgraph
 Memgraph configuration is available in `/etc/memgraph/memgraph.conf`. After
 changing the configuration, Memgraph needs to be restarted.
 
+#### RPM Package Installation
+
+If you downloaded the RPM package of Memgraph, you can install it by running
+the following command.
+
+```
+rpm -U /path/to/memgraph-<version>.rpm
+```
+
+After the successful installation, Memgraph can be started as a service. To do
+so, type the following command.
+
+```
+systemctl start memgraph
+```
+
+To verify that Memgraph is running, run the following command.
+
+```
+journalctl --unit memgraph
+```
+
+It is expected to see something like the following output.
+
+```
+Nov 23 13:40:13 hostname memgraph[14654]: Starting 8 workers
+Nov 23 13:40:13 hostname memgraph[14654]: Server is fully armed and operational
+Nov 23 13:40:13 hostname memgraph[14654]: Listening on 0.0.0.0 at 7687
+```
+
+Memgraph is now ready to process queries, you may now proceed to
+[querying](#querying). To shutdown Memgraph server, issue the following
+command.
+
+```
+systemctl stop memgraph
+```
+
+Memgraph configuration is available in `/etc/memgraph/memgraph.conf`. After
+changing the configuration, Memgraph needs to be restarted.
+
 ### Querying
 
 Memgraph supports the openCypher query language which has been developed by
@@ -142,7 +188,7 @@ To learn more about the openCypher language, visit **openCypher Query
 Language** chapter in this document. For real-world examples of how to use
 Memgraph visit **Examples** chapter. If you wish to use a programming language
 to execute queries on Memgraph, go to the **Drivers** chapter. Details on what
-can be stored in Memgraph are in **Storable Data Types** chapter.
+can be stored in Memgraph are in **Data Storage** chapter.
 
 We *welcome and encourage* your feedback!
 

docs/user_technical/storage.md

@@ -1,3 +1,33 @@
+## Durability and Data Recovery
+
+*Memgraph* uses two mechanisms to ensure the durability of the stored data:
+
+  * write-ahead logging (WAL) and
+  * taking periodic snapshots.
+
+Write-ahead logging works by logging all database modifications to a file.
+This ensures that all operations are done atomically and provides a trace of
+steps needed to reconstruct the database state.
+
+Snapshots are taken periodically during the entire runtime of *Memgraph*. When
+a snapshot is triggered, the whole data storage is written to disk. The
+snapshot file provides a quicker way to restore the database state.
+
+Database recovery is done on startup from the most recently found snapshot
+file. Since the snapshot may be older than the most recent update logged in
+the WAL file, the recovery process will apply the remaining state changes
+found in the said WAL file.
+
+NOTE: Snapshot and WAL files are not (currently) compatible between *Memgraph*
+versions.
+
+Behaviour of the above mechanisms can be tweaked in the configuration file,
+usually found in `/etc/memgraph/memgraph.conf`.
+
+In addition to the above mentioned data durability and recovery, a
+snapshot file may be generated using *Memgraph's* import tools. For more
+information, take a look at **Import Tools** chapter.
+
 ## Storable Data Types
 
 Since *Memgraph* is a *graph* database management system, data is stored in