update

20210323005824-distributed_systems.org

@@ -3,8 +3,51 @@
 A [[file:20210310182044-software_architecture.org][Software Architecture]] consisting of multiple independant pieces of software
 coordinating via message passing.
 
+* A "Monolithic" Web application as a distributed system
+
+A monolith presents itself as a single black box with no external dependencies or race conditions to contend with. But, is it?
+
+- There's probably some sort of database to store state.
+- If it's a web application, its clients are distributed!
+
+#+begin_src plantuml :file distributed-web-application.svg :exports results
+  actor Browser
+  participant WebApp
+  database Database
+
+  Browser -> WebApp : Request Page
+  WebApp -> Database : Load Data
+  Database -> WebApp : Return Data
+  WebApp -> Browser : Render Page
+  ...
+  Browser -> WebApp : Save Form
+  WebApp -> Database : Write Data
+  Database -> WebApp : Data Stored
+  WebApp -> Browser : Done!
+#+end_src
+
+#+RESULTS:
+[[file:distributed-web-application.svg]]
+
+- Myths
+  + The database is local, therefore the connection is guaranteed to be fast and
+    stable.
+  + Data is isolated to the user accessing it, there's no possibility for race
+    conditions / having to merge data.
+  + The behavior of one user won't affect the experience of another.
+- Any one of those lines of communication could fail for some reason
+  + Bad request :: Something the user sent could cause an error
+  + Bad state :: A service could be in a bad state and fail to react as
+    expected. The web server or database may be overloaded. Invalid data could
+    be persisted, breaking further requests.
+  + Network issues :: Even on the same host, the other service could be down or
+    inaccessible.
+- Any of those activities could occur /concurrently/
+  + What happens when two different users try to save changes to the same data?
+    Or the same user, from two different clients or browser tabs?
 * [[file:20210323010700-process_isolation.org][Process Isolation]]
 - Monolith vs. Services
+
 * [[file:20210323012431-concurrent_and_parallel_programming.org][Concurrent and Parallel Programming]]
 
 #+begin_quote