Adds "when not to" section

Addresses #1

README.md

@@ -21,18 +21,24 @@ If you don't really now what a middleware is, be at ease, nobody really knows.
 Nowadays, you may have eared of their latest avatar: *web services*.
 As our programmer is going to realize, one now have *two* problems.
 The burden of writing, using, and maintaining code using middleware is always huge.
-Because they are made to handle a **tremendous** number of complex situations,
+Because they are made to handle a **tremendous** number of features and complex situations
-most of which involve adversary users, users being bots, or both.
+—most of which involve adversary users, users being bots, or both.
 
 But most of the time, the actual problem does not really involve these situations.
-At least not at the beginning (which means probably never).
+At least not at the beginning (… which means probably never).
-If you are building up (firsts versions of) communicating programs
+People familiar with middleware history would argue that their key feature
+is not just *messages passing* but *remote call*, which involves *object serialization*.
+But most of the time, the messages are pretty simple anyway, and using a middleware
+to implement a serialization of a list of instances having three members of fundamental types
+is not a good use of your time.
+
+If you are building up (firsts versions of) two communicating programs
 that will run on a (safe) local network,
-and for which the exchanged messages are known,
+and for which the exchanged messages are known and simple,
 then I have good news:
 **you don't have to use web services** (or any kind of middleware).
 
-**You just need to know how to read/write from/to (special) files**.
+**YOU JUST NEED TO KNOW HOW TO READ/WRITE FROM/TO (SPECIAL) FILES**.
 
 
 ### Overview
@@ -49,7 +55,7 @@ in which you read/write.
 
 Once you made your service on top of named pipes,
 it is easy to wrap it within an interface made with other languages/tools.
-For instance, it is very easy to expose it on the network using common tools like `socat`.
+For instance, it is very easy to expose it on the network using common tools like `socat` (see below).
 
 Be warned that this is not secure, though, you should only use this for testing
 purpose in a secured local network.
@@ -67,13 +73,13 @@ The theoretical principle can be represented by this UML sequence diagram:
     │      │       │       │
     │      │       │┌──────╢
     │      │  block││ wait ║
-    │ask   │       │└─────→║
+    │ask   │       │└─────>║
-    ├─────────────→│       │
+    ├─────────────>│       │
-    ╟─────┐│       ├──────→│
+    ╟─────┐│       ├──────>│
     ║wait ││block  │       ║process
-    ║←────┘│       │       ║
+    ║<────┘│       │       ║
-    │      │←──────────────┤
+    │      │<──────────────┤
-    │←─────┤       │   tell│
+    │<─────┤       │   tell│
     │      │       │       │
 ```
 
@@ -84,6 +90,29 @@ Notes:
   for the sake of simplicity, but you may just as well use only one.
 
 
+### When NOT to use named pipes
+
+To be completely honest, here are a list of cases that —**if they are all true**—
+may lead you to consider that maybe it would be a good idea
+to think about how you may eventually end up
+looking for a solution that might be something that's close to a middleware:
+
+- ☒ your service takes time to compute something,
+- ☒ you have one service, but an unknown (large) number of clients,
+- ☒ all clients expect the same interface,
+- ☒ which involves answering to the server,
+- ☒ with *complex* data structures,
+- ☒ you absolutely need to serve them all as fast as possible,
+- ☒ over the internet,
+- ☒ and you are *certain* that no one will want *another* middleware in the next project.
+
+I your use case don't match all of this checklist but you still want to
+use a middleware, maybe you should just consider making a side software
+that will expose/transliterate the data going through the named pipe.
+That way, your service stays simple and you can easily
+exchange one middleware for another without even touching it.
+
+
 Build and run
 -------------
 

service1.py

@@ -0,0 +1,21 @@
+#!/usr/bin/env python
+
+import sys
+
+if __name__ == "__main__":
+    print("Start server")
+
+    while True:
+        with open(sys.argv[1]) as fin:
+            datas = fin.readline()
+
+        data = datas.strip()
+        print("Received: <",data,">", file=sys.stderr)
+
+        with open(sys.argv[2], 'w') as fout:
+            fout.write(data)
+
+        if data == "exit":
+            break
+
+    print("Stop server", file=sys.stderr)

service2.py

@@ -0,0 +1,93 @@
+#!/usr/bin/env python
+
+from enum import Enum
+import threading
+import stat
+import sys
+import os
+
+class ERROR(Enum):
+    NOT_FIFO = 1
+
+class Service:
+    def __init__(self, context: str, data: str, out: str) -> None:
+        self._has_current_context: bool = False
+        self._mutex = threading.Lock()
+        self._file_current_context: str = context
+        self._file_data: str = data
+        self._out: str = out
+        self._current_context: str = ""
+
+    def get_has_current_context(self) -> bool:
+        return self._has_current_context
+
+    def set_has_current_context(self, flag: bool) -> None:
+        self._mutex.acquire()
+        self._has_current_context = flag
+        self._mutex.release()
+
+    def update_current_context(self) -> None:
+        while True:
+            print("Wait for context...", file = sys.stderr)
+            has_error: bool = False
+            try:
+                with open(self._file_current_context) as if_current_context:
+                    self._current_context: str = if_current_context.readline().strip()
+            except:
+                has_error = True
+
+            if not has_error:
+                self.set_has_current_context(True)
+                print("\tReceived context:", self._current_context, file=sys.stderr)
+
+    def handle_data(self) -> None:
+        while True:
+            if self.get_has_current_context():
+                print("Wait for data...", file=sys.stderr)
+                has_error: bool = False
+                try:
+                    with open(self._file_data) as if_data:
+                        data: str = if_data.readline().strip()
+                except:
+                    has_error = True
+
+                if not has_error:
+                    print("\tReceived data:",data, file=sys.stderr)
+
+                    print("Do stuff...", file=sys.stderr)
+                    result: str = self._current_context + ":" + data
+                    print("\tdone", file=sys.stderr)
+
+                    print("Output...", file=sys.stderr)
+                    with open(self._out, 'w') as out:
+                        out.write(result)
+
+                    print("\tdone", file=sys.stderr)
+
+def is_named_pipe_fifo(filename: str):
+    st = os.stat(filename)
+    return stat.S_ISFIFO(st.st_mode)
+
+if __name__ == "__main__":
+
+    assert(len(sys.argv) == 4)
+
+    for i in range(1,4):
+        if not is_named_pipe_fifo(sys.argv[i]):
+            print("ERROR:", sys.argv[i], "is not a named pipe FIFO", file=sys.stderr)
+            sys.exit(ERROR.NO_FIFO)
+
+    print("Start server", file=sys.stderr, flush=True)
+    server = Service(sys.argv[1], sys.argv[2], sys.argv[3])
+
+    do_current_context = threading.Thread( target = server.update_current_context )
+    do_tasks           = threading.Thread( target = server.handle_data )
+
+    do_current_context.start()
+    do_tasks.start()
+
+    do_current_context.join()
+    do_tasks.join()
+
+    print("End", file=sys.stderr)
+