An LFE (Lisp Flavoured Erlang) application, designed and intended to be run as a microservice,
implementing a simple urban bus routing prototype
Rationale: This project is a direct LFE port of the earlier developed urban bus routing prototype, written in Erlang/OTP using Cowboy web server library, and tailored to be run as a microservice in a Docker container. The following description of the underlying architecture and logics has been taken from here as is, without any modifications or adjustment.
Consider an IoT system that aimed at planning and forming a specific bus route for a hypothetical passenger. One crucial part of such system is a module, that is responsible for filtering bus routes between two arbitrary bus stops where a direct route is actually present and can be easily found. Imagine there is a fictional urban public transportation agency that provides a wide series of bus routes, which covered large city areas, such that they are consisting of many bus stop points in each route. Let's name this agency Trans-RoutE-Townish Co., Ltd. or in the Net representation — transroutownish.com, hence the name of the project.
A module that is developed here is dedicated to find out quickly, whether there is a direct route in a list of given bus routes between two specified bus stops. It should immediately report back to the IoT system with the result true if such a route is found, i.e. it exists in the bus routes list, or false otherwise, by outputting a simple JSON structure using the following format:
{
"from" : <starting_bus_stop_point>,
"to" : <ending_bus_stop_point>,
"direct" : true
}
<starting_bus_stop_point> and <ending_bus_stop_point> above are bus stop IDs: unique positive integers, taken right from inputs.
A bus routes list is a plain text file where each route has its own unique ID (positive integer) and a sequence of its bus stop IDs. Each route occupies only one line in this file, so that they are all representing something similar to a list — the list of routes. The first number in a route is always its own ID. Other consequent numbers after it are simply IDs of bus stops in this route, up to the end of line. All IDs in each route are separated by whitespace, usually by single spaces or tabs, but not newline.
There are some constraints:
- Routes are considered not to be a round trip journey, that is they are operated in the forward direction only.
- All IDs (of routes and bus stops) must be represented by positive integer values, in the range
1 .. 2,147,483,647. - Any bus stop ID may occure in the current route only once, but it might be presented in any other route too.
The list of routes is usually mentioned throughout the source code as a routes data store, and a sample routes data store can be found in the data/ directory of this repo.
Since the microservice architecture for building independent backend modules of a composite system are very prevalent nowadays, this seems to be natural for creating a microservice, which is containerized and run as a daemon, serving a continuous flow of HTTP requests.
This microservice is intended to be built locally and to be run like a conventional daemon in the VM environment, as well as a containerized service, managed by Docker.
One may consider this project has to be suitable for a wide variety of applied areas and may use this prototype as: (1) a template for building a similar microservice, (2) for evolving it to make something more universal, or (3) to simply explore it and take out some snippets and techniques from it for educational purposes, etc.
The microservice is known to be built and run successfully under Ubuntu Server (Ubuntu 22.04.3 LTS x86-64). Install the necessary dependencies (erlang-nox, erlang-dev, rebar3, make, docker.io):
$ sudo apt-get update && \
sudo apt-get install erlang-nox erlang-dev make docker.io -y
...
Rebar3 is preferred to install as the following:
$ curl -sO https://s3.amazonaws.com/rebar3/rebar3 && \
chmod -v 700 rebar3 && ./rebar3 local install && \
export PATH=/home/<username>/.cache/rebar3/bin:$PATH && \
rm -vf rebar3
...
The LFE (Lisp Flavoured Erlang) distribution can be downloaded and installed automatically as a dependency of the Rebar3 LFE plugin rebar3_lfe. For that to be done, it simply needs to launch the $ rebar3 command without any arguments or options. The LFE distribution will be fetched and installed into the ./_build/default/plugins/lfe/ directory.
The only prerequisite for that is to have Rebar3 config with the actual version of the Rebar3 LFE plugin specified in it: put the following into the ~/.config/rebar3/rebar.config file, then run the $ rebar3 command:
{plugins, [
{rebar3_lfe, "0.4.7"}
]}.
(In this snippet the version 0.4.7 of the LFE plugin was chosen as an example, but the latest release is generally considered to be a better choice.)
Build the microservice using Rebar3 (and its LFE plugin):
$ rebar3 lfe clean; \
rebar3 as prod lfe clean
...
$ rebar3 compile; \
rebar3 as prod compile
...
$ rebar3 lfe release; \
rebar3 as prod lfe release
...
Or build the microservice using GNU Make (optional, but for convenience — it covers the same Rebar3 build workflow under the hood):
$ make clean
...
$ make # <== Compilation phase.
...
$ make all # <== Assembling releases of the microservice.
...
The following command given is for demonstrational purposes only — Rebar3 will always fetch necessary dependencies during a one of their building phases, even at the cleaning phase:
$ rebar3 tree
===> Fetching rebar3_lfe v0.4.4
===> Fetching rebar_cmd v0.4.0
===> Fetching rebar3_hex v6.11.4
...
===> Fetching lfe v2.1.1
===> Fetching ltest v0.13.4
===> Fetching erlang_color v1.0.0
...
===> Verifying dependencies...
===> Fetching cowboy v2.10.0
===> Fetching jsx v3.1.0
===> Fetching syslog v1.1.0
===> Fetching pc v1.14.0
===> Analyzing applications...
...
└─ bus─0.3.3 (project app)
├─ cowboy─2.10.0 (hex package)
│ ├─ cowlib─2.12.1 (hex package)
│ └─ ranch─1.8.0 (hex package)
├─ jsx─3.1.0 (hex package)
└─ syslog─1.1.0 (hex package)
Build a Docker image for the microservice:
$ # Pull the Erlang image first, if not already there:
$ sudo docker pull erlang:alpine
...
$ # Then build the microservice image:
$ sudo docker build -ttransroutownish/buslfe .
...
Run the microservice using its startup script along with the foreground command, that is meant "Start release with output to stdout":
$ ./_build/prod/rel/bus/bin/bus foreground; echo $?
...
The microservice then can be stopped, again by using its startup script along with the stop command, that is meant "Stop the running node". It should be issued in another terminal session, not the current one:
$ ./_build/prod/rel/bus/bin/bus stop; echo $?
0
To identify, which commands are available and what they mean, the startup script can be run without specifying a command or arguments:
$ ./_build/prod/rel/bus/bin/bus
Usage: bus [COMMAND] [ARGS]
Commands:
foreground Start release with output to stdout
remote_console Connect remote shell to running node
...
stop Stop the running node
restart Restart the applications but not the VM
...
daemon Start release in the background with run_erl (named pipes)
...
daemon_attach Connect to node started as daemon with to_erl (named pipes)
...
Thus, to run the microservice as a daemon, in the background, the daemon command should be used instead:
$ ./_build/prod/rel/bus/bin/bus daemon; echo $?
0
The daemon_attach command then allows connecting to the microservice to make interactions with them. But the latter is not required at all regarding the true purpose of the microservice. And it can be stopped again with the stop command in the same terminal session.
Run a Docker image of the microservice, deleting all stopped containers prior to that:
$ sudo docker rm `sudo docker ps -aq`; \
export PORT=8765 && sudo docker run -dp${PORT}:${PORT} --name buslfe transroutownish/buslfe; echo $?
...
The following is not necessary but might be considered interesting — to look up into the running container, and check out that the microservice's startup script, application BEAMs, log, and routes data store are at their expected places and in effect:
$ sudo docker ps -a
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
<container_id> transroutownish/buslfe "bus/bin/bus foregro..." About an hour ago Up About an hour 0.0.0.0:8765->8765/tcp, :::8765->8765/tcp buslfe
$
$ sudo docker exec -it buslfe sh; echo $?
/var/tmp #
/var/tmp # bus/erts-14.1.1/bin/erl -version
Erlang (SMP,ASYNC_THREADS) (BEAM) emulator version 14.1.1
/var/tmp #
/var/tmp # ls -al
total 20
drwxrwxrwt 1 root root 4096 Dec 13 16:41 .
drwxr-xr-x 1 root root 4096 Nov 30 09:32 ..
drwxr-xr-x 1 root root 4096 Dec 13 16:50 bus
/var/tmp #
/var/tmp # ls -al bus/
total 32
drwxr-xr-x 1 root root 4096 Dec 13 16:50 .
drwxrwxrwt 1 root root 4096 Dec 13 16:41 ..
drwxr-xr-x 2 root root 4096 Dec 13 16:41 bin
drwxr-xr-x 3 root root 4096 Dec 13 16:41 erts-14.1.1
drwxr-xr-x 14 root root 4096 Dec 13 16:41 lib
drwxr-xr-x 2 root root 4096 Dec 13 16:50 log
drwxr-xr-x 3 root root 4096 Dec 13 16:41 releases
/var/tmp #
/var/tmp # ls -al bus/bin/ bus/lib/bus-0.3.3/ebin/ bus/lib/bus-0.3.3/priv/data/ bus/log/
bus/bin/:
total 112
drwxr-xr-x 2 root root 4096 Dec 13 16:41 .
drwxr-xr-x 1 root root 4096 Dec 13 16:50 ..
-rwxr-xr-x 1 root root 35983 Dec 13 16:41 bus
-rwxr-xr-x 1 root root 35983 Dec 13 16:41 bus-0.3.3
-rw-r--r-- 1 root root 14214 Dec 13 16:41 install_upgrade.escript
-rw-r--r-- 1 root root 6695 Dec 13 16:41 no_dot_erlang.boot
-rw-r--r-- 1 root root 7560 Dec 13 16:41 nodetool
bus/lib/bus-0.3.3/ebin/:
total 32
drwxr-xr-x 2 root root 4096 Dec 13 16:41 .
drwxr-xr-x 4 root root 4096 Dec 13 16:41 ..
-rw-r--r-- 1 root root 1934 Dec 13 16:41 aux.beam
-rw-r--r-- 1 root root 1207 Dec 13 16:41 bus-app.beam
-rw-r--r-- 1 root root 951 Dec 13 16:41 bus-controller.beam
-rw-r--r-- 1 root root 1626 Dec 13 16:41 bus-handler.beam
-rw-r--r-- 1 root root 519 Dec 13 16:41 bus-sup.beam
-rw-r--r-- 1 root root 704 Dec 13 16:41 bus.app
bus/lib/bus-0.3.3/priv/data/:
total 56
drwxr-xr-x 2 root root 4096 Dec 13 16:41 .
drwxr-xr-x 3 root root 4096 Dec 13 16:41 ..
-rw-rw-r-- 1 root root 46218 Jan 15 2023 routes.txt
bus/log/:
total 16
drwxr-xr-x 2 root root 4096 Dec 13 16:50 .
drwxr-xr-x 1 root root 4096 Dec 13 16:50 ..
-rw-r--r-- 1 root root 5480 Dec 13 16:50 bus.log
/var/tmp #
/var/tmp # netstat -plunt
Active Internet connections (only servers)
Proto Recv-Q Send-Q Local Address Foreign Address State PID/Program name
tcp 0 0 0.0.0.0:43939 0.0.0.0:* LISTEN 1/bus
tcp 0 0 0.0.0.0:8765 0.0.0.0:* LISTEN 1/bus
tcp 0 0 0.0.0.0:4369 0.0.0.0:* LISTEN 51/epmd
tcp 0 0 :::4369 :::* LISTEN 51/epmd
/var/tmp #
/var/tmp # ps ax
PID USER TIME COMMAND
1 root 0:02 {beam.smp} /var/tmp/bus/bin/bus -Bd -K true -A30 -- -root /var/tmp/bus -bindir /var/tmp/bus/er
51 root 0:00 /var/tmp/bus/erts-14.1.1/bin/epmd -daemon
79 root 0:00 [epmd]
80 root 0:00 [epmd]
113 root 0:00 erl_child_setup 1048576
132 root 0:00 sh
144 root 0:00 ps ax
/var/tmp #
/var/tmp # exit # Or simply <Ctrl-D>.
0
All the routes are contained in a so-called routes data store. It is located in the data/ directory. The default filename for it is routes.txt, but it can be specified explicitly (if intended to use another one) in the apps/bus/src/bus.app.src file.
Identify, whether there is a direct route between two bus stops with IDs given in the HTTP GET request, searching for them against the underlying routes data store:
| HTTP request param | Sample value | Another sample value | Yet another sample value |
|---|---|---|---|
from |
4838 |
82 |
2147483647 |
to |
524987 |
35390 |
1 |
The direct route is found:
$ curl 'http://localhost:8765/route/direct?from=4838&to=524987'
{"direct":true,"from":4838,"to":524987}
The direct route is not found:
$ curl 'http://localhost:8765/route/direct?from=82&to=35390'
{"direct":false,"from":82,"to":35390}
The microservice has the ability to log messages to a logfile and to the Unix syslog facility. When running under Ubuntu Server (not in a Docker container), logs can be seen and analyzed in an ordinary fashion, by tailing the _build/prod/rel/bus/log/bus.log logfile:
$ tail -f _build/prod/rel/bus/log/bus.log
...
[2023-12-12|10:35:09.383551+03:00][info] Server started on port 8765
[2023-12-12|10:35:09.383631+03:00][info] Application: bus. Started at: bus@localhost.
[2023-12-12|10:37:05.416196+03:00][debug] from=4838 | to=524987
[2023-12-12|10:37:05.416322+03:00][debug] 1 = 1 2 3 4 5 6 7 8 9 987 11 12 13 4987 415 ...
...
[2023-12-12|10:37:18.498899+03:00][debug] from=82 | to=35390
[2023-12-12|10:37:18.499017+03:00][debug] 1 = 1 2 3 4 5 6 7 8 9 987 11 12 13 4987 415 ...
...
[2023-12-12|10:40:06.647772+03:00][info] Server stopped
Messages registered by the Unix system logger can be seen and analyzed using the journalctl utility:
$ journalctl -f
...
Dec 12 10:35:08 <hostname> bus[<pid>]: Starting up
Dec 12 10:35:09 <hostname> bus[<pid>]: Server started on port 8765
Dec 12 10:37:05 <hostname> bus[<pid>]: from=4838 | to=524987
Dec 12 10:37:18 <hostname> bus[<pid>]: from=82 | to=35390
Dec 12 10:40:06 <hostname> bus[<pid>]: Server stopped
Dec 12 10:40:07 <hostname> run_erl[<pid>]: Erlang closed the connection.
Inside the running container logs might be queried also by tailing the bus/log/bus.log logfile:
/var/tmp # tail -f bus/log/bus.log
...
[2023-12-13|16:50:22.271789+00:00][info] Server started on port 8765
[2023-12-13|16:50:22.272542+00:00][info] Application: bus. Started at: bus@<container_id>.
[2023-12-13|17:00:37.153458+00:00][debug] from=4838 | to=524987
[2023-12-13|17:00:37.154489+00:00][debug] 1 = 1 2 3 4 5 6 7 8 9 987 11 12 13 4987 415 ...
...
[2023-12-13|17:00:44.935333+00:00][debug] from=82 | to=35390
[2023-12-13|17:00:44.935875+00:00][debug] 1 = 1 2 3 4 5 6 7 8 9 987 11 12 13 4987 415 ...
...
[2023-12-13|17:05:07.684377+00:00][notice] SIGTERM received - shutting down
[2023-12-13|17:05:07.686099+00:00][info] Server stopped
And of course Docker itself gives the possibility to read log messages by using the corresponding command for that:
$ sudo docker logs -f buslfe
...
[2023-12-13|16:50:22.271789+00:00][info] Server started on port 8765
[2023-12-13|16:50:22.272542+00:00][info] Application: bus. Started at: bus@<container_id>.
[2023-12-13|17:00:37.153458+00:00][debug] from=4838 | to=524987
[2023-12-13|17:00:37.154489+00:00][debug] 1 = 1 2 3 4 5 6 7 8 9 987 11 12 13 4987 415 ...
...
[2023-12-13|17:00:44.935333+00:00][debug] from=82 | to=35390
[2023-12-13|17:00:44.935875+00:00][debug] 1 = 1 2 3 4 5 6 7 8 9 987 11 12 13 4987 415 ...
...
[2023-12-13|17:05:07.684377+00:00][notice] SIGTERM received - shutting down
[2023-12-13|17:05:07.686099+00:00][info] Server stopped
When the query string passed in a request, contains inappropriate input, or the URI endpoint doesn't contain anything else at all after its path, the microservice will respond with the HTTP 400 Bad Request status code, including a specific response body in JSON representation, like the following:
$ curl 'http://localhost:8765/route/direct?from=qwerty4838&to=-i-.;--089asdf../nj524987'
{"error":"Request parameters must take positive integer values, in the range 1 .. 2,147,483,647. Please check your inputs."}
Or even simpler:
$ curl http://localhost:8765/route/direct
{"error":"Request parameters must take positive integer values, in the range 1 .. 2,147,483,647. Please check your inputs."}