Factor: server programming challenges

An TCP based echo server.

One it's extra requirements

• with no data, it should immediately close connection.
• it should handle half-duplex shutdowns, terminating it, instead of passing on the half-duplex shutdown

A server that talks JSON, accepting multiple requests in json format per connection.

$ { jo method=isPrime number=3; jo method=isP; jo method=isPrime number=10; } | nc 127.0.0.1 1234
{"method":"isPrime","prime":true}
{"error":"r2r failed!"}
{"method":"isPrime","prime":false}

A per connection, in-memory db, controlled by a custom network binary protocol.

Byte:  |   0   |  1     2     3     4  |  5     6     7     8  |
Type:  |  char |         int32 (BE)    |         int32 (BE)    |
Value: |  'I'  |       timestamp       |         price         |
Value: |  'Q'  |        mintime        |        maxtime        |

Lessons learned

• You can only print byte-arrays on a binary encoded threaded-server. Nothing else or it will silently fail.
• Read the docs about binary streams.
• You can capture tcp traffic data with Wireshark to later replay it for a way to test your code offline.

A TCP based ascii chatroom. With nicks and message status on join/leave.

Unlike the other ones I did this one offline. No second tries.

Luckly Factor has the necessary primitives for this. I choose mailboxes and threads. There are also channels available but, inlike mailboxes they are 2sides blocking.

An UDP based key/value store.

Can't use <threaded-server> with UDP. But found an article implementing a simple udp server.

Unfortunately Factor seems unable to receive empty UDP packets/datagrams. Which are a requirement for this challenge.

An evil tcp proxy for the chat protocol made on exercise #3. It rewrites some very specific text content.

Hit a couple of "fun" challenges.

• Ensuring both sides of the connection get closed correctly. For that I learned about Factor's destructors.
• TODO Ensuring messages are separated by a newline. This is a more strict requirement than on exercise #3. Seems like is not possible by readln.
  • Golang's bufio ReadString('\n') handles this a bit more correctly as it threats EOF without \n as an error.
  • See $ echo -n foobarbaz | socat - TCP:127.0.0.1:4321.
• My code still pollutes process stdout with exceptions for closed sockets exceptions not handled…but YOLO.

External access setup using pinggy.io.

• For TCP: ssh -p 443 -R0:127.0.0.1:1234 qr+tcp@free.pinggy.io
• For UDP:
  • Tried pinggy: pinggy -p 443 -R0:127.0.0.1:1234 udp@free.pinggy.io
  • But doesn't seem to work for the edge-case of empty UDP packets. I get an ICMP port unreachable for it.

• For TCP/UDP: nc or socat. I prefer socat due being non-interactive.

  $ echo -n "" | nc 127.0.0.1 1234
  $ echo -n "" | socat - TCP:127.0.0.1:1234
  $ echo -n "" | socat - UDP:127.0.0.1:1234
  

• For UDP: I found nmap or nping useful to craft custom packets and see it's output.

  • nping: in particular, can show the data sent and received back in one command.

  $ sudo nping -v3 --bpf-filter='udp and dst port 53' -c 1 --data-string="foo" --udp -p 1234 127.0.0.1
  
  Starting Nping 0.7.80 ( https://nmap.org/nping ) at 2025-11-13 14:12 -03
  SENT (0.0107s) UDP [127.0.0.1:53 > 127.0.0.1:1234 len=11 csum=0x275F] IP [ver=4 ihl=5 tos=0x00 iplen=31 id=21414 foff=0 ttl=64 proto=17 csum=0x2926]
  0000   45 00 00 1f 53 a6 00 00  40 11 29 26 7f 00 00 01  E...S...@.)&....
  0010   7f 00 00 01 00 35 04 d2  00 0b 27 5f 66 6f 6f     .....5....'_foo
  RCVD (0.0111s) UDP [127.0.0.1:1234 > 127.0.0.1:53 len=15 csum=0xFE22] IP [ver=4 ihl=5 tos=0x00 iplen=35 id=48370 flg=D foff=0 ttl=64 proto=17 csum=0x7fd5]
  0000   45 00 00 23 bc f2 40 00  40 11 7f d5 7f 00 00 01  E..#..@.@.......
  0010   7f 00 00 01 04 d2 00 35  00 0f fe 22 66 6f 6f 3d  .......5..."foo=
  0020   62 61 72                                          bar
  
  Max rtt: N/A | Min rtt: N/A | Avg rtt: N/A
  Raw packets sent: 1 (31B) | Rcvd: 1 (35B) | Lost: 0 (0.00%)
  Tx time: 0.00115s | Tx bytes/s: 26979.98 | Tx pkts/s: 870.32
  Rx time: 1.00159s | Rx bytes/s: 34.94 | Rx pkts/s: 1.00
  Nping done: 1 IP address pinged in 1.02 seconds