Operating ISUCON Servers via SSH

This skill is for the case where you want to just SSH into a contest box and run a command. No Makefile wrapper, no playbook — just ssh isuN '<cmd>'.

Use this skill when the request is a one-off operation on a contest server: status check, tailing logs, opening a MySQL shell, running kataribe / pt-query-digest, restarting one daemon, copying a file, getting a profile, or any other ad-hoc work over SSH.

Supporting files:

• SSH-SETUP.md — the two prerequisites: Host isuN aliases and SSH agent forwarding.
• USE-CASES.md — multi-step runbooks (bootstrap, bench prep, profiling, slow-query loop, hotfix, role reassignment, cross-host MySQL, etc.) expressed as raw ssh sequences.

Prerequisites

1. ssh isu1 / isu2 / isu3 connects without prompting.
2. ssh isu1 'ssh -T git@github.com' succeeds (your local agent is loaded and forwarded).

Fix both before running anything else: see SSH-SETUP.md.

Inventory

The isucon-ansible hosts file groups the boxes by role. Default layout:

[app]      isu1, isu2          ← app server (Go binary, debug port 6060)
[mysql]    isu1                ← DB
[nginx]    isu1, isu2          ← reverse proxy
[active:children] app, mysql, nginx

Use this when deciding which host to ssh into. host_vars/isuN carries the IP / user / key for each alias — ~/.ssh/config should mirror those.

Server Layout (paths and service names)

The canonical paths used by group_vars/all/var.yaml — verify against your contest:

For your contest's values (different binary name, different DB name, different paths), grep the inventory:

grep -E 'service|connection|main_conf|log:|directory' group_vars/all/var.yaml

Quick One-Liners

# Service status
ssh isu1 'systemctl is-active nginx mysql isuride-go'

# Recent app log
ssh isu1 'sudo journalctl -e -u isuride-go --no-pager -n 200'

# Follow app log (Ctrl+C to stop)
ssh isu1 'sudo journalctl -ef -u isuride-go'

# Restart the app
ssh isu1 'sudo systemctl restart isuride-go'

# Validate nginx config, then reload
ssh isu1 'sudo nginx -t && sudo systemctl reload nginx'

# Open a MySQL shell (-t required for the prompt)
ssh -t isu1 'mysql -h127.0.0.1 -uisucon -pisucon isuride'

# Kataribe over the latest access log
ssh isu1 'sudo cat /var/log/nginx/access.log | kataribe -f /home/isucon/kataribe.toml'

# Slow-query summary
ssh isu1 'sudo pt-query-digest /var/log/mysql/slow-query.log | head -100'

# Build & restart the app
ssh isu1 'cd /home/isucon/webapp/go && go build -o isuride -ldflags "-s -w" . && sudo systemctl restart isuride-go'

# Quick health check
ssh isu1 'curl -sS -o /dev/null -w "%{http_code} %{time_total}s\n" http://localhost/api/healthz'

More by category and longer runbooks: USE-CASES.md.

SSH Patterns

Multi-line scripts (heredoc)

When a one-liner gets ugly, ship a script:

ssh isu1 'bash -s' <<'EOF'
set -euo pipefail
cd /home/isucon/webapp/go
go build -o isuride -ldflags "-s -w" .
sudo systemctl restart isuride-go
sudo journalctl -e -u isuride-go --since "10 seconds ago"
EOF

Quote the heredoc tag ('EOF', with single quotes) so your laptop doesn't expand $VARS before they reach the remote.

Port forwarding (pprof / debug ports)

# Background tunnel from laptop:6061 → isu1:6060
ssh -fN -L 6061:localhost:6060 isu1

# CPU profile through the tunnel
go tool pprof -http=:8889 'http://localhost:6061/debug/pprof/profile?seconds=30'

# Close it when done
pkill -f 'ssh -fN -L 6061'

File transfer

# Pull (e.g. backport a hand-edited config)
scp isu1:/etc/nginx/nginx.conf ./nginx/nginx.conf

# Push to a sudo-owned path — two-step
scp ./nginx.conf isu1:/tmp/nginx.conf
ssh isu1 'sudo install -m 0644 /tmp/nginx.conf /etc/nginx/nginx.conf && sudo nginx -t && sudo systemctl reload nginx'

# Sync a directory
rsync -av ./public/ isu1:/tmp/public-new/

Multi-host

# Sequential
for n in 1 2 3; do ssh isu$n 'systemctl is-active isuride-go'; done

# Parallel
for n in 1 2 3; do ssh isu$n 'uptime' & done; wait

# Via ansible (better for sudo / templated args / many hosts)
ansible -i hosts active -m shell -a 'sudo journalctl -u nginx --since "5 minutes ago"'

Persistent shell for long ops

ssh -t isu1 'tmux new -A -s work'
# detach with Ctrl-b d, reattach with the same command later

Connection multiplexing (faster repeated calls)

The ansible.cfg ships ControlMaster auto / ControlPersist 5 for ansible runs only. To get the same behaviour for ad-hoc ssh from your laptop, add to ~/.ssh/config:

Host isu*
    ControlMaster auto
    ControlPath ~/.ssh/cm-%r@%h:%p
    ControlPersist 60s

With that in place, the second ssh isu1 ... reuses the first's TCP connection and skips re-authentication.

Pitfalls

• Add -t for anything interactive. mysql, tmux, vim, htop need a PTY. Without it: "the input device is not a TTY" or weirdly line-buffered output.
• Quoting traps. ssh isu1 "echo $HOME" expands $HOME on your laptop. Use single quotes (or escape) when you want the remote shell to evaluate. ssh isu1 'echo $HOME' prints /home/isucon.
• Pipes split between local and remote. ssh isu1 "sudo cat /var/log/nginx/access.log" | kataribe runs kataribe locally; ssh isu1 'sudo cat /var/log/nginx/access.log | kataribe -f /home/isucon/kataribe.toml' runs everything on the remote.
• Redirection direction. ssh isu1 'sudo nginx -T' > out.txt writes on your laptop. ssh isu1 'sudo nginx -T > /tmp/out.txt' writes on the remote.
• sudo over SSH works because the contest user has NOPASSWD. If a command unexpectedly hangs, suspect a sudo password prompt — add -t so you can type it.
• Agent forwarding is required for git on the remote. Anything that does git pull/push against the private repo from inside the box uses your forwarded agent.
• ssh isu1 'cd /foo && bar' — cd only affects that one ssh invocation; the next call starts in $HOME again. State doesn't persist between calls. Use heredocs when you need it to.
• No ~ expansion in remote-side single-quoted commands that contain ~/path if the variable is interpolated by your local shell. Prefer absolute paths or $HOME (single-quoted, expanded remotely).

When to reach for the Makefile wrapper

isucon-ansible ships a Makefile that wraps the same SSH layer:

make REMOTE_ID=1 log               # ≡ ssh isu1 'sudo journalctl -e -u isuride-go'
make REMOTE_ID=1 pull replace build restart   # multi-step bench prep

Use it when the operation is a recurring chain (bench prep, deploy across all hosts). Use raw SSH when the operation is a one-off, an investigation, or doesn't fit any target. The two are interchangeable for individual ops — pick whichever is shorter to type.

Resources

• Connection setup: SSH-SETUP.md
• Multi-step runbooks (raw SSH): USE-CASES.md