Booting

You are an expert in deploying applications with Kamal. This skill helps you control Kamal's boot strategy — how many hosts boot the new container at a time, and how long to pause between batches — so a deploy across a large fleet rolls out gradually instead of restarting everything at once.

When You Need This

By default, Kamal boots new containers on all hosts in parallel. That is fast and fine for small fleets. It becomes a problem when you have many servers and a simultaneous restart causes:

• A thundering-herd reconnect against a shared database, cache, or queue.
• A capacity dip while every host is briefly cycling its container.
• Load spikes from cold caches or warm-up work happening everywhere at once.

The boot configuration lets you boot hosts in groups, with an optional pause between each group, so the fleet drains and refills gradually.

How Kamal Boots by Default

On kamal deploy, Kamal builds and pushes the image, then on each host starts the new container, waits for it to pass its health check, routes traffic to it via kamal-proxy, and stops the old container. With no boot configuration, that per-host cycle happens on every host simultaneously.

The boot block changes only the pacing of that rollout across hosts — not what happens on each host.

Walk-Through: Add a Boot Strategy

Step 1 — Read the existing config

Open the user's config/deploy.yml and look at the servers: section. Note:

• How many hosts are in the deploy (count them, or run kamal config to list resolved hosts and roles).
• Whether any host carries multiple roles (e.g. both web and a workers role) — that determines whether parallel_roles is relevant.

The bigger the fleet, the more a boot limit helps. For two or three identical web servers, the default parallel boot is usually fine.

Step 2 — Choose a boot limit

Add a boot: block and set limit to the size of each batch. It can be an integer (a fixed number of hosts) or a percentage string:

boot:
  limit: 25%   # boot a quarter of the hosts at a time

boot:
  limit: 3     # boot 3 hosts at a time

Kamal boots one group of that size, finishes it, then moves to the next group until all hosts are done.

Step 3 — Add a wait between groups

If you want the fleet to settle before the next batch cycles (let caches warm, connections rebalance, alarms clear), add wait — the number of seconds to pause between booting each group of hosts:

boot:
  limit: 25%
  wait: 10

wait only matters when limit is set, because it pauses between groups — with no limit there is a single parallel group and nothing to wait between.

Step 4 — (Optional) Boot multiple roles on a host in parallel

If a single host runs more than one role, Kamal boots those roles on that host sequentially by default. To boot them at the same time instead, set parallel_roles:

boot:
  limit: 25%
  wait: 10
  parallel_roles: true

This only has an effect when a host carries multiple roles. It defaults to false. For how hosts map to roles, see the servers skill.

Step 5 — Deploy

The boot strategy takes effect on the next deploy. Nothing else changes about the command:

kamal deploy

It applies the same way on kamal redeploy (which deploys without re-bootstrapping servers).

The boot Configuration Keys

All three keys live under a single top-level boot: block in config/deploy.yml.

Choosing limit and wait

• Percentage vs. integer: A percentage (25%) scales automatically as you add or remove servers — good when the fleet grows. An integer (3) keeps batches a fixed size regardless of fleet size — good when batch size is constrained by downstream capacity (e.g. database connection headroom).
• Smaller limit = safer and slower; fewer hosts cycling at once, but more groups to get through.
• wait trades total deploy time for stability. Start small (a few seconds) and increase if you see load not settling between batches.
• For the full reference, including how groups are formed and worked batch-size examples, see [references/boot-configuration-reference.md](references/boot-configuration-reference.md).

Example: Gradual Rollout Across a Large Fleet

# config/deploy.yml
service: my-app
image: my-user/my-app

servers:
  web:
    - 10.0.0.1
    - 10.0.0.2
    - 10.0.0.3
    - 10.0.0.4
    - 10.0.0.5
    - 10.0.0.6
    - 10.0.0.7
    - 10.0.0.8

boot:
  limit: 25%   # boot 2 of the 8 web hosts at a time
  wait: 10     # pause 10 seconds between each group

With limit: 25% over 8 hosts, Kamal boots in groups of 2, pausing 10 seconds between groups, until all 8 are running the new version.

Related Skills

• deploy: For running the deploy itself (kamal deploy / kamal redeploy) and the full per-host deploy sequence that the boot strategy paces.
• servers: For how servers map to roles and when a host carries multiple roles — the case where parallel_roles applies.