brew services restart versus parallel version install for a small Homebrew service upgrade

Direct answer

For a background service that you want to upgrade without a downtime window, you have two practical options:

• Upgrade the existing formula and run brew services restart. This stops the current instance, then starts a new one using the upgraded binary.
• Install a separate, version‑specific formula (e.g., foo@2.3) if the maintainer has published one, then point the service to that prefix. This allows the old and new binaries to coexist, but you still need to stop the service before switching the symlink to avoid running a process that was started with a different binary.

Homebrew does not provide an atomic, zero‑downtime mechanism for a single formula. The choice depends on whether a versioned formula is available for your package.

Confirmed facts

• Homebrew installs formulae into /usr/local/Cellar (or /opt/homebrew/Cellar) and exposes the executables through a single set of symlinks in /usr/local/bin (or /opt/homebrew/bin).
• Running brew services restart <formula> sends a stop signal to the launchd or systemd unit, waits for the process to exit, and then starts the unit again. The unit file points to the symlinked binary, so the new instance uses the upgraded executable.
• Parallel installation is only possible when the formula has an explicit versioned variant. Most core formulae do not expose side‑by‑side installs.
• Changing the symlink while a service is running does not affect the already‑running process; new processes will use the new binary, but the existing process keeps the old binary in memory.

Likely explanation

Because Homebrew keeps a single symlink for each binary, the system cannot automatically switch an already‑running service to a new binary without restarting it. If a versioned formula exists, you can install it to a separate prefix, but you still need to stop the service before updating the symlink that the unit file references.

Steps for this case

1. Check for a versioned formula:

   brew search <service-name> | grep '@'
   

2. If a versioned formula exists, install it:

   brew install <service-name>@<version>
   

3. Verify the new prefix:

   brew --prefix <service-name>@<version>
   

4. Switch the symlink (this is safe only after stopping the service):

   brew services stop <service-name>
   brew unlink <service-name>
   brew link <service-name>@<version> --force
   brew services start <service-name>
   

5. If no versioned formula exists, upgrade the current one:

   brew upgrade <service-name>
   brew services restart <service-name>
   

Question for you

Does the service you are upgrading provide a versioned Homebrew formula (e.g., service@2.4)? Knowing this will determine whether the parallel‑install path is viable.