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A reliable handler design depends on five details: the notifying task must return changed: true, names or topics must match, repeated notifications are merged, definition order controls execution, and handlers can be flushed early when later tasks depend on their effects.
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What an Ansible handler is
A handler is a task with deferred, change-triggered execution. It looks like an ordinary Ansible task, but it is declared under handlers: and does not run on every playbook execution. A task elsewhere in the play sends a notification when it changes the managed host.
This pattern prevents unnecessary restarts and reloads. A template task can rewrite a web-server configuration and notify a restart handler; if the rendered file is identical to the existing file, the template reports no change and the service is left running. The same mechanism works for daemon reloads, cache refreshes, configuration validation, or any other follow-up action that should happen only after a change.
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Handlers belong to the play’s handler scope. Give each handler a unique name, because handlers from roles and other play components are inserted into that scope and a later duplicate can shadow an earlier definition.
The smallest working playbook
The following play writes an Apache configuration and restarts the service only when the template task changes the destination file:
- hosts: webservers
tasks:
- name: Write the Apache configuration
ansible.builtin.template:
src: httpd.j2
dest: /etc/httpd.conf
notify: Restart apache
handlers:
- name: Restart apache
ansible.builtin.service:
name: httpd
state: restartedThe task’s notify value must identify the handler’s name exactly. The handler is not called immediately after the template task. Ansible records the notification and normally runs the handler after the play’s task sections have finished.
What happens on an unchanged run
If /etc/httpd.conf already matches the rendered template, the template task is ok, not changed. No notification is queued, so Restart apache does not run. This idempotent behavior is the principal reason to use handlers instead of placing a restart task directly after every configuration task.
Handler execution, step by step
- An ordinary task executes. Modules such as
template,copy, andlineinfilecalculate whether they changed the host. - A changed result queues notifications. A task that returns
changed: truequeues each handler name or topic listed undernotify. A task that returns no change queues nothing. - Notifications are coalesced. If several tasks notify the same handler during one play, Ansible runs that handler once rather than once per notification.
- Handlers run at the normal handler point. Ansible processes the queued handlers after the play’s task sections. They are executed in handler definition order, not in the order in which tasks notified them or in the order shown in a particular
notifylist. - The queue is cleared for that execution point. A later change can notify the handler again, and an explicit flush can run newly queued notifications at a chosen point.
| Situation | Result |
|---|---|
Notifying task reports ok |
No handler is queued. |
One task reports changed |
Each matching handler or topic is queued. |
| Three tasks notify one handler | The handler runs once at the handler execution point. |
| Two handlers are notified in reverse order | They still run in the order in which they are defined. |
| A handler is defined but never notified | It does not run. |
Connecting tasks with notify
Use a handler’s exact name for a direct notification. A task may notify one handler:
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- name: Install the game-server configuration
ansible.builtin.template:
src: server.conf.j2
dest: /etc/game-server/server.conf
notify: Restart game serverOr it may notify several handlers:
- name: Deploy the proxy configuration
ansible.builtin.template:
src: proxy.conf.j2
dest: /etc/proxy/proxy.conf
notify:
- Validate proxy configuration
- Reload proxyThe list order above does not determine execution order. If Reload proxy is defined before Validate proxy configuration in the handlers: section, reload runs first. Arrange handler definitions in the safe operational order you need.
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Using listen topics to fan out one event
A handler can listen for a topic that is different from its task name. Tasks notify the topic, and every handler declaring that topic runs. This is useful when one configuration change needs several responses or when you want a stable event vocabulary that is independent of implementation-specific handler names.
- name: Write the application configuration
ansible.builtin.template:
src: app.conf.j2
dest: /etc/myapp/app.conf
notify: Restart application stack
handlers:
- name: Restart application service
ansible.builtin.service:
name: myapp
state: restarted
listen: Restart application stack
- name: Refresh application proxy
ansible.builtin.service:
name: myapp-proxy
state: reloaded
listen: Restart application stackHere the task knows only the event topic. Both listening handlers are queued when the template changes. Handler definition order still controls which listener runs first.
| Design choice | Direct handler name | listen topic |
|---|---|---|
| Coupling | The task depends on a specific handler name. | The task depends on an event topic; implementation names can change independently. |
| Fan-out | List several handler names explicitly. | One notification reaches every handler listening to the topic. |
| Collision risk | Names must remain unique across the play and inserted roles. | Topic names provide an event vocabulary, but listening handlers still need unique names. |
| Best fit | Small, local playbooks with one obvious response. | Shared roles or events that intentionally trigger multiple responses. |
| Templating | Handler names are templated early. | listen topics cannot be templated. |
Forcing handlers to run before the play ends
Most handlers should remain deferred until the normal handler point. Sometimes later tasks must see the effect immediately—for example, a configuration must be applied before a migration or a validation command can succeed. Insert the ansible.builtin.meta task with flush_handlers:
- name: Install the new application configuration
ansible.builtin.template:
src: app.conf.j2
dest: /etc/myapp/app.conf
notify: Restart application
- name: Run handlers before continuing
ansible.builtin.meta: flush_handlers
- name: Perform a check against the running application
ansible.builtin.command: /usr/local/bin/check-myappflush_handlers runs handlers that have been notified up to that point. Notifications generated by later tasks remain pending until the next handler execution point or another flush. Use flushes deliberately: an early restart can interrupt a deployment, and flushing too often removes the batching benefit that handlers provide.
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Making command and shell tasks notify correctly
Modules that manage files and services generally know when they changed. A command or shell task may not, because Ansible cannot infer the command’s intended state. Set changed_when so the result reflects the command’s actual effect.
- name: Generate the server index
ansible.builtin.command: /usr/local/bin/build-index
register: build_index
changed_when:
- build_index.rc == 0
- build_index.stdout is search('updated')
notify: Reload game serverWhen changed_when is a list, its expressions have implicit and semantics: every expression must be true for the task to be marked changed. If the command reports success but did not update anything, the handler is not notified. If your condition is too broad, the handler will run on every playbook execution; if it is too narrow, required reloads will be skipped.
Loops can trigger broader handler actions
A loop is one Ansible task, even though it processes multiple items. If any loop item changes, the task is marked changed and all handlers notified by that task are queued. This can be surprising when each item represents an independent configuration file but the handler restarts a shared service.
- name: Install server drop-in files
ansible.builtin.template:
src: '{{ item }}.j2'
dest: '/etc/myapp/{{ item }}.conf'
loop:
- limits
- logging
- networking
notify: Restart applicationA change to only logging still queues Restart application. If you need different follow-up behavior per item, separate the work into distinct tasks or design separate notifications rather than assuming Ansible will invoke a handler once per changed item.
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Keep names unique
Handler names share play scope. A duplicate name can make a later-loaded handler shadow an earlier one, producing a result that looks like a failed notification even though the task changed correctly. Use descriptive, specific names such as Reload nginx after gateway config rather than generic names such as Restart service in a large collection.
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Role handlers are not private by default
Handlers defined by roles are inserted into the global handler scope and can be used outside the role in which they were defined. If a role-qualified notification is required, use the role-qualified form:
- name: Apply the role-provided configuration
ansible.builtin.template:
src: app.conf.j2
dest: /etc/myapp/app.conf
notify: 'web_role : Restart application'The role-qualified form is especially useful when different roles expose similarly named operations. It also makes the intended provider explicit when a play combines multiple roles.
Put distribution differences in module parameters
Do not rely on templated handler names to select a distribution-specific service. Handler names are templated early, which can create ordering and resolution surprises. Keep the stable handler name or topic, and template variables inside the handler’s module parameters instead:
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handlers:
- name: Restart web service
ansible.builtin.service:
name: '{{ web_service_name }}'
state: restartedFor event grouping, remember that listen topics cannot be templated. Choose a stable literal topic and vary the handler’s parameters with ordinary variables.
Includes, imports, and handlers
Dynamic task includes
A dynamic task include can itself be notified as a handler. When the notification fires, Ansible executes the tasks supplied by that include. This lets one event activate a variable set of tasks at runtime.
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handlers:
- name: Apply database maintenance actions
ansible.builtin.include_tasks: database-maintenance.ymlStatic imports
A static import is expanded before execution. The imported tasks become part of the playbook during parsing, so the import statement itself cannot be notified as a handler. If the operation must be deferred behind a notification, use a dynamic include instead.
Meta tasks as handlers
Since ansible-core 2.14, meta tasks can be used as handlers. The special flush_handlers action cannot itself be used as a handler; it is the mechanism that executes handlers already queued at a chosen point. When portability across older ansible-core versions matters, verify the behavior against the version installed in your execution environment.
Why a handler did not run: a diagnostic checklist
Work through these checks in order:
- Did the notifying task report a change? A task result of
okdoes not queue a handler. For command and shell tasks, inspectchanged_when. - Does the notification match? Compare spelling, capitalization, punctuation, and whitespace in a direct handler name. For topics, confirm the task notifies the literal value used by
listen. - Is the handler in scope? Confirm it is under the relevant play’s
handlers:section or supplied by a role that is loaded for that play. - Was a duplicate name loaded later? A later definition can shadow an earlier handler. Rename handlers or use a role-qualified notification where appropriate.
- Did a loop behave differently than expected? Any changed item marks the task changed and can trigger every handler notified by that task.
- Was execution deferred? A notification normally waits until the play’s handler point. Add
ansible.builtin.meta: flush_handlersonly when the handler must run before subsequent tasks. - Are you using an include or import? A dynamic include can be notified; a static import is expanded before execution and the import statement cannot be notified.
- Are variables being templated in the wrong place? Keep variable-driven service names and paths in module parameters, not in handler names or
listentopics.
Design patterns that keep handlers predictable
Use one event for one operational outcome
Name a direct handler after the action it performs, such as Reload proxy or Restart worker pool. Use a topic when the event has several legitimate consumers, such as a configuration deployment that must reload both a service and its proxy.
Keep validation and activation separate
A configuration-writing task can notify a validation handler and an activation handler. Define them in the order required by your operation. If validation must happen before activation, place the validation handler first in the handler section; the order of the notify list cannot override that definition order.
Batch related changes
Let several template and copy tasks notify the same restart handler. Ansible coalesces those notifications, so a play that changes five related files still performs one restart at the normal handler point. This reduces service churn while preserving idempotence.
Flush only at a real dependency boundary
Use a flush when a later task must interact with the newly restarted or reloaded service. Otherwise, leave notifications deferred so all changes in the play can be applied together.
A complete example with validation, a topic, and an early flush
- hosts: game_servers
tasks:
- name: Deploy the server configuration
ansible.builtin.template:
src: server.conf.j2
dest: /etc/game-server/server.conf
notify: game server configuration changed
- name: Deploy the access-control configuration
ansible.builtin.template:
src: access.conf.j2
dest: /etc/game-server/access.conf
notify: game server configuration changed
- name: Apply configuration before checking readiness
ansible.builtin.meta: flush_handlers
- name: Check the running server
ansible.builtin.command: /usr/local/bin/game-server --check-ready
handlers:
- name: Validate game-server configuration
ansible.builtin.command: /usr/local/bin/game-server --validate-config
listen: game server configuration changed
- name: Restart game-server service
ansible.builtin.service:
name: game-server
state: restarted
listen: game server configuration changedBoth file tasks publish the same event topic. If neither file changes, neither listener runs. If one or both change, validation and restart are each queued once, in the order shown in the handler section. The explicit flush makes the running service reflect the new configuration before the readiness command executes.
Version and portability considerations
Most handler behavior—change-triggered execution, coalescing, definition order, direct names, and listening topics—is foundational Ansible behavior. The details around meta tasks and include handling are version-sensitive. In particular, meta tasks are documented as usable handlers beginning with ansible-core 2.14, while flush_handlers remains an execution control task rather than a handler target. When a playbook must run across multiple ansible-core versions, test these constructs against the oldest supported version and keep version-specific assumptions documented near the affected task.
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