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| # How to manage service dependencies | ||
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| Pebble takes service dependencies into account when starting and stopping services. When Pebble starts a service, it also starts the services which that service depends on (configured with `required`). Conversely, when stopping a service, Pebble also stops services which depend on that service. | ||
| If you are using Pebble to manage your services, chances are, you've got more | ||
| than one service to manage. | ||
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| For example, if service `nginx` requires `logger`, `pebble start nginx` will start both `nginx` and `logger` (in an undefined order). Running `pebble stop logger` will stop both `nginx` and `logger`; however, running `pebble stop nginx` will only stop `nginx` (`nginx` depends on `logger`, not the other way around). | ||
| And when orchestrating more services, things can and will get trickier, because | ||
| more often than not, those services depend on each other to function together. | ||
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| When multiple services need to be started together, they're started in order according to the `before` and `after` configuration, waiting 1 second for each to ensure the command doesn't exit too quickly. The `before` option is a list of services that this service must start before (it may or may not `require` them). Or if it's easier to specify this ordering the other way around, `after` is a list of services that this service must start after. | ||
| This document shows how to manage a set of services that have dependencies on | ||
| each other to function properly. | ||
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| Note that currently, `before` and `after` are of limited usefulness, because Pebble only waits 1 second before moving on to start the next service, with no additional checks that the previous service is operating correctly. | ||
| ## Demo web application | ||
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| We are using a simple web application to demonstrate the instructions in this | ||
| guide. | ||
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| ### Setup | ||
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| The web application has the following setup: | ||
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| - a database listening on port 3306 | ||
| - a backend server listening on port 8081, which talks to the database | ||
| - a frontend server listening on port 8080, which talks to the backend | ||
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| The relationship between the frontend server, backend server and database can be | ||
| simplified as: | ||
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| `frontend (8080) -> backend (8081) -> database (3306)` | ||
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| These components (or services) are all dependent on one another; if any | ||
| component fails to start or starts with errors, the web application won't | ||
| function properly. | ||
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| For example, if the backend server fails to start, or is unable to | ||
| communicate with the database, the frontend service will not run successfully. | ||
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| ### Layer configuration | ||
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| The web application is initially configured with the Pebble layer below: | ||
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| ```{code-block} yaml | ||
| services: | ||
| frontend: | ||
| override: replace | ||
| command: python3 -m http.server 8080 | ||
| startup: enabled | ||
| backend: | ||
| override: replace | ||
| command: python3 -m http.server 8081 | ||
| startup: enabled | ||
| database: | ||
| override: replace | ||
| command: python3 -m http.server 3306 | ||
| startup: enabled | ||
| ``` | ||
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| ```{note} | ||
| We are using Python's http module to mock the servers. | ||
| ``` | ||
| ### Problem with setup | ||
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| If we start the Pebble daemon (``pebble run``): | ||
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| ```{terminal} | ||
| :input: pebble run | ||
| 2024-06-28T02:17:23.347Z [pebble] Started daemon. | ||
| [backend database frontend] | ||
| 2024-06-28T02:17:23.353Z [pebble] POST /v1/services 2.613042ms 202 | ||
| 2024-06-28T02:17:23.356Z [pebble] Service "backend" starting: python3 -m http.server 8081 | ||
| 2024-06-28T02:17:24.363Z [pebble] Service "database" starting: python3 -m http.server 3306 | ||
| 2024-06-28T02:17:25.370Z [pebble] Service "frontend" starting: python3 -m http.server 8080 | ||
| 2024-06-28T02:17:26.385Z [pebble] Started default services with change 1. | ||
| ``` | ||
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| Ideally we would expect all three services to start up successfully | ||
| (``pebble services``): | ||
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| ```{terminal} | ||
| :input: pebble services | ||
| Service Startup Current Since | ||
| backend enabled active today at 10:17 CST | ||
| database enabled active today at 10:17 CST | ||
| frontend enabled active today at 10:17 CST | ||
| ``` | ||
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| However, this configuration does not account for the service dependencies. | ||
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| For example, the output below shows the ``database`` service failing to start | ||
| ("inactive"), but the ``backend`` and ``frontend`` services starting | ||
| successfully ("active"): | ||
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| ```{terminal} | ||
| :input: pebble run | ||
| 2024-06-28T02:20:03.337Z [pebble] Started daemon. | ||
| [backend database frontend] | ||
| 2024-06-28T02:20:03.343Z [pebble] POST /v1/services 2.763792ms 202 | ||
| 2024-06-28T02:20:03.346Z [pebble] Service "backend" starting: python3 -m http.server 8081 | ||
| 2024-06-28T02:20:03.346Z [pebble] Service "database" starting: python3 -m http.server 3306 | ||
| 2024-06-28T02:20:03.347Z [pebble] Service "frontend" starting: python3 -m http.server 8080 | ||
| 2024-06-28T02:20:03.396Z [pebble] Change 1 task (Start service "database") failed: cannot start service: exited quickly with code 1 | ||
| 2024-06-28T02:20:04.353Z [pebble] Started default services with change 1. | ||
| ``` | ||
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| ```{terminal} | ||
| :input: pebble services | ||
| Service Startup Current Since | ||
| backend enabled active today at 10:20 CST | ||
| database enabled inactive - | ||
| frontend enabled active today at 10:20 CST | ||
| ``` | ||
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| We can configure the layer so that Pebble only starts a given service if all | ||
| other services it is dependent on are running successfully to avoid consuming | ||
| resources unnecessarily. | ||
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| ## Define service dependencies | ||
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| To create service dependencies in Pebble, use the ``requires`` key with | ||
| ``before`` / ``after`` in the | ||
| [service definition](../reference/layer-specification.md). | ||
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| ### Specify dependent services | ||
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| To specify one or more services that a given service requires to run | ||
| successfully, use the ``require`` key. | ||
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| ```{code-block} yaml | ||
| :emphasize-lines: 6, 7, 12, 13 | ||
| services: | ||
| frontend: | ||
| override: replace | ||
| command: python3 -m http.server 8080 | ||
| startup: enabled | ||
| requires: | ||
| - backend | ||
| backend: | ||
| override: replace | ||
| command: python3 -m http.server 8081 | ||
| startup: enabled | ||
| requires: | ||
| - database | ||
| database: | ||
| override: replace | ||
| command: python3 -m http.server 3306 | ||
| startup: enabled | ||
| ``` | ||
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| In the layer configuration above, the ``backend`` service is dependent on the | ||
| ``database`` service, and the ``frontend`` service is dependent on the | ||
| ``backend`` service. | ||
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| ### Specify start order for dependent services | ||
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| To specify the order in which one or more dependent services must start | ||
| successfully, relative to a given service, use the ``before`` or ``after`` keys. | ||
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| ```{code-block} yaml | ||
| :emphasize-lines: 8, 9, 16, 17 | ||
| services: | ||
| frontend: | ||
| override: replace | ||
| command: python3 -m http.server 8080 | ||
| startup: enabled | ||
| requires: | ||
| - backend | ||
| after: | ||
| - backend | ||
| backend: | ||
| override: replace | ||
| command: python3 -m http.server 8081 | ||
| startup: enabled | ||
| requires: | ||
| - database | ||
| after: | ||
| - database | ||
| database: | ||
| override: replace | ||
| command: python3 -m http.server 3306 | ||
| startup: enabled | ||
| ``` | ||
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| In the updated layer above, the ``backend`` service requires the ``database`` | ||
| service to be started before it, and the ``frontend`` service requires the | ||
| ``backend`` service to be started before it. | ||
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| ```{note} | ||
| Currently, ``before`` and ``after`` are of limited usefulness, because Pebble only waits 1 second before moving on to start the next service, with no additional checks that the previous service is operating correctly. | ||
| If the configuration of `requires`, `before`, and `after` for a service results in a cycle or "loop", an error will be returned when attempting to start or stop the service. | ||
| ``` | ||
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| % Does it only check that the status is "Active"? So what does `require` do? | ||
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| ## Verify service dependencies | ||
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| To verify the start order of services, start the Pebble daemon again: | ||
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| ```{terminal} | ||
| :input: pebble run | ||
| 2024-06-28T03:53:23.307Z [pebble] Started daemon. | ||
| [database backend frontend] | ||
| 2024-06-28T03:53:23.313Z [pebble] POST /v1/services 3.103291ms 202 | ||
| 2024-06-28T03:53:23.317Z [pebble] Service "database" starting: python3 -m http.server 3306 | ||
| 2024-06-28T03:53:24.324Z [pebble] Service "backend" starting: python3 -m http.server 8081 | ||
| 2024-06-28T03:53:25.332Z [pebble] Service "frontend" starting: python3 -m http.server 8080 | ||
| 2024-06-28T03:53:26.337Z [pebble] GET /v1/changes/1/wait 3.023563626s 200 | ||
| 2024-06-28T03:53:26.338Z [pebble] Started default services with change 1. | ||
| ``` | ||
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| The start order is now ``database`` -> ``backend`` -> ``frontend`` and all | ||
| services are "active". | ||
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| ```{terminal} | ||
| :input: pebble services | ||
| Service Startup Current Since | ||
| backend enabled active today at 10:17 CST | ||
| database enabled active today at 10:17 CST | ||
| frontend enabled active today at 10:17 CST | ||
| ``` | ||
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| To further verify the service dependencies, we can force fail a service that is | ||
| required by another service. | ||
| For example, if we force the ``database`` service to fail, the output for the | ||
| ``pebble run`` command should be similar to: | ||
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| ```{terminal} | ||
| :input: pebble run | ||
| 2024-06-28T02:28:06.569Z [pebble] Started daemon. | ||
| [database backend frontend] | ||
| 2024-06-28T02:28:06.575Z [pebble] POST /v1/services 3.212375ms 202 | ||
| 2024-06-28T02:28:06.578Z [pebble] Service "database" starting: python3 -m http.server 3306 | ||
| 2024-06-28T02:28:06.627Z [pebble] Change 1 task (Start service "database") failed: cannot start service: exited quickly with code 1 | ||
| 2024-06-28T02:28:06.633Z [pebble] GET /v1/changes/1/wait 57.610375ms 200 | ||
| 2024-06-28T02:28:06.633Z [pebble] Started default services with change 1. | ||
| ``` | ||
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| Since a required service fails to start, all services that are dependent on it | ||
| should not start ("inactive") accordingly: | ||
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| ```{terminal} | ||
| :input: pebble services | ||
| Service Startup Current Since | ||
| backend enabled inactive - | ||
| database enabled inactive - | ||
| frontend enabled inactive - | ||
| ``` | ||
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| You can use the `changes and tasks`_ commands to get more details about the | ||
| failed run. | ||
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| ## See more | ||
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| - [pebble services command](../reference/cli-commands/services.md) | ||
| - [pebble start command](../reference/cli-commands/start.md) | ||
| - [pebble stop command](../reference/cli-commands/stop.md) | ||
| - [Changes and tasks](../reference/changes-and-tasks/changes-and-tasks.md) | ||
| - [Layer specification](../reference/layer-specification.md) |