# Charm architecture Synapse is a drop in replacement for other chat servers like Mattermost and Slack. It integrates with [PostgreSQL](https://www.postgresql.org/) as its database. The charm design leverages the [sidecar](https://kubernetes.io/blog/2015/06/the-distributed-system-toolkit-patterns/#example-1-sidecar-containers) pattern to allow multiple containers in each pod with [Pebble](https://juju.is/docs/sdk/pebble) running as the workload container’s entrypoint. Pebble is a lightweight, API-driven process supervisor that is responsible for configuring processes to run in a container and controlling those processes throughout the workload lifecycle. Pebble `services` are configured through [layers](https://github.com/canonical/pebble#layer-specification), and the following containers represent each one a layer forming the effective Pebble configuration, or `plan`: 1. An [NGINX](https://www.nginx.com/) Pebble layer, which can be used to efficiently serve static resources, as well as be the incoming point for all web traffic to the pod. 2. The [Synapse](https://github.com/matrix-org/synapse) container itself, which has Synapse installed and configured. As a result, if you run a `kubectl get pods` on a namespace named for the Juju model you’ve deployed the Synapse charm into, you’ll see something like the following: ```bash NAME READY STATUS RESTARTS AGE synapse-0 3/3 Running 0 6h4m ``` This shows there are 3 containers - the two named above, as well as a container for the charm code itself. All containers will have the command `/charm/bin/pebble`. Pebble is responsible for service management and process startup, as explained above. ## OCI images We use [Rockcraft](https://documentation.ubuntu.com/rockcraft/en/latest/) to build OCI Image for Synapse. The image is defined in [Synapse rock](https://github.com/canonical/synapse-operator/tree/main/synapse_rock) and is published to [Charmhub](https://charmhub.io/), the official repository of charms. This is done by publishing a resource to Charmhub as described in the [Juju SDK How-to guides](https://juju.is/docs/sdk/publishing). ## Container Configuration files for the container can be found in the respective directory that define the rock, see the section above. ### NGINX NGINX is configured as a Pebble Layer and is the entry point for all web traffic to the pod (on port `8080`). Serves static files directly and forwards non-static requests to the Synapse container (on port `8008`). NGINX provides static content cache, reverse proxy, and load balancer among multiple application servers, as well as other features. It can be used in front of Synapse server to significantly reduce server and network load. ### Synapse Synapse is a Python application run by the `start.py` script. Synapse listens to non-TLS port `8008` serving by default. NGINX can then forward non-static traffic to it. The workload that this container is running is defined in the [Synapse rock](https://github.com/canonical/synapse-operator/tree/main/synapse_rock). If Synapse is integrated with PostgreSQL, [Synapse Stats Exporter](https://github.com/canonical/synapse_stats_exporter) will be enabled. Synapse Stats Exporter listens to non-TLS port `9877` and will be configured as a target if the charm is integrated with Prometheus. It will provide two metrics: number of rooms and number of users. ## Integrations See [Integrations](https://canonical.com/juju/docs/synapse-charm/latest/reference/integrations/index.html.md#reference-integrations). ## Charm code overview The `src/charm.py` is the default entry point for a charm and has the `SynapseOperatorCharm` Python class which inherits from the `CharmBase`. CharmBase is the base class from which all Charms are formed, defined by [Ops](https://documentation.ubuntu.com/ops/latest/) (Python framework for developing charms). See more information in [Charm](https://canonical.com/juju/docs/juju-cli/3.6/reference/charm/#charm). The `__init__` method guarantees that the charm observes all events relevant to its operation and handles them. Take, for example, when a configuration is changed by using the CLI. 1. User runs the command ```bash juju config synapse server_name=myserver.myserver.com ``` 1. A `config-changed` event is emitted 2. Event handlers are defined in the charm’s framework observers. An example looks like the following: ```python self.framework.observe(self.on.config_changed, self._on_config_changed) ``` 1. The method `_on_config_changed` will take the necessary actions. The actions include waiting for all the relations to be ready and then configuring the containers.