feat(rlog): use rlog as the default db backend

.ci/docker-compose-file/docker-compose-emqx-cluster-mnesia.override.yaml

@@ -0,0 +1,29 @@
+x-default-emqx: &default-emqx
+    env_file:
+      - conf.cluster.env
+    healthcheck:
+      test: ["CMD", "/opt/emqx/bin/emqx_ctl", "status"]
+      interval: 5s
+      timeout: 25s
+      retries: 5
+
+services:
+  emqx1:
+    <<: *default-emqx
+    container_name: node1.emqx.io
+    restart: on-failure
+    environment:
+      - "EMQX_HOST=node1.emqx.io"
+      - "EMQX_DB__BACKEND=mnesia"
+      - "EMQX_DB__ROLE=core"
+
+  emqx2:
+    <<: *default-emqx
+    container_name: node2.emqx.io
+    depends_on:
+      - emqx1
+    restart: on-failure
+    environment:
+      - "EMQX_HOST=node2.emqx.io"
+      - "EMQX_DB__BACKEND=mnesia"
+      - "EMQX_DB__ROLE=core"

.ci/docker-compose-file/scripts/run-emqx.sh

@@ -5,12 +5,18 @@ set -euxo pipefail
 export _EMQX_DOCKER_IMAGE_TAG="$1"
 _EMQX_TEST_DB_BACKEND="${2:-${_EMQX_TEST_DB_BACKEND:-mnesia}}"
 
-if [ "$_EMQX_TEST_DB_BACKEND" = "rlog" ]
-then
-  CLUSTER_OVERRIDES="-f .ci/docker-compose-file/docker-compose-emqx-cluster-rlog.override.yaml"
-else
-  CLUSTER_OVERRIDES=""
-fi
+case "$_EMQX_TEST_DB_BACKEND" in
+  rlog)
+    CLUSTER_OVERRIDES=".ci/docker-compose-file/docker-compose-emqx-cluster-rlog.override.yaml"
+    ;;
+  mnesia)
+    CLUSTER_OVERRIDES=".ci/docker-compose-file/docker-compose-emqx-cluster-mnesia.override.yaml"
+    ;;
+  *)
+    echo "ERROR: Unknown DB backend: ${_EMQX_TEST_DB_BACKEND}"
+    exit 1
+    ;;
+esac
 
 {
   echo "HOCON_ENV_OVERRIDE_PREFIX=EMQX_"
@@ -39,10 +45,9 @@ is_cluster_up() {
     is_node_listening node2.emqx.io
 }
 
-# shellcheck disable=SC2086
 docker-compose \
   -f .ci/docker-compose-file/docker-compose-emqx-cluster.yaml \
-  $CLUSTER_OVERRIDES \
+  -f "$CLUSTER_OVERRIDES" \
   -f .ci/docker-compose-file/docker-compose-python.yaml \
   up -d
 

apps/emqx_conf/src/emqx_conf_schema.erl

@@ -362,11 +362,12 @@ fields("db") ->
     [ {"backend",
        sc(hoconsc:enum([mnesia, rlog]),
           #{ mapping => "mria.db_backend"
-           , default => mnesia
+           , default => rlog
            , desc => """
 Select the backend for the embedded database.<br/>
-<code>mnesia</code> is the default backend, that offers decent performance in small clusters.<br/>
-<code>rlog</code> is a new experimental backend that is suitable for very large clusters.
+<code>rlog</code> is the default backend, a new experimental backend
+that is suitable for very large clusters.<br/>
+<code>mnesia</code> is a backend that offers decent performance in small clusters.
 """
            })}
     , {"role",
@@ -391,7 +392,10 @@ to <code>rlog</code>.
            , desc => """
 List of core nodes that the replicant will connect to.<br/>
 Note: this parameter only takes effect when the <code>backend</code> is set
-to <code>rlog</code> and the <code>role</code> is set to <code>replicant</code>.
+to <code>rlog</code> and the <code>role</code> is set to <code>replicant</code>.<br/>
+This values needs to be defined for manual or static cluster discovery mechanisms.<br/>
+If an automatic cluster discovery mechanism is being used (such as <code>etcd</code>),
+there is no need to set this value.
 """
            })}
     , {"rpc_module",