Add README for db-sync component

components/ee/db-sync/README.md

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+# db-sync
+
+This component runs only in Gitpod SaaS and is responsible for bidirectional synchronisation of data between two or more mysql databases.
+
+### Commands
+
+`db-sync` has two subcommands: `db-sync run` and `db-sync export` .
+
+- `db-sync run`: Runs the sync process repeatedly
+- `db-sync export`: Exports a SQL file containing the sync operations
+
+### Config
+
+`db-sync` uses a config file (passed as the `--config` argument to `db-sync run`). There is an [example config](https://github.com/gitpod-io/gitpod/blob/main/components/ee/db-sync/example-config.json) in the repository and a production config file looks like this:
+
+```yaml
+{
+  "disableTransactions": false,
+  "replicationLogDir": "/var/log/db-sync",
+  "roundRobin": true,
+  "syncPeriod": 10000,
+  "tableSet": "gitpod",
+  "targets": [
+    {
+      "database": "gitpod",
+      "host": "localhost",
+      "name": "...",
+      "password": "...",
+      "port": 3300,
+      "user": "gitpod"
+    },
+    {
+      "database": "gitpod",
+      "host": "localhost",
+      "name": "...",
+      "password": "...",
+      "port": 3301,
+      "user": "gitpod"
+    }
+  ]
+}
+```
+
+### Modes
+
+`db-sync` runs in “round robin mode” in production (`”roundRobin”: true` in the above config).
+
+Round robin mode:
+  - Connects to each target database in the config and creates a “replicator” for each.
+  - Calls `synchronize` on each replicator in turn, to sync each database with every other one.
+  - The synchronisation loop (running each replicator) happens every `syncPeriod` milliseconds (so every 10s in the above config).
+
+`db-sync` attempts a graceful shutdown by handing `SIGINT` and waiting for any currently running sync to complete before terminating. In Kubernetes this should mean that when `db-sync` is terminated it has 30 seconds (the default termination grace period) in which to complete synchronisation before being forcibly terminated with a `SIGTERM`.
+
+### Synchronisation
+
+- The list of tables that `db-sync` considers for synchronisation is defined by the `GitpodTableDescriptionProvider` in [tables.ts](https://github.com/gitpod-io/gitpod/blob/243ee21379f90f3c70f79f0317723006270493bf/components/gitpod-db/src/tables.ts#L47-L309).
+- Each table to be synchronised must specify a `timeColumn` of type `timestamp`
+- Tables can specify some columns as ignored by `db-sync` (via `[ignoreColumns](https://github.com/gitpod-io/gitpod/blob/243ee21379f90f3c70f79f0317723006270493bf/components/gitpod-db/src/tables.ts#L21)`)
+
+All rows in considered tables with a `timeColumn` later than the start time of the current sync are inserted into the target database(s).
+
+If a table specifies a `deletionColumn` all rows marked as deleted in the source are hard-deleted from the target database(s) *as well as the source database*.
+
+### gitpod_replication table
+
+`db-sync` uses the `gitpod_replication` db table. This table holds a single row, continuously updated, containing the last time `db-sync` performed a sync. This timestamp is used as the start_date for the next sync. This start_date can also be ignored to force a full sync.
+
+### Logging
+
+`db-sync` writes logs to `config.replicationLogDir` where each timestamped logged file contains the SQL run on each database during that synchronisation.