Add documentation for oplog (FerretDB#3960)

website/docs/configuration/oplog-support.md

@@ -0,0 +1,57 @@
+---
+sidebar_position: 4
+slug: /configuration/oplog-support/
+---
+
+# OpLog support
+
+FerretDB currently has a basic implementation of the OpLog (operations log).
+
+The OpLog is a special capped collection which stores all operations that modify your data.
+A capped collection is a fixed-sized collection that overwrites its entries when it reaches its maximum size.
+Naturally, OpLog is a capped collection so as to ensure that data does not grow unbounded.
+
+:::note
+At the moment, only basic OpLog tailing is supported.
+Replication is not supported yet.
+:::
+
+Oplog support is critical for the Meteor framework to build real-time applications.
+Such applications require notifications on real-time events and can use the OpLog to build a simple pub/sub system.
+
+## Enabling OpLog functionality
+
+FerretDB will not create the oplog automatically; you must do so manually.
+
+To enable OpLog functionality, manually create a capped collection named `oplog.rs` in the `local` database.
+
+```js
+// use local
+db.createCollection('oplog.rs', { capped: true, size: 536870912 })
+```
+
+You may also need to set the replica set name using [`--repl-set-name` flag / `FERRETDB_REPL_SET_NAME` environment variable](flags.md#general).
+
+:::tip
+**`--repl-set-name` flag / `FERRETDB_REPL_SET_NAME`** environment variable allow clients and drivers to perform an initial replication handshake.
+We do not perform any replication but clients and drivers will assume that the replication protocol is being used.
+The purpose of this flag is to allow access to the OpLOg.
+:::
+
+```sh
+docker run -e FERRETDB_REPL_SET_NAME=rs0 ...
+```
+
+To query the OpLog:
+
+```js
+db.oplog.rs.find()
+```
+
+To query OpLog for all the operations in a particular namespace (`test.foo`), run:
+
+```js
+db.oplog.rs.find({ ns: 'test.foo' })
+```
+
+If something does not work correctly or you have any question on the OpLog functionality, [please inform us here](https://github.com/FerretDB/FerretDB/issues/new?assignees=ferretdb-bot&labels=code%2Fbug%2Cnot+ready&projects=&template=bug.yml).

website/versioned_docs/version-v1.18/configuration/oplog-support.md

@@ -0,0 +1,57 @@
+---
+sidebar_position: 4
+slug: /configuration/oplog-support/
+---
+
+# OpLog support
+
+FerretDB currently has a basic implementation of the OpLog (operations log).
+
+The OpLog is a special capped collection which stores all operations that modify your data.
+A capped collection is a fixed-sized collection that overwrites its entries when it reaches its maximum size.
+Naturally, OpLog is a capped collection so as to ensure that data does not grow unbounded.
+
+:::note
+At the moment, only basic OpLog tailing is supported.
+Replication is not supported yet.
+:::
+
+Oplog support is critical for the Meteor framework to build real-time applications.
+Such applications require notifications on real-time events and can use the OpLog to build a simple pub/sub system.
+
+## Enabling OpLog functionality
+
+FerretDB will not create the oplog automatically; you must do so manually.
+
+To enable OpLog functionality, manually create a capped collection named `oplog.rs` in the `local` database.
+
+```js
+// use local
+db.createCollection('oplog.rs', { capped: true, size: 536870912 })
+```
+
+You may also need to set the replica set name using [`--repl-set-name` flag / `FERRETDB_REPL_SET_NAME` environment variable](flags.md#general).
+
+:::tip
+**`--repl-set-name` flag / `FERRETDB_REPL_SET_NAME`** environment variable allow clients and drivers to perform an initial replication handshake.
+We do not perform any replication but clients and drivers will assume that the replication protocol is being used.
+The purpose of this flag is to allow access to the OpLOg.
+:::
+
+```sh
+docker run -e FERRETDB_REPL_SET_NAME=rs0 ...
+```
+
+To query the OpLog:
+
+```js
+db.oplog.rs.find()
+```
+
+To query OpLog for all the operations in a particular namespace (`test.foo`), run:
+
+```js
+db.oplog.rs.find({ ns: 'test.foo' })
+```
+
+If something does not work correctly or you have any question on the OpLog functionality, [please inform us here](https://github.com/FerretDB/FerretDB/issues/new?assignees=ferretdb-bot&labels=code%2Fbug%2Cnot+ready&projects=&template=bug.yml).