# 5.1. Compaction

The compaction operation is the way to reduce disk space usage by removing unused and old data from database or view index files. This operation is very similar to the vacuum (SQLite ex.) operation available for other database management systems.

During compaction of the target CouchDB creates new file with the `.compact` extension and transfers only actual data into. Because of this, CouchDB checks first for the available disk space - it should be twice greater than the compacted file’s data.

When all actual data is successfully transferred to the compacted file CouchDB replaces the target with the compacted file.

## 5.1.1. Database Compaction

Database compaction compresses the database file by removing unused file sections created during updates. Old documents revisions are replaced with small amount of metadata called tombstone which are used for conflicts resolution during replication. The number of stored revisions (and their tombstones) can be configured by using the `_revs_limit` URL endpoint.

Compaction is manually triggered operation per database and runs as a background task. To start it for specific database there is need to send HTTP `POST /{db}/_compact` sub-resource of the target database:

    curl -H "Content-Type: application/json" -X POST http://localhost:5984/my_db/_compact

On success, HTTP status 202 Accepted is returned immediately:

    HTTP/1.1 202 Accepted
    Cache-Control: must-revalidate
    Content-Length: 12
    Content-Type: text/plain; charset=utf-8
    Date: Wed, 19 Jun 2013 09:43:52 GMT
    Server: CouchDB (Erlang/OTP)

    {"ok":true}

Although the request body is not used you must still specify Content-Type header with application/json value for the request. If you don’t, you will be aware about with HTTP status 415 Unsupported Media Type response:

    HTTP/1.1 415 Unsupported Media Type
    Cache-Control: must-revalidate
    Content-Length: 78
    Content-Type: application/json
    Date: Wed, 19 Jun 2013 09:43:44 GMT
    Server: CouchDB (Erlang/OTP)

    {"error":"bad_content_type","reason":"Content-Type must be application/json"}

When the compaction is successful started and running it is possible to get information about it via database information resource:

    curl http://localhost:5984/my_db

    HTTP/1.1 200 OK
    Cache-Control: must-revalidate
    Content-Length: 246
    Content-Type: application/json
    Date: Wed, 19 Jun 2013 16:51:20 GMT
    Server: CouchDB (Erlang/OTP)

    {
        "committed_update_seq": 76215,
        "compact_running": true,
        "data_size": 3787996,
        "db_name": "my_db",
        "disk_format_version": 6,
        "disk_size": 17703025,
        "doc_count": 5091,
        "doc_del_count": 0,
        "instance_start_time": "0",
        "purge_seq": 0,
        "update_seq": 76215
    }

Note that `compaction_running` field is `true` indicating that compaction is actually running. To track the compaction progress you may query the `_active_tasks` resource:

    curl http://localhost:5984/_active_tasks

    HTTP/1.1 200 OK
    Cache-Control: must-revalidate
    Content-Length: 175
    Content-Type: application/json
    Date: Wed, 19 Jun 2013 16:27:23 GMT
    Server: CouchDB (Erlang/OTP)

    [
        {
            "changes_done": 44461,
            "database": "my_db",
            "pid": "<0.218.0>",
            "progress": 58,
            "started_on": 1371659228,
            "total_changes": 76215,
            "type": "database_compaction",
            "updated_on": 1371659241
        }
    ]

## 5.1.2. Views Compaction

Views are also need compaction like databases, unlike databases views are compacted by groups per design document. To start their compaction there is need to send HTTP `POST /{db}/_compact/{ddoc}` request:

    curl -H "Content-Type: application/json" -X POST http://localhost:5984/dbname/_compact/designname

    {"ok":true}

This compacts the view index from the current version of the specified design document. The HTTP response code is 202 Accepted (like compaction for databases) and a compaction background task will be created.

### Views cleanup

View indexes on disk are named after their MD5 hash of the view definition. When you change a view, old indexes remain on disk. To clean up all outdated view indexes (files named after the MD5 representation of views, that does not exist anymore) you can trigger a view cleanup:

    curl -H "Content-Type: application/json" -X POST http://localhost:5984/dbname/_view_cleanup

    {"ok":true}

## 5.1.3. Automatic Compaction

While both database and views compactions are required be manually triggered, it is also possible to configure automatic compaction, so that compaction of databases and views is automatically triggered based on various criteria. Automatic compaction is configured in CouchDB’s configuration files.

The `daemons/compaction_daemon` is responsible for triggering the compaction. It is enabled by default and automatically started. The criteria for triggering the compactions is configured in the `compactions` section.