4.5. Clustered Purge — Apache CouchDB® 3.5 Documentation

# 4.5. Clustered Purge

The primary purpose of clustered purge is to clean databases that have multiple deleted tombstones or single documents that contain large numbers of conflicts. But it can also be used to purge any document (deleted or non-deleted) with any number of revisions.

Clustered purge is designed to maintain eventual consistency and prevent unnecessary invalidation of secondary indexes. For this, every database keeps track of a certain number of historical purges requested in the database, as well as its current `purge_seq`. Internal replications and secondary indexes process database’s purges and periodically update their corresponding purge checkpoint documents to report `purge_seq` processed by them. To ensure eventual consistency, the database will remove stored historical purge requests only after they have been processed by internal replication jobs and secondary indexes.

## 4.5.1. Internal Structures

To enable internal replication of purge information between nodes and secondary indexes, two internal purge trees were added to a database file to track historical purges.

    purge_tree: UUID -> {PurgeSeq, DocId, Revs}
    purge_seq_tree: PurgeSeq -> {UUID, DocId, Revs}

Each interactive request to `_purge API`, creates an ordered set of pairs on increasing `purge_seq` and purge_request, where purge_request is a tuple that contains docid and list of revisions. For each purge_request uuid is generated. A purge request is added to internal purge trees: a tuple `{UUID -> {PurgeSeq, DocId, Revs}}` is added to `purge_tree`, a tuple is `{PurgeSeq -> {UUID, DocId, Revs}}` added `to purge_seq_tree`.

## 4.5.2. Compaction of Purges

During the compaction of the database the oldest purge requests are to be removed to store only `purged_infos_limit` number of purges in the database. But in order to keep the database consistent with indexes and other replicas, we can only remove purge requests that have already been processed by indexes and internal replications jobs. Thus, occasionally purge trees may store more than `purged_infos_limit` purges. If the number of stored purges in the database exceeds `purged_infos_limit` by a certain threshold, a warning is produced in logs signaling a problem of synchronization of database’s purges with indexes and other replicas.

## 4.5.3. Local Purge Checkpoint Documents

Indexes and internal replications of the database with purges create and periodically update local checkpoint purge documents: `_local/purge-{type}-{hash}`. These documents report the last `purge_seq` processed by them and the timestamp of the last processing. These documents are only visible in `_local_docs` when you add a `include_system=true` parameter, so e.g. `/test-db/_local_docs?include_system=true`. An example of a local checkpoint purge document:

    {
      "_id": "_local/purge-mrview-86cacdfbaf6968d4ebbc324dd3723fe7",
      "type": "mrview",
      "purge_seq": 10,
      "updated_on": 1540541874,
      "ddoc_id": "_design/foo",
      "signature": "5d10247925f826ae3e00966ec24b7bf6"
    }

The below image shows possible local checkpoint documents that a database may have.

Local Purge Checkpoint Documents

## 4.5.4. Internal Replication

Purge requests are replayed across all nodes in an eventually consistent manner. Internal replication of purges consists of two steps:

1. Pull replication. Internal replication first starts by pulling purges from target and applying them on source to make sure we don’t reintroduce to target source’s docs/revs that have been already purged on target. In this step, we use purge checkpoint documents stored on target to keep track of the last target’s `purge_seq` processed by the source. We find purge requests occurred after this `purge_seq`, and replay them on source. This step is done by updating the target’s checkpoint purge documents with the latest process `purge_seq` and timestamp.

2. Push replication. Then internal replication proceeds as usual with an extra step inserted to push source’s purge requests to target. In this step, we use local internal replication checkpoint documents, that are updated both on target and source.

Under normal conditions, an interactive purge request is already sent to every node containing a database shard’s replica, and applied on every replica. Internal replication of purges between nodes is just an extra step to ensure consistency between replicas, where all purge requests on one node are replayed on another node. In order not to replay the same purge request on a replica, each interactive purge request is tagged with a unique `uuid`. Internal replication filters out purge requests with UUIDs that already exist in the replica’s `purge_tree`, and applies only purge requests with UUIDs that don’t exist in the `purge_tree`. This is the reason why we needed to have two internal purge trees: 1) `purge_tree`: `{UUID -> {PurgeSeq, DocId, Revs}}` allows to quickly find purge requests with UUIDs that already exist in the replica; 2) `purge_seq_tree`: `{PurgeSeq -> {UUID, DocId, Revs}}` allows to iterate from a given `purge_seq` to collect all purge requests happened after this `purge_seq`.

## 4.5.5. Indexes

Each purge request will bump up `update_seq` of the database, so that each secondary index is also updated in order to apply the purge requests to maintain consistency within the main database.

## 4.5.6. Config Settings

These settings can be updated in the default.ini or local.ini:

Field | Description | Default
--- | --- | ---
max_revisions_number | Allowed maximum number of accumulated revisions in one purge request | infinity
allowed_purge_seq_lag | Beside purged_infos_limit, allowed additional buffer to store purge requests | 100
index_lag_warn_seconds | Allowed durations when index is not updated for local purge checkpoint document | 86400

During a database compaction, we check all checkpoint purge docs. A client (an index or internal replication job) is allowed to have the last reported `purge_seq` to be smaller than the current database shard’s `purge_seq` by the value of `(purged_infos_limit + allowed_purge_seq_lag)`. If the client’s `purge_seq` is even smaller, and the client has not checkpointed within `index_lag_warn_seconds`, it prevents compaction of purge trees and we have to issue the following log warning for this client:

    Purge checkpoint '_local/purge-mrview-9152d15c12011288629bcffba7693fd4’
    not updated in 86400 seconds in
    <<"shards/00000000-1fffffff/testdb12.1491979089">>

If this type of log warning occurs, check the client to see why the processing of purge requests is stalled in it.

There is a mapping relationship between a design document of indexes and local checkpoint docs. If a design document of indexes is updated or deleted, the corresponding local checkpoint document should be also automatically deleted. But in an unexpected case, when a design doc was updated/deleted, but its checkpoint document still exists in a database, the following warning will be issued:

    "Invalid purge doc '<<"_design/bar">>' on database
    <<"shards/00000000-1fffffff/testdb12.1491979089">>
    with purge_seq '50'"

If this type of log warning occurs, remove the local purge doc from a database.