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CouchDB Quirks

Mango indexes

Exists operator

The $exists operator can be used with a mango index for the true value, but not for the false value. For false, a more heavy solution is required: a partial index.

Index selection

CouchDB may accept or refuse to use a mango index for a query, with obsure reasons. In general, you can follow these two rules of thumb:

  1. An index on the fields foo, bar, baz can be used only to fetch documents where foo, bar, and baz exist. It means that a query that filters only on the value on foo won’t use the mango index, because it can miss a document where foo has the expected value but without bar or baz. If you know that all the documents that you want have the bar and baz fields, you can just add two filters $exists: true (one for bar, the other for baz).

  2. You should use exactly the same sequence of fields for creating the index and the sort operator of the query. If you have an index on os, browser, ip for the io.cozy.sessions.logins, and you want to have all the documents for a login from windows, sorted by browser, you can use the index, but you should use os, browser, ip for the sort (or at least os, browser, even if it is seems to weird to sort on os when all the sorted documents will have the same value, windows). Please note that using use_index on a request, the results will be sorted by default according to this rule. So, you can omit the sort operator on the query (except if you want the descending order).

Comparison of strings

Comparison of strings is done using ICU which implements the Unicode Collation Algorithm, giving a dictionary sorting of keys. This can give surprising results if you were expecting ASCII ordering. Note that:

  • All symbols sort before numbers and letters (even the “high” symbols like tilde, 0x7e)
  • Differing sequences of letters are compared without regard to case, so a < aa but also A < aa and a < AA
  • Identical sequences of letters are compared with regard to case, with lowercase before uppercase, so a < A.

Old revisions

CouchDB keeps for each document a list of its revision (or more exactly a tree with replication and conflicts).

It’s possible to ask the list of the old revisions of a document with GET /db/{docid}?revs_info=true. It works only if the document has not been deleted. For a deleted document, a trick is to query the changes feed to know the last revision of the document, and to recreate the document from this revision.

With an old revision, it’s possible to get the content of the document at this revision with GET /db/{docid}?rev={rev} if the database was not compacted. On CouchDB 2.x, compacts happen automatically on all databases from times to times.

A purge operation consists to remove the tombstone for the deleted documents. It is a manual operation, triggered by a POST /db/_purge.

Conflicts

It is possible to create a conflict on CouchDB like it does for the replication by using new_edits: false, but it is not well documented to say the least. The more accurate description was in the old wiki, that no longer exists. Here is a copy of what it said:

The replicator uses a special mode of _bulk_docs. The documents it writes to the destination database already have revision IDs that need to be preserved for the two databases to be in sync (otherwise it would not be possible to tell that the two represent the same revision.) To prevent the database from assigning them new revision IDs, a “new_edits”:false property is added to the JSON request body.

Note that this changes the interpretation of the _rev parameter in each document: rather than being the parent revision ID to be matched against, it’s the existing revision ID that will be saved as-is into the database. And since it’s important to retain revision history when adding to the database, each document body in this mode should have a _revisions property that lists its revision history; the format of this property is described on the HTTP document API. For example:

curl -X POST -d '{"new_edits":false,"docs":[{"_id":"person","_rev":"2-3595405","_revisions":{"start":2,"ids":["3595405","877727288"]},"name":"jim"}]}' "$OTHER_DB/_bulk_docs"

This command will replicate one of the revisions created above, into a separate database OTHER_DB. It will have the same revision ID as in DB, 2-3595405, and it will be known to have a parent revision with ID 1-877727288. (Even though OTHER_DB will not have the body of that revision, the history will help it detect conflicts in future replications.)

As with _all_or_nothing, this mode can create conflicts; in fact, this is where the conflicts created by replication come from. In short, it’s a PUT /doc/{id}?new_edits=false with _rev the new revision of the document, and _revisions the parents of this revision in the revisions tree of this document.

Conflict example

Here is an example of a CouchDB conflict.

Let’s assume the following document with the revision history [1-abc, 2-def] saved in database:

{
  "_id": foo,
  "_rev": 2-def,
  "bar": "tender",
  "_revisions": {
    "ids": [
      "def",
      "abc"
    ]
  }
}

The _revisions block is returned when passing revs=true to the query and gives all the revision ids, which the revision part after the dash. For instance, in 2-def, 2 is called the “generation” and def the “id”.

We update the document with a POST /bulk_docs query, with the following content:

{
    "docs": [
        {
            "_id": "foo",
            "_rev": "3-ghi",
            "_revisions": { "start": 3, "ids": ["ghi", "xyz", "abc"] }
            ,
            "bar": "racuda"
        }
    ],
    "new_edits": false
}

This produces a conflict bewteen 2-def and 2-xyz: the former was first saved in database, but we forced the latter to be a new child of 1-abc. Hence, this document will have two revisions branches: 1-abc, 2-def and 1-abc, 2-xyz, 3-ghi.

Sharing

In the sharing protocol, we implement this behaviour as we follow the CouchDB replication model. However, we prevent CouchDB conflicts for files and directories: see this explanation

Design docs in _all_docs

When querying GET /{db}/_all_docs, the response include the design docs. It’s quite difficult to filter them, particulary when pagination is involved. We have added an endpoint GET /data/:doctype/_normal_docs to the stack to help client side applications to deal with this.