Sub-Document Operations with the Python SDK

Starting with Couchbase Server 4.5 you can atomically and efficiently update and retrieve parts of a document.

These parts are called Sub-Documents.

While full-document retrievals retrieve the entire document and full document updates require sending the entire document, Sub-Document retrievals only retrieve relevant parts of a document and Sub-Document updates only require sending the updated portions of a document.

You should use Sub-Document operations when you are modifying only portions of a document, and full-document operations when the contents of a document is to change significantly.

In order to use Sub-Document operations you need to specify a path indicating the location of the Sub-Document. The path follows N1QL syntax. Considering the document:

The paths name, addresses.billing.country and purchases.complete[0] are all valid paths.

The lookup_in operations query the document for certain path(s); these path(s) are then returned. You have a choice of actually retrieving the document path using the get Sub-Document operation, or simply querying the existence of the path using the exists Sub-Document operation. The latter saves even more bandwidth by not retrieving the contents of the path if it is not needed.

Multiple operations can be combined:

Mutation operations modify one or more paths in the document. The simplest of these operations is upsert, which, similar to the fulldoc-level upsert, will either modify the value of an existing path or create it if it does not exist:

Likewise, the insert operation will only add the new value to the path if it does not exist:

Dictionary values can also be replaced or removed, and you may combine any number of mutation operations within the same general mutate_in API. Here’s an example of one which replaces one path and removes another.

The array_prepend and array_append operations are true array prepend and append operations. Unlike fulldoc append/prepend operations (which simply concatenate bytes to the existing value), array_append and array_prepend are JSON-aware:

If your document only needs to contain an array, you do not have to create a top-level object wrapper to contain it. Simply initialize the document with an empty array and then use the empty path for subsequent Sub-Document array operations:

If you wish to add multiple values to an array, you may do so by passing multiple values to the array_append, array_prepend, or array_insert operations. Be sure to know the difference between passing a collection of multiple elements (in which case the collection is inserted as a single element in the array, as a sub-array) and passing multiple elements (in which case the elements are appended individually to the array):

Note that passing multiple values to a single array_append operation results in greater performance increase and bandwidth savings than simply specifying a single array_append for each element.

If you wish to create an array if it does not exist and also push elements to it within the same operation you may use the create_parents option:

Limited support also exists for treating arrays like unique sets, using the array_addunique command. This will do a check to determine if the given value exists or not before actually adding the item to the array:

Note that currently the array_addunique will fail with a SubdocPathMismatchException if the array contains JSON floats, objects, or arrays. The array_addunique operation will also fail with SubdocCantInsertValueException if the value to be added is one of those types as well.

Note that the actual position of the new element is undefined, and that the array is not ordered.

New elements can also be inserted into an array. While append will place a new item at the end of an array and prepend will place it at the beginning, insert allows an element to be inserted at a specific position. The position is indicated by the last path component, which should be an array index. For example, to insert "cruel" as the second element in the array ["Hello", "world"], the code would look like:

Note that the array must already exist and that the index must be valid (i.e. it must not point to an element which is out of bounds).

Counter operations allow the manipulation of a numeric value inside a document. These operations are logically similar to the counter operation on an entire document:

The increment and decrement operations perform simple arithmetic against a numeric value. The updated value is returned.

The existing value for counter operations must be within range of a 64 bit signed integer. If the value does not exist, the operation will create it (and its parents, if create_path is enabled).

Note that there are several differences as compared to the full-document counter operations:

Multiple Sub-Document operations can be executed at once on the same document, allowing you to retrieve or modify several Sub-Documents at once. When multiple operations are submitted within the context of a single lookup_in or mutate_in command, the server will execute all the operations with the same version of the document.

When submitting multiple mutation operations within a single mutate_in command, those operations are considered to be part of a single transaction: if any of the mutation operations fail, the server will logically roll-back any other mutation operations performed within the mutate_in, even if those commands would have been successful had another command not failed.

When submitting multiple retrieval operations within a single lookup_in command, the status of each command does not affect any other command. This means that it is possible for some retrieval operations to succeed and others to fail. While their statuses are independent of each other, you should note that operations submitted within a single lookup_in are all executed against the same version of the document.

Sub-Document mutation operations such as upsert or insert will fail if the immediate parent is not present in the document. Consider:

Looking at the some_field field (which is really level_0.level_1.level_2.level_3.some_field), its immediate parent is level_3. If we were to attempt to insert another field, level_0.level_1.level_2.level_3.another_field, it would succeed because the immediate parent is present. However if we were to attempt to insert to level_1.level_2.foo.bar it would fail, because level_1.level_2.foo (which would be the immediate parent) does not exist. Attempting to perform such an operation would result in a Path Not Found error.

By default the automatic creation of parents is disabled, as a simple typo in application code can result in a rather confusing document structure. Sometimes it is necessary to have the server create the hierarchy however. In this case, the create_path option may be used.

Concurrent Sub-Document operations on different parts of a document will not conflict. For example the following two blocks can execute concurrently without any risk of conflict:

Even when modifying the same part of the document, operations will not necessarily conflict. For example, two concurrent array_append operations to the same array will both succeed, never overwriting the other.

So in some cases the application will not need to supply a CAS value to protect against concurrent modifications.

If CAS is required then it can be provided like this:

Couchbase’s traditional 'client verified' durability, using PersistTo and ReplicateTo, is still available, particularly for talking to Couchbase Server 6.0 and earlier. Refer to the API docs for the legacy API.

In Couchbase Server 6.5 and up, this is built upon with Durable Writes, which uses the concept of majority to indicate the number of configured Data Service nodes to which commitment is required:

Sub-Document operations have their own set of errors. When programming with Sub-Document, be prepared for any of the full-document errors (such as DocumentDoesNotExistException) as well as special Sub-Document errors which are received when certain constraints are not satisfied. Some of the errors include:

If a Sub-Document command fails a top-level error is reported (MultiMutationException), rather than an individual error code (e.g. PathNotFoundException). When receiving a top-level error code, you should traverse the results of the command to see which individual code failed.

Path syntax largely follows N1QL conventions: A path is divided into components, with each component referencing a specific level in a document hierarchy. Components are separated by dots (.) in the case where the element left of the dot is a dictionary, or by brackets ([n]) where the element left of the bracket is an array and n is the index within the array.

As a special extension, you can indicate the last element of an array by using an index of -1, for example to get the last element of the array in the document

Use some.array[-1] as the path, which will return the element 0.

Each path component must conform as a JSON string, as if it were surrounded by quotes, and any character in the path which may invalidate it as a JSON string must be escaped by a backslash (\). In other words, the path component must match exactly the path inside the document itself. For example:

must be referenced as literal\"quote.array.

If the path also has special path characters (i.e. a dot or brackets) it may be escaped using N1QL escapes. Considering the document

A path such as `literal[]bracket`.`literal.dot`. You can use double-backticks (``) to reference a literal backtick.

If you need to combine both JSON and path-syntax literals you can do so by escaping the component from any JSON string characters (e.g. a quote or backslash) and then encapsulating it in backticks (`path`).

Extended Attributes (also known as XATTRs), built upon the Sub-Document API, allow developers to define application-specific metadata that will only be visible to those applications that request it or attempt to modify it. This might be, for example, meta-data specific to a programming framework that should be hidden by default from other frameworks or libraries, or possibly from other versions of the same framework. They are not intended for use in general applications, and data stored there cannot be accessed easily by some Couchbase services, such as Search.