Sub-Document Operations

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 SQL++ syntax. Considering the document:

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

The lookupIn 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.

The examples use the following imports:

Operations in the Scala SDK return a Try to indicate success or failure. It can be hard to read multiple nested Try so you may prefer to use either flatMap:

or a for-comprehension:

Multiple operations can be combined, and this can be most neatly done with a for-comprehension:

The Scala SDK provides three APIs for all operations. There’s the simple blocking one you’ve already seen, then this asynchronous variant that returns Scala Future:

And a third that uses reactive programming primitives from Project Reactor:

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 mutateIn API. Here’s an example of one which replaces one path and removes another.

Mutation operations can take primitives (String, Int, etc.), along with any type for which an Encodable can be found. This includes the built-in JSON library (JsonObject and JsonArray), many type from third-party JSON libraries, and Scala case classes.

See the Key Value Operations page for more details.

The arrayPrepend and arrayAppend operations are true array prepend and append operations. Unlike fulldoc append/prepend operations (which simply concatenate bytes to the existing value), arrayAppend and arrayPrepend 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 create an array if it does not exist and also push elements to it within the same operation you may use the createPath option:

Limited support also exists for treating arrays like unique sets, using the arrayAddUnique 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 arrayAddUnique will fail with a PathMismatchException if the array contains JSON floats, objects, or arrays. The arrayAddUnique operation will also fail with CannotInsertValueException 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 increment and decrement full-document operations:

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 createPath 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 lookupIn or mutateIn command, the server will execute all the operations with the same version of the document.

When submitting multiple mutation operations within a single mutateIn 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 mutateIn, even if those commands would have been successful had another command not failed.

When submitting multiple retrieval operations within a single lookupIn 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 lookupIn 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 createPath option may be used.

Couchbase Server 7.6 and later support Sub-Doc lookup from replicas, which could be used to, for example, increase availability during node failover scenarios.

The collection.lookupInAnyReplica() method reads from all replicas and the active, and returns the first response.

If no replica or active returns a successful result, a Failure(DocumentUnretrivableException) is returned. This includes if the document does not exist.

The collection.lookupInAllReplicas() fetches all available replicas (and the active), and returns all responses.

Errors from the replicas and the active are ignored, so if all fail (including if the document does not exist), an empty stream is returned.

You may want to use lookupInAllReplicas to build a consensus, but it’s more likely that you’ll make use of lookupInAnyReplica as a fallback to a lookupIn, when the active node times out.

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 arrayAppend 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 durability, using PersistTo and ReplicateTo, is still available, particularly for talking to Couchbase Server 6.0 and earlier:

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 DocumentNotFoundException) 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 SQL++ 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 SQL++ 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.