This document describes how to develop and deploy Document Processors, often called docproc in this documentation. Document processing is a framework to create chains of configurable components, that read and modify document operations.
The input source splits the input data into logical units called documents. A feeder application sends the documents into a document processing chain. This chain is an ordered list of document processors. Document processing examples range from language detection, HTML removal and natural language processing to mail attachment processing, character set transcoding and image thumbnailing. At the end of the processing chain, extracted data will typically be set in some fields in the document.
The motivation for document processing is that code and configuration is atomically deployed, as like all Vespa components. It is also easy to build components that access data in Vespa as part of processing.
To get started, see the sample application. Read indexing to understand deployment and routing. As document processors are chained components just like Searchers, read Searcher Development. For reference, see the Javadoc, and services.xml.
Refer to album-recommendation-docproc to get started, LyricsDocumentProcessor.java is a document processor example. Add the document processor in services.xml, and then add it to a chain. The type of processing done by the processor dictates what chain it should be part of:
An example that adds a general document processor to the "default" chain, and an indexing related processor to the chain for a particular content cluster:
<services> <container version="1.0" id="default"> <document-processing> <chain id="default"> <documentprocessor id="ai.vespa.example.album.LyricsDocumentProcessor" bundle="albums-docproc"/> </chain> <chain id="my-indexing" inherits="indexing"> <documentprocessor id="ai.vespa.example.album.PostIndexingDocumentProcessor" bundle="albums-docproc"/> </chain> </document-processing> </container> <content version="1.0" id="content"> <documents> ... <document-processing cluster="default" chain="my-indexing" /> </documents> </content> </services>
The "default" chain, if it exists, is run by default, before the chain used for indexing. The default indexing chain is called "indexing", and must be inherited by any chain that is to replace it.
To run through any chain, specify a route which includes the chain.
For example, the route
default/chain.my-chain indexing would route feed operations
through the chain "my-chain" in the "default" container cluster, and then to the "indexing" hop,
which resolves to the specified indexing chain for each content cluster the document should be sent to.
More details can be found in indexing:
A document processor is a component extending
All document processors must implement
public Progress process(Processing processing);
When the container receives a document operation,
it will create a new
Processing, and add the
DocumentRemoves to the
The latter is useful also where a processing should be stopped
say for blocklist use, to stop a
Furthermore, the call stack of the document processing chain in question will
be copied to
so that document processors may freely modify the flow of control for this processing
without affecting all other processings going on.
After creation, the
Processing is added to an internal queue.
A worker thread will retrieve a
Processing from the input queue,
and run its document operations through its call stack.
A minimal, no-op document processor implementation is thus:
process() method should loop through all
document operations in
whatever it sees fit to them, and return a Progress:
||Returned if a document processor has successfully processed a
Processing failed and the input message should return a fatal
failure back to the feeding application,
meaning that this application will not try to re-feed this document operation.
Return an error message/reason by calling
See execution model.
The document processor wants to release the calling thread and be called again later.
This is useful if e.g. calling an external service with high latency.
The document processor may then save its state in the
By the use of
||Processing failed and the input message should return a transient failure back to the feeding application, meaning that this application may try to re-feed this document operation.|
Throwing any other
The call stack mentioned above is another name for a document processor chain.
Document processor chains are a special case of the general
component chains -
to avoid confusion some concepts are explained here as well.
A document processor chain is nothing more than a list of document processor instances,
having an id, and represented as a stack.
The document processor chains are typically not created for every processing,
but are part of the configuration.
Multiple ones may exist at the same time,
the chain to execute will be specified by the message bus destination of the incoming message.
The same document processor instance may exist in multiple document processor chains,
which is why the
CallStack of the
is responsible for knowing the next document processor to invoke in a particular message.
The execution order of the document processors in a chain are not ordered explicitly, but by ordering constraints declared in the document processors or their configuration.
The Document Processing Framework works like this:
Progress.FAILEDor thrown an exception.
There is a single instance of each document processor chain. In every chain, there is a single instance of each document processor - unless a chain is configured with multiple, identical document processors - this is a rare case.
As is evident from the model above,
multiple worker threads execute the document processors in a chain concurrently.
Thus, many threads of execution can be going through
process() in a
document processor, at the same time.
This model places an important constraint on document processor classes: instance variables are not safe. They must be eliminated, or made thread-safe somehow.
Also see Resource management,
deconstruct() in order to not leak resources.
The execution model outlined above also shows one important restriction:
If a document processor performs any high-latency operation in its process() method,
a docproc worker thread will be occupied.
With all n worker threads blocking on an external resource,
throughput will be limited.
This can be fixed by saving the state in the Processing object,
A document processor doing a high-latency operation should use a pattern like this:
Progress.LATER. This Processing is the appended to the back of the input queue, and we will be called again later.
As is evident, this will let the finite set of document processing threads to do more work at the same time.
Any state in the document processor for the particular Processing should be kept as local variables in the process method, while state which should be shared by all Processings should be kept as member variables. As the latter kind will be accessed by multiple threads at any one time, the state of such member variables must be thread-safe. This critical restriction is similar to those of e.g. the Servlet API. Options for implementing a multithread-safe document processor with instance variables:
Processing has a map
String -> Object
that can be used to pass information between document processors.
It is also useful when using
Progress.LATER to save the state of a processing - see
get/setVariable and more.
The sample application uses such context variables, too.
Ordering of feed operations is not guaranteed. Operations on different documents will be done concurrently and are therefore not ordered. However, Vespa guarantees that operations on the same document are processed in the order they were fed if they enter vespa at the same feed endpoint.
Document operations that are produced inside a document processor obey the same rules as at feed time. If you either split the input into other documents or into multiple operations to the same document, Vespa will ensure that operations to the same document id are sequenced and are delivered in the order they enter.
Consider the following configuration:
<?xml version="1.0" encoding="utf-8" ?> <services version="1.0"> <container version="1.0" id="default"> <document-processing> <chain id="default"> <documentprocessor id="SomeDocumentProcessor"> <config name="foo.something"> <variable>value</variable> </config> </documentprocessor> </chain> </document-processing> </container> </services>
Changing chain ids, components in a chain, component configuration and schema mapping all takes effect after vespa activate - no restart required. Changing a cluster name (i.e. the container id) requires a restart of docproc services after vespa activate.
Note when adding or modifying a processing chain in a running cluster; if at the same time deploying a new document processor (i.e. a document processor that was unknown to Vespa at the time the cluster was started), the container must be restarted:
$ vespa-sentinel-cmd restart container
The framework core supports asynchronous processing, processing one or multiple documents or document updates at the same time, document processors that makes dynamic decisions about the processing flow and passing of information between processors outside the document or document update:
Docproc Servicesmay be created. One of the services is the default.
DocumentOperationfor processing, meaning
DocumentRemoves. It has a
Call Stackwhich lists the calls to make to various
DocumentProcessorsto process each DocumentOperation handed to the service.
Calls, which refer to the Document Processor instance to call.
Processingfor its duration (instead of in a thread or process). A Document Processor may make some asynchronous calls (typically to remote services) and return to the framework that it should be called again later for the same Processing to handle the outcome of the calls.
context, which is a Map of named values which can be used to pass arguments between processors.