# Creating a new kind of servable

This document explains how to extend TensorFlow Serving with a new kind of servable. The most prominent servable type is SavedModelBundle, but it can be useful to define other kinds of servables, to serve data that goes along with your model. Examples include: a vocabulary lookup table, feature transformation logic. Any C++ class can be a servable, e.g. int, std::map<string, int> or any class defined in your binary -- let us call it YourServable.

## Defining a Loader and SourceAdapter for YourServable

To enable TensorFlow Serving to manage and serve YourServable, you need to define two things:

1. A Loader class that loads, provides access to, and unloads an instance of YourServable.

2. A SourceAdapter that instantiates loaders from some underlying data format e.g. file-system paths. As an alternative to a SourceAdapter, you could write a complete Source. However, since the SourceAdapter approach is more common and more modular, we focus on it here.

The Loader abstraction is defined in core/loader.h. It requires you to define methods for loading, accessing and unloading your type of servable. The data from which the servable is loaded can come from anywhere, but it is common for it to come from a storage-system path. Let us assume that is the case for YourServable. Let us further assume you already have a Source<StoragePath> that you are happy with (if not, see the Custom Source document).

In addition to your Loader, you will need to define a SourceAdapter that instantiates a Loader from a given storage path. Most simple use-cases can specify the two objects concisely via the SimpleLoaderSourceAdapter class (in core/simple_loader.h). Advanced use-cases may opt to specify Loader and SourceAdapter classes separately using the lower-level APIs, e.g. if the SourceAdapter needs to retain some state, and/or if state needs to be shared among Loader instances.

There is a reference implementation of a simple hashmap servable that uses SimpleLoaderSourceAdapter in servables/hashmap/hashmap_source_adapter.cc. You may find it convenient to make a copy of HashmapSourceAdapter and then modify it to suit your needs.

The implementation of HashmapSourceAdapter has two parts:

1. The logic to load a hashmap from a file, in LoadHashmapFromFile().

2. The use of SimpleLoaderSourceAdapter to define a SourceAdapter that emits hashmap loaders based on LoadHashmapFromFile(). The new SourceAdapter can be instantiated from a configuration protocol message of type HashmapSourceAdapterConfig. Currently, the configuration message contains just the file format, and for the purpose of the reference implementation just a single simple format is supported.

Note the call to Detach() in the destructor. This call is required to avoid races between tearing down state and any ongoing invocations of the Creator lambda in other threads. (Even though this simple source adapter doesn't have any state, the base class nevertheless enforces that Detach() gets called.)

## Arranging for YourServable objects to be loaded in a manager

Here is how to hook your new SourceAdapter for YourServable loaders to a basic source of storage paths, and a manager (with bad error handling; real code should be more careful):

First, create a manager:

std::unique_ptr<AspiredVersionsManager> manager = ...;


Then, create a YourServable source adapter and plug it into the manager:

auto your_adapter = new YourServableSourceAdapter(...);


Lastly, create a simple path source and plug it into your adapter:

std::unique_ptr<FileSystemStoragePathSource> path_source;
// Here are some FileSystemStoragePathSource config settings that ought to get
// it working, but for details please see its documentation.
FileSystemStoragePathSourceConfig config;
// We just have a single servable stream. Call it "default".
config.set_servable_name("default");
config.set_base_path(FLAGS::base_path /* base path for our servable files */);
config.set_file_system_poll_wait_seconds(1);
TF_CHECK_OK(FileSystemStoragePathSource::Create(config, &path_source));


## Accessing loaded YourServable objects

Here is how to get a handle to a loaded YourServable, and use it:

auto handle_request = serving::ServableRequest::Latest("default");
ServableHandle<YourServable*> servable;
Status status = manager->GetServableHandle(handle_request, &servable);
if (!status.ok()) {
LOG(INFO) << "Zero versions of 'default' servable have been loaded so far";
return;
}
// Use the servable.
(*servable)->SomeYourServableMethod();


## Advanced: Arranging for multiple servable instances to share state

SourceAdapters can house state that is shared among multiple emitted servables. For example:

• A shared thread pool or other resource that multiple servables use.

• A shared read-only data structure that multiple servables use, to avoid the time and space overhead of replicating the data structure in each servable instance.

Shared state whose initialization time and size is negligible (e.g. thread pools) can be created eagerly by the SourceAdapter, which then embeds a pointer to it in each emitted servable loader. Creation of expensive or large shared state should be deferred to the first applicable Loader::Load() call, i.e. governed by the manager. Symmetrically, the Loader::Unload() call to the final servable using the expensive/large shared state should tear it down.