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A server for multilanguage, composable NLP API in Python
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1. Charade


A server for multilanguage, composable NLP API in Python.

1.1. Philosophy

Charade was born as a container where multiple independent natural language services can coexist and interact with each other. In order to develop on Charade, it may be useful to understand the reasons behind its implementation.

  • multiple analyses can be run over a single text - for instance named entity recognition and sentiment detection - so a request from a user should be able to specify what kind of tasks should be performed on the provided text
  • to avoid repeting work and ensure consistency, one task may be dependent on another: for instance, if both the NER and sentiment analysis rely on the same parsing stage, they will get to see the same tokens, something which would not be guaranteed if the two analyses performed tokenization internally
  • a single task could have many coexisting implementations, so that a developer would be free to experiment with new models without having to interfere with existing ones. The user consuming the service could then be able to request a particular implementation of a task by specifying its name
  • multiple implementations of a single task should offer a consistent interface, in order to ensure that clients or other downstream services can switch between them freely
  • the server should not be restricted to a single (natural) language, and various implementations should be free to decide what languages to support
  • developers implementing various models should be able to choose freely what technology to use, so various services can be implemented on top of NLTK, spaCy, pyTorch, TensorFlow, GenSim... Charade should make it easy to use any of these libraries to implement a particular model, without forcing other developers to adopt the same library
  • one should be able to implement as many tasks and models as desired, while choosing at deploy time which one are supported by the server - i.e. the server should be composable from Lego pieces

Therefore, the process of deploying Charade servers works as follows. The developers write various models to perform some tasks, possibly trying competing implementations in parallel. Various kind of models are already provided with Charade, but you should not shy from writing your own.

Once the models are ready, one writes an entry point script that actually loads only the ones that will be used in production. At every point of the process, one has available an API offering the existing models, and a user interface to try them.

1.2. What Charade is and is not

Charade is a framework that helps teams experimenting with multiple approaches to tackle some custom NLP task. It is meant to leverage existing NLP libraries, such as NLTK or spaCy, and not to replace them. A team using Charade can develop and evolve a suite of NLP capabilities - say NER, sentiment analysis and so on - while maintaining the possibility to customize them on particular datasets, and compose servers where only the relevant capabilities are deployed.

Charade is not itself a library for NLP tasks, although it provides some examples of models developed using various libraries. It is not a ready-made component either: while some of the models provided can be useful, we expect that teams using Charade will develop and customize their own models. The provided ones can serve as example, or can provide some capabilities in a larger deployment.

1.3. Installing

NB If you are on MacOS Mojave, make sure to have the XCode headers installed

xcode-select --install
open /Library/Developer/CommandLineTools/Packages/macOS_SDK_headers_for_macOS_10.14.pkg

Also, OpenMP is required by PyTorch, on MacOS it can be installed by

brew install libomp

1.3.1. Using Pipenv (recommended)

Install Pipenv if needed (pip install pipenv). An introduction to Pipenv can be found here.

Create a virtual environment related to this project by running pipenv shell from inside the top directory in the project.

  1. If you want to develop Charade, you can install dependencies with this command:
pipenv install --dev

If you also make the iPython kernel for Charade visible to other environments, you can use

python -m ipykernel install --user --name="charade"

In this way, you can use any installation of Jupyter to launch the charade kernel.

  1. If instead you want to try Charade without developing, then run
pipenv install --ignore-pipfile

to install all dependencies.

  1. In both cases, download the models for spacy, allen-nlp and nltk via
python -m spacy download en
python -m spacy download it
python -m spacy download de

python -m nltk.downloader averaged_perceptron_tagger
python -m nltk.downloader maxent_ne_chunker
python -m nltk.downloader words

mkdir -p models/allen/pretrained
wget -O models/allen/pretrained/ner-model-2018.12.18.tar.gz Common errors

NB If you get an error that you don't have the right version of Pyhton, you can manage that through PyEnv. To install PyEnv, see the installation instructions. On MacOS just run brew install pyenv. After having install PyEnv, install the required version of Pyhton, for instance pyenv install 3.6.8.

After this step, pipenv should detect the version of pyenv automatically.

1.3.2. Using Conda and Pip

If you don't need to develop Charade itself, you can create a virtual environment in Conda by running something like conda create -n charade python=3.6, then activate it with source activate charade (any other name will do). Then install dependencies with Pip inside the environment:

pip install -r requirements.txt

Finally, update spacy models via

python -m spacy download en
python -m spacy download it

NB The requirements.txt file is autogenerated by Pipenv with the command pipenv lock --requirements > requirements.txt - do not edit this file by hand.

1.4. Running

Just define the server in src/, then run

python src/

The existing file only contains those models that do not require a custom training step. The other models are commented. You can launch any of the traning scripts - they are ready, but may be trained on toy datasets, so be ready to adjust them to your needs - and then uncomment the resulting models in the main script.

Once you have a running server, you can try some queries. An example query can be sent using examples/ You can pass a parameter to select a particular request, for instance

examples/ reprise

You can see available examples with ls examples.

Also, there is a frontend available at http://localhost:9000/app.

1.5. Docker running

The docker can be built by using scripts/ Then, to run the docker container simply do

docker-compose up

NB Since both uwsgi and some services (e.g. pytorch) make use of multiple threads, this can cause deadlocks. To avoid them, we need to run the uwsgi command with the option --lazy-apps as specified in the Dockerfile (see for an explanation of this mechanism). Note that if the uwsgi option --processes is > 1, each worker will load the full application and thus the server startup may require a lot of time and memory. By employing multiple threads and a single process instead (e.g. --processes 1 --threads 4) the server startup is fast enough.

1.6. Endpoints

A Charade server had just two endpoints:

  • GET /: returns a JSON describing the available services
  • POST /: post a request with a text and some services to be performed

1.7. Architecture

A Charade server is defined by instantiating and putting together various services. Each service is defined by

  • a task
  • a service name
  • optional dependencies
  • an actual implementation.

Tasks are used to denote interchangeable services. For instance, there may exist various NER models, possibly using different libraries and technologies. In this case, we will define a ner task, with the only requirement that if there are various implementations of ner, they need to abide to the same interface.

Names are used to distinguish different implementations of the same task. The task/name pair should identify a unique service. For instance, one could have deployed ner services named allen, nltk, pytorch-crf, pytorch-crf-2.

Dependencies can be used to avoid repeating the same task over and over. For instance, a ner implementation may (or may not) depend on some implementation of the parse task, which takes care of tokenization. At runtime, the server will ensure that the parse task is executed before ner.

The precise mechanism is as follows. The user request contains a field called tasks, which contains the list of tasks to be executed on the given chunk of text. For instance:

  "tasks": [
    {"task": "parse", "name": "spacy"},
    {"task": "ner", "name": "allen"},
    {"task": "dates", "name": "misc"}

Tasks are executed in the order requested by the user. The objects returned by the various tasks populate corresponding fields in a response dictionary. For instance, for this request, the response object will have the shape

  "parse": ...,
  "ner": ...,
  "dates": ...

Each service can look at the request object and the response object (the part that has been populated so far). In this way, a service can look at the output produced by other services that come before.

If a dependency for a service has not been requested explicitly by the user, the server will choose any implementation of the dependency task and execute it before the dependent task. For instance, say one has a ner service called custom which depends on parse. If the user request contains

  "tasks": [
    {"task": "ner", "name": "custom"},
    {"task": "dates", "name": "misc"}

then the server will choose any implementation of parse and perform it before ner. This has two advantages:

  • duplication is reduced, for instance the parsing and tokenization of the text can be done just once and many other services can consume it
  • one has the guarantee that all services rely on the same tokenization, giving a better consistency.

Implementations are defined by writing a class that inherits from services.Service. The methods to override are, response) and Service.describe() (optional, but recommended). The former has access to

  • the user request
  • the part of the response constructed so far

and has to return a dictionary containing the service output. This method can raise services.MissingLanguage if the language of the request is not supported in the given service. The class should load any needed model in its constructor, to avoid reloading models for each request.

For instance, a trivial parser that just splits sentences on period and tokens on whitespace may look like this:

from services import Service

class SimpleParser(Service):
    def __init__(self):

    def run(self, request, response):
        text = request['text']
        debug = request.get('debug', False)
        result = []
        start = 0
        end = 0
        for sentence in text.split('\.'):
            tokens = []
            for token in sentence.split(' '):
                start = end + 1
                end += start + len(token)
                if debug:
                        'text': token,
                        'start': start,
                        'end': end
                        'start': start,
                        'end': end
        return result

1.8. Requests

The user requests have the following fields:

  • text: required, the text to be analyzed
  • debug: optional flag, default False. Services can use this flag to decide to include additional information. Also, when this flag is set, the response contains an additional field debug with general information, such as timing of the services and the resolved ordering among tasks.
  • lang: 2 letter language of the text, optional. Default: autodetect
  • previous: see Resumable requests
  • tasks: a list of requested tasks, with the shape
  "tasks": [
    {"task": "parse", "name": "spacy"},
    {"task": "ner", "name": "allen"},
    {"task": "dates", "name": "misc"}

plus possibly other service-dependent fields.

1.8.1. Resumable requests

Say there are two tasks, task A and task B. Task A has a dependency on B, which is much slower. When trying various implementations for A, it does not make sense to recompute the result of task B again and again. In this case, one may want to issue a request for task B, and then a second request for task A, passing the result of the previous request. In this way, there will be no need to recompute the result of task B.

In this case, one can put a field called previous in the request. The content of the field must match the response for the previous request. In this case, the server will resume computation from that point. For instance, a user request may look like this:

  "text": "Ulisse Dini (Pisa, 14 novembre 1845 ...",
  "tasks": [
    {"task": "names", "name": "misc"}
  "previous": {
    "ner": [
        "text": "Ulisse Dini",
        "start": 0,
        "end": 11,
        "label": "PER"

In this example, the ner step is already computed, and does not need to be recomputed again.

1.9. Describing services

Each service can be self describing by ovverriding the method describe(self) of the Service class. This can be used to report information about supported languages, dependencies, additional parameters needed in the request, trained models and so on. The class Service already defines a basic implementation, while services can add more specific information. Some standard keys to use for this purpose are:

  • langs: the supported languages; use ['*'] if any languages are supported
  • extra-params: an optional list of additional parameters of the request accepted by the service (see example)
  • models: a dictionary containing the information about the models used by the service

For each models, the following parameters are standardized:

  • pretrained: indicates that the model is included in the library
  • trained-at: datetime in ISO format
  • training-time: as format HH:mm:ss
  • datasets: list of datasets on which the model is trained
  • metrics: a dictionary of metrics that measure the performance of the model
  • params: a dictionary of parameters that were used to train the model

A complete example of response could look like this:

  'task': 'some-task',
  'name': 'my-name',
  'deps': ['parse'],
  'optional_deps': ['ner'],
  'langs': ['it', 'en'],
  'extra-params': [
      'name': 'some-param1',
      'type': 'string',
      'required': False
      'name': 'some-param2',
      'type': 'int',
      'required': True
      'name': 'some-param3',
      'type': 'string',
      'choices': ['value1', 'value2'],
      'required': True
  'models': {
    'it': {
      'pretrained': False,
      'trained-at': '2019-03-27T16:00:49',
      'training-time': '02:35:23',
      'datasets': ['some-dataset'],
      'metrics': {
        'accuracy': 0.935,
        'precision': 0.87235,
        'recall': 0.77253
      'params': {
        'learning-rate': 0.001,
        'momentum': 0.8,
        'num-epochs': 50
    'en': {
      'pretrained': True

You can use the extra-params field to describe additional parameters that are required (or optional) for a specific service. Each extra parameter can take the shape

  'name': <string>,
  'type': <string>,
  'choices': <string list?>,
  'required': <bool>

where type can take the values "string" or "int", and choices can be used to optionally constrain the valid values for the parameter.

1.10. Services

The following services are defined. To read the interface: output types are written inside <>. A trailing ? denotes that the field is only present when debug is True in the user request.

1.10.1. Parsing

Splits the text into sentences and the sentences into tokens. The interface requires that the output has the shape

    {'start': <int>, 'end': <int>, 'text': <string?>},

1.10.2. NER

Finds people, organizations, dates, places and other entities in the text. The interface requires that the output has the shape

  {'start': <int>, 'end': <int>, 'text': <string?>, 'label': <string>},

1.10.3. Date extraction

Finds and parses dates in the text. The interface requires that the output has the shape

  {'start': <int>, 'end': <int>, 'text': <string?>, 'date': <string>},

where date is formatted as yyyy-MM-dd.

1.10.4. Codes extraction

Finds common codes in the text. The interface requires that the output has the shape

  {'start': <int>, 'end': <int>, 'text': <string>, 'type': <string>, 'lang': <lang code>},

1.10.5. Fiscal codes

Extracts information from fiscal codes. The interface requires that the output has the shape

  {'start': <int>,
   'end': <int>,
   'text': <string>,
   'type': <string>,
   'lang': <lang code>,
   'correct': <bool>, # if the fiscal code is formally correct
   'sex': <sex code>,
   'birthdate' <string>

1.10.6. Extractive summarization

Extracts the sentences from the text that best summarize it. The interface requires that the output has the shape

  {'start': <int>, 'end': <int>, 'text': <string?>},

where the sentences are in order from most informative to least informative.

It can require additional (optional) parameters in the request:

  • num-extractive-sentences: the number of sentences to extract

1.10.7. Keyword extraction

Extracts the most relevant keywords from the text. The interface requires that the output has the shape

  {'text': <string>},

where the keywords are in order from most to least relevant. Here we do not use spans, since the important information is the keyword, which is probably repeated many times across the text.

It can require additional (optional) parameters in the request:

  • num-keywords: the number of keywords to extract

1.10.8. Sentiment detection

Detects the sentiment used in various sentences of the text. The interface requires that the output has the shape

  {'start': <int>, 'end': <int>, 'sentiment': <float>, 'text': <string?>},

where there is an entry for each sentence, and sentiment ranges from 0 (extremely negative) to 1 (extremely positive).

1.10.9. Names

Extract names and surnames of people mentioned in the text. It is a more refined version of NER, which just retrieves entities of type PER.

The interface requires that the output has the shape

  {'start': <int>, 'end': <int>, 'name': <string?>, 'surname': <string?>},

1.10.10. Topic modeling

Does a soft clustering of text (for instance using LDA or similar techniques). This means that the text is associated to a distribution over topics. Topics themselves are discovered as a word mixture from the training data. The interface requires that the output has the shape

  'distribution': <array[float]>,
  'best-topic': <int>,
  'best-score': <float>,
  'topics': <array[array[string]]?>,

where each topic is represented with the arrary of its most representative words. The topics field is only present in debug mode.

It can require additional (optional) parameters in the request:

  • lda-model: the name of a pretrained LDA model

1.10.11. Classification

Does a classification of the text in a pre-trained and finite set of possible classes. This means that the text is associated to a distribution over possible classes, of which we only output the most fitting. The interface requires that the output has the shape

  'category': <string>,
  'category_probability': <float>,
  'distribution': <map[string, float]?>

The distribution field is only present in debug mode.

1.11. How to create a new service

Create a new class in a file inside src/services which inherits from services.Service. In this class, make sure to call the Service constructor to register the service, like this:

class SomeService(Service):
    def __init__(self, langs):
        Service.__init__(self, 'some-task', 'some-name', [], []) # first required deps, then optional deps

Override the method def run(self, request, response) which implements the logic for your service. The return type for the service should be any dictionary.

Also, override the method describe(self) to return information about the service itself. A basic implementation of describe is in the Service class, so a standard implementation would look like:

def describe(self):
    result = super().describe()
    result['key'] = value
    # more keys
    return result

For the common keys, see the section on Describing services.

Be sure to check out the following things:

  • The return type of run should be JSON serializable
  • If your service defines a new task, make sure to document it in the README
  • Otherwise, follow the type convention of existing services for the same task
  • If your service requires some previous step (e.g. parsing), try to add it as a dependency and do not hardcode it inside the service
  • If your service may benefit of some previous step (e.g. extra hints), you can add it as optional dependency; the main task will be performed whether or not the optional dependency is already scheduled, but if the optional dependency is scheduled anyway, it will be executed first.
  • If your service requires an optional parameter in the request, add it in the schema validator in src/
  • If you cannot handle a certain language, raise services.MissingLanguage
  • If you have a model that needs a training step, follow the conventions under Organization
  • If you need an additional library, pipenv install the-library, then commit the new Pipfile and Pipfile.lock. Also remember to keep the requirements file up to date with pipenv lock --requirements > requirements.txt.
  • Add tests as needed

1.12. Testing

Tests are written with nose. If you have installed Charade in development mode (pipenv install --dev), you can run tests with the nosetests command.

Tests for a particular service should put under tests/services/ The naming convention is so that Nose autodiscovery will find them when running nosetests. Classes and methods should also follow this naming convention:

class TestTheThing(TestCase):
    def test_something(self):

You can also test here classes and functions under common. If you need to test something which is only used in training, put it under common as well.

Tests for Charade itself are placed under tests without further nesting.

1.13. Style guide

  • Follow PEP-8
  • Prefer long names such as request, result, token over req, res, tok
  • But be consistent with libraries: for instance, spacy defines document.ents Iterate over that as for ent in documents.ents:
  • Do not use trailing commas
  • Do not commit models or data - commit scripts to retrieve them
  • All bash scripts use set -e, set -u
  • Make sure that bash scripts can be called from anywhere (see the existing one for examples)

1.14. Organization

Follow a tree similar to the following

├── Pipfile
├── Pipfile.lock
├── data
│   └── ner
│       └── ...
├── examples
│   ├── request.json
│   ├──
│   ├── request2.json
│   └── request3.json
├── models
│   └── pytorch
│       └── ner
│           └── ...
├── requirements.txt
├── resources
│   ├── names
│   │   └── it.txt
│   ├── stopwords
│   │   └── en.txt
│   └── surnames
│       └── it.txt
├── scripts
│   └── pytorch
│       └── ner
│           └── it
│               ├──
│               ├──
│               └──
├── src
│   ├──
│   ├── common
│   │   ├──
│   │   └── pytorch
│   │       ├──
│   │       └── ner
│   │           ├──
│   │           └──
│   ├──
│   ├──
│   ├── services
│   │   ├──
│   │   ├──
│   │   ├──
│   │   ├──
│   │   ├──
│   │   ├──
│   │   └──
│   └── training
│       └── pytorch
│           └── ner
│               ├──
│               └──
└── tests
    ├── services
    │   ├──
    │   └──

It should be clear what goes where: data, models, resources, training and so on. When in doubt, follow existing conventions. The directory common holds code that should be shared at inference and training time.

Under data, only put data that is needed at training time - everything that is needed at inference time goes under models. If some data file is needed also at inference time, either

  • store the content of the file as a field inside the model, or
  • make sure that the training scripts copy the necessary files from data to models.
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