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Adapting New Services to Harmony

Please reach out in #harmony-service-providers (EOSDIS Slack) for additional guidance on any adaptation needs, and especially with any feedback that can help us improve.

Quick Start

Fully setting up a service can be overwhelming for first time service providers. We now provide a script bin/generate-new-service to generate much of the scaffolding to get services ready for local integration testing with harmony quickly. The scaffolding provides the following:

  1. Updates to env-defaults files to add needed environment variables for running the service
  2. Updates to services.yml to fill in a new service definition to call the service
  3. Updates to local environment variables in .env to ensure the service is deployed
  4. Updates to local environment variables in .env to bypass needing to have a UMM-S record ready to go in UAT and collections associated
  5. A new directory at the same level as the harmony repo that contains:
    1. Dockerfile defining an image that includes some common libraries used by service providers
    2. A script to build the service image
    3. Python wrapper code to make use of the harmony-service-library with hooks identified for places to add the custom service code

Setting up a new service

Prior to setting up a new service be sure to get harmony fully functional and tested with harmony-service-example by following the Quickstart in the main README. Then come back to this section to set up the new service.

  1. Run bin/generate-new-service and fill in values when prompted.
  2. Read the output after the script completes and follow the instructions provided in the terminal to finish setting up the service.

Using the script will help to see the files that need to be changed in order to test with harmony and many of the defaults will just work. Once you have finished testing things out be sure to follow the steps outlined in the rest of this document to ensure the service is ready to be integrated into other harmony test environments.

Note that the service chain that is generated in services.yml will define a service chain that queries for granules from the CMR and then invokes a single service image. If setting up a more complex service chain be sure to modify the entry.

Table of Contents

Requirements for Harmony Services

In order for a service to run in Harmony several things need to be provided as covered in the following sections. A simple reference service, harmony-service-example, provides examples of each. For examples of services that are being used in production see harmony-netcdf-to-zarr and podaac-l2ss-py. This document describes how to fulfill these requirements in depth.

1. Allowing Harmony to invoke services

Harmony provides a Python library, harmony-service-lib-py, to ease the process of adapting Harmony messages to service code. It provides helpers for message parsing, command line interactions, data staging, reading and writing STAC catalogs, and Harmony callbacks. Full details as well as an example can be found in the project's README and code. This is the preferred way for services to interact with Harmony as it handles much of the work for the service and makes it easy for services to stay up-to-date with Harmony.

2. Accepting Harmony requests

When invoking a service, Harmony provides an input detailing the specific operations the service should perform and the URLs of the data it should perform the operations on. Each new service will need to adapt this message into an actual service invocation, typically transforming the JSON input into method calls, command-line invocations, or HTTP requests. See the latest Harmony data-operation schema for details on Harmony's JSON input format.

Ideally, this adaptation would consist only of necessary complexity peculiar to the service in question. Please let the team know if there are components that can make this process easier and consider sending a pull request or publishing your code if you believe it can help future services.

3. Sending results to Harmony

This is handled automatically by the service library using the output of the service invocation.

4. Canceled requests

Canceled requests are handled internally by Harmony. Harmony will prevent further work from being sent to a service on behalf of a canceled request, but will not otherwise interact with a service that is already processing data on behalf of a request. For services employing the service library nothing needs to be done to support request cancellation.

5. Error handling

The best way to trigger a service failure when an unrecoverable condition is encountered is to extend HarmonyException (a class provided by the service library) and throw an exception of that type. The service library may also throw a HarmonyException when common errors are encountered.

Exceptions of this type (HarmonyException or a subclass) which are meant to be bubbled up to users should not be suppressed by the service. The exception will automatically be caught/handled by the service library. The exception message will be passed on to Harmony and bubbled up to the end user (accessible via the errors field in the job status Harmony endpoint (/jobs/<job-id>) and in many cases via the final job message). When possible services should strive to give informative error messages that give the end user some sense of why the service failed, without revealing any internal details about the service that could be exploited.

Here is a good example:

"""This module contains custom exceptions specific to the Harmony GDAL Adapter
    service. These exceptions are intended to allow for clearer messages to the
    end-user and easier debugging of expected errors that arise during an
    invocation of the service.
"""

from harmony.exceptions import HarmonyException

class HGAException(HarmonyException):
    """Base class for exceptions in the Harmony GDAL Adapter."""

    def __init__(self, message):
        super().__init__(message, 'nasa/harmony-gdal-adapter')


class DownloadError(HGAException):
    """ This exception is raised when the Harmony GDAL Adapter cannot retrieve
        input data.
    """
    def __init__(self, url, message):
        super().__init__(f'Could not download resource: {url}, {message}')


class UnknownFileFormatError(HGAException):
    """ This is raised when the input file format is one that cannot by
        processed by the Harmony GDAL Adapter.
    """
    def __init__(self, file_format):
        super().__init__('Cannot process unrecognised file format: '
                         f'"{file_format}"')


class IncompatibleVariablesError(HGAException):
    """ This exception is raised when the dataset variables requested are not
    compatible, i.e. they have different projections, geotransforms, sizes or
    data types.
    """
    def __init__(self, message):
        super().__init__(f'Incompatible variables: {message}')


class MultipleZippedNetCDF4FilesError(HGAException):
    """ This exception is raised when the input file supplied to HGA is a zip
        file containing multiple NetCDF-4 files, as these cannot be aggregated.
    """
    def __init__(self, zip_file):
        super().__init__(f'Multiple NetCDF-4 files within input: {zip_file}.')

Services can fail for other unforeseen reasons, like running out of memory, in which case Harmony will make an effort to provide a standardized error message. Just because a service invocation results in failure does not mean that the entire job itself will fail. Other factors that come into play are retries and cases where a job completes with errors (partial success). Retries happen automatically (up to a Harmony-wide configured limit) on failed data downloads and service failures.

6. Defining environment variables in env-defaults

Add environment variables specific to the service to env-defaults. See the harmony-service-example for an example of the environment variables needed:

HARMONY_SERVICE_EXAMPLE_IMAGE=harmonyservices/service-example:latest
HARMONY_SERVICE_EXAMPLE_REQUESTS_CPU=128m
HARMONY_SERVICE_EXAMPLE_REQUESTS_MEMORY=128Mi
HARMONY_SERVICE_EXAMPLE_LIMITS_CPU=128m
HARMONY_SERVICE_EXAMPLE_LIMITS_MEMORY=512Mi
HARMONY_SERVICE_EXAMPLE_INVOCATION_ARGS='python -m harmony_service_example'

Be sure to prefix the entries with the name of your service. Set the value for the INVOCATION_ARGS environment variable. This should be how you would run your service from the command line. For example, if you had a python module named my-service in the working directory, then you would run the service using:

python -m my-service

So your entry for INVOCATION_ARGS would be

MY_SERVICE_INVOCATION_ARGS='python -m my-service'

7. Registering services in services.yml

Add an entry to services.yml under each CMR environment that has umm-s appropriate to the service and send a pull request to the Harmony team, or ask a Harmony team member for assistance. It is important to note that the order that service entries are placed in this file can have an impact on service selection. In cases where multiple services are capable of performing the requested transformations, the service that appears first in the file will handle the request.

The structure of an entry in the services.yml file is as follows:

- name: harmony/service-example    # A unique identifier string for the service, conventionally <team>/<service>
  data_operation_version: '0.20.0' # The version of the data-operation messaging schema to use
  has_granule_limit: true          # Optional flag indicating whether we will impose granule limts for the request. Default to true.
  default_sync: false              # Optional flag indicating whether we will force the request to run synchrously. Default to false.
  type:                            # Configuration for service invocation
      <<: *default-turbo-config    # To reduce boilerplate, services.yml includes default configuration suitable for all Docker based services.
      params:
        <<: *default-turbo-params  # Always include the default parameters for docker services
        env:
          <<: *default-turbo-env   # Always include the default docker environment variables and then add service specific env
          STAGING_PATH: public/harmony/service-example # The S3 prefix where artifacts generated by the service will be stored
  umm_s: S1234-EXAMPLE            # Service concept id for the service. It is a required field and must be a string.
  collections:                    # Optional, should not exist in most cases. It is only used when there are granule_limit or variables applied to collections of the service.
    - id: C1234-EXAMPLE
      granule_limit: 1000         # A limit on the number of granules that can be processed for the collection (OPTIONAL - defaults to no limit)
      variables:                  # A list of variables provided by the collection (OPTIONAL)
        - v1
        - v2
  maximum_sync_granules: 1        # Optional limit for the maximum number of granules for a request to be handled synchronously. Defaults to 1. Set to 0 to only allow async requests.
  capabilities:                   # Service capabilities
    subsetting:
      bbox: true                  # Can subset by spatial bounding box
      temporal: true              # Can subset by a time range
      variable: true              # Can subset by UMM-Var variable
      multiple_variable: true     # Can subset multiple variables at once
    averaging:
      time: true                  # Can perform averaging over time
      area: true                  # Can perform averaging over area
    output_formats:               # A list of output mime types the service can produce
      - image/tiff
      - image/png
      - image/gif
    reprojection: true            # The service supports reprojection
  steps:
      - image: !Env ${QUERY_CMR_IMAGE} # The image to use for the first step in the chain
      - image: !Env ${HARMONY_EXAMPLE_IMAGE}     # The image to use for the second step in the chain

Each harmony service must have one and only one umm-s concept-id configured via the umm-s field in services.yml. Collections on which a service works are specified via creating a UMM-S/UMM-C association in the CMR with the configured umm-s concept. See this wiki link and the Service Configuration notebook for further UMM-S guidance with respect to Earthdata Search.

NOTE: collections field can only have value when granule_limit or variables need to be configured for specific collections for the service.

If you intend for Harmony job results that include this collection to be shareable, make sure that guests have read permission on the collection (via CMR ACLs endpoints), and if no EULAs are present that the harmony.has-eula tag is associated with the collection and set to false via the CMR /search/tags/harmony.has-eula/associations endpoint. Example request body: [{"concept_id": "C1233860183-EEDTEST", "data": false}]. All collections used in the Harmony job must meet these two requirements in order for the job to be shareable.

The last part of this entry defines the workflow for this service consisting of the query-cmr service (CMR_GRANULE_LOCATOR_IMAGE) followed by the docker_example service (DOCKER_EXAMPLE_IMAGE). For single service (excluding query-cmr) workflows, one need only list the steps. For more complicated workflows involving chained services (once again not counting the query-cmr service) one can list the operations each service in the chain provides along with a list of conditions under which the service will be invoked.

The following steps entry is for a chain of services including the PODAAC L2 Subsetter followed by the Harmony netcf-to-zarr service:

steps:
  - image: !Env ${QUERY_CMR_IMAGE}
  - image: !Env ${PODAAC_L2_SUBSETTER_IMAGE}
    operations: ['spatialSubset', 'variableSubset']
    conditional:
      exists: ['spatialSubset', 'variableSubset']
  - image: !Env ${HARMONY_NETCDF_TO_ZARR_IMAGE}
    operations: ['reformat']
    conditional:
      format: ['application/x-zarr']

First we have the query-cmr service (this service is the first in every current workflow). This is followed by the PODAAC L2 Subsetter service, which provides the 'spatialSubset' and 'variableSubset' operations and is only invoked if the user is requesting one or both of those. Finally, we have the Harmony netcdf-to-zarr service which provides the 'reformat' operation and is only invoked if the request asks for 'zarr' output.

There is also a conditional option on umm-c native_format that compares with the value of the collection UMM-C field: ArchiveAndDistributionInformationType.FileArchiveInformation.Format when the sibling FormatType = 'Native'. Here is an example of its usage:

steps:
  - image: !Env ${QUERY_CMR_IMAGE}
  - image: !Env ${NET_2_COG_IMAGE}
    conditional:
      umm_c:
        native_format: ['netcdf-4']
  - image: !Env ${HYBIG_IMAGE}

Here we have the query-cmr service (this service is the first in every current workflow). This is followed by the optional NetCDF to COG service, which will only be invoked when the collection's UMM-C native format is one of the values that are defined (case insensitive) in the steps configuration (i.e. [netcdf-4]). Finally, we have the HyBIG service that converts the GeoTIFF inputs from the previous step to Global Imagery Browse Services (GIBS) compatible PNG or JPEG outputs. See 10. Service chaining for more info.

Aggregation Steps

Services that provide aggregation, e.g., concatenation for CONCISE, require that all inputs are available when they are run. Harmony infers this from the operations field in the associated step. Currently the only supported aggregation operation is concatenate.

There are limits to the number of files an aggregating service can process as well as the total number of bytes of all combined input files. To support larger aggregations Harmony can partition the output files from one service into batches to be passed to multiple invocations of the next (aggregating) step. Whether or not a service should have its input batched, the maximum number of input files to include in each batch, and total combined file sizes to allow can be set in the aggregating service's step definition using the is_batched, max_batch_inputs, and max_batch_size_in_bytes fields.

The following steps entry is an example one might use for an aggregating service:

steps:
  - image: !Env ${QUERY_CMR_IMAGE}
  - image: !Env ${EXAMPLE_AGGREGATING_SERVICE_IMAGE}
    is_batched: true
    max_batch_inputs: 100
    max_batch_size_in_bytes: 2000000000
    operations: ['concatenate']

There are default limits set by the environment variables MAX_BATCH_INPUTS and MAX_BATCH_SIZE_IN_BYTES. Providers should consult the env-defaults.ts file to obtain the current values of these variables.

The settings in services.yml take precedence over these environment variables. If a provider wishes to use larger values (particularly for max_batch_size_in_bytes) that provider should contact the Harmony team first to make sure that the underlying Kubernetes pods have enough resources allocated (disk space, memory).

8. Docker Container Images

The service and all necessary code and dependencies to allow it to run should be packaged in a Docker container image. Docker images can be staged anywhere Harmony can reach them, e.g. ghcr.io, Dockerhub or AWS ECR. If the image cannot be made publicly available, contact the harmony team to determine how to provide access to the image.

Harmony will run the Docker image, passing the following command-line parameters:

--harmony-action <action> --harmony-input <input> --harmony-sources <sources-file> --harmony-metadata-dir <output-dir>

<action> is the action Harmony wants the service to perform. Currently, Harmony only uses invoke, which requests that the service be run and exit. The service library Harmony provides also supports a start action with parameter --harmony-queue-url <url>, which requests that the service be started as a long running service that reads requests from an SQS queue. This is likely to be deprecated.

<input> is a JSON string containing the details of the service operation to be run. See the latest Harmony data-operation schema for format details.

<sources-file> file path that contains a STAC catalog with items and metadata to be processed by the service. The intent of this file is to allow Harmony to externalize the potentially very long list of input sources to avoid command line limits while retaining the remainder of the message on the command line for easier manipulation in workflow definitions.

<output-dir> is the file path where output metadata should be written. The resulting STAC catalog will be written to catalog.json in the supplied dir with child resources in the same directory or a descendant directory.

The Dockerfile in the harmony-service-example project serves as a minimal example of how to set up Docker to accept these inputs using the ENTRYPOINT declaration.

In addition to the defined command-line parameters, Harmony can provide the Docker container with environment variables as set in services.yml by setting service.type.params.env key/value pairs. See the existing services.yml for examples.

9. Recommendations for service implementations

Note that several of the following are under active discussion and we encourage participation in that discussion.

In order to improve user experience, metrics gathering, and to allow compatibility with future development, Harmony strongly encourages service implementations to do the following:

  1. Provide provenance information in output files in a manner appropriate to the file format and following EOSDIS guidelines, such that a user can recreate the output file that was generated through Harmony. The following fields are recommended to include in each output file. Note that the current software citation fields include backend service information; information on Harmony workflow is forthcoming. For NetCDF outputs, information specific to the backend service should be added to the history global attribute, with all other fields added as additional global attributes. For GeoTIFF outputs, these fields can be included under metadata as TIFFTAG_SOFTWARE. See the NASA ESDS Data Product Development Guide for Data Producers for more guidance on provenance information.
Field Name Field Example Field Source
Service Title podaac-subsetter UMM-S Long Name
Service Version v1.0.0 UMM-S Version
Service Publisher NASA Physical Oceanography Distributed Active Archive Center UMM-S Service Organization Long Name
Access Date 2020-08-26 00:00:00 Time stamp of file generation
Input granule identifier SMAP_L3_SM_P_E_20200824_R16515_001 Filename of input granule
File request source https://harmony.uat.earthdata.nasa.gov/C1233800302-EEDTEST/ogc-api-coverages/1.0.0/collections/all/coverage/rangeset?subset=lat(-5:5)&subset=lon(-10:10) Harmony request
  1. Preserve existing file metadata where appropriate. This includes file-level metadata that has not changed, layer metadata, and particularly provenance metadata that may have been generated by prior transformations.
  2. Log request callback URLs, which serve as unique identifiers, as well as Earthdata Login usernames when available to aid in tracing requests and debugging.
  3. Proactively protect (non-Docker) service endpoints from high request volume or computational requirements by using autoscaling with maximum thresholds, queueing, and other methods to avoid outages or non-responsiveness.
  4. Use the latest available data-operation schema. As harmony development continues the schema will evolve to support new features, which will require services be compatible with the new schema in order to take advantage of those features. In addition code to support older versions of the schema can be retired once all services have updated to later versions of the schema.
  5. Name files according to the established conventions. Using the Harmony-provided Python library makes this automatic for cases where the file corresponds to a single granule. Files subset to a single variable should be suffixed with underscore followed by the variable name. Files that have been regridded should be suffixed with _regridded. Files that have been subsetted should be suffixed with _subsetted. Finally, files should have the conventional file extension according to their format, e.g. .zarr.
  6. The stagingLocation property of the Harmony message contains a prefix of a recommended place to stage your output. If your service is running in the Harmony account or has access to the Harmony staging bucket, we recommend you place results under that location, as it will allow users to access your data via the S3 API and ensure correct retention policies and access logging as features are added to Harmony. It is not mandatory that you make use of this location, but highly recommended if your service produces files that need to be staged.

10. Service chaining

In order to support service-chaining--a pipeline of two or more services that process data--Harmony uses a STAC Catalog that describes the output of one service, and the input to the next service in the workflow chain (or pipeline).

In the following Harmony workflow, we show a series of services (in boxes) and STAC Catalogs between services which describe the data available for the next service. First, the Granule Locator queries CMR using the criteria specified in the Harmony request. It then writes a STAC Catalog describing the Granules and their source URLs, and this STAC Catalog is provided to the fictional Transmogrifier Service. After transmogrification is done (not an easy task), the service writes a STAC Catalog describing its outputs with URLs. Again, the fictional Flux Decoupler service does its thing (easier than it sounds) and writes a STAC Catalog. Finally, this is provided to the Results Handler which stages the final output artifacts in S3 and Harmony provides a STAC Catalog describing the final output of the original Harmony request.

 -----------------------------
 |     Granule Locator       |
 -----------------------------
              |
       (STAC Catalog)
              |
 -----------------------------
 |  Transmogrifier Service   |
 -----------------------------
              |
       (STAC Catalog)
              |
 -----------------------------
 |  Flux Decoupler Service   |
 -----------------------------
              |
       (STAC Catalog)
              |
 -----------------------------
 |     Results Handler       |
 -----------------------------
              |
       (STAC Catalog)

The Harmony Service Library automatically handles reading and writing the STAC catalogs.