Advanced features of the REST API

This section of the Orthanc Book is a complement to the description of the REST API of Orthanc. It explains some advanced uses of the API.

Jobs

Since Orthanc 1.4.0, a jobs engine is embedded within Orthanc. Jobs are high-level tasks to be processed by Orthanc. Jobs are first added to a queue of pending tasks, and Orthanc will simultaneously execute a fixed number of jobs (check out configuration option ConcurrentJobs). Once the jobs have been processed, they are tagged as successful or failed, and kept in a history (the size of this history is controlled by the JobsHistorySize option).

By default, Orthanc saves the jobs into its database (check out the SaveJobs option). If Orthanc is stopped then relaunched, the jobs whose processing was not finished are automatically put into the queue of pending tasks. The command-line option --no-jobs can also be used to prevent the loading of jobs from the database upon the launch of Orthanc.

Note that the queue of pending jobs has a maximum size (check out the LimitJobs option). When this limit is reached, the addition of new jobs is blocked until some job finishes.

Synchronous vs. asynchronous calls

Some calls to the REST API of Orthanc need time to be executed, and thus result in adding a job to the processing queue. This notably includes the following URIs:

Such REST API calls can be configured to be executed in a synchronous or an asynchronous mode:

  • Synchronous calls wait for the end of the execution of their associated job. This is in general the default behavior.
  • Asynchronous calls end immediately and return a handle to their associated job. It is up to the caller to monitor the execution by calling the jobs API (e.g. to know whether the job has finished its execution).

The choice between synchronous and asynchronous modes is done by setting the Synchronous field (or indifferently the Asynchronous field) in the POST body of the call to the REST API. Note that the transfers accelerator only runs in asynchronous mode.

An integer number (possibly negative) can be specified in the Priority field of the POST body. Jobs with higher priority will be executed first. By default, the priority is set to zero.

Despite being more complex to handle, the asynchronous mode is highly recommended for jobs whose execution time can last over a dozen of seconds (typically, the creation of an archive or a network transfer). Indeed, synchronous calls can be affected by timeouts in the HTTP protocol if they last too long.

Monitoring jobs

The list of all jobs can be retrieved as follows:

$ curl http://localhost:8042/jobs
[ "e0d12aac-47eb-454f-bb7f-9857931e2904" ]

Full details about each job can be retrieved:

$ curl http://localhost:8042/jobs/e0d12aac-47eb-454f-bb7f-9857931e2904
{
  "CompletionTime" : "20190306T095223.753851",
  "Content" : {
    "Description" : "REST API",
    "InstancesCount" : 1,
    "UncompressedSizeMB" : 0
  },
  "CreationTime" : "20190306T095223.750666",
  "EffectiveRuntime" : 0.001,
  "ErrorCode" : 0,
  "ErrorDescription" : "Success",
  "ID" : "e0d12aac-47eb-454f-bb7f-9857931e2904",
  "Priority" : 0,
  "Progress" : 100,
  "State" : "Success",
  "Timestamp" : "20190306T095408.556082",
  "Type" : "Archive"
}

Note that the /jobs?expand URI will retrieve this information in one single REST query. The Content field contains the parameters of the job, and is very specific to the Type of job.

The State field can be:

  • Pending: The job is waiting to be executed.
  • Running: The job is being executed. The Progress field will be continuously updated to reflect the progression of the execution.
  • Success: The job has finished with success.
  • Failure: The job has finished with failure. Check out the ErrorCode and ErrorDescription fields for more information.
  • Paused: The job has been paused.
  • Retry: The job has failed internally, and has been scheduled for re-submission after a delay. As of Orthanc 1.5.7, this feature is not used by any type of job.

In order to wait for the end of an asynchronous call, the caller will typically have to poll the /jobs/...` URI (i.e. make periodic calls), waiting for the ``State field to become Success or Failure.

Interacting with jobs

Given the ID of some job, one can:

  • Cancel the job by POST-ing to /jobs/.../cancel.
  • Pause the job by POST-ing to /jobs/.../pause.
  • Resume a job in Paused state by POST-ing to /jobs/.../resume.
  • Retry a job in Failed state by POST-ing to /jobs/.../resubmit.

The related state machine is depicted in the implementation notes.

PDF

Among many different types of data, DICOM files can be used to store PDF files. The /tools/create-dicom URI can be used to upload a PDF file to Orthanc. The following scripts perform such a DICOM-ization; They convert the HelloWorld2.pdf file to base64, then perform a POST request with JSON data containing the converted payload.

Using bash:

# create the json data, with the BASE64 data embedded in it
(echo -n '{"Tags" : {"PatientName" : "Benjamino", "Modality" : "CT"},"Content" : "data:application/pdf;base64,'; base64 HelloWorld2.pdf; echo '"}') > /tmp/foo

# upload it to Orthanc
cat /tmp/foo | curl -H "Content-Type: application/json" -d @- http://localhost:8042/tools/create-dicom

Using Powershell:

# create the BASE64 string data
$fileInBase64 = $([Convert]::ToBase64String((gc -Path "HelloWorld2.pdf" -Encoding Byte)))

# create the json data
$params = @{Tags = @{PatientName = "Benjamino";Modality = "CT"};Content= "data:application/pdf;base64,$fileInBase64"}

# upload it to Orthanc and convert the result to json
$reply = Invoke-WebRequest -Uri http://localhost:8042/tools/create-dicom -Method POST -Body ($params|ConvertTo-Json) -ContentType "application/json" | ConvertFrom-Json

# display the result
Write-Host "The instance can be retrieved in PDF at http://localhost:8042$($reply.Path)/pdf"

Please note that the /tools/create-dicom API call will return the Orthanc instance ID of the newly created DICOM resource.

You can use the /instances/.../pdf URI to retrieve an embedded PDF file.

Instrumentation with Prometheus

Orthanc publishes its metrics according to the text-based format of Prometheus (check also the OpenMetrics project), onto the /tools/metrics-prometheus URI of the REST API. For instance:

$ curl http://localhost:8042/tools/metrics-prometheus
orthanc_count_instances 1 1551868380543
orthanc_count_patients 1 1551868380543
orthanc_count_series 1 1551868380543
orthanc_count_studies 1 1551868380543
orthanc_disk_size_mb 0.0135002136 1551868380543
orthanc_jobs_completed 1 1551868380543
orthanc_jobs_failed 0 1551868380543
orthanc_jobs_pending 0 1551868380543
orthanc_jobs_running 0 1551868380543
orthanc_jobs_success 1 1551868380543
orthanc_rest_api_active_requests 1 1551868380543
orthanc_rest_api_duration_ms 0 1551868094265
orthanc_storage_create_duration_ms 0 1551865919315
orthanc_storage_read_duration_ms 0 1551865943752
orthanc_store_dicom_duration_ms 5 1551865919319
orthanc_uncompressed_size_mb 0.0135002136 1551868380543

Note that the collection of metrics can be statically disabled by setting the global configuration option MetricsEnabled to false, or dynamically disabled by PUT-ing 0 on /tools/metrics:

$ curl http://localhost:8042/tools/metrics
1
$ curl http://localhost:8042/tools/metrics -X PUT -d '0'
$ curl http://localhost:8042/tools/metrics
0

Here is a sample configuration for Prometheus (in the YAML format):

scrape_configs:
  - job_name: 'orthanc'
    scrape_interval: 10s
    metrics_path: /tools/metrics-prometheus
    basic_auth:
      username: orthanc
      password: orthanc
    static_configs:
      - targets: ['192.168.0.2:8042']

Obviously, make sure to adapt this sample with your actual IP address. Thanks to Docker, you can easily start a Prometheus server by writing this configuration to, say, /tmp/prometheus.yml, then type:

$ sudo docker run -p 9090:9090 -v /tmp/prometheus.yml:/etc/prometheus/prometheus.yml --rm prom/prometheus:v2.7.0