Querying an OBC for Telemetry Data

At some point, users will want to fetch data which was written to the telemetry database (for an example of this, see the mission application tutorial).

This tutorial will walk the user through the process of communicating with the telemetry database service in order to retrieve a desired set of telemetry information.

Pre-Requisites

Setup

In order to have something to query, we’ll need to seed the database. We’ll use curl to create some HTTP requests which will add data points to the telemetry database.

Note

For all commands, replace the 10.0.2.20 value with the IP address of your OBC. The example commands will also be referencing the telemetry database service’s default port, 8006

From your instance of the SDK, run the following:

$ curl 10.0.2.20:8006 -H "Content-Type: application/json" --data "{\"query\":\"mutation {insert(subsystem:\\\"eps\\\",parameter:\\\"voltage\\\",value:\\\"5.0\\\"){success}}\"}"
$ curl 10.0.2.20:8006 -H "Content-Type: application/json" --data "{\"query\":\"mutation {insert(subsystem:\\\"eps\\\",parameter:\\\"voltage\\\",value:\\\"5.0\\\"){success}}\"}"
$ curl 10.0.2.20:8006 -H "Content-Type: application/json" --data "{\"query\":\"mutation {insert(subsystem:\\\"eps\\\",parameter:\\\"current\\\",value:\\\"0.1\\\"){success}}\"}"
$ curl 10.0.2.20:8006 -H "Content-Type: application/json" --data "{\"query\":\"mutation {insert(subsystem:\\\"eps\\\",parameter:\\\"current\\\",value:\\\"0.1\\\"){success}}\"}"
$ curl 10.0.2.20:8006 -H "Content-Type: application/json" --data "{\"query\":\"mutation {insert(subsystem:\\\"gps\\\",parameter:\\\"voltage\\\",value:\\\"3.3\\\"){success}}\"}"

Each of these commands should return the following:

{"data":{"insert":{"success":true}}}

GraphiQL

Please navigate to http://{ip}:8006/graphiql in order to communicate with the telemetry service for this tutorial.

More information about the GraphiQL interface can be found here.

Schema

The telemetry query can be used to fetch a certain selection of data from the telemetry database. It will return an array of database entries.

The query has the following schema:

query {
    telemetry(timestampGe: Float, timestampLe: Float, subsystem: String, parameter: String, limit: Integer): [{
        timestamp: Float!
        subsystem: String!
        parameter: String!
        value: String!
    }]
}

Each of the input arguments acts as a filter for the database query:

  • timestampGe - Return entries with timestamps occurring on or after the given value
  • timestampLe - Return entries with timestamps occurring on or before the given value
  • subsystem - Return entries which match the given subsystem name
  • parameter - Return entries which match the given parameter name
  • limit - Return only the first n entries found

Using the telemetry query without specifying any of the input arguments will result in all entries in the telemetry database being returned.

The query returns an array of entries, each of which has the following response fields:

  • timestamp - The system time when the entry was recorded, in milliseconds since UNIX epoch
  • subsystem - The subsystem which generated the entry data
  • parameter - The specific data point the entry corresponds to
  • value - The value of the data point

Querying a Specific Subsystem

In order to return only telemetry generated by a specific subsystem, we’ll send a query using the subsystem input parameter.

For example, to get only telemetry from our (fake) EPS subsystem, we’ll send the following query:

{
  telemetry(subsystem: "eps") {
    timestamp
    subsystem
    parameter
    value
  }
}

The request should return something like this:

{
  "data": {
    "telemetry": [
      {
        "timestamp": 730.855591753,
        "subsystem": "eps",
        "parameter": "current",
        "value": "0.1"
      },
      {
        "timestamp": 730.21144042,
        "subsystem": "eps",
        "parameter": "current",
        "value": "0.1"
      },
      {
        "timestamp": 721.187618335,
        "subsystem": "eps",
        "parameter": "voltage",
        "value": "5.0"
      },
      {
        "timestamp": 688.450861332,
        "subsystem": "eps",
        "parameter": "voltage",
        "value": "5.0"
      }
    ]
  }
}

Querying a Time Frame

If we want to fetch the telemetry from a particular time frame, for example all telemetry occurring between 1pm and 2pm, we’ll use a combination of the timestampGe and timestampLe input arguments.

Note

In order to select a timeframe, it is helpful to know the current system time. This can be found by running the following command on the OBC: date +%s. The command gets the current number of seconds since UNIX epoch, which matches the telemetry timestamp units.

Our request should look like this:

{
  telemetry(timestampGe: 730.0, timestampLe: 731.0) {
    timestamp
    subsystem
    parameter
    value
  }
}

This queries the system for telemetry in a 1 second window between 730 and 731, inclusive. You will need to update the query for your specific timestamp range.

The response should look like this:

{
  "data": {
    "telemetry": [
      {
        "timestamp": 730.855591753,
        "subsystem": "eps",
        "parameter": "current",
        "value": "0.1"
      },
      {
        "timestamp": 730.21144042,
        "subsystem": "eps",
        "parameter": "current",
        "value": "0.1"
      }
    ]
  }
}

Limiting the Results

Let’s say we only want to know the most recent voltage reading from our EPS subsystem.

We’ll specify the subsystem and parameter input argument to narrow the filters down to the subsystem and parameter we want. We’ll then also use the limit input argument to tell the telemetry database service to only return the single most-recent entry.

Our request should look like this:

{
  telemetry(subsystem: "eps", parameter: "voltage", limit: 1) {
    timestamp
    subsystem
    parameter
    value
  }
}

The response should look like this:

{
  telemetry(subsystem: "eps", parameter: "voltage", limit: 1) {
    timestamp
    subsystem
    parameter
    value
  }
}