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Jaahas.Json.TimeSeriesExtractor

A C# library for extracting time series data from JSON using System.Text.Json. This can be useful when e.g. JSON data is received from an IoT device and the data must be separated into individual series for storage in a database.

Getting Started

Add a NuGet package reference to Jaahas.Json.TimeSeriesExtractor to your project.

Usage

The TimeSeriesExtractor class is the entry point for the library.

Call TimeSeriesExtractor.GetSamples to extract values from a JSON string or a JsonElement:

const string json = @"{ ""timestamp"": ""2021-05-30T09:47:38Z"", ""temperature"": 24.7, ""pressure"": 1021.3, ""humidity"": 33.76 }";

// GetSamples uses lazy evaluation; use the ToArray extension from 
// System.Linq to force eager evaluation.
var samples = TimeSeriesExtractor.GetSamples(json).ToArray();

The JSON document must represent an object or an array of objects. If the document is an array of objects, each object in the array is processed as if it were a separate JSON document. You can customise the extraction behaviour by passing a TimeSeriesExtractorOptions object when calling the method.

Data Samples

Properties on the JSON objects are converted to instances of TimeSeriesSample. Although the type of the Value property on TimeSeriesSample is object, in practical terms the value will either be null, or one of the following types:

  • double
  • string
  • bool

Selecting the Timestamp

The TimestampProperty on TimeSeriesExtractorOptions defines the JSON Pointer path that is used to retrieve the timestamp for the samples extracted from the JSON object. By default, this property is set to /time, but can overridden if required:

new TimeSeriesExtractorOptions() {
  TimestampProperty = "/metadata/utcSampleTime"
}

If the TimestampProperty is null, or does not exist in the document, the delegate assigned to the GetDefaultTimestamp property on the TimeSeriesExtractorOptions is called to request a fallback timestamp to use. If no delegate has been specified, DateTimeOffset.UtcNow is used.

You can use the TimestampSource property on a TimeSeriesSample instance to determine how the timestamp for the sample was obtained.

Timestamps will be automatically parsed as follows:

  • If the raw JSON value is a string, the timestamp will be retrieved using JsonElement.TryGetDateTimeOffset.
  • If an integer value can be obtained from the JSON value using JsonElement.TryGetInt64, it is assumed to represent milliseconds since midnight UTC on 01 January 1970.
  • If a timestamp cannot be inferred from either of the above approaches, the default timestamp will be used instead.

Custom timestamp parsing is described in the next section. Additional timestamp parsing behaviour can be specified when recursive mode is enabled. See below for more details.

Custom Timestamp Parsing

Timestamp parsing can be overridden by setting the TimestampParser property on the TimeSeriesExtractorOptions. The property is a delegate that receives a JsonElement representing the timestamp property and returns a DateTimeOffset?. The delegate should return null if the timestamp cannot be parsed.

For example, to parse timestamps where the timestamp property is the number of seconds since midnight UTC on 01 January 1970 instead of the number of milliseconds:

new TimeSeriesExtractorOptions() {
  TimestampParser = element => element.ValueKind == JsonValueKind.Number
    ? DateTime.UnixEpoch.AddSeconds(element.GetInt64())
    : null
}

Selecting the Properties to Handle

By default, TimeSeriesExtractor will create a sample for each property on the object except for the configured timestamp property. To customise if a JSON element will be processed or not, you can assign a delegate to the CanProcessElement property on the TimeSeriesExtractorOptions instance passed to the extractor. The delegate returns a Boolean value indicating if the element should be processed. For example:

new TimeSeriesExtractorOptions() {
  CanProcessElement = (TimeSeriesExtractorContext context, JsonPonter pointer, JsonElement element) => {
    // Check if the current pointer is allowed
    switch (pointer.ToString()) {
      case "/temperature":
      case "/pressure":
      case "/humidity":
        return true;
      default:
        return false;
    }
  }
}

If you have a known list of properties to include or exclude, you can use one of the TimeSeriesExtractor.CreateJsonPointerMatchDelegate method overloads to create a compatible delegate that can be assigned to the CanProcessElement property. For example:

var matcher = TimeSeriesExtractor.CreateJsonPointerMatchDelegate(new JsonPointerMatchDelegateOptions() {
    PointersToInclude = ["/temperature", "/pressure", "/humidity"]
});

var options = new TimeSeriesExtractorOptions() {
  CanProcessElement = matcher
}

Pattern Matching using Wildcards

TimeSeriesExtractor.CreateJsonPointerMatchDelegate supports using single- and multi-character wildcards (? and * respectively) in JSON pointers when the TimeSeriesExtractorOptions.AllowWildcardExpressions property is true. Pattern matching wildcard expressions do not need to be valid JSON pointers.

Example 1: process all elements that are descendents of a data property anywhere in the document:

var matcher = TimeSeriesExtractor.CreateJsonPointerMatchDelegate(new JsonPointerMatchDelegateOptions() {
    AllowWildcardExpressions = true,
    PointersToInclude = ["*/data/*"]
});

var options = new TimeSeriesExtractorOptions() {
  CanProcessElement = matcher
}

Example 2: exclude all properties named metadata anywhere in the document:

var matcher = TimeSeriesExtractor.CreateJsonPointerMatchDelegate(new JsonPointerMatchDelegateOptions() {
    AllowWildcardExpressions = true,
    PointersToExclude = ["*/metadata"]
});

var options = new TimeSeriesExtractorOptions() {
  CanProcessElement = matcher
}

Pattern Matching using MQTT-style Match Expressions

In addition to matching using wildcard characters, TimeSeriesExtractor.CreateJsonPointerMatchDelegate also supports using MQTT-style match expressions in JSON pointers when the TimeSeriesExtractorOptions.AllowWildcardExpressions property is true. Note that, unlike with pattern matching expressions, MQTT-style match expressions must be valid JSON pointers.

Example 1: process all elements that are descendents of /data/instrument-1:

var matcher = TimeSeriesExtractor.CreateJsonPointerMatchDelegate(new JsonPointerMatchDelegateOptions() {
    AllowWildcardExpressions = true,
    PointersToInclude = ["/data/instrument-1/#"]
});

var options = new TimeSeriesExtractorOptions() {
  CanProcessElement = matcher
}

Example 2: include all temperature properties that are grandchildren of /data:

var matcher = TimeSeriesExtractor.CreateJsonPointerMatchDelegate(new JsonPointerMatchDelegateOptions() {
    AllowWildcardExpressions = true,
    PointersToInclude = ["/data/+/temperature"]
});

var options = new TimeSeriesExtractorOptions() {
  CanProcessElement = matcher
}

Note that, if the JSON pointer includes single- or multi-character wildcards, regular pattern matching will always be used instead of MQTT-style match expressions.

Data Sample Keys

Each TimeSeriesSample has a Key property that is used to identify the JSON property that was used to generate the sample. The key is generated from the Template property on the TimeSeriesExtractorOptions class. Consider the following example template:

devices/{deviceId}/instruments/{$prop}

The parts of the template enclosed in { and } are placeholders that will be replaced at runtime; {deviceId} will be replaced with the value of the property named deviceId on the same object as the property being processed, and {$prop} will be replaced with the JSON Pointer path of the property itself, without the leading /.

Here is an example JSON document:

{
  "deviceId": 7,
  "temperature": 28.9
}

When processing the temperature property on the JSON object using the template above, the key that would be generated would be devices/7/instruments/temperature.

The default key template if one is not specified is simply {$prop} (i.e. the JSON Pointer path to the property).

Built-In Template Placeholders

The following built-in template placeholders are available:

Placeholder Description Example
{$prop} The JSON Pointer path to the property being processed, without the leading /. data/device-1/temperature
{$prop-local} The local property name of the property being processed. temperature
{$prop-path} The full JSON Pointer path to the parent element of the property being processed, without the leading /. data/device-1

All other placeholders are resolved using the JSON document, or using a default placeholder value (see below). Note that, when recursive mode is disabled, the {$prop} and ${prop-local} template placeholders are functionally identical, and the {$prop-path} placeholder will always be replaced with an empty string.

Default Template Placeholder Values

Default template replacement values can be provided via the GetTemplateReplacement property on TimeSeriesExtractorOptions; these will be used if a property name referenced in the template does not exist on the JSON object being processed. For example:

Template:

devices/{deviceId}/instruments/{$prop}

JSON:

{
  "temperature": 97.3
}

Options:

new TimeSeriesExtractorOptions() {
  GetTemplateReplacement = name => {
    switch (name) {
      case "deviceId":
        return "A-001";
      default:
        return null;
    }
  }
}

When processing the temperature property on the JSON object using the template above, the key that would be generated would be devices/A-001/instruments/temperature.

Handling Unresolved Template Replacements

By default, unresolved template replacements are ignored; the final key will contain the placeholder text. To skip JSON properties when one or more template replacements cannot be fulfilled, set the AllowUnresolvedTemplateReplacements property on the TimeSeriesExtractorOptions object to false.

Recursive Processing

The default behaviour of TimeSeriesExtractor is to process top-level properties on the specified JSON object only. For example, consider the following JSON:

{
  "temperature": 28.1,
  "pressure": 1020.99,
  "acceleration": {
    "x": -0.876,
    "y": 0.516,
    "z": -0.044
  }
}

The default behaviour of TimeSeriesExtractor would be to emit 3 samples, despite the acceleration property defining a nested structure containing additional data:

  • temperature: 28.1
  • pressure: 1020.99
  • acceleration: "{ \"x\": -0.876, \"y\": 0.516, \"z\": -0.044 }"

In this circumstance, it is desirable to recursively process the nested object and emit a sample for each of the sub-properties. This can be done by setting the Recursive property on TimeSeriesExtractorOptions to true. If the PathSeparator property on TimeSeriesExtractorOptions is configured to anything other than /, the path delimiters in the {$prop} substitution value will be replaced with the specified PathSeparator.

With recursive mode enabled, the same JSON above would result in 5 samples being emitted:

  • temperature: 28.1
  • pressure: 1020.99
  • acceleration/x: -0.876
  • acceleration/y: 0.516
  • acceleration/z: -0.044

When recursive mode is enabled, the extractor will also iterate over nested arrays. For example:

{
  "temperatures": [
    37.7,
    38.1,
    37.9
  ]
}

The samples emitted would be:

  • temperatures/0: 37.7
  • temperatures/1: 38.1
  • temperatures/2: 37.9

It is also possible to omit array indexes from the sample keys when recursive mode is enabled; see below for more details.

A Note on Recursive Mode Template Replacements

In recursive mode, template replacements other than the built-in placeholders are resolved using all objects in the hierarchy from the root to the current object, and the matches are concatenated together with the configured path separator. Consider the following JSON:

{
  "location": "System A",
  "measurements": {
    "location": "Subsystem 1",
    "temperature": 57.6
  }
}

Given a template of {location}/{$prop}, the key generated for the nested temperature property would be System A/Subsystem 1/measurements/temperature.

If you want to use the local property name in the generated key instead of the full JSON Pointer path, you can specify {$prop-local} in the template instead of {$prop}.

Using the above example JSON and a template of {location}/{$prop-local}, the key for the nested temperature property would be System A/Subsystem 1/temperature instead of System A/Subsystem 1/measurements/temperature.

Remember: when recursive mode is disabled, the {$prop} and ${prop-local} template placeholders are functionally identical, and the {$prop-path} placeholder will always be replaced with an empty string.

Enabling Nested Timestamps

When recursive mode is enabled, the TimeSeriesExtractorOptions.AllowNestedTimestamps property controls whether timestamps can be extracted from nested objects. By default, this property is set to false, meaning that the timestamp is resolved by evaluating the timestamp property against the root of the JSON document. If set to true, the timestamp pointer is evaluated against each ancestor of the property being processed, and the first successful match is used for the sample. For example, consider the following JSON:

{
  "time": "2021-05-30T09:47:38Z",
  "temperature": 24.7,
  "pressure": 1021.3,
  "humidity": 33.76,
  "acceleration": {
    "time": "2021-05-30T09:47:37Z",
    "x": -0.876,
    "y": 0.516,
    "z": -0.044
  }
}

If the configured timestamp property is /time and nested timestamps and recursive mode are enabled, the timestamp for the x, y and z acceleration samples would be 2021-05-30T09:47:37Z instead of 2021-05-30T09:47:38Z.

Omitting Array Indexes From Sample Keys

When recursive mode is enabled, the TimeSeriesExtractorOptions.IncludeArrayIndexesInSampleKeys property controls whether array indexes in the JSON Pointer path are included in the generated sample keys. By default, this property is set to true. If set to false, the array indexes will be omitted from the generated sample keys. The TimeSeriesExtractorOptions.IncludeArrayIndexesInSampleKeys is typically used in conjunction with the TimeSeriesExtractorOptions.AllowNestedTimestamps property to allow multiple samples with identical keys but different timestamps to be extracted from an array. For example, consider the following JSON:

{
  "device-1": {
    "data": [
      {
        "time": "2021-05-30T09:47:38Z",
        "temperature": 24.7
      },
      {
        "time": "2021-05-30T09:47:39Z",
        "temperature": 24.8
      },
      {
        "time": "2021-05-30T09:47:40Z",
        "temperature": 24.9
      }
    ] 
  }
}

If array indexes are omitted from the sample keys, three samples would be extracted with a key of device-1/data/temperature. With array indexes included in the sample keys, three samples would be extracted with keys of device-1/data/0/temperature, device-1/data/1/temperature and device-1/data/2/temperature.

Specifying a Custom Processing Start Position

By default, processing starts at the root of the JSON document. However, in some scenarios it may be desirable to customise the start position. For example, consider the following example response from the Airthings API:

{
  "data": {
    "battery": 100,
    "co2": 650.0,
    "humidity": 26.0,
    "pm1": 0.0,
    "pm25": 0.0,
    "pressure": 1028.7,
    "radonShortTermAvg": 2.0,
    "temp": 24.6,
    "time": 1686421947,
    "voc": 58.0,
    "relayDeviceType": "hub"
  }
}

When processing the above document, it may be preferable to start at the data property instead of at the root of the document. This can be achieved by setting the TimeSeriesExtractorOptions.StartAt property to specify a JSON pointer to the desired start position:

new TimeSeriesExtractorOptions() {
  StartAt = "/data",
  TimestampParser = element => element.ValueKind == JsonValueKind.Number
    ? DateTime.UnixEpoch.AddSeconds(element.GetInt64())
    : null
}

Note that specifying a start position means that all features that use JSON pointers (including the configured timestamp property and the {$prop} placeholder for a given sample) become relative to the start position instead of the root of the JSON document. For example, in the above document the {$prop} placeholder for the humidity reading would be humidity instead of data/humidity.

Building the Solution

The repository uses Cake for cross-platform build automation. The build script allows for metadata such as a build counter to be specified when called by a continuous integration system such as TeamCity.

A build can be run from the command line using the build.ps1 PowerShell script. For documentation about the available build script parameters, see build.cake.