​
Initialize a Tracker

As outlined in the policy configuration guide, you define your trackers in the policy.json file for your project. Below is an example policy to illustrate the examples that will follow.
policy.json
{
  "Policy": "Tracker Example Policy",
  "PolicyType": "open",
  "CallingFunctions": [
    {
      "name": "mint(uint256 amount)",
      "functionSignature": "mint(uint256 amount)",
      "encodedValues": "uint256 amount"
    }
  ],
  "ForeignCalls": [],
  "Trackers": [
    {
      "name": "TradingVolume",
      "type": "uint256",
      "initialValue": 0
    }
  ],
  "MappedTrackers": [],
  "Rules": [
    // purposefully empty for now
  ]
}
There are two ways that you can utilize trackers in your project.
  1. Trackers can be used as input values in rule expressions.
  2. Trackers can be updated as effects of rules.
Let’s see how to accomplish these two usage options with standard and mapped trackers

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Usage in Rules

First, we need to add at least one rule to the policy that will use the named trackers.

​
Tracker

This rule condition ensures that the total volume so far and the new amount does not exceed 1 billion tokens.
policy.json
{
  "Policy": "Tracker Example Policy",
  "PolicyType": "open",
  "CallingFunctions": [
    {
      "name": "mint(uint256 amount)",
      "functionSignature": "mint(uint256 amount)",
      "encodedValues": "uint256 amount"
    }
  ],
  "ForeignCalls": [],
  "Trackers": [
    {
      "name": "TradingVolume",
      "type": "uint256",
      "initialValue": 0
    },
    {
      "name": "TradeCountPerUser",
      "type": "uint256",
      "initialValue": 0
    }
  ],
  "MappedTrackers": [],
  "Rules": [
    {
      "condition": "(TR:TradingVolume + amount) < 1_000_000_000 AND TR:TradeCountPerUser(from) < 10",
      "positiveEffects": [],
      "negativeEffects": ["revert(\"Trading Volume Max Reached\")"],
      "callingFunction": "mint(uint256 amount)"
    }
  ]
}

​
Mapped Tracker

This rule condition ensures that the sender has not exceeded 10 transactions.
policy.json
{
  "Policy": "Tracker Example Policy",
  "PolicyType": "open",
  "CallingFunctions": [
    {
      "name": "mint(uint256 amount)",
      "functionSignature": "mint(uint256 amount)",
      "encodedValues": "uint256 amount"
    }
  ],
  "ForeignCalls": [],
  "Trackers": [],
  "MappedTrackers": [
    {
      "name": "TradeCountPerUser",
      "keyType": "address",
      "valueType": "uint256",
      "initialKeys": ["0xB7f8BC63BbcaD18155201308C8f3540b07f84F5e"],
      "initialValues": [0]
    }
  ],
  "Rules": [
    {
      "condition": "TR:TradeCountPerUser(from) < 10",
      "positiveEffects": ["TRU:TradeCountPerUser(from) += 1"],
      "negativeEffects": ["revert(\"Trading Transaction Max Reached\")"],
      "callingFunction": "mint(uint256 amount)"
    }
  ]
}
Note that this rule is applied to the internal _update function. This ensures the policy is applied to both the transfer and transferFrom function calls.

​
Tracker Update Effects

As of now, this rule will always succeed because the tracker value is never updated. Below we add a positiveEffect that updates the tracker value whenever a transfer call is successful.

​
Tracker

policy.json
//...
    "Rules": [
      {
        "condition": "(TR:TradingVolume + amount) < 1_000_000_000",
        "positiveEffects": [
          "TRU:TradingVolume += amount"
        ],
        "negativeEffects": ["revert(\"Trading Volume Max Reached\")"],
        "callingFunction": "mint(uint256 amount)"
      }
    ]
//...

​
Mapped Tracker

policy.json
//...
    "Rules": [
      {
        "condition": "TR:TradeCountPerUser(from) < 10",
        "positiveEffects": [
          "TRU:TradeCountPerUser(from) += 1"
        ],
        "negativeEffects": ["revert(\"Trading Transaction Max Reached\")"],
        "callingFunction": "mint(uint256 amount)"
      }
    ]
//...
Notice that when referencing a tracker value in a rule condition or as a read value in an effect, you precede the name with TR as in TR:TradingVolume.When referencing in an update you must precede it with TRU as in TRU:TradingVolume

​
Taking it Further

The rule defined thus far is a good start, but needs additional configuration to achieve the desired outcome. First, as soon as 1_000_000_000 in trading volume is reached the token will no longer be transferrable. Fixing this will requires resetting the TradingVolume tracker when the defined duration is reached. To do this we need another tracker to record the timestamp and mark the beginning of a rolling 24 hour period. Then we test for that condition in a new rule and reset the tracker values when the condition is true.
policy.json
{
  "Policy": "Tracker Example Policy",
  "PolicyType": "open",
  "CallingFunctions": [
    {
      "name": "mint(uint256 amount)",
      "functionSignature": "mint(uint256 amount)",
      "encodedValues": "uint256 amount"
    }
  ],
  "ForeignCalls": [],
  "Trackers": [
    {
      "name": "TimeStamp",
      "type": "uint256",
      "initialValue": 0
    },
    {
      "name": "TradingVolume",
      "type": "uint256",
      "initialValue": 0
    }
  ],
  "MappedTrackers": [
    {
      "name": "UserTimeStamp",
      "keyType": "address",
      "valueType": "uint256",
      "initialKeys": ["0xB7f8BC63BbcaD18155201308C8f3540b07f84F5e"],
      "initialValues": [0]
    },
    {
      "name": "TradeCountPerUser",
      "keyType": "address",
      "valueType": "uint256",
      "initialKeys": ["0xB7f8BC63BbcaD18155201308C8f3540b07f84F5e"],
      "initialValues": [0]
    }
  ],
  "Rules": [
    {
      "condition": "(GV:BLOCK_TIMESTAMP - TR:TimeStamp) >= 86400",
      "positiveEffects": ["TRU:TimeStamp = GV:BLOCK_TIMESTAMP", "TRU:TradingVolume = 0"],
      "negativeEffects": [],
      "callingFunction": "mint(uint256 amount)"
    },
    {
      "condition": "(TR:TradingVolume + amount) < 1_000_000_000",
      "positiveEffects": ["TRU:TradingVolume += amount"],
      "negativeEffects": ["revert(\"Trading Volume Max Reached\")"],
      "callingFunction": "mint(uint256 amount)"
    },
    {
      "condition": "(GV:BLOCK_TIMESTAMP - TR:UserTimeStamp(from)) >= 86400",
      "positiveEffects": [
        "TRU:UserTimeStamp(from) = GV:BLOCK_TIMESTAMP",
        "TRU:TradeCountPerUser = 0"
      ],
      "negativeEffects": [],
      "callingFunction": "mint(uint256 amount)"
    },
    {
      "condition": "TR:TradeCountPerUser(from) < 10",
      "positiveEffects": ["TRU:TradeCountPerUser(from) += 1"],
      "negativeEffects": ["revert(\"Trading Transaction Max Reached\")"],
      "callingFunction": "mint(uint256 amount)"
    }
  ]
}
You may have noticed the GV:BLOCK_TIMESTAMP variable above. The Rules Engine enables you to access the block level details with the GV prefix.