---
id: core-difficulty-adjustment
title: Difficulty Adjustment
keywords:
  - core
  - difficulty
  - adjustment
description: Core protocol design - difficulty adjustment.
---

---

Zilliqa has a dynamic difficulty level adjustment mechanism. The goal of this
mechanism is to adjust the difficulty level according to the number of PoW
submissions received.

When the network receives a low number of PoW submissions, the mechanism will
reduce the difficulty so that more nodes can join and maintain the network. On
the other hand, when the network receives a high number of PoW submissions, the
mechanism will increase the difficulty level, making it harder for Sybil attacks
to be executed.

## Adjustment Procedure

1. When we bootstrap the system, every node reads the initial difficulty level
   from `constants.xml`. After that, the nodes will update the difficulty level
   at the first transaction epoch of each DS epoch
1. At the beginning of each DS epoch, every DS node receives PoW submissions and
   records them. When the DS leader proposes the next DS block, it calls
   `CalculateNewDifficulty()` and `CalculateNewDSDifficulty()` to calculate the
   new difficulty levels. These difficulty levels are placed in the
   `m_dsDifficulty` and `m_difficulty` fields in the DS block. Finally, the DS
   leader announces the DS block to start the consensus
1. The DS backup nodes receive the announcement, and calculate the new
   difficulty by similarly calling `CalculateNewDifficulty()` and
   `CalculateNewDSDifficulty()` then comparing the results with those proposed
   by the DS leader
1. Once the DS committee completes consensus over the DS block containing the
   new difficulty level, the committee broadcasts the block to the shards. The
   shard nodes accept the new difficulty and use it to do PoW for the next DS
   epoch
1. New nodes that are not part of the network can get the latest difficulty from
   the lookup nodes by retrieving the latest DS block

## Adjustment Formula and Parameters

There are 4 parameters in `constants.xml` that are used to calculate the
difficulty:

- `POW_CHANGE_TO_ADJ_DIFF`
- `POW_CHANGE_TO_ADJ_DS_DIFF`
- `EXPECTED_SHARD_NODE_NUM`
- `NUM_DS_ELECTION`

Additionally, there are two dynamic parameters that are used to calculate the
difficulty:

- The number of shard PoW submissions
- The number of DS PoW submissions

The basic formulas used to calculate the new difficulty are: