Epoch vs Period

Understanding the distinction between Epoch and Period is critical for grasping the underlying mechanics of the system. Both terms are used to represent time intervals but serve different purposes and are calculated differently.

1. What is an Epoch?

An epoch is a fixed time unit measured in blocks. It represents the core timing mechanism for operations within the blockchain.

  • Definition:
    • An epoch is defined as a set number of blocks.
  • Configuration:
    • In this system, 1 epoch = 200 blocks.
  • Approximate Duration:
    • Each block takes approximately 3 seconds to generate.
    • Thus, 1 epoch ≈ 10 minutes.

Example:

  • Blocks 0–199 belong to epoch 0.
  • Blocks 200–399 belong to epoch 1.
  • Blocks 400–599 belong to epoch 2, and so on.

Epochs are used for:

  • Validator rotation.
  • Slashing mechanisms.
  • Reward distribution calculations.

2. What is a Period?

A period is a broader time unit measured in timestamps, typically aligned with calendar days. It serves as a higher-level grouping of epochs.

  • Definition:
    • A period ideally starts at 00:00 UTC each day.
  • Challenges in Blockchain:
    • Blockchain works with block numbers, not timestamps, making it difficult to align perfectly with real-world time.
    • Instead, the system determines a new period when the timestamp of the current epoch exceeds the next day’s threshold.

Key Notes:

  • A period starts between 00:00 UTC → 00:15 UTC, depending on block generation timing.
  • Period duration is variable and influenced by network performance.

3. Key Differences

FeatureEpochPeriod
Measurement UnitNumber of blocksTimestamps (real-world time)
DurationFixed (200 blocks ≈ 10 minutes)Variable (aligned with calendar)
PurposeOperational tasks (e.g., record stats, validator rotation)High-level grouping of epochs (e.g., reward distribution, randomization, slashing)
Start ConditionBased on block numberBased on UTC timestamp

4. Interplay Between Epochs and Periods

Periods are determined by epochs, and the transition is triggered as follows:

  1. At the end of each epoch, the system checks the timestamp of the last block.
  2. If the timestamp indicates a new calendar day, a new period begins.
  3. Validator roles, random beacons, and other operations are changed at the period boundary.

Key Implications:

  • A period may contain fewer epochs if the network is slow (fewer blocks are produced per day).
  • Epoch-based calculations remain deterministic, while period-based calculations adapt to timestamp variability.

5. Practical Example

Consider the following scenario:

  • Block production rate: ~1 block per 3 seconds.
  • Daily period duration: 24 hours = 86,400 seconds.
  • Expected epochs per period: $$\approx (\frac{86,400}{(200 \times 3)} $$

6. Visual Representation

+--------+-----------------------------------------------+-------------------------------------------------------------+
| Period |                       1                       |                              2                              |
+========+=======+=========+=========+=====+=============+=============+=============+=============+=====+=============+
|  Epoch |   0   |    1    |    2    | ... |     239     |     240     |     241     |     242     |     |             |
+--------+-------+---------+---------+-----+-------------+-------------+-------------+-------------+-----+-------------+
|  Block | 0-199 | 200-399 | 400-599 | ... | 47800-47999 | 48000–48199 | 48200–48399 | 48400–48599 | ... | 95800–95999 |
+--------+-------+---------+---------+-----+-------------+-------------+-------------+-------------+-----+-------------+

In this table:

  • Each period consists of multiple epochs.
  • Each epoch is defined by a range of block numbers.
  • A new period starts when the UTC timestamp exceeds 00:00 UTC.