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Capella Light Client -- Fork Logic

Introduction

This document describes how to upgrade existing light client objects based on the Altair specification to Capella. This is necessary when processing pre-Capella data with a post-Capella LightClientStore. Note that the data being exchanged over the network protocols uses the original format.

Upgrading light client data

A Capella LightClientStore can still process earlier light client data. In order to do so, that pre-Capella data needs to be locally upgraded to Capella before processing.

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def upgrade_lc_header_to_capella(pre: bellatrix.LightClientHeader) -> LightClientHeader:
    return LightClientHeader(
        beacon=pre.beacon,
    )

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def upgrade_lc_bootstrap_to_capella(pre: bellatrix.LightClientBootstrap) -> LightClientBootstrap:
    return LightClientBootstrap(
        header=upgrade_lc_header_to_capella(pre.header),
        current_sync_committee=pre.current_sync_committee,
        current_sync_committee_branch=pre.current_sync_committee_branch,
    )

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def upgrade_lc_update_to_capella(pre: bellatrix.LightClientUpdate) -> LightClientUpdate:
    return LightClientUpdate(
        attested_header=upgrade_lc_header_to_capella(pre.attested_header),
        next_sync_committee=pre.next_sync_committee,
        next_sync_committee_branch=pre.next_sync_committee_branch,
        finalized_header=upgrade_lc_header_to_capella(pre.finalized_header),
        finality_branch=pre.finality_branch,
        sync_aggregate=pre.sync_aggregate,
        signature_slot=pre.signature_slot,
    )

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def upgrade_lc_finality_update_to_capella(pre: bellatrix.LightClientFinalityUpdate) -> LightClientFinalityUpdate:
    return LightClientFinalityUpdate(
        attested_header=upgrade_lc_header_to_capella(pre.attested_header),
        finalized_header=upgrade_lc_header_to_capella(pre.finalized_header),
        finality_branch=pre.finality_branch,
        sync_aggregate=pre.sync_aggregate,
        signature_slot=pre.signature_slot,
    )

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def upgrade_lc_optimistic_update_to_capella(pre: bellatrix.LightClientOptimisticUpdate) -> LightClientOptimisticUpdate:
    return LightClientOptimisticUpdate(
        attested_header=upgrade_lc_header_to_capella(pre.attested_header),
        sync_aggregate=pre.sync_aggregate,
        signature_slot=pre.signature_slot,
    )

Upgrading the store

Existing LightClientStore objects based on Altair MUST be upgraded to Capella before Capella based light client data can be processed. The LightClientStore upgrade MAY be performed before CAPELLA_FORK_EPOCH.

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def upgrade_lc_store_to_capella(pre: bellatrix.LightClientStore) -> LightClientStore:
    if pre.best_valid_update is None:
        best_valid_update = None
    else:
        best_valid_update = upgrade_lc_update_to_capella(pre.best_valid_update)
    return LightClientStore(
        finalized_header=upgrade_lc_header_to_capella(pre.finalized_header),
        current_sync_committee=pre.current_sync_committee,
        next_sync_committee=pre.next_sync_committee,
        best_valid_update=best_valid_update,
        optimistic_header=upgrade_lc_header_to_capella(pre.optimistic_header),
        previous_max_active_participants=pre.previous_max_active_participants,
        current_max_active_participants=pre.current_max_active_participants,
    )