# SIP-91: Debt Cache Contract

Author Anton Jurisevic https://research.synthetix.io/t/sip-91-debt-cache-contract/213 Implemented 2020-10-20

## Simple Summary

Separates the debt snapshot logic out of Issuer, and into a new DebtCache contract.

## Abstract

This SIP proposes adding a new DebtCache contract, along with a RealtimeDebtCache implementation inheriting DebtCache, which will be used on L2 where debt snapshots are not required. All debt cache related functions that currently exist in Issuer will be moved into the new contracts.

## Motivation

The modifications applied in SIP-83 to cache the total system debt increased the size of the Issuer contract to the point that its compiled bytecode was too large to be deployed on the L2 OVM. As the L2 deployment of Synthetix has no execution costs, debt snapshots are unnecessary in this context. By separating out the snapshot logic, the Issuer and DebtCache contracts will individually fit into the contract size limit on L2, and the DebtCache itself can be individually replaced with a RealtimeDebtCache version which does not require cache synchronisation.

This structure will also allow debt cache logic to be upgraded without modifying the Issuer, which when redeployed requires a time-consuming process of re-adding all Synths to the system.

## Specification

### Rationale

Although in principle one could simply replace the entire Issuer contract with a different version for L2, this would require modifying the internal structure of that contract, and entail multiple versions of the core debt snapshot code to exist. This would increase the overhead of making contract modifications in the future. By separating out the logic into a new contract, this overhead is reduced, and the Issuer contract's size is brought down much further below the fundamental size limits imposed by the OVM, allowing more headroom to extend its functionality.

Many functions will be renamed in this refactor; although in many cases no functionality will change, these modifications to the interface are in service of making it clearer, more consistent, and more complete.

### Technical Specification

Issuer will no longer be responsible for maintaining the debt cache, but will instead pass this responsibility on to the new DebtCache contract.

#### Interface Modifications

The following functions, and a number of supporting internal functions will be moved from the Issuer and into a new contract called DebtCache. These functions will be renamed to clarify the resulting interface. Several new functions will added to round out its functionality and simplify the implementation.

Issuer Function DebtCache Function Description
debtSnapshotStaleTime debtSnapshotStaleTime Reports the current snapshot stale time.
currentSNXIssuedDebtForCurrencies currentSynthDebts Reports the debt values for a set of synths at current prices and supply.
cachedSNXIssuedDebtForCurrencies cachedSynthDebts Reports the cached debt values for a set of synths.
currentSNXIssuedDebt currentDebt Reports the current total system debt value across all synths.
cachedSNXIssuedDebtInfo cacheInfo Reports the cached system debt, when a snapshot was last taken, and the cache's invalidity and stale status
cacheSNXIssuedDebt takeDebtSnapshot Takes completely fresh debt snapshot, updating the cache, timestamp, and validity status.
updateSNXIssuedDebtForCurrencies updateCachedSynthDebts Modifies the cached debt value with the deltas from a specific set of synths.
purgeDebtCacheForSynth purgeCachedSynthDebt Admin function to purge the cached value of a specific Synth if it was not added/removed from the system properly after an upgrade.
updateSNXIssuedDebtForSynth updateCachedSynthDebtWithRate Allows the issuer and exchanger contracts to update a synth's cached debt without refetching its price
updateSNXIssuedDebtOnExchange Deleted Exchange-specific logic will be moved into the Exchanger contract; the same functionality will be implemented with the new updateCachedSynthDebtsWithRates function.
None updateCachedSynthDebtsWithRates As updateCachedSynthDebtWithRate, but for a set of synths.
None updateDebtCacheValidity Allows the issuer to invalidate teh cache when adding or removing synths.
None cachedDebt Reports the cached system debt.
None cacheTimestamp The timestamp that the cache was last refreshed with a full snapshot.
None cacheInvalid True if the cache has been invalidated by, or since, the last full snapshot.
None cacheStale True if the cache timestamp is older than the stale time.

The DebtCacheSynchronised event will be renamed to DebtCacheSnapshotTaken.

In addition, the issuer itself will also gain a new function, Issuer.synthAddresses(bytes32[] memory currencyKeys) returns (ISynth[] memory), which will be used by the debt cache to obtain several synth addresses in a single function call in order to fetch the total supply of each.

#### Flexible Storage Removal

To simplify implementation, offset the gas cost of the additional function call, the cached debt values will be stored in the debt cache contract itself rather than in flexible storage. There is less of a necessity to persist this information since it no longer rides along with the issuer, but in addition storing the cache in the contract will decrease gas consumption for any account calling takeDebtSnapshot, which will improve gas costs for the snapshot keeper, which must run regularly to keep the debt snapshot from going stale.

#### L2 Realtime Debt Cache

On L2 DebtCache will be replaced by RealtimeDebtCache, which is a drop-in replacement that shares an identical interface; but its semantics will be altered as follows:

• cacheInfo will report realtime values from currentDebt for the debt and invalidity. The cache timestamp will always report as the current block timestamp, and the cache will never be stale.
• cachedSynthDebts will report realtime values from currentSynthDebts.
• All mutative functions in the interface such as takeDebtSnapshot will become no-ops.

While the Issuer is being modified, functions to add and remove multiple Synths at once will also be added, which will speed up redeployments of the Issuer (among other operations) by batching Synth migrations rather than performing them one by one.