Reference: Network Sync

Library of Exile has a built-in sync mechanism for registry data. It is automatic for all ON_LOGIN types and requires no code from dependent mods.

`SyncTime`

public enum SyncTime {
    ON_LOGIN,  // Data is sent to the client when the player logs in or /reload fires
    NEVER      // Data stays on the server only
}

How `ON_LOGIN` sync works

After each datapack reload — ExileRegistryContainer.onAllDatapacksLoaded() serializes all entries from that container to bytes and caches the result in cachedBuf. This step happens once regardless of how many players are online.
On player login or /reload — OnDatapackSyncEvent fires (Forge). CommonInit responds by calling Database.sendPacketsToClient(player, SyncTime.ON_LOGIN) for every ON_LOGIN type. The pre-cached bytes are sent as an EfficientRegistryPacket for each type.
On the client — RegistryPackets receives each packet, deserializes the JSON entries, and re-registers them via ClientSyncRegistration.INSTANCE. This re-populates the client's Database with the same data the server has.

Performance note: The serialization-to-bytes step runs once per reload, not once per login. This keeps login fast on servers with many connected players.

What is synced

All entries that came from datapacks (i.e. isFromDatapack() == true) are included in the sync packet. Hardcoded entries (registered in Java) are not synced — clients are expected to register those via their own FMLCommonSetupEvent flow.

This means: if you add a hardcoded entry to a ON_LOGIN type, it must be registered in the dependent mod's client-side code too, or the client's Database will be missing it.

`ON_LOGIN` types (built-in)

Type	Why synced
`MOB_AFFIX`	Clients display affix names on mob tooltips
`MAP_FINISH_RARITY`	Clients display map reward tiers
`MAP_CONTENT`	Clients show map content possibilities in UI
`RELIC_STAT`	Clients render relic stat values on items
`RELIC_TYPE`	Clients display relic type names
`RELIC_AFFIX`	Clients show affix descriptions on relics
`RELIC_RARITY`	Clients display rarity colors/names
`ITEM_MOD`	Client item modification display
`ITEM_REQ`	Client item requirement display
`CURRENCY`	Currency display in client UI
`ORB_EDIT`	Orbs of Crafting mod: client display

`NEVER` types (built-in)

Type	Why server-only
`MAP_DATA_BLOCK`	Map generation is purely server-side
`MOB_LIST`	Mob spawning is purely server-side
`LEAGUE`	League evaluation is server-side

Registering your own `ON_LOGIN` type

When you register a new ExileRegistryType with SyncTime.ON_LOGIN, the sync mechanism picks it up automatically for JSON entries. No extra registration is needed.

Your content class must implement IAutoGson<T> so the serialization layer knows how to write and read it.

Client-side access

On the client, Database.getRegistry(type) works the same way as on the server. After ON_LOGIN packets are applied, the client's Database contains all synced entries.

The client never receives NEVER entries. Calling Database.get(LibDatabase.MOB_LIST, ...) on a client will return the default empty entry.

Reference: Network Sync

SyncTime

How ON_LOGIN sync works