The Client <--> Server division

The Minecraft code can be divided into two �sides� � Client and Server.

• The Server side is responsible for maintaining the master copy of the world - updating the blocks and entities based on packets received from the client, and sending updated information to all the clients.

• The Client side is primarily responsible for reading input from the player and for rendering the screen.

There is one server, and a number of clients which connect to it. Even in single player mode, the server and client code are running simultaneously (in separate threads).

Some parts of the code are used by both Client and Server code, for example Block methods.  If you are writing code which might be called by either Client or Server, how can you tell which side has called the code?
First preference: if your method has been passed a World object, or the Class you are extending has a World field (typically called worldObj):

• if World.isRemote is true, this is Client side
• if World.isRemote is false, this is Server side.

If your code has been passed a NetHandler:

• NetHandler.isServerHandler() returns true if Server side.

As a last resort:

• FMLcommonHandler.instance().getEffectiveSide() returns Side.SERVER or Side.CLIENT.

Client and Server run in parallel

Communication between the Client and Server sides takes place through Packets, which are sent back and forth using NetClientHandler and NetServerHandler.  The basic steps are shown in the diagram below.  There are a large number of different packets used:

* Packets from Client to Server

* Packets from Server to Client

o   Changes to Entities

o   Changes to the World /blocks

o   Managing Graphical User Interface for Containers

o   Miscellaneous

Some guidelines on the Client<-->Server communication here.

Communication from Client to Server

Communication from Server to Client

If you need to send packets in your mod code, Forge has thoughtfully provided PacketDispatcher to send packets to the client or the server without having to retrieve a NetServerHandler or NetClientHandler.

�          .sendPacketToServer()

�          .sendPacketToPlayer()

�          .sendPacketToAllAround()

�          .sendPacketToAllInDimension()

�          .sendPacketToAllPlayers()

If the existing packets don�t meet your needs, you can create a custom packet Packet250CustomPayload.  More details here.

In many cases, you won�t need to create custom packets to keep your client and server code synchronised.  The vanilla code and Forge code do this in a number of ways already:

• Creation, Removal, Movement, health, actions etc for Entities.  This is accomplished for new Entities by Forge packet EntitySpawnPacket.  You can define a custom handler for this using EntityRegistration.setCustomSpawning().
• Removal or placement of Blocks
• Adding DataWatcher variables to your Entity (see here)
• Containers (Inventory, furnace, etc)

Server Packets: Changes to Entities

The following Packets are used to inform the client about changes to Entities.  These are usually generated by

• EntityTrackerEntry (keeps track of entities near to each player)
• EntityPlayerMP (keeps track of player status, position, health, inventory, etc

The Client generally handles these packets by calling methods in

• EntityClientPlayerMP(for changes to the user / controlling player)
• WorldClient (addition or removal of Entities)
• Entity (for status changes to entities in general, eg health, position, etc)

	Typical server origin	NetClientHandler (Client processing)	Called by Client during processing
Packet5PlayerInventory Change the equipped item or armour of an entity (player, other players, mob)	EntityLivingBase.onUpdate() EntityTrackerEntry	.handlePlayerInventory()	Entity.setCurrentItemOrArmor()
Packet6SpawnPosition Tells the client where the player will respawn next.	ServerConfigurationManager	.handleSpawnPosition()	EntityClientPlayerMP.setSpawnChunk() WorldInfo.setSpawnPosition()
Packet8UpdateHealth Sends updated health and food status to the client.	EntityPlayerMP.onUpdateEntity()	.handleUpdateHealth()	EntityClientPlayerMP
Packet9Respawn Tell the client to respawn the player at the given location.	ServerConfigurationManager .transferPlayerToDimension() .respawnPlayer()	.handleRespawn() Change dimension, create new WorldClient, change game type	Minecraft.setDimensionAndSpawnPlayer PlayerControllerMP.setGameType
Packet10Flying, Packet11PlayerPosition, Packet12PlayerLook, Packet13PlayerLookMove	NetServerHandler .setPlayerLocation() Only Packet13 is ever sent from server to client.  It is sent once per second, to keep client and server synchronised.  Used as a signal to the client that the world reload after respawning has completed.	.handleFlying() Update position and/or look to the new values.  Then send the same Packet with updated values (in particular the foot and eye y-values) back to the server.  Get rid of respawn screen.	EntityClientPlayerMP .setPositionAndRotation()
Packet16BlockItemSwitch Changes the selected hotbar slot (0..8)	ServerConfigurationManager	handleBlockItemSwitch() Change inventory.currentitem to the nominated slot.
Packet17Sleep Put a player to sleep	EntityPlayerMP.sleepInBedAt	handleSleep() Put a player to sleep at the specified location, if possible.	Entityplayer.sleepInBedAt
Packet18Animation Entity performs an action 1 = swing item 2 = gets hurt (not used) 3 = wake up 6 = critical hit 7 = magic critical hit	EntityPlayerMP EntityLivingBase	handleAnimation() Starts the appropriate animation (entity) or EffectRenderer	EffectRenderer.addEffect() Entity EntityLivingBase EntityPlayer <![if !supportLineBreakNewLine]> <![endif]>
Packet20NamedEntitySpawn Spawn an Other Player on the client	EntityTrackerEntry	handleNamedEntitySpawn() Spawn the other player on the client, set its position, look, current item, and other metadata (Datawatcher objects)	EntityOtherPlayerMP WorldClient.addEntityToWorld()
Packet22Collect Collect an item from the ground	EntityLivingBase .onItemPickup()	handleCollect() Removes the EntityItem, plays a sound effect, renders the pickup effect.  Does not actually give an item to the entity.	WorldClient() EffectRenderer()
Packet23VehicleSpawn Spawns a non-living Entity (boat, fireball, firework, etc)	EntityTrackerEntry .getPacketForThisEntity() FMLNetworkHandler .getEntitySpawningPacket()	handleVehicleSpawn() Create the Entity, set position, pitch & yaw.  If thrown, set velocity.	WorldClient.addEntityToWorld()
Packet24MobSpawn Spawn a mob	EntityTrackerEntry .getPacketForThisEntity() FMLNetworkHandler. getEntitySpawningPacket()	handleMobSpawn() Create the Entity, set position, pitch & yaws, and other metadata (Datawatcher objects).	WorldClient.addEntityToWorld()
Packet25EntityPainting	EntityTrackerEntry .getPacketForThisEntity()	handleEntityPainting() Create a painting	WorldClient.addEntityToWorld()
Packet26EntityXPOrb	EntityTrackerEntry .getPacketForThisEntity()	handleEntityExpOrb() Create an XP orb.	WorldClient.addEntityToWorld()
Packet28EntityVelocity	EntityTrackerEntry .sendLocationToAllClients()	handleEntityVelocity() Change the Entity�s velocity	Entity.setVelocity()
Packet29DestroyEntity Remove one or more entities from client world, i.e. client doesn�t need to track them anymore.	EntityPlayerMP.onUpdate()	handleDestroyEntity()	WorldClient.removeEntityFromWorld
Packet30Entity, Packet31RelEntityMove, Packet32EntityLook, Packet33RelEntityMoveLook Tell the client the relative movement and/or yaw+pitch of the entity.	EntityTrackerEntry .sendLocationToAllClients()	.handleEntity() Update position (based on relative movement) and/or yaw/pitch.	Entity.setPositionAndRotation2()
Packet34EntityTeleport Move the Entity to the specified position, yaw+pitch	EntityTrackerEntry .sendLocationToAllClients() Performs teleport instead of relative movement: once every 20 seconds, if riding, or if movement is too big.	.handleEntityTeleport() Set entity position to the specified position, yaw+pitch	Entity.setPositionAndRotation2()
Packet35EntityHeadRotation	EntityTrackerEntry .sendLocationToAllClients()	.handleEntityHeadRotation() Sets the entity�s head to the given yaw	Entity. setRotationYawHead
Packet38EntityStatus 2 = hurt, 3 = dead	EntityPlayerMP.onItemUseFinish() WorldServer.setEntityState()	.handleEntityStatus() calls Entity.handleHealthUpdate(), which plays appropriate sound and calls appropriate client method	Entity .attackEntityFrom() .onDeath()
Packet39AttachEntity Sends mounting / leashing info for horse, boat, etc.	EntityPlayerMP.mountEntity EntityTrackerEntry EntityLiving	.handleAttachEntity() mounts / leashes or unmounts / unleashes entity � horse, boat, cart, etc	EntityLiving EntityBoat
Packet40EntityMetadata Copy all changed metadata (WatchedObjects) to client (and optionally, clear the �changed� flag .watched for each)	EntityTrackerEntry	.handleEntityMetadata() Transmit all metadata for the entity that has been changed (�.watched� flag is true).  The �.watched� flag for each is optionally cleared when the packet is created.	Entity.DataWatcher .updateWatchedObjectsFromList
Packet41EntityEffect Apply the potion effect to the entity	EntityTrackerEntry EntityPlayerMP ServerConfigurationManager	.handleEntityEffect() Apply the potion effect to the entity	EntityLivingBase.addPotionEffect()
Packet42RemoveEntityEffect Remove a potion effect from the entity	EntityPlayerMP	.handleRemoveEntityEffect()	EntityLivingBase. removePotionEffectClient
Packet43Experience Set player experience	EntityPlayerMP ServerConfigurationManager	.handleExperience() sets the client player experience & level	EntityClientPlayerMP.setXPstats
Packet44UpdateAttributes Send all Attributes for this entity to the client.	EntityTrackerEntry	.handleUpdateAttributes() set the attributes of the client entity	EntityLivingBase.getAttributeMap() (the elements in the map are overwritten).
Packet200Statistic Send statistics (achievement, distance walked, or similar)	EntityPlayerMP	.handleStatistic()	EntityClientPlayerMP .incrementStat
Packet201PlayerInfo Send network info about a player to client (name, connected?, ping)	ServerConfigurationManager	.handlePlayerInfo()	NetClientHandler
Packet202PlayerAbilities Inform the client of the player�s abilities (can fly, walk speed, etc)	EntityPlayerMP ServerConfigurationManager	.handlePlayerAbilities() NB - .allowEdit is ignored	EntityClientPlayerMP

There are also two FMLpackets used for custom entities:

	Typical Server origin	FMLClientHandler (Client processing)	Called by Client during processing
EntitySpawnPacket	FMLNetworkHandler .getEntitySpawningPacket (from EntityTrackerEntry)	EntitySpawnPacket.execute()	FMLClientHandler .spawnEntityIntoClientWorld()
EntitySpawnAdjustmentPacket	FMLNetworkHandler .makeEntitySpawnAdjustment (from EntityTrackerEntry)	EntitySpawnAdjustmentPacket.execute()	FMLClientHandler .adjustEntityLocationOnClient()

Server Packets: Miscellaneous

Miscellaneous packets that I didn�t know where else to put.

	Typical server origin	NetClientHandler (Client processing)	Called by Client during processing
Packet3Chat Chat messages including commands	EntityPlayerMP	.handleChat() Print to GUI	FMLNetworkHandler.handleChatMessage() Forge Event: ClientChatReceivedEvent GUINewChat.printChatMessage()
Packet4UpdateTime Synchronise Client time from server	MinecraftServer ServerConfigurationManager	.handleUpdateTime() Synchronise world�s time to server
Packet203AutoComplete Server sends possible autocompletions for the given string fragment previously sent by the client	NetServerHandler .handleAutoComplete()	.handleAutoComplete() inserts the possible autocompletions into the GUIchat	GUIchat
Packet206SetObjective to Packet209SetPlayerTeam	ServerScoreboard	Handle teams and objectives, not sure what these are for.
Packet250CustomPayload Channel (string) Length Rawdata (byte stream)	Vanilla: ServerConfigurationManager EntityPlayerMP NetClientHandler ForgePacket FML.common.network	.handleCustomPayload() Channels starting with MC| are handled by vanilla code (Request texture pack load; login information; merchant transaction information) Channels starting with FML are sent to handleFMLPacket. All others are sent to NetworkRegistry.handleCustomPacket (mod can register a custom handler)	FMLNetworkHandler.handlePacket250Packet handleFMLPacket; handleVanilla250Packet
Packets 252+ : connection maintenance

There are also a number of packets used by Forge - see FMLPacket.
The most relevant ones are

• EntitySpawnPacket - which gets passed to a user-supplied function when the entity spawns.
• EntitySpawnAdjustmentPacket - used to adjust an Entity's location
• OpenGuiPacket - opens a Graphical User Interface screen on the client or server.