Minecraft:Block update

A block update is an in-game mechanism that occurs when a block is modified in some way. Limited to the computing power of a computer, the game can not process all blocks at all times, which is why the game needs such mechanism. When a block changes (due to a player, an entity, or a chunk tick, etc.), it notifies nearby blocks that they should check if they need to respond to the change.

Java Edition

Template:IN, there are two main types of block updates: post placement (PP) and neighbor changed (NC).

Priorities

This article uses the priorities listed in the table below to categorize the various block updates. However, keep in mind that these values are not actually present in the game's code.

Priority	Description	Method in code
-3	Updates before the change.	N/A
-2	Some updates after the change.	Blockstate.Template:CdTemplate:More info
-1	Some updates after the change.	Template:Jbcp.Template:Cd
0	General NC updates.	Template:Jcp.Template:Cd
1	General comparator updates.	Template:Jbcp.Template:Cd
2	General PP updates.	Template:Jbcp.Template:Cd
3	Later updates.	N/A

Most players only use -3 to 3 priority values to describe scheduled ticks, so it is recommended not to use these values for the update sequence in order to avoid ambiguity with scheduled tick priorities.

Post placement update

A post placement (PP) update (aka update post placement, update neighbor shapes, update shape) is the most common type of block updates, mainly used for connected blocks changing their shapes (e.g. stairs, fences) and attached blocks dropping into items (e.g. torches, cactus, wheat).

Sending

General PP updates

When a block is placed, destroyed, replaced, or its blockstate is changed, the game sends PP update to its immediate neighbors, in this order: west, east, north, south, down, up.

Exceptions

Some blocks have unique behaviors:

• Lighting a nether portal does not send PP update.
• A nether portal generating does not send PP update.
• Changing block state with debug stick does not send PP update.
• Shulker boxes sends general PP updates when beginning to open/close or finished opening/closing.
• When a redstone wire is placed, destroyed, replaced, or its blockstate is changed, for any block that the redstone wire points horizontally to, if it is not a redstone wire, then the game also sends PP updates to the block above it and the block below it. However, it does not send PP update to an observer.
• Pistons and beds have more complex updates.

Receiving

When a block receives a PP update, the direction of the PP update may be considered. For example, a Minecraft:wheat block accepts only PP updates from beneath.

Only the following blocks can receive PP updates:

Block	What to do when receiving a PP update
Support-requiring blocks (e.g. torches, snow, carpets, doors, fire, etc.)	Checks if it should break and drop as an item.
Connective blocks (e.g. fences, stairs, chests, redstone wires, nether portals, etc.)	Checks if it should change its shape.
Template:BlockLink	Checks for being locked or unlocked.
Template:BlockLink	Tries to update its instrument type.
Template:BlockLink	Calculates the distance to logs.
Template:BlockLink	Solidifies if contacted with water.
Template:BlockLink and waterlogged blocks	Requests a scheduled tick for flowing.
Template:BlockLink	Requests a scheduled tick for pulsing.
Template:EntityLink	Requests a scheduled tick for checking for falling.
Template:BlockLink	Requests a scheduled tick for checking whether it is covered by a solid block.
Template:BlockLink
Template:BlockLink	Requests a scheduled tick for checking if it should drop.
Template:BlockLink	Requests a scheduled tick for checking whether it is outside of water.
Template:BlockLink	Tries to request a scheduled tick to generate bubble columns.
Template:BlockLink

Neighbor changed update

Neighbor changed (NC) updates (aka neighbor changed, update neighbors, redstone update) are the most widely known type of block updates (for historical reasons), and are mainly used for redstone components.

Sending

General NC updates

Most blocks send NC updates to their immediate neighbors when they are placed, destroyed, or replaced, in this order: west, east, down, up, north, south.

Exceptions

Some changes do not produce General NC updates:

• Template:BlockLink
  • Thickens when the upper bamboo thickens.
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  • Grows into a bamboo.
• Template:BlockLink and Template:BlockLink
  • Convert to each other.
• Template:BlockLink, Template:BlockLink, Template:BlockLink and Template:BlockLink
  • Turn into a golem.
• Plants
  • Turn into a double-block plant.
• Template:BlockLink
  • Its water level changes.
• Template:BlockLink and Template:BlockLink
  • Grow, ripen or change their shape.
• Template:BlockLink
  • Grows.
• Template:BlockLink
  • Becomes lit or unlit.
• Template:BlockLink
  • Its signal or its mode is changed.
• Template:BlockLink, Template:BlockLink, and Template:BlockLink
  • Die.
• Template:BlockLink, Template:BlockLink, Template:BlockLink, and Template:BlockLink
  • Grow.
• Template:BlockLink, Template:BlockLink, and Template:BlockLink
  • Opes, close, activate, or deactivate.
• Template:BlockLink
  • Transports to destination.
• Template:BlockLink
  • Gets hydrated or dry.
• Template:BlockLink
  • Melts gradually (but has not turned into water).
• Template:BlockLink
  • Inserts or ejects records.
• Template:BlockLink
  • Grows.
• Template:BlockLink
  • Becomes lit or unlit.
• Template:BlockLink
  • Becomes lit or unlit.
• Template:BlockLink
  • Increases its number using bone meal.
• Template:BlockLink
  • Absorbs water.
• Template:BlockLink and Template:BlockLink
  • Grow.
  • Melon or pumpkin is removed from them.
• Template:BlockLink
  • Grows.
  • Its fruits are collected.
• Template:BlockLink
  • Its number decreases.
• Template:BlockLink
  • Grows, changes its shape, grows out a new vine block.
• Template:BlockLink, Template:BlockLink, and Template:BlockLink
  • Turn into a wither.
• Template:BlockLink
  • Gets eaten by a sheep.
• Template:BlockLink
  • Get eaten by a rabbit.
• Template:BlockLink
  • Gets damaged.
• Template:BlockLink
  • The Eye of Ender is placed.
• Template:BlockLink
  • Generates.
• Template:BlockLink
  • Bottles or potions are added into it.
• Template:BlockLink and Template:BlockLink
  • Change their mode.
• Template:BlockLink and Template:BlockLink
  • Grow.
• Template:BlockLink
  • Changes its mode.
• Template:BlockLink, Template:BlockLink, and Template:BlockLink
  • Becomes activated or deactivated.
• Template:BlockLink
  • Its age or shape changes.
• Template:BlockLink
  • Increase their stage.
• Template:BlockLink
  • Its "disarmed" value changes.
• Template:BlockLink
  • Become occupied or unoccupied.
• Template:BlockLink and Template:BlockLink
  • Generates as a nether portal.
• Template:BlockLink
  • Add or remove hay bale below it.
• Template:BlockLink, Template:BlockLink, Template:BlockLink, Template:BlockLink, Template:BlockLink, and Template:BlockLink
  • Its shape changes.
• Template:BlockLink, Template:BlockLink, and Template:BlockLink
  • Be covered or uncovered by snow.
• Template:BlockLink, Template:BlockLink, and Template:BlockLink
  • Grow.
  • Add or remove kelp/vines above/below it.
• Template:BlockLink
  • Its signal changes.
• All blocks
  • Change their blockstate using debug stick.
  • Placed by a command.
• Template:BlockLinks and Template:BlockLink
  • Have more complex behavior.

Other NC updates

Some blocks (mainly redstone components) send more NC updates other than general NC updates when changing.

Block	Change	Priority	Range and order	Schematic diagram
Lever / Button / Pressure plate / Tripwire hook	Destroy an activated lever/button/pressure plate/tripwire hook.	-2	Their immediate neighbors (in order of west, east, down, up, north, south). Then the immediate neighbors (in order of west, east, down, up, north, south) of the block they're attached to.
	Active or deactive them. Change pressure plate's signal. Tripwire hook connects to a tripwire.	3
Repeater and Comparator	Place or its blockstate changes.	-1	The block they're facing. Then the immediate neighbors (without the repeater/comparator itself. In order of west, east, down, up, north, south) of the block they're facing.
	Remove.	-2
	Comparator's signal changes.	3
Chest	Begin to be open/closed	N/A (the block is not changed)	Its immediate neighbors (in order of west, east, down, up, north, south).
Trapped chest	Begin to be open/closed	N/A (the block is not changed)	Its immediate neighbors (in order of west, east, down, up, north, south). Then the immediate neighbors (in order of west, east, down, up, north, south) of the block below it.
Observer	Lit or unlit.	3	The block it's facing. Then the immediate neighbors (without the observer itself. In order of west, east, down, up, north, south) of the block it's facing.
	Destroy or replace an activated observer.	-2
	Place an activated observer.	-1	Set itself to unactivated without any update. Then update the block it's facing. Then the immediate neighbors (excluding the observer itself. In order of west, east, down, up, north, south) of the block it's facing.
Redstone torch	Place or its blockstate changes.	-1	The immediate neighbors (in order of west, east, down, up, north, south) of the immediate neighbors (in order of down, up, north, south, west, east) of the torch.
	Destroy, replace, or its blockstate changes.	-2
Redstone wire	Destroy or replace.	-2	The immediate neighbors (in order of west, east, down, up, north, south) of the immediate neighbors (in order of down, up, north, south, west, east) of the wire. Then updates the immediate neighbors (in order of west, east, down, up, north, south) and the immediate neighbors (in order of west, east, down, up, north, south) of the immediate neighbors (in order of down, up, north, south, west, east) of adjacent redstone wires (in order of north, east, south, west). Then updates the immediate neighbors (in order of west, east, down, up, north, south) and the immediate neighbors (in order of west, east, down, up, north, south) of the immediate neighbors (in order of down, up, north, south, west, east) of redstone wires above the solid blocks adjacent to the redstone wire (in order of north, east, south, west). Then updates the immediate neighbors (in order of west, east, down, up, north, south) and the immediate neighbors (in order of west, east, down, up, north, south) of the immediate neighbors (in order of down, up, north, south, west, east) of redstone wires below the non-solid blocks adjacent to the redstone wire (in order of north, east, south, west).
	Place.	-1	Updates the immediate neighbors (in order of west, east, down, up, north, south) of the block above the wire. Then updates the immediate neighbors (in order of west, east, down, up, north, south) of the block below the wire. Then updates the immediate neighbors (in order of west, east, down, up, north, south) and the immediate neighbors (in order of west, east, down, up, north, south) of the immediate neighbors (in order of down, up, north, south, west, east) of adjacent redstone wires (in order of north, east, south, west). Then updates the immediate neighbors (in order of west, east, down, up, north, south) and the immediate neighbors (in order of west, east, down, up, north, south) of the immediate neighbors (in order of down, up, north, south, west, east) of redstone wires above the solid blocks adjacent to the redstone wire (in order of north, east, south, west). Then updates the immediate neighbors (in order of west, east, down, up, north, south) and the immediate neighbors (in order of west, east, down, up, north, south) of the immediate neighbors (in order of down, up, north, south, west, east) of redstone wires below the non-solid blocks adjacent to the redstone wire (in order of north, east, south, west).
	Its signal changes	3	Update the immediate neighbors (in order of west, east, down, up, north, south) of the datum points. Datum points include the wire and the immediate neighbors of the wire, in random order.
	Changes between dot and cross	3	Update the immediate neighbors (without the wire itself, and in order of west, east, down, up, north, south) of the solid block next to the wire (in order of north, east, south, west).
Rails	Template:Info needed
Any block	Placed with a command.	3	Update the immediate neighbors (in order of west, east, down, up, north, south).

Receiving

When an NC update is received, the block that caused that update may taken into consideration. For example, a rail accepts only NC updates from power components.

Only the following blocks can react to NC updates:

Block	What to do when receiving an NC update
Template:BlockLink	Tries to melt.
Template:BlockLink	Tries to change its state.
Template:BlockLink
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Template:BlockLink
Template:BlockLink	Tries to change itself to be in the correct state.
Template:BlockLink
Template:BlockLink	Tries to absorb water.

Comparator update

Template:Expand section Template:Schematic The comparator update (i.e. Block entity updates, Update neighbor for output signal) is a subtype of the neighbor update that is only sent to comparators. Minecraft:Redstone comparators can read certain blocks and produce an output signal, and in turn, these blocks can send comparator updates when changing, to notify the comparators. Comparators can also read some entities (Minecraft:item frames and Minecraft:glow item frames), as such, these entities can send comparator updates too, even though they are not blocks.

When a block that can be detected by comparators changes, it sends updates to surrounding comparators (including comparators separated by a solid block), in this order: north, east, south, west.

After receiving a comparator update, comparators attempt to calculate and change their output signal.

Special blocks

• Template:BlockLink: during the start of the extension/retraction process, every time a moving piston block gets placed and every time one of the normal blocks<ref>"Normal block" is intended as not a "moving piston" block.</ref> is replaced/removed, PP updates are sent to these blocks' neighbors. Once all moving piston blocks have been placed, the game will send NC updates to the replaced/removed normal blocks' neighbors.

Minecraft:Sticky pistons, however, do not send NC updates around their head when they start to pull blocks: only the NC updates caused by the moving piston block turning back into a normal block are sent, once the retraction process is over. If a sticky piston fails to retract a Minecraft:slime block or a Minecraft:honey block due to the pull limit, no updates will be sent around the head whatsoever.

Block updates and DFS

Block updates follow the principle of Template:W (DFS).

For example, when Block A changes, it maybe sends block updates to B and C one by one. Receiving the update, B maybe changes and sends updates to D and E. Receiving update from B, D changes and sends updates to some block. Then E receives the update from B. Then C receives the update from A.

Self update

When a block is placed, it checks and adapts itself to its surroundings by Blockstate.onPlace() or similar methods. Strictly speaking, this isn't a block update; however when a block is placed or changed, if it changes again when self-updated, then the former change does not produce any general update (this is a bug).

Here is a list of self updates that can cause this bug.

• Template:BlockLink
  • When being placed, if it should be powered, becomes powered.
• Template:BlockLink
  • If in the nether, it turns into an dry sponge.
• Template:BlockLink
  • When being placed, it absorbs water and turns into an wet sponge.
• Template:BlockLink and Template:BlockLink
  • When a powered target or observer is placed, it turns into an unpowered state.
• Template:BlockLink
  • When being placed, it updates the signal and connection.
• Template:BlockLink
  • When being placed, if it should be extinguished, removes itself.
• Template:BlockLink
  • When being placed, if it should explode, removes itself.
• Template:BlockLink, Template:BlockLink, Template:BlockLink, Template:BlockLink
  • When being placed, it updates its shape.
• Template:BlockLink
  • When being placed, it checks whether it is pressed and updates its state.
• Template:BlockLink
  • When being placed, if it should turn into a golem, removes itself.

History