Ticking area

A ticking area is a player-specified group of chunks that continues to be updated even when there is no player nearby. Because the chunks remain active, processes such as growth, decay, spawning, movement, and redstone functions in them aren't suspended when no players are present, provided at least one player remains somewhere in the dimension.

Behavior
Normally, only the chunks closely surrounding a player are updated by the game. These chunks are said to be in the player's chunk update range. As the player moves through the world, new chunks that come into range are added to the list of updated chunks and older chunks that pass out of range are removed. Only the chunks in the list are updated to reflect the passage of time. This means, for instance, that if a player spends a lot of time exploring or in another dimension, his crops won't grow and any cooking, smelting, or automated farming underway when he left will still be unfinished when he returns.

Ticking areas solve this problem by keeping player-designated chunks from being suspended. Each chunk in a ticking area is updated exactly as if it were perpetually in a player's chunk update range.

Note that in other editions of Minecraft, spawn chunks play a similar role of keeping some chunks active in the absence of a player. Pocket Edition does not use the concept of spawn chunks.

Events Processed
The following time-related events occur without interruption in ticking areas, provided there is at least one player in the same dimension (the Overworld, The Nether, or The End) as the ticking area:
 * Water flows and breaks non-solid blocks.
 * Lava flows, sets fire to nearby flammable blocks, destroys item entities, and interacts with water.
 * Fire spreads to adjacent flammable blocks.
 * Dropped items fall and will despawn after 5 minutes if not picked up or collected by a hopper.
 * Passive mobs spawn, grow up, and move around normally.
 * Sheep eat grass and grow wool.
 * Sand, gravel, and concrete powder fall if the block beneath them is moved or broken.
 * Primed TNT falls and explodes.
 * Minecarts function normally.
 * Redstone mechanisms and devices function normally, provided they are entirely contained within ticking areas.
 * Crops, mushrooms, saplings, and nether wart grow to maturity if the circumstances are suitable.
 * Farmland changes its hydration in response to available water.
 * Leaves decay if not connected to a log.
 * Grass and mycelium spread to adjacent blocks.
 * Ice forms and melts in response to changes in lighting.
 * Rain, snow, and lightning occur and have their usual effects.
 * Hostile mobs spawn and move around as usual.

Suspended Events
Many of the above events involve entities moving or spreading, which raises the possibility of updates in a ticking area affecting blocks in outside chunks. If an outside chunk is within any player's chunk update range, of course, the chunk is active and these events will proceed as usual. However, if the outside chunk is inactive the effects are suspended in most cases. Specifically,
 * Flowing water or lava can spread to the first adjacent block in an outside chunk, but the flow becomes suspended there until the outside chunk becomes active.
 * Fire can spread to the first adjacent flammable block outside the ticking area. Like water and lava, it becomes suspended there; although visible, its animation does not run and it cannot spread further until the outside chunk becomes active.
 * Grass and mycelium can spread to the first adjacent block in an outside chunk, but the affected block will not actually change its appearance until its chunk becomes active; it will then change instantly. Grass and mycelium cannot spread beyond the first such block, nor from such a block into the ticking area, until the outside chunk becomes active.
 * Pumpkin and melon stems growing on the edge of a ticking area can place their fruits on an adjacent block in an outside chunk.
 * An entity (mob, minecart, arrow, etc.) that attempts to move into an outside chunk becomes suspended as soon as it leaves the ticking area. It remains visible but motionless. When the outside block becomes active, the entity resumes moving.
 * Exploding TNT can damage or destroy blocks in an inactive chunk, and unlike other events its effects are not limited to adjacent blocks. However, secondary effects in the outside chunk are suspended until the chunk becomes active. For instance, if an explosion destroys a block that supported sand or gravel, the sand or gravel does not fall immediately. The same thing happens with items that were attached to destroyed blocks, such as item frames and redstone torches; they do not drop until the chunk is activated.
 * Primed TNT that has been launched into an inactive chunk is suspended in mid-air within the first outside block it enters. It disappears until the outside chunk becomes active, at which time it resumes its flight and countdown.

Spawning
Spawning in ticking area chunks follows the same rules as for chunks within a player's chunk update range. It observes mob caps, light requirements, and rules about where specific types of mobs may spawn and how much overhead space they require. However, if no actual player is within range of a ticking area chunk the rules about distance from a player do not apply: Mobs can spawn anywhere within the ticking area and do not instantly despawn if no player is within 128 blocks. This means that when you return to your ticking area after a journey, you may find a large number of hostile mobs waiting for you, especially in or near unlit areas and areas protected from daylight. To prevent this, special care should be taken to light your ticking areas and to cover surfaces with non-spawnable blocks such as slabs and carpets.

Creation and Management
A player creates and manages ticking areas by executing a command. Up to ten ticking areas, each containing up to 100 chunks, can exist at one time in a world. Note that commands can only be used in a world if cheats are enabled. This makes the world ineligible for earning achievements, so ticking areas and earning achievements are mutually exclusive.

To create a ticking area, specify its location and size in a  command. There are two forms: In either case the ticking area consists of all chunks that overlap the defined area. Optionally, you can also specify a name for the ticking area to identify it in subsequent commands.
 * In the first form, you specify two pairs of coordinates in the world. The coordinates specify the opposite corners—either northeast and southwest or northwest and southeast—of a rectangular area of the world.
 * In the second form, you specify a center coordinate and a radius of 1 to 4 chunks. These arguments define a circular area of the world.

Note that the Y coordinate is checked for validity but otherwise ignored. This is because a chunk encompasses an entire column of blocks from the bottom of the world to its top, so it includes every possible Y value.

To display a list of current ticking areas, execute a  command. This shows each ticking area's name (if provided) and corner coordinates (for the first form) or center coordinates and radius (for the second form). The coordinates shown in the list may not match those you entered because the game adjusts them to the actual corner or center coordinates of a chunk. By default, this command only lists ticking areas in your current dimension; you can use the optional  argument to see all ticking areas in the world.

To remove a ticking area from the list, execute a  command specifying either the ticking area's name or the coordinates of a point in the world. If you specify a name, only that ticking area is deleted. If you specify a point, all ticking areas in the current dimension that contain that point are deleted. An optional  argument removes all ticking areas in all dimensions.