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Pool aliases allow you to dynamically substitute template pools during structure generation, enabling powerful variations and randomization.

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Overview

Pool aliases replace references to one template pool with another at generation time. This is particularly useful for:
  • Creating structure variants (different wood types, stone styles, etc.)
  • Randomizing Trial Chamber layouts
  • Swapping mob spawner contents
  • Conditional pool selection based on biome or other factors

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RandomEntries

The lithostitched:internal/random_entries alias type randomly selects matching pools from holder sets.
This is an internal system primarily used for Trial Chambers. The documentation describes it as “hacky” and not recommended for general use.

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Configuration

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type
string
required
Must be "lithostitched:internal/random_entries"
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aliases
ResourceKey<StructureTemplatePool>[]
required
List of pool resource keys that will be aliased (replaced).
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pools
HolderSet<StructureTemplatePool>[]
required
List of holder sets containing pools to substitute. Must be the same length as aliases.Each holder set must contain the same number of pools.

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Example: Trial Chamber Spawners

From Lithostitched’s Trial Chambers configuration: 
{
  "type": "lithostitched:set_pool_aliases",
  "structures": "minecraft:trial_chambers",
  "pool_aliases": [
    {
      "type": "lithostitched:internal/random_entries",
      "aliases": [
        "minecraft:trial_chambers/spawner/contents/ranged",
        "minecraft:trial_chambers/spawner/contents/slow_ranged"
      ],
      "pools": [
        "#lithostitched:trial_spawner/ranged",
        "#lithostitched:trial_spawner/slow_ranged"
      ]
    },
    {
      "type": "lithostitched:internal/random_entries",
      "aliases": [
        "minecraft:trial_chambers/spawner/contents/melee"
      ],
      "pools": [
        "#lithostitched:trial_spawner/melee"
      ]
    },
    {
      "type": "lithostitched:internal/random_entries",
      "aliases": [
        "minecraft:trial_chambers/spawner/contents/small_melee"
      ],
      "pools": [
        "#lithostitched:trial_spawner/small_melee"
      ]
    }
  ],
  "append": false
}
Source: /home/daytona/workspace/source/src/common/main/resources/data/lithostitched/lithostitched/worldgen_modifier/set_trial_chambers_pool_aliases.json:1

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How RandomEntries Works

  1. Validation: Ensures all holder sets have the same number of pools
  2. Random Selection: Picks a random index based on world seed
  3. Synchronized Replacement: All aliases use the same random index, maintaining consistency
public void forEachResolved(RandomSource random, 
    BiConsumer<ResourceKey<StructureTemplatePool>, 
    ResourceKey<StructureTemplatePool>> consumer) {
    int index = random.nextInt(pools.getFirst().size());
    for (int i = 0; i < pools.size(); i++) {
        consumer.accept(aliases.get(i), pools.get(i).get(index).unwrapKey().get());
    }
}
Source: /home/daytona/workspace/source/src/common/main/java/dev/worldgen/lithostitched/worldgen/poolalias/RandomEntries.java:46

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Pool Alias Tags

Define holder sets using tags for pool organization: File: data/lithostitched/tags/worldgen/template_pool/trial_spawner/melee.json 
{
  "values": [
    "lithostitched:trial_spawner/melee/variant_1",
    "lithostitched:trial_spawner/melee/variant_2",
    "lithostitched:trial_spawner/melee/variant_3"
  ]
}
This tag groups related pools that can be randomly selected.

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Setting Pool Aliases

Use the lithostitched:set_pool_aliases worldgen modifier to apply aliases:
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type
string
required
Must be "lithostitched:set_pool_aliases"
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structures
ResourceKey<Structure> | HolderSet<Structure>
required
Structure(s) to apply aliases to. Can be a single structure key or a tag.
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pool_aliases
PoolAliasBinding[]
required
List of alias bindings to apply.
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append
boolean
default:"false"
If true, appends to existing aliases. If false, replaces all aliases for this structure.

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Example: Biome-Specific Variants

Create different village styles by biome: 
{
  "type": "lithostitched:set_pool_aliases",
  "structures": "minecraft:village",
  "pool_aliases": [
    {
      "type": "lithostitched:internal/random_entries",
      "aliases": [
        "minecraft:village/common/houses"
      ],
      "pools": [
        "#mymod:village_variants/houses"
      ]
    }
  ],
  "append": false
}

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Validation Rules

The RandomEntries codec validates:
  1. Equal List Lengths: aliases.size() == pools.size()
  2. Consistent Pool Sizes: All holder sets must have the same number of pools
private static DataResult<RandomEntries> validate(RandomEntries entry) {
    if (entry.pools.size() == entry.aliases.size()) {
        Integer size = null;
        
        for (HolderSet<StructureTemplatePool> pool : entry.pools) {
            if (size != null) {
                if (pool.size() != size) 
                    return DataResult.error(() -> 
                        "Each template pool set should have the same number of entries");
            }
            size = pool.size();
        }
        
        return DataResult.success(entry);
    }
    return DataResult.error(() -> 
        "List of aliases and list of pools should be the same length");
}
Source: /home/daytona/workspace/source/src/common/main/java/dev/worldgen/lithostitched/worldgen/poolalias/RandomEntries.java:28

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Common Patterns

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Synchronized Variants

Replace multiple related pools while keeping them synchronized: 
{
  "type": "lithostitched:internal/random_entries",
  "aliases": [
    "mymod:dungeon/floor_basic",
    "mymod:dungeon/floor_special",
    "mymod:dungeon/walls"
  ],
  "pools": [
    "#mymod:dungeon_style/basic_floors",
    "#mymod:dungeon_style/special_floors",
    "#mymod:dungeon_style/walls"
  ]
}
All three pools use the same random index, ensuring consistent theming.

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Single Pool Randomization

Randomize one pool independently: 
{
  "type": "lithostitched:internal/random_entries",
  "aliases": [
    "mymod:spawner/contents"
  ],
  "pools": [
    "#mymod:mob_sets/all"
  ]
}

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Limitations

Pool aliases have some important limitations:
  • Generation Time Only: Aliases are resolved during structure generation, not at datapack load time
  • World Seed Based: Same world seed produces same alias resolutions
  • No Nesting: Alias resolution doesn’t support recursive pool substitution
  • Internal System: The RandomEntries implementation is marked as “hacky” and may change

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Best Practices

Ensure all holder sets in a RandomEntries binding have equal sizes:
// Good: Both tags have 3 pools
"pools": [
  "#mymod:set_a",  // Contains 3 pools
  "#mymod:set_b"   // Contains 3 pools
]

// Bad: Mismatched sizes will cause validation error
"pools": [
  "#mymod:set_a",  // Contains 3 pools
  "#mymod:set_b"   // Contains 5 pools - ERROR!
]
Organize variant pools with tags for easier management:
data/mymod/tags/worldgen/template_pool/
  stone_variants/
    cobblestone.json
    stone_bricks.json
    andesite.json
Pool aliases create implicit dependencies. Document which pools are aliased:
// This aliases minecraft:trial_chambers/spawner/contents/melee
// Ensure custom mob pools are compatible with Trial Chamber spawner format

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Java Interface

public record RandomEntries(
    List<ResourceKey<StructureTemplatePool>> aliases,
    List<HolderSet<StructureTemplatePool>> pools
) implements PoolAliasBinding {
    
    @Override
    public void forEachResolved(RandomSource random, 
        BiConsumer<ResourceKey<StructureTemplatePool>, 
            ResourceKey<StructureTemplatePool>> consumer);
    
    @Override
    public Stream<ResourceKey<StructureTemplatePool>> allTargets();
}
Source: /home/daytona/workspace/source/src/common/main/java/dev/worldgen/lithostitched/worldgen/poolalias/RandomEntries.java:21

Template Pools

Configure structure template pools

Processors

Transform blocks during placement

Structure Types

Structure generation systems

Worldgen Modifiers

Modify world generation

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