Dynamic Tasks Example

This example demonstrates how to generate tasks dynamically from a data list using Nix's map and builtins.listToAttrs. It models a monorepo with multiple microservices where each service gets its own build and deploy tasks generated automatically.

Task Graph

deploy-all
├── deploy-api-gateway
│   └── build-api-gateway
│       └── generate-protos
├── deploy-order-service
│   └── build-order-service
│       └── generate-protos
└── deploy-user-service
    └── build-user-service
        └── generate-protos

generate-protos is a shared dependency — nix-tasks only runs it once even though all three build tasks depend on it.

How It Works

A services list drives all task generation:

services = [
  { name = "api-gateway";  port = 8080; path = "cmd/api-gateway"; }
  { name = "user-service"; port = 8081; path = "cmd/user-service"; }
  { name = "order-service"; port = 8082; path = "cmd/order-service"; }
];

Adding or removing an entry from this list automatically updates every generated task and the aggregate build-all / deploy-all targets.

Per-service tasks (builtins.listToAttrs + map)

buildTasks = builtins.listToAttrs (map (svc: {
  name = "build-${svc.name}";
  value = lib.mkTask {
    description = "Build ${svc.name}";
    deps = [ "go" ];
    depends = [ "task:generate-protos" ];
    commands = [ "echo 'Building ${svc.name}...'" ];
  };
}) services);

Aggregate fan-in tasks

build-all = lib.mkTask {
  description = "Build all services";
  depends = map (svc: "task:build-${svc.name}") services;
  commands = [ "echo 'All services built.'" ];
};

Running

# List all generated tasks
nix-tasks list

# Build everything (runs generate-protos once, then all builds in parallel)
nix-tasks run build-all

# Deploy everything (builds first, then deploys in parallel)
nix-tasks run deploy-all

# Build/deploy a single service
nix-tasks run build-api-gateway
nix-tasks run deploy-user-service

Key Patterns

Pattern	Technique
Generate N tasks from a list	builtins.listToAttrs + map
Fan-in aggregate task	depends = map (svc: "task:build-${svc.name}") services
Shared dependency (run once)	Multiple tasks depends on the same task
Merge task sets	sharedTasks // buildTasks // deployTasks // aggregateTasks
Graceful empty list	if services == [] then [] else ...